HomeMy WebLinkAboutLUA-06-083_MiscD A T E :
TO:
FROM:
SlJB.JECT:
PLANNING/BUILDING/ M\\; ·
PUBLIC WORKS DEPARTMENT
M E M O R A N D U M
i\ug u st 25, 2 006
Jill Ding, De ve lopment Serv ice s Senio r Pl a nn e r
T o m M alphrus . Water lJtility Eng ineer (ex t. 7313 ) '/ /11
Emergency Power Generation Facilities, Shoreline Review
Encl osed for shoreline exemption perm it re qui rements , p lease find o ne o ri g ina l
Emerge nc y Pc)\,ver Ci e ncrati o n faciliti e s 2006 -S hore line Revie w .
Pl ease co ntac t me, if you n eed additi o nal information to compl et e a shore line exemption
permit for th e Emergency PO\ver Generatio n F a cilities 2006 proj e ct. Tha nk yo u fo r yo ur
as si st an ce \Vith thi s project.
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cc : Abdou! Ga fou r. Wa ter Lt ili ty Superv iso r
1-1 :\l·ile S ys\.WT R -Drinking Wa ter Ut il ity\WTR-27 -Water Project Fiks\WTR-27 -3239 -Lmcrge ncy l'cm cr Grncrat ion Fac il it ies
2006\SE P/1-La nd Usc\Sh o rcl inc l >wg X 111 it tal .doc\TVl lp
-ENGINEERING GEOl-JOGY REPORT-
CITY OF RENTON
NORTH TALBOT AND MT. OLIVET
EMERGENCY GENERATOR FACILITIES
Prepared by RH2 Engineering
for City of Renton
~
November 2005
This r eport is based on si t e 1nvest19atJons during 200.5 .
Tins report provides a n alvsis, inter ore t a tion and
e\'a lu ation of site g eo logvl h VL11 ogeology and eng,neenng
geoiogy specific to th e design and cons truc tabil1t y of tw o
ernergency general 01 fao/1ties .
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RH2 Prqjed: IlliJ\: 105.0 7 9.01. 70 2
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DEV ELOPMENT PLANNING
CITY OF RENTON
JUN 3 0 2006
RECEIVED
City of Renton
North Talbot and 11ft. Olivet Emergency Generator Facilities
REPORT ON ENGINEERING GEOLOGY
November 2005
Report on 1'tfa_v and October, 2005 Site Investigations
RH2 Engineering (RI-L2) has prepared this Report for the cxclusivt' use of the Cir, of
Renton specifically to :-;upport the design of rv,:o etnergency generator facilities located at t\VO
Ciry-mvned properties in Renton, Y\-'ashington. This Report has been divided into t\VO
separate sections, one for each of the proposed sites. L'se of this report by others, or for
another project, is at the user's sole risk. \\lith.in the limitations of the Scope of \''(;'ork,
Schedule and Budget, RH2 has completed geologic explorations to gain information
necessary for de,-eloptng specific recommendations for the design of the facilities. The
geologic services ha,.-e been conducted in accordance with the locally accepted practices of a
Licensed professional engineering geologist and per the elements of RCW 18.220 and \'C-\C
308.15 that are included in the Scope of \\/ork. The conclusions and recommendations
contained m this report are based upon surfact' anJ subsurface geologic exploration of the
earth :~oil ;md sediments) and groundwater conditions at the site and previous studies and
maps of the region.
Based on the explorations cotnpleted under the Scope of \Vork, RH2 predicts that the types
of earth encountered during excavation and construction of the facility \"\ill be glacial drift
(predominantly till at North Talbot and out\vash at lv!t. Oli-..-ct) and fill. The lateral continuity
of drift and fill and their compositions may be highly variable. Zones \\rith more fine or
coarse sediment, or more dense or open textures, may occur laterally across the site or at
depth. Site inspection by RH2 during excavation is strongly recommended to determine the
significance of ,·ariations in the geology and hydrogeology.
RH2 Engineering shoulJ be notified \vhen excavation begins as well as for inspection of the
subgrades prior to the initial placen1enr of the foundations to ensure thar the earth and water
conditions arc consistent "\vith those prcdicte<l in this report and to determine if the exposed
nanve earth meers the design requirements. If unsuitable earth 1s exposed.
recommendations for correcting the problems must start with a field investigation. If
conditions change dut' to new construction at or adjact'nt to the project, RI I2 should inspect
those changes prior ro construction. \Ve look for,:vard to assisting and supporting rhe City to
ensure successful construction of the facilit\".
Sincerely~
SIGNED l l/!6/05
Geoffrey Clayton,
Licensed Geologist. Fngineering Geologist and Hydrogeologist
RH2 ENGINEERING
1 H,/:::,.111(, 2·2";":+l P.\!
I
11/16/05
EXPIRES 7/7/07 EXPIRES 2/25/07
11/16105
EXPIRES 8/2/06
This report is a final and complete response to all elements, which are contained in the Scope
of Work and Contnct agreement between Citv of Renton and RH2 Engineering.
2 l(,/21111(, :::27·-+l P\I
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ENGINEERING GEOLOGY REPORT FOR THE NORTH TALBOT EMERGENCY
GENERATOR BUILDINGS
NORTH TALBOT SITE
LOCATION & SITE DESCRIPTION
The North Talbot site is located at Talbot Park in Renton, Washington. Talbot Park is
located on the southeast corner of Talbot Road South and South 19'" Streer and it comprises
three parcels (see Figure 1). The parcel numbers are 7222000130, 7222000061 and
7222000121. The northernmost parcel is approximately 0.9 acres and contains a parking lot,
small brick building and a grassed area. The two southern parcels are each approximatelv 0.9
acres and contain a fully buried 6-milli6n gallon reserrnir overlain bv tennis courts. The
proposed EG facility will be located on the northernmost parcel in the grassy area just south
of the existing brick building. The grassy area is relatively flat with one small mounded area
near the center of the parcel.
REVIEW OF EXISTING INFORMATION
The geotechnical engineering report prepared by CH2'vl Hill for the buried Talbot Hill
Reservoir was obtained from the City of Renton and was reviewed. Regional geologic maps,
aerial photography and LiDAR data were also reviewed.
REGIONAL GEOLOGY
Regional maps foi; the Renton area were reviewed for stratigraphic, tectonic, structural and
geomorpb.ic information relevant to the project. The effects of the advance and retreat of
the Vashon ice sheet during the Frazer glaciation, about 18,000 to 13,000 vears before
present (13P), dominate the surficial geology. The stratigraphic section contains Vashon
lodgement till (Qgt) overlying older glacial drift (likelv including Qga, compact Vashon
advance outwash sands) and interglacial sediments. The till in th.is region is very dense, and
typically, so is the underlying glacial drift. The till and underlving sediments were compacted
by the weight of more than 3,000 feet of ice when the Vashon ice sheet over-rode the area.
The bedrock that underlies the glacial sediments is the Renton Formation. The Renton
l'ormation is an Eocene (55 to 34 million vears ago) sedimentarv unit consisting of
mudstonc, siltstone and sandstone. The Renton Formation lies far below the proposed
excavation for the EG building and will not be encountered during construction.
The EG building will likely be supported bv till, a strong sedin1ent that is highly favorable
for supporting strucnues and can be reaclilv excavated with heavv equipment. Till
commonlv has the strength of controlled-densin,-fi]] (CDF) but is rippable and, therefore,
less costlv to excavate than bedrock.
Till resists deep-seated mass wasting and the l1ow of groundwater, so the site will be stable
and excavations will require minimal dewatering.
Tectonicallv, the Talbot site lies between an oblique convergent plate boundan that begins
about 7 5 miles off the Pacific coastline of Washington and the nsmg and volcanically active
Cascade '.\fountain Range. The slab of oceanic rocks sliding beneath western Washington is
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Proposed Generator
Building Location
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Parcel No :
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Parcel No :
7222000061
Parcel No :
722200012 1
521st St
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0 100 Feet Figure 1 North Talbot Site Plan
Rev,s,on Date: 10/1 112005 By A FM
J \data \REN \ 105-0 79\GEO\G IS\Figure 1 -No Talb ot S ite Plan.mxd
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North Talbot and Mt. Olivet
Emergency Generator Facilities
November 2005
Engineering Geology Report
called the Juan de Fuca plate. Tius plate slides beneath western \Vashington in a subduction
zone that dips from the ocean bottom off the coast to a depth of about 6S miles beneath the
crest of the Cascades. ,\t a depth of about 60 miles, it becomes so hot that molten rock is
formed, which may nugrate to the surface and form volcanoes. The Juan de Fuca plate is
sliding beneath Renton at a depth of about 40 nules. f n the continental crust above the
subduction zone, the oblique collision with the Juan de Fuca Plate causes the bedrock below
the Renton to be compressed and pushed northward. This tectonic setting results in
sigiuficant seisnuc activity and, if the design life for the EG facility is 100 vears, the
probabilitv is high that ir will experience a deep subduction earthquake, an intermediate
crustal earthquake (like the Nisqually earthquake) and/ or a shallow earthquake that breaks
the ground surface (e.g. along the recently discovered Seattle fault). ,\pproximate 50-year
probabilities for Puget Sound earthquakes are:
•Cascadia M9:
•Seattle f<ault M 26.5:
•Deep~[ 26.5:
•Random shallow 1\[ 26.5
10-14%
5% (from slip rate, GR model; 1000 vear return time)
84% (from 1949, 1965, 2001)
15% for entire Puget Sound area including, Tacoma,
South Whidbe1· or other fault zones
The risk of liquefaction depends upon the composition, texture (compaction/ density),
stn1cnue (stratigraphic thickness and orientation) and moisnire content of the earth in
question. Regionallv, tl1e liquefaction potential for Qvt, Vashon till, is \'ery low because it is
very dense and contains gravel and cobbles embedded in a compact matrix of silt and sand.
Even when saturated, the permeability is so low that \vatcr cannot migrate to cause
liquefaction.
The geomorphology of the property is donunated by glacial shaping of the landscape,
primarilv deposition of rill which was molded into small ridges aligned parallel to the north
to south during the advance of the Vashon ice sheet. The sue lies close ro the crest of a
small ridge that is elongated nortl1-south. The slopes to the east and west of the ridge crest
are generally gentle except where locally steepened by postglacial erosion. 'lbe risk of slope
failure on this glaciallv carved hill is very low in the project area because the slopes are gentle
and tl1e till is strong and impermeable, and thus, they arc not susceptible to weakening by the
build-up of groundwater pore pressure.
SITE GEOLOGY
The property was studied for geomorphic evidence of mass wasting bv analvsis of LiDAR
data and by performing a reconnaissance of the site. The analysis of LiD,.\R and
reconnaissance of the property revealed no evidence of past mass wasting or slope instability
problems at the site. The risk of mass wasting adverselv impacting the site is negligible.
Boulders typically occur in till and the proposed excaYation may require rernoval of several
large and verv hard boulders.
The CH2M Hill geotechnical engineering report for the existing Talbot Hill Reservoir was
reviewed for information regarding the sire geology and geotcchnical parameters. Five
borings and two test pits logs \Vere inclt1ded in the report. Sieve analyses, n1oisture content,
in-situ densitv, and standard penetration tests were also performed as part of the CH2~l
Hill's work. The test pit and boring logs indicate that the reservoir site was generally
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North Talbot and Mt. Olivet
Emergency Generator Facilities
November 2005
Engineering Geology Report
underlain by 2 to 6 feet of dark brown gravelly, sandy loam underlain by verv dense blue-
grey Vashon till. The report states that groundwater was not encountered in anv of the
explorations.
It appears the site has been previously graded for the construction of the facilities that
currentlv exist on site. Based on contours noted in the CH2M Hill report and contours in
the existing survey, it appears material exported during the excavation for the existing
reservoir mav have placed oYer the proposed site. The thickness of the fill is unknown but is
likelv between 2 and 4 feet.
ENGINEERING GEOLOGY
Based on the review of the existing CH2M Hill geotechnical report and well-established
relationships between the strength and character of glacial till, RH2 concludes that the earth
encountered during excavations will have sufficient strength to support the proposed EG
building.
RH2 predicts that three different types of earth or gcotechnical zones will be found within
tl1e site. These zones can be readilv distinguished based on compositional and textural
differences. Listed from shallowest to deepest, these zones are: l) fill, 2) soil horizons
developed by weathering oi glacial till, and 3) glacial till.
1) If fill is encountered it will likely be medium-loose and consist of material excavated
from the reservoir site. lt will likely be a mL'l:ture of the grey grawllv, sandy, silty
glacial till and the brown loamy material described in the CH2M Hill boring and test
pit logs. It is unknown whetl1er all tl1e material excavated for the construction of the
reservoir was exported or whether son1e of it was spread over the site. It is also
unknown whether organic tnate_rial was stockpiled separately or if it \Vas rnixcd in
with the fill. Because of uncertainties about the composition and strength of the fill,
it should be completelv stripped and removed from areas that will support structures.
2) Below the fill, excavations will likelv encounter soil created bv the weathering of
glacial till. Soil developed by the weathering of "a parent" composed of glacial till
typically consists of four zones or horizons (0, ,\, Band C) that are distinguishable
based on color, composition and texture. The soil grades from the surficial CJ-
horizon, which is loose and mainlv organic material, to the organic-rich and strongh·
bioturbated mineral soil of the "\-horizon. The soil of these two upper horizons
cannot be used as structural fill but mav be stockpiled for use as topsoil 1f screened
to remove large rocks, roots and stumps. The B-horizon is predominantly a mineral
soil penetrated bv roots and enriched in iron oxide ( orange colored) and clav. The B-
horizon may be used as topsoil but should first be amended with compost on a 1: 1
volume basis. The C-horizon is a mineral soil composed of disaggregated till with
iron oxide staining resulting frotn the percolation of oxygenated ground\vater
through it. The soil of tl1e C-horizon grades downward into unweathered grav till.
Commonly, the earth of the C-horizon can support relativclv light loads like spread
footings for single-familv homes. However, it is nor suitable for supporting tl1e loads
and vibrations of an emergency generator facilitv unless rhe floor slab is strengtl1ened
to minimize the risk of differential settlement.
3) The graY, un-weathered, non-sorted, non-stratified glacial till will likely be hard and
capable of supporting 5,000 pounds per square foot with negligible long-term or
North Talbot and :Vlt. Olivet
Emergencv Generator Facilities
!\' ovember 2005
Engineering Geology Report
differential settlement. Till contains cobbles and boulders that mav be difficult to
excavate and require tiger teeth and or a hoe-ram to remove. Cobbles and boulders
that occur in rhe floor or \Valis of the excavation tnay require over-excavation to
remove.
CONCLUSIONS AND RECOMMENDATIONS
, \ subsurface investigation of the site was not performed. . \ll conclusions and
recommendations in this Report are based on our extensi,·e knowledge of the geology of the
area and the geotechnical engioeering report prepared for the existing reservoir located near
the proposed EG building location. It is recommended that RH2 be involved through the
construction phase of this project in order to verify that the recommendations in this report
are followed and that conditions encountered reflect those described in the report.
Geologic Hazards
• The risks and hazards of landslides, mass wasting and volcanism are too small to
rnertt consideration for mitigation. l',;o flood-way, floodplain or channel migration
hazards exist within the project area.
• l\:o changes in types or rates of surficial geologic processes arc expected due to the
proposed de,·dopment modifications. Ir is not anticipated that the project will
pemrnnentlv change surface water flows or quality.
• No permanent changes to surface (storm) water management will be necessarv.
• .-\11 foundations should be buried a minimum of 18 inches to protect against frost
hea\·ing.
• The risk of earthquakes is high at this site, but typical of the Puget Sound region.
111c site class definition for this site is Site Class C, per the 2003 International
Building Code (1615.1.1). Designing the EG buildings per today's stringent seismic
standards should be adequate for seistnic risk mitigation.
• Risks and hazards of liquefaction are negligible due to the density of the native
glacial till and absence of groundwater.
Water & Wet Weather Earthwork
• Significant temporary erosion and sediment control (TFSC) will be needed if the
work is performed during wet weatl1er. Wet weather work should be avoided to
ensure that there will be no nmoff of sediment-laden water from the site and to
protect the subgradc from becoming wet and unsuitable for t,rnndation placement.
• If construction or.curs during \Vet weather, the area of earth exposed to precipitation
should be minimized to prevent the added cost and time required to overexcarnte
and replace till, soil or fill that has become too loose or wcr.
• Wet fill and tl1e soils developed on the till arc unsuitable for support1ng strucrures
and construction activities.
• Glacial till is vcrv moisture sensiriYc and \vhen \Vet it mav becon1e unsuitable for . .
supporting foundations and concrete slabs. \\7hen exposed to precipitation, freezing
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:\/orth Talbot and l\lt. Olivet November 2005
Emergency Generator Facilities Engineering Geology Report
or disturbance b,· construction activities, till will become loose, slippery and nearly
impossible to re-compact.
• Excavations into the in-situ till should not encounter any significant ground\vater.
There is a slight chance that small zones of perched water will be encountered just
above the dense unweathered glacial till. Groundwater seepage from these zones, if
encountered, should diminish after a few hours. Precipitation will increase seepage.
• It is reconnnended that footing drains be installed around the perimeter of the
foundation. They should be installed in such a manner that water will flow by
gravity and discharge where the \Vater will not cause erosion, mass \vasting or
degrade water qualirY.
Excavation
• There will not be any permanent cut or fill slopes resulting from this project. "Jo
retaining walls are needed.
• Temporary cut slopes should comply with WA.C 296-155 Part N for benching,
slopes and shoring. The contractor should have a Competent Person on site at all
times to classifr the earth encountered and monitor the temporary excavations as
described in the W .\ C.
• Temporary cuts in till will hold slopes of 1 H: 1 V or steeper if all earth loosened or
disturbed by excavation is scraped off. However, slopes steeper than 2H: IV will be
sub1ect to raveling and exfoliation, especially during rainfall and freeze thaw cycles.
All cobbles and boulders should be removed from the cut faces so thev cannot roll
do'\\.-n cut surfaces and cause injury or damage.
• Tcmporarv vertical cuts in till mav also be stable. The stability of vertical cuts must
be inspected regularly for stability by a competent person. Because till transmits
water verY slowly, and the sand and gra\·el are bound tn a silt-clay matrix, it is far less
susceptible to caving, sloughing or running than sediments that are entirely sand or
silt.
• Till can be compacted to form an excellent structural fill if the excavated till can be
kept close to its optimum moisture content and all rocks larger than 4 inches in
diameter are removed. However, structural fill composed of till should not be sloped
steeper than 2H: 1 \'.
• The excavation for the proposed stn1Cture n1av require die removal of large, hard
boulders.
• Where fill and soi.I are excavated, thev should be exported or stockpiled for use as
non-structural fill.
• Wet earth must be dried for use on site or it should be exported.
Subgrade Preparation & Bearing Capacity
• llased on the review of existing information, the net allowable bearing capacity for
the proposed EG building should not exceed 5,000 psf. Th.is is assuming all
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North Talbot and Mt. Olivet
J•:mergency Generator Facilities
November 2005
Engineering Geology Report
foundations are placed on dense unweathered native till and that all topsoil, fill, soil
horizons, and organic materials arc stripped and removed from the site. This
allowable bearing capacity also assumes that footings are buried at least 18 111cbes
and the minimum footing width is not less than 18 inches.
• The allowable bearing capacitv may be increased by 1/3 for wind or seismic (shorr-
term duration) loads to a net allowable bearing capacity of 6,500 psf.
• It is very important to have a licensed Engineering Geologist inspect the subgrade
prior to anv compaction efforts or placement of fill to confirm that the materials
encountered during the excavation are consistent with those described in this Report.
• During the dry season, pouring footings and slabs directly on in-situ earth is possible.
Crushed rock should be placed on the native subgrade during the wet season to help
ensure the stability of the subgrade.
• If the floor slab is to be poured directlv on in-situ earth, then all boulders and
cobbles larger than 4 inches in diameter should be removed from the base of the
excavation. Voids created bv the removal of cobbles and boulders should be
backfilled with Crnshed Surfacing Base Course in six-inch lifts to a maximum depth
of 12 inches. Vmds deeper than 12 inches that are created by removal of boulders
should be backfilled with controlled density fill (CD!"}
• To ensure that the subgrade supporting footings and slabs ts firm and unyielding,
RH2 recommends placement of a leveling course of Crushed Surfacing Base Course
(')vSDOT 9-03.9(3)) tl1at is a minimum of 6 inches thick and a maximum of ! -foot
thick below all building foundations. The leveling course should be compacted to 95
percent of its maximum drv dcnsitv, as determined by i\STM Dl 557 (modified
proctor). Lifts should not exceed 4 inches loose thickness. Cobbles and boulders
embedded in the subgrade should not protrude more than 2 inches into this leveling
course.
• Ji structural fill is required below the leveling course to achieve a certain elevation,
the net allowable bearing capacitv should be reduced to 3,000 psf. The thickness of
the structural fill should not be more than 3 feet. lf more than 3 feet of fill is
required below the leveling course, CDF or crushed rock should be used. All fill
placed should be compacted to meet 95 percent of the maximum dn density as
determined by .-\ST!\! 01557 (modified proctor). Lifts should not exceed 6 inches in
loose thickness.
• .-\n excavator with tiger teeth will likely be required to excavate through the dense
glacial till. Howe,·er, it is difficult to remove all material disturbed during excavation
with a bucket with tiger teeth. The contractor should have a bucket without teeth on
site or have some other method to ren1ove all 1naterial disturbed during excavation at
the level of the in-situ subgrade. There should be no more than 1 to 2 inches of
loose, disturbed material on the subgrade when it is inspected by the Engineering
Geologist.
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North Talbot and Mt. Olivet
Emergency Generator Facilities
Backfill
November 2005
Engineering Geology Report
• Rl-!2 anticipates chat unweathered glacial till will be suitable for use as structural fill.
1-JowcYer, if this mate1-ial is to be used as structural fill, it must be kept within 2
percent of optimum 1noisture content so compaction requirements can be met. All
cobbles greater than 4 inches in diameter and boulders should be rcmm·ed if till is
used as structural fill.
!'.lg<: 111 nf 21
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ENGINEERING GEOLOGY REPORT FOR THE MT. OLIVET EMERGENCY
GENERATOR BUILDING
LOCATION & SITE DESCRIPTION
The Mt. Olivet site is located on the northeast corner of NE Third Street and Bronson Wav
N.E. in Renton, Washington (See Figure 2). The parcel number is 1723059130. The parcel
is approximatelv 3.8 acres and the Cit\· has already built a water reservoir and booster pump
station on the site. The proposed emergencv generator (EG) facility will be located east of
the existing reservoir in a grassy area at rhe base of the steep slope. The building will have a
5-foot wide sidewalk around most of its perimeter, except in the southeast corner. 'l'he
grassy area and area around the reservoir are relativelv flat. The eastern half of the propertv
slopes up steeply to residential housing. The northeast corner of the proposed building will
cut into this steep slope and a retaining wall will be required to prevent mass wasting and
slope instability .
. \ second reservoir mav be constructed on the site in the iuture. Based on information
provided bv the City, the second reservoir would likelv be located southeast of the existing
reservoir. The southern portion of the property slopes down steeplv to N .E. 3'J Street and
this will constrain the size and location of the second reservoir. See Figure 3 for
approximate locations of iuture reservoir and proposed EG building.
REVIEW OF EXISTING INFORMATION
No existing geotechnical engineering reports for the site could be found. Regional geologic
maps, aerial photography and LiD.-\R data were reviewed.
REGIONAL GEOLOGY
Regional maps for the Renton area were reviewed for stratigraphic, tectonic, structural and
geomorphic information relevant to the project. Like the 1'iorth Talbot site, the effects of
the advance and retreat of the Vashon ice sheet from 18,000 to 13,000 years before present
(BP) dominate the surficial geology. However, the stratigraphic section for this site differs
from the North Talbot site.
The '.lt. Ohnt site is mapped as Vashon outwash (Qgo). Figure 4 is a geologic map of the
Mt. Oliwt site and the surrounding region. There are two types of glacial outwash: 1)
advance outwash; and 2) recessional outwash. ,-\dvance outwash is deposited by meltwatcr
from an advancing glacier or iee sheet. It is commonly underlain by glacial lacustrine or
transitional beds composed predominantly of silt. Vashon advance oul\vash in the Renton
area has been overridden by the Vashon ice sheet; therefore, it has usuallv been compacted
to a Yetv dense srate due to the weight' of the more than 3,000 feet of ice that covered
Renton during the glacial maximum. Recessional oul\vash is deposited from the meltwater of
a retreating glacier. T t has not been overridden bv the weight of the glacier or ice sheet;
therefore, it is not as dense as advance outwash. Typically recessional out\vash is underlain
bv dense glacial till and/ or advance oul\vash and older glacial sediments and nonglacial
0 100 Feet Figure 2 -Mt. Olivet Site Plan
Revision Date: 10/26/200 5 By AFM
J:ldata\RE N\105-079\GEO\GIS\Fig ure 2 -Mt Olivet Site Plan .mxd
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SYSTEM TO BE REUSED.
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I
/
EXISTING PAAl(]NG ARfA
POSSIBLE FUTU RE RESERVO IR
-,
\
'
FIGURE 3 -MT. OLIVET DETAILED SITE PLAN Rf'VlS!ON OATE :I(]{ 15. 200S
J.'°"iA\RfHUOS-079\0,.')\G(:ij.P-(,£0,rlGJ -----·-· ·---1": )0'
N
A Qa Qgo
Mt. Olivet Site
Qa
,
Qg
I
0 1,500 Feet Figure 4
Qgt
Qgpc
)
Legend
~ a pprox . flow direction of glacial meltwater
~ and recess io nal outwash sediments '-Stream
Waterbody
DNR 1 :100,000 Digital Geology
( -. 1, Qa -alluvium
~ Ols -mass-wasting deposits, mostly landslides
'::.) Ogo -continental glacial outwash, Fraser-age; mostly Vashon Stade
c:3 Ogt -contine ntal glacial lill, Fraser-age; mostly Vashon Stade
• ~. 1 Ogpc -contin e ntal glacial drift, pre-Fraser, and no nglacial deposits
OEm -marine sedimentary rocks
Ec(2r) -c ontine ntal sedime ntary deposits or rocks; Renton Formation, Puget Group
Mt. Olivet Geologic Mae isio nDate: 10/26/2005 ByAFM
J:\data\REN\105-079\GEOIGIS\Figure 4 -Mt O livet Geologic Map.mxd
North Talbot and Mt. Olivet
Emergency Generator Facilities
November 2005
Engineering Geology Report
deposits at greater depths. Thus, glacial outwash in Renton can vary between loose to very
dense, depending on when and how it was deposited.
The EG building will be supported by recessional outwash, which was likely deposited as a
delta by a very large river of meltwater flowing in front of the Vashon ice sheet as it retreated
north of Renton between 14,000 and 13,000 years BP. This meltwater river was many times
larger than the present Cedar River, even at flood stage, and it likely carved the Cedar River
valley catastrophically within a few years or decades. It is very likely that excavations for the
proposed EG building will be entirely in the recessional outwash deposits. The recessional
outwash observed at the site is medium-loose to medium-dense and is favorable for
supporting the proposed EG building. The outwash can be easily excavated with an
excavator.
Tectonically, the Mt. Olivet site lies between an oblique convergent plate boundary that
begins about 75 miles off the Pacific coastline of Washington and the rising and volcanically
active Cascade Mountain Range. The slab of oceanic rocks sliding beneath western
Washington is called the .Juan de Puca plate. This plate slides beneath western Washington in
a subduction zone that dips from the ocean bottom off the coast to a depth of about 65
miles beneath the crest of the Cascades. At a depth of about 60 miles it becomes so hot that
molten rock is formed which may migrate to the surface and form volcanoes. The .Juan de
Puca plate is sliding beneath Renton at a depth of about 40 miles. In the continental crust
above the subduction zone, the oblique collision with the Juan de Puca Plate causes the
bedrock below the Renton area to be compressed and pushed northward. This tectonic
setting results in significant seismic activity and, if the design life for the EG facility is 100
years, the probability is high that it will experience a deep subduction earthquake, an
intermediate crustal earthquake (like the Nisqually earthquake) and/ or a shallow earthquake
that breaks the ground surface (e.g. along the recently discovered Seattle fault). Approximate
50 year probabilities for Puget Sound earthquakes are:
•Cascadia M9:
•Seattle Fault M 2':6.5:
•Deep M 2':6.5:
•Random shallow M 2':6.5
10-14%
5% (from slip rate, GR model; 1000 yr return time)
84% (from 1949, 1965, 2001)
15% for entire Puget Sound area including, Tacoma,
South Whidbey or other fault zones
The risk of liquefaction depends upon the composition, texture (compaction/density),
structure (stratigraphic thickness and orientation) and moisture content of the earth in
question. Regionally, the liquefaction potential for Qvo, Vashon recessional outwash, can be
quite high because it is commonly loose and silty. The key to determining the liquefaction
potential is its moisture content. At the Mt. Olivet site, the outwash appears to be well-
drained; therefore, the risk of liquefaction is low.
The geomorphology of the region is dominated by glacial shaping of the landscape followed
by human disturbance of the land. As the glaciers receded, recessional outwash was
deposited by rivers of glacial meltwater flowing across a large plateau, which at that time, was
continuous from Maplewood Heights to Fairwood. Erosion of the meltwater into this
plateau created a shallow east-west valley that initially built a delta in the Mt. Olivet area that
is much higher than the floor of the present-day Cedar River valley. As massive discharges of
meltwater continued to cut the valley deeper, the delta of recessional outwash also became
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Emergency Generator Facilities
November 2005
Engineering Geology Report
larger and lower and grew westward and northward. The EG building will be supported by
the glacial outwash sediments that formed this delta.
During the last 13,000 years, the Cedar River has continued to cut and widen its valley and
deposit sediments carried downstream (Qa on Figure 4)). The slopes in the Cedar River
valley have been oversteepened due to this erosion, and this has resulted in slope failures
(Qls on Figure 4). However, RH2 does not interpret the steep slopes above the Mt. Olivet
site as being the result of post-glacial erosion.
SITE GEOLOGY
The property was studied for geomorphic evidence of mass wasting by analysis of LiDAR
data and by performing a reconnaissance and subsurface investigation of the site. The
analysis of LiDAR and reconnaissance of the property revealed no evidence of past mass
wasting or slope instability problems at the site. The risk of mass wasting adversely
impacting the site is negligible.
The origin of the steep slope along the south side of the property is likely from excavation
for the construction of N.E. 3'd Street. This slope has experienced several small slope failures
and is presently undergoing creep. The steep slope on the east side of the property is likely
due to excavation and grading that was part of quarrying operations or preparation of the
site for the existing reservoir. This steep slope does not appear to be subject to a deep mass
wasting, but it is experiencing creep. Precautions should be taken to ensure the construction
of the new EG building will not increase the risk of slope instability.
In addition to the site reconnaissance, RH2 performed a subsurface investigation of the site.
The subsurface investigation consisted of observing the excavation of three test pits to
determine the depth of fill, the thickness of soil horizons and the composition and origin of
the parent material on the site. See Appendix A for test pit logs.
Test pits TP1 and TP2 were located south of the existing reservoir adjacent to the southern
steep slope at the first site proposed for the EG facility. Test pit TP3 was located near the
footprint of the second site proposed for the EG building. Test pits TP1 and TP2 generally
exposed approximately 2.5 feet of fill underlain by very fine silty sand with minor gravel. In
TP1 there was a buried soil horizon. Test pit TP3 generally exposed approximately 0.5 to 1
feet of fill overlying a soil horizon that was approximately 1-foot thick. Below the soil
horizon was a medium to medium-dense, fine to medium sand with zones of silt and zones
of gravel. The sand in TP3 had bedding planes that dipped to the northwest. Groundwater
was not encountered in any of the excavations.
The site has been previously graded for the construction of the facilities that currently exist
on site and it appears that fill was spread over most of the site.
ENGINEERING GEOLOGY
RH2 concludes that the earth encountered during excavations will have sufficient strength to
support the proposed EG building.
RH2 anticipates that three to four geotechnical zones, listed from shallowest to deepest, will
be found within the site. These will be readily discriminated based on compositional and
textural differences. The four zones are: 1) fill; 2) weathered glacial outwash, 3) medium-
loose to medium-dense fine sand with minor gravel (glacial outwash in TPl and TP2), and 4)
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Emergency Generator Facilities
November 2005
Engineering Geology Report
medium-dense fine-to-medium sand with silt and gravel (glacial outwash in TP3).
1) Fill was encountered in all three test pits and ranged from approximately 1 to 2.5 feet
in thickness. The fill encountered varied between a brown to orange-brown, fine
sandy silt to silty fine to medium sand with gravel, roots and wood debris. Pieces of
broken asphalt were encountered in TPl. All fill encountered during construction
should be stripped and removed from the site. RH2 anticipates approximately 1 foot
to 1.5 feet of fill must be removed in the location of the proposed EG building.
2) Below the fill is soil created by weathering processes of the parent underlying sandy
outwash. The soil overlying glacial outwash typically consists of four zones or
horizons (0, A, B and C) that are distinguishable based on color, composition and
texture. The soil grades from the surficial 0-horizon, which is loose and mainly
organic material, to the organic-rich and strongly bioturbated mineral soil of the A-
horizon. The B-horizon is predominantly a mineral soil penetrated by roots and
enriched in iron oxide (orange colored) and clay. The C-horizon is a mineral soil
composed of disaggregated outwash with iron oxide staining resulting from the
percolation of oxygenated groundwater through it. The disaggregated and partially
oxidized C-horizon grades downward into well bedded and unweathered gray
outwash. The combined thickness of the soil horizons varied from approximately 1
to 3 feet in all of the test pits. The O through C soil horizons should be stripped and
.removed from the site. The O and A horizons may be reused as top-soil and non-
structural fill. RH2 anticipates that, taken together, these soil horizons will be 1 to 2
feet in thickness. This soil must be removed in the location of the proposed EG
building.
3) In TPl and TP2, below the soil horizons described above, is a light brown, silty fine
sand with minor gravel and cobbles (glacial outwash). The sand appeared to coarsen
slightly with depth and the test pits held very stable walls. The proposed location of
the EG building has been moved from near TP1 and TP2 to TP3. However, if the
location of the EG building is moved back to this area, the earth should be capable
of supporting 2,500 pounds per square foot. It will hold temporary cut slopes of
1.5H:1 V, but these slopes will be subject to raveling and erosion if left exposed
during rainfall and freeze thaw cycles. All cobbles, boulders and roots should be
removed.
4) The proposed EG building will be located close to TP3. Below the soil horizons,
(described in No. 2 above), TP3 exposed a medium to medium-dense, grey
unweathered, sorted and stratified, sandy glacial outwash. The excavation walls in
TP3 held temporary vertical cut faces through the duration of the exploration.
However, sloughing and raveling may occur rapidly at slopes of 1H:1V or steeper,
especially if left exposed to precipitation. The contractor should anticipate using
temporary shoring and/ or shielding to protect workers as necessary from slope
hazards. The outwash is well drained and not as moisture sensitive as glacial till. This
unit is easily excavated.
CONCLUSIONS AND RECOMMENDATIONS
Conclusions and recommendations in this Report are based on RH2's knowledge of the
geology of the area and our subsurface investigation of the site. It is recommended that RH2
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November 2005
Engineering Geology Report
be involved through the construction phase of this project in order to verify
recommendations in this Report are followed and that conditions encountered reflect those
described in this Report.
Geologic Hazards
• No changes in types and rates of surficial geologic processes are expected due to the
proposed development modifications. It is not anticipated that the project will
permanently change surface water flows or quality.
• The risks and hazards of volcanism are too small to merit consideration or
mitigation. No flood-way, floodplain or channel migration hazards exist within the
project area.
• Existing risks and hazards of landslides and mass wasting are very small. However,
recommendations in this Report should be followed to prevent the location and
construction of the proposed EG building from increasing these risks.
• All foundations should be buried a minimum of 18 inches to protect against frost
heaving.
• No permanent changes to surface (storm) water management will be necessary.
• The risk of earthquakes is high at this site, but typical of the Puget Sound region.
The site class definition for this site is Site Class D, per the 2003 International
Building Code (1615.1.1). Designing the EG buildings per today's stringent seismic
standards should be adequate for seismic risk mitigation.
• Risks and hazards of liquefaction are negligible due to the granular character of the
earth and the absence of groundwater.
Water & Wet Weather Earthwork
• Temporary erosion and sediment control (TESC) will be needed, especially if the
work is performed during wet weather. Wet weather work should be avoided to
ensure that there will be no runoff of sediment-laden water from the site and to
protect the subgrade from becoming wet and unsuitable for foundation placement.
• If construction occurs during wet weather, the area of earth exposed to precipitation
should be minimized to prevent the added cost and time required to over-excavate
and replace till, soil or fill that has become too loose or wet.
• Excavations into the in-situ outwash should not encounter any groundwater.
• A footing drain will not be required for the proposed building due to the
permeability of the outwash. Instead, a French drain or infiltration trench should be
constructed that is a minimum of 1-foot wide and 18-inches deep. This trench
should contain a perforated pipe buried in clean washed rock. This drain should be
constructed around the perimeter of the EG building so that it captures roof and
sidewalk runoff and distributes the infiltration of that water in a manner that mimics
the existing condition. The french drain can be eliminated from the east side of the
building if roof runoff is routed away from the east side.
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November 2005
Engineering Geology Report
• RH2 recommends sloping the roof of the proposed EG building towards the north
and west, away from the steep slope to minimize the amount of water routed to the
toe of the steep slope. Alternatively, the downspouts on the east side of the building
should be tightlined and discharged well away from steep slopes.
• Excavation into the hillside should be minimized both to reduce the risk of slope
failure and the height and cost of retaining walls. Therefore, the 5-foot wide
concrete sidewalk shown on preliminary plans along portions of the eastern walls of
the EG building should be removed.
Excavation
• Excavation for the EG building will require an approximately 10-to 12-foot vertical
cut into the steep slope on the east side of the property. This steep slope is already at
or near its angle of repose, and excavation could undermine the slope and induce
slope failures far above the excavation. A retaining wall will be required along the
east comer of the building. See the section titled "Retaining Wall Alternatives" below
for discussion of the retaining wall recommendations. No other permanent cut and
fill slopes will be required for this project.
• Based on the current layout of the proposed building (Figure 3), RH2 recommends
that approximately 3 feet of earth be removed to expose medium to medium-dense
unweathered glacial outwash.
• Wet earth should be dried for use on site or exported.
• Temporary cut slopes should comply with WAC 296-155 Part N for benching,
slopes and shoring. The contractor should have a Competent Person on site at all
times to classify the earth encountered and monitor the temporary excavations as
described in the WAC.
Subgrade Preparation & Bearing Capacity
• Based on the review of existing information, the net allowable bearing capacity for
the proposed EG building shall not exceed 3,000 psf. This is assuming all
foundations are placed on medium to medium-dense unweathered native grey
outwash and that all fill, soil horizons, and organic materials are stripped and
removed from the subgrade. This allowable bearing capacity also assumes that
footings are buried at least 18 inches and the minimum footing width is not less than
18 inches.
• The allowable bearing capacity may be increased by 1/3 for ,vind or seismic (short-
term duration) loads to a net allowable bearing capacity of 4,000 psf.
• It is very important to have a licensed Engineering Geologist inspect the subgrade
prior to any compaction efforts or placement of fill to confirm that the materials
encountered during the excavation are consistent with those described in this report.
• All boulders and cobbles larger than 4 inches in diameter should be removed from
the base of the excavation. Voids created by the removal of boulders or over
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Emergency Generator Facilities
November 2005
Engineering Geology Report
excavation of soft zones should be backfilled with Crushed Surfacing Base Course in
6-inch lifts.
• To ensure that the subgrade supporting footings and slabs is firm and unyielding,
RH2 recommends placement of a leveling course of Crushed Surfacing Base Course
(WSDOT 9-03.9(3)) that is a minimum of 6 inches thick and a maximum of 1 foot
thick below all building foundations. The leveling course should be compacted to 95
percent of its maximum dry density as determined by ASTM D1557 (modified
proctor). Lifts should not exceed 4 inches loose thickness. Cobbles and boulders
embedded in the subgrade should not protrude more than 2 inches into this leveling
course.
• If structural fill is required below the leveling course to achievr,:11 certain elevation,
the net allowable bearing capacity should be reduced tc j,000.psf The thickness of
the structural fill should not be more than three feet. Ir m0r<: than 3 feet of fill is
required below the leveling course, CDF or crushed rock should be used. All fill
placed should be compacted to meet 95 percent of the maximum dry density as
determined by ASTM Dl 557 (modified proctor). Lifts should not exceed 6 inches in
loose thickness.
• The contractor should have a bucket without teeth on-site or have some other
method to remove all material disturbed during excavation at the level of the in-situ
subgrade. There should be no more than 1 to 2 inches of loose, disturbed material
on the subgrade when it is inspected by the Engineering Geologist.
Temporary Shoring and Permanent Retaining Wall Altematives
• It is very likely that temporary shoring will be required to construct the proposed EG
building. The contractor should be prepared to design and install shoring measures
or shielding as needed to construct the proposed improvements.
• Cuts into the steep slope to the east should be minimized by:
o Locating and designing the EG building and associated improvements so
that minimal excavation into the steep slope is required.
o Designing the roof so that storm water does not spill towards the base of the
steep slope.
o Eliminating in the paved sidewalk where it would require cuts into the slope
and replacing it with washed rock or some other permeable and flexible
material that will provide drainage and prevent muddy water from splashing
on the building.
o Minimizing the distance between all permanent retaining walls and slopes
and the proposed EG building, and thus minimizing the volume of earth at
the toe of the slope that must be removed and the height of cuts into the
slope.
o Building the walls of the EG building so that they can resist lateral earth
pressures and thus eliminate the need for deeper cuts into the slope.
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Emergency Generator Facilities
November 2005
Engineering Geology Report
• All permanent slopes should be graded no steeper than 2H: 1 V and should be
vegetated immediately.
• All temporary slopes should be graded no steeper than 0. 7 SH: 1 V and be subject
to engineering geologist or geotechnical engineer inspection and approval.
Temporary slopes should be protected from rain-induced erosion with plastic
sheeting and up slope interception of all storm water so that it flows around the
excavation.
• A rockery may be used for all permanent slopes 6 feet tall or less. All rockeries
greater than 4 feet in height will require a design by a professional engineer.
Rockeries should follow these recommendations:
o Maximum wall height should be 6 feet or less
o Batter of the rockery walls should not be steeper thanl 1H:6V
o A drain should be installed behind the rockery facing to prevent the build-up
of pore water pressure. The drain should be a 4-to 6-inch diameter,
perforated pipe and should be covered with Gravel Backfill for Drains
(WSDOT 9-03.12(4)). The perforated pipe and Gravel Backfill for Drains
shall be placed in a 1-foot-wide by 18-inch-tall trench. The Gravel Backfill
for Drains and perforated pipe should be wrapped in geotextile fabric.
Backfill above the drain should consist of 2-to 4-inch quarry spalls up to the
ground surface, 1 foot in width.
o Foundation soils must be inspected by an engineering geologist and/ or
geotechnical engineer. The earth supporting the wall shall be in-siru medium
dense to dense and prepared to firm and unyielding surface prior to rock
placement. Compaction may be necessary to achieve firm and unyielding
condition by use of jumping jack, hoe pack, plate compactor or other heavy
compaction equipment.
o The base of the wall shall be keyed into soil a minimum of 18-inches.
o Materials and construction shall meet the requirements of 8-24 Rock and
Gravity Block Wall and Gabion Cribbing of the WSDOT Standard
Specifications 2004.
• Cast-in-place concrete retaining walls are recommended for all permanent cuts
greater than 6 feet tall. Cast-in-place concrete retaining walls may be incorporated
into the proposed building structural design. RH2 recommends using one of the two
following retaining wall alternatives for minimizing the wall height and required
excavation into the slope.
o The first alternative is to extend concrete wing walls out from the northeast
and southeast walls of the proposed structure to retain the slope. This
alternative would remove the eastern corner of the wallcway / french drain
around the building but would still allow access to the doors in the southeast
side of the building. The wing walls could be transitioned into rockeries
when the wall height reaches 6 feet or less.
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Engineering Geology Report
o The second alternative is to construct a concrete retaining wall in
approximately a north-south direction (or parallel to the slope) along the east
corner of the proposed building. The wall should be located as close to the
building as possible to minimize the total wall height. The wall could be
transitioned into a rockery once the wall height is 6-feet or less.
• The following parameters may be used for designing a cast-in-place concrete
retaining wall or rockery.
Back.ill
o A unit weight of 125 pounds per cubic foot (pd) may be used for the native
outwash.
o The following allowable at-rest lateral earth pressures shall be used for
concrete walls with counterforts and/ or wing wall supports integral with
generator set structure.
•
•
59 pcf (triangular distribution) .
37(H) psf, where H 1s the total wall
(uniform distribution along full wall height). This
approximate slope of 1H:1 V above the wall.
height in feet
accounts for the
o The following allowable active lateral earth pressures shall be used for
rockery wall and concrete cantilever walls not integral with generator set
structure.
•
•
38 pcf (triangular distribution) .
24(H) psf where H IS the total wall height m feet
(uniform distribution along full wall height). This accounts for the
1H:1V slope above wall.
o Allowable passive lateral earth pressures for a key below a cantilever wall is
271 pcf (triangular distribution). The upper two feet of passive lateral earth
pressures should be ignored.
o Additional loads due to earthquake induced forces on wall shall be 19 pcf
(inverted triangle distribution).
o The coefficient of friction between soil and concrete is 0.42.
o A surcharge of 100 pounds per square foot with uniform distribution along
the wall height should be used. This surcharge assurues construction
equipment will be above the wall.
o A drain will be required behind all walls. Recommendations for at rest and
active earth pressures above assurue drained conditions behind the walls.
o A bearing capacity of 3,000 psf static and 4,000 psf for short term loads
(earthquake and wind) may be used for the base of the walls.
• Native earth may be used as structural fill as long as organic and deleterious materials
are not present Weather may affect the ability of the contractor to adequately
compact the native earth. Stockpiled fill and earth should be kept close to optimum
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Engineering Geology Report
moisture content and not exposed to rain especially if tbey have high silt contents.
Native silts to be compacted to 95 percent of modified proctor maximum dry
density (ASTM D1557) must be within + /-2 percent of optimum moisture content.
• Clean native sands witb minimal silt content will be suitable for use as structural fill
and will be much less sensitive to moisture for achieving compaction.
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Test Pit Logs -Mt. Olivet EG Building
Appendix A -Test Pit Logs
City of Renton
Mt. Olivet Emergency Generator Building
Three test pits were excavated on October 20, 2005 using a CASE 580 rubber-tired cxtend-
a-hoe provided and operated by City of Renton. See Figure 2 for approximate test pit
locations. The excavation was observed and test pits were logged by Andrea Mast on
October 20, 2005. The objective was to determine tbe deptb of stripping required for tbe
design and construction of a proposed emergency generator facility.
TEST PIT 1 (TP1)
Located annrox. 2 feet south of SW buildine stake.
Depth Soil Interpretation
0-6 in Brown silt witb fine sand and minor gravel, roots, loose, dty to moist
(WEATHERED FILL)
6 in -2.5 ft Orange-brown, verv fine sandv silt witb <>tavel, asphalt debris, drv (FILL)
2 ft -2.5 ft Dark brown, silt witb fine sand and subrounded gravel, roots, medium loose
(0-Horizon)
2.5 ft -3.5 Brown, fine sandy silt witb subrounded gravel, roots, medium loose (A&B
ft Horizons)
3 ft-5 ft Grey-brown fine to coarse sand witb sub angular gravel and silt, occasional
large gravel or small cobble, dry, medium loose to medium dense, no visible
bedding, stable pit walls
5 ft-9 ft Light brown-grey, very fine silty sand witb occasional gravel and cobbles,
occasional roots, dty, medium loose to medium dense, no visible bedding,
stable pit walls (GLACIAL OUTWASH SEDIMENTS)
Notes: Test pit completed at approx. 9 feet. No groundwater seepage or caving
observed. No bedrock encountered. Easy excavation. Test pit observed and
'°''"ed by Andrea Mast on 10/21/2005.
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Test Pit Lois -Mt. Olivet EG Bui/di,{~
T es t Pit T P I -F ill overlying buried soil h o ri zon
T est Pit T P2 -F ill over na tive fin e o u twa sh sands
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Test Pit Logs-Mt. Olivet EC Building
TEST PIT 2 (TP2)
Located avvrox. 2 feet east of NE building stake.
Devth Soil Intemretation
0-8 in Dark brown, fine siltv sand with roots, loose, moist (WE1\ THERED FILL)
6 in -2.5 ft Orange-brown, very silty sand, roots and burnt wood, drv (FILL)
2.5 ft -9 ft Llght brown, Silty fine sand with occasional gravel and cobbles. Roots.
Coarsens with depth to a medium-fine silty sand with occastional gravel and
cobbles. No visual bedding, dry, medium loose to medium dense. Less roots
below 5.5' (GU\ClAL OUTWASH SEDIMENTS)
Notes: Test pit completed at approx. 9'. No groundwater seepage or caving observed.
No bedrock encountered. Easy excavation. Test pit observed and logged by
Andrea Mast on 10/21/2005.
TEST PIT 3 (TP3)
Located avvrox. 26 feet south of northern fence
Devth Soil Intemretation
0-1 ft Brown, silty fine to medium sand with minor gravel. Bioturbatcd, roots, moist
(FILL)
0.5 ft -1.3 Orange-brown, fine sandy silt/ silty sand witb minor gravel. Minor roots, moist.
ft (A & B HORIZON)
1.3 ft-1.8 Brown fine sandy silt/ silty sand with minor gravel and roots, moist. (C-
ft HORIZON)
1.7 ft-6.5 Grey, fine to medium sand with silt and coarse sand and fine to coarse sub-
ft rounded to sub-angular gravel interbedded with silty fine sand. Visual bedding
planes. Bedding dipping approximately 15 to 20 degrees to the northwest. Moist
to dry. (GLACL\L OUTWASH)
Notes: Test pit completed at approx. 6.5 feet. No groundwater seepage or caving
observed. No bedrock encountered. Easy excavation. Test pit observed and
lo""ed by Andrea Mast on 10/21/2005.
2/16/2006 2:31 PM Pg 3 of 4 J:\data\REN\! 05-079\GEO\Reporl\Print Me\Figure.s & Appendices\Appendix A -1
T est Pit Lo~~s -Mt. Olivet EC Building
Tesr Pit T P 3 -Th.in la yer of ftll , soil h o riz o n s and unweathered o utwa sh sands.
2/16/2()()6 2 :.1 1 PM Pg 4 or 4 J :\data\REN\10.'i-0 791(.,H )\Rcpo rr \Print Me\Figun:s & A ppendi(es\A ppendix A -l
APPENDIX B Calculations 1
REN 105.079 Earth Pressures -Equivalent Fluid Pressure
Sandy Silt in Area Excavated and used as backfill
Effective Fnction Angle 32 degrees Medium to Medium Dense Sand -Lower end of Medium dense Sand, minor gravel
Unit Weight r 125 lbs/ft"
Lateral At Rest Pressure for Sto.e_ed Conditions:
K:, 0.47 At Rest Coefficient
P,
P,
59 lbs/ft3
402 lbslft2
No Slope At Rest Earth Pressure
11' Wall w/ 1H:1V Up Unlfonn Load Addition
Coeffectent of Friction at Tank to Earth Interface:
0.42 Robert Day Reference
Active Earth Pressures:
K,
P,
P,
0.31 Active Earth Coefficient (level backfill)
38 lbs/ft3 Active Earth Coefficient (level backfill)
263 lbs/ft2 11'Wall w/ 1H:1V U_e_ Unlfonn Load Addition
Allowable Passive Earth Pressures·
K,
_I',_
1.5 Factor of Safety
3.3 Passive Earth Coefficient
407 lbs/ft3
Rankine Theory
EG Building Walls
Mt.Olivet
From Day, MacNab, Hausmann
Surcharge for Upslope Earth
Area 1
Area 2
Total Area
11 ft
6.10 ft
45
4.31 fl
4.31 fl
Wall Height
Width Surcharge of Soil Acting on Wall
Rise (XX V)
Run (XX H)
Slope Degrees
Ht
Lgth
9.29 sq fl Area
4.31 fl
16.00
15.04 fl
Ht
Angle (top slope to intercept of 45-phr/2)
Lgth
32.41 sq fl Area
41.71 sqflperfl
I Pe 271 lbs/ft3 Safety Factor -For Key I Rankine Theory with Safety Factor I Total Weight
Earthquake Earth Pressures:
P19. 19 lbs/ft Inverted Triangle Distribution JRobert Day Reference
Resultant Force at 0.6 of backfilled wall height (H)
Surcharge Pressures
IP•P 100 lbs/ft~ (300 lbs/ft2 • Ka) !Robert Day Reference
11/15/2005 3-37 PM
5213.57 lbs perfoo!
Total Weight per Unit Width Distributed
855.05 _esf From MacNab
Wall Height has linear relationship with surcharge
77.73 psf Vertical Load per foot of wall height
36.54 psf At Rest Load per foot of wall height
23.88 psf Active Load per foot of wall height
J \data\REN\105-079\GEO\Ren Mt Olivel Gea Cales.xis
TECHNICAL INFORMATION
REPORT
CITY OF RENTON
NORTH TALBOT AND MT. OLIVET
EMERGENCY GENERATOR FACILITIES
Prepared by RH2 Engineering
for City of Renton
March 2006
RH2 Project: REI\1 !05.079.01.103
r--""'li-n""~ Bothell
East Wenatchee
Port Orchard
Bellingham
Tacoma
DEVELOPMENT PLANNING
CITY OF RENTON
JUN 3 0 2006
RECEIVED
TECHNICAL INFORMATION
REPORT
CITY OF RENTON
NORTH TALBOT AND MT. OLIVET
EMERGENCY GENERATOR FACILITIES
Prepared by RH2 Engineering
for City of Renton
. SIGNED:
.3 /.;i._ ,jot,
EXPIRES 7 /14,06
March 2006
RH2 Project: REI\ 105.079.01.103
Bothell
East Wenatchee
Port Orchard
Bellingham
~~i:±j Tacoma
EXPIRES I 0/1 /06
SIGNED: d/.;2/c,r:,
North Talbot and Mt. Olivet
Emergency Gencrat,or_F_a_c_ili_· n_· e_s __________ _
Section 1 -Project Overview
March 2006
Stormwater Drainage Report
The Emergency Power Generation project proposes the construction of two concrete
masonry buildings to house engine generators on two sites. The first building (North Talbot
Generator Project Site) will be located on Parcel No. 7222000130 within the southeast
quarter of Section 19, Township 23 N., Range 5 E. on a 0.88-acre parcel located at the
southeast corner of the intersection of South 19"' Street and Talbot Road. The other
building (Mt. ( )liver Generator Project Site) will be located on Parcel No. 1723059130 within
the northeast quarter of Section 17, Township 23 N., Range 5 E. on a 3.79-acre parcel
located at the northeast comer of the intersection of Northeast 3'J Street and Bronson \)lay.
/\ vicinity map is included on the cover sheet of the plans and included in Appendix A.
North Talbot Generator Project Site Existing Conditions
The existing site is currently utilized for the North Talbot Booster Pump Station. Portions
of the site arc also used as parking, open space park and tennis courts that ate utilized by the
public. The area proposed for improvements is currently a well-maintained lawn. The
pro1ect site is generally flat with slopes less than one percent. Refer to the plans for an
existing site plan and photos of the existing site.
Mt. Olivet Generator Project Site Existing Conditions
The existing site is currently occupied by a reservoir structure, a booster pump station and
several miscellaneous utility vaults. There is also an asphalt parkmg lot in the northeast
corner of the site. These impervious surfaces account for approximately 13 percent of the
total 3.79-acre site. A small portion of land around the circumference of the existing
reservoir is a well-maintained grass lawn. The remainder of the site is generally forested.
'[he site is roughly triangular in shape with the easternmost boundary oriented north/south.
The proposed project site is relatively flat and situated in the northeast corner of the central
portion of the site. This area is at the base of a steep hillside which ranges in slope from 35
percent to 100 percebt. The top of the slope terminates at the wesrern edge of the
developed property to the east. The topo6>raphy immediately west of the project site is flat
with contours falling off to the north, west and south near the propeny limits of the site.
Much of the precipitation impacting the site infiltrates inro the soil. :\ny runoff from the
site ts collected in a catch basin/pipe network along the soutl1 edge of Bronson Way and the
North edge of NE 3'J Street which, border the site.
rigure I -TIR Worksheet and 'Figure 2 -Site T Awtion have been included for the Mt. Olivet site
in Appendix A of this Report. Figure 3 -Draint,ge Basins. Subbasins and Site Characlenstics has
been excluded from this report because the Nortl1 Talbot Generator Building is not subject
to drainage review and the Mt. ( )livet Generator Building has minimal ptoJect site area and
requests exemptions for Flow Control and Water Quality (refer to the remaining sections of
this report for additional infom1ation). An E~gineen·n,g Geology Report.Jar the City of Renton North
Talbot und Mt. Olivet Generator Facilities has heen prepared by RH2 Engineenng Inc. for the
design and construction of these buildings, and therefore a separate figure was not generated
for the .-\ppendix. The Engineering Geology Report identifies that the Mt. Olivet site is
mapped as Vashon outwash (Qgo) soil, which is classified within hydrologic soil group 'A'
and is expected to be conducive to infiltration.
P:1gc ~ uf"7
,/'.:'1 /21111(1 1.07.24 P\f S:\Jata\lll•'.N\ \1)5 -f!79\St(mn\"!'lR \"l'n:hrnul lnforma.tlon Rcp1>rt.J(K
North Talbot and Mt. Olivet
Emergency Generator Facilities
March 2006
Stormwater Drainage Report
Section 2 -Conditions and Requirements Summary
"!be stormwater management design for this project has been prepared to conform to the
King County, Washington Surface Water Design l'vfanua!, January 2005 Edition (I(CS\VDM).
The North Talbot Generator Building Site currently contains less than 15 percent
tmpervtous surface and therefore rhis project does not meet the defmition of
"redevelopment" according the KCS\X'DM. The North Talbot Generator Project Site
proposes to construct less th,m 2,000 square feet of new impervious surface and disturb less
than 7,000 square feet of land. Therefore, the North Talbot Generator Project is not subject
to Drainage Review according to KCS\X'DM Section 1.1.1 -l'ro;ects Requiring Drainage Review.
It is therefore not included for the remainder of this report, but will be incorporated in the
required erosion and sedimentation control measures.
Tbe .\fr Olivet Generator Building Site currently contains less than 15 percent impervious
surface, and therefore this development does not meet the definition of "redevelopment"
according the KCSWDM. The Mt. Olivet project will add more than approximately 3,230
square feet of new in1pervious surface and replace 180 square feet of existing impervious
(pavement). Therefore, the Mt. Olivet Generator Project is subject to Drainage Review
according to KCS\vTIM Section 1.1. l -Projeds Requiri"~ Drain'{~' Review and will be the focus
of the remainder of this report.
Adopted CritJcal Drainage Areas, Master Drainage PIJlns, Basin Plans, Salmon Conservation
Plans, Stormwater Compliance Plans, Lake Management Plans, Flood Hazard Reduction
Plans and Shared Facility Drainage Plans are not known to exist for the proposed project
sites.
It is anticipated the proposed projects will require a Building Permit.
Section 3 -Offsite Analysis
It is anticipated the increase in new in1pervious surface of slightly over 3,200 square feet will
not have a significant adverse impact on the downstrean1 and/ or upstream drainage systems.
The project's roof and pavement runoff will generally be infiltrated into the soils using
infiltration trenches. Therefore, we request an exemption from providing a formal offsite
analysis for the !\fr. ()livet Site based on the minor increase in impervious area and
infiltration being sufficient evidence that the project will not have a significant adverse
impact.
Section 4 -Flow Control and Water Quality Facility Analysis and Design
The proposed Mt. Olivet project site is within a Conservation Flow Control Area. Using
historic site conditions (i.e. forested), the existing site will not generate runoff during the
peak 100-year event for the 0.0783-acre project site. Under developed conditions, the
proposed project site will generate 0.037 cfs during the peak 100-year event. "[berefore,
since the result ts less than a 0.1-cfs difference between the existing conditions and the
developed conditions, the facility requirement may be waived (refer to KCS\X'DM Section
1.2.3 -Core Requirement #3: Flow Control, pg. 1-35). Also, the implementation of infiltration
trenches for roof downspouts and paved surfaces will serve to further reduce impacts.
Please refer to Appendix B for stormwater modeling results using King County Runoff
Time Series.
rag(.: _', or 7
',/21 /20IJ() 1:IJ7.24 PM S: \<lala\R[•'.N\ I \)S-(l7(J\S1cmn\"l"JR \l"cchmcal !nfonnauon Rt.>p!1rt.d()C
North Talbot wd Mt. Olivet
Emergency Generator Facilities
--~-~-----------------·-.,~--~---~-
March 2006
Stormwater Drainage Report
A formal water yuality facility design is not proposed for this project. An exemption from
providing water guality control is requested according to the surface area exemption:
a) Less than 5,000 sguare feet of new PGIS that is not fully dispersed will be added, AND
b) Less than 5,000 syuare feet of new plus replaced PGIS that is not fully dispersed will be
created as part of a redevelopment project, ,\ND
c) Less thw 35,000 square feet of new PGPS that is not fully dispersed will be added.
Infiltration trenches arc to be gravel-filled trenches designed according to KCSWDM
Section C:.2.2 --Full lnfiltration. The existing soils at the Mt. Olivet site are medium swds, and
therefore a minimum of 30 feet of trench is provided per 1,000 syuare feet of impervious to
be served. There is approximately 1,400 syuare feet of impervious area (roofs and
maintenance access roads/paths) to be served by the infiltration trench, and therefore a
minimum of 102 linear feet of infiltration trench is reyuired.
Section 5 -Conveyance System Analysis and Design
The project docs not propose lo construct a formal convcyann~ system and therefore this
section does not apply.
Section 6 -Special Reports and Studies
An Engineering Geology Report was prepared for this project as identified in the Project
Overview Section above. Other reports and studies were not identified as needed for the
projects.
Section 7 -Other Permits
Other permits have not been identified as needed for the projects.
Section 8 -ESC CSWPPP Analysis and Design
ESC Plan Analysis and Design (Part A)
'[he proposed proJect sites are both relatively small, disturbing less than 7,000 square feet of
area to construct. The following are the anticipated practices to be in1plemented to meet the
reyuirements. .A.11 Best Management Practices (BMPs) listed cw be found in Appendix C
and the BMPs will be included in the project specifications developed for the public works
project.
Clearing Limits
The clearing limits have been identified on the plans. Prior to beginning land disturbing
activities, the contractor shall ckarly lnark all clearing limits with plastic, tndal, or stake wire
fence. The duff layer, native top soil, and natural vegetation shall be retained in an
undisturbed state to the maximum extent practicable. Where the duff layer is removed, it
should be stockpiled onsite, covered to prevent erosion, wd used as topsoil for final site
stabilization.
Applicable BMPs:
BMP C:101: Preserving Natural Vegetation
BMl' C:103: High Visibility Plastic or Metal Fence
BMP Cl04: Stake and Wire Fence
3/21/2006 I :07.24 P\! S:\Jata \RJ•:N \ I fb-1l79\Storm\l'!R\'l'c.:chn1c1I lnt<mnalH!fi Report.doc
North Talbot and Mt. Olivet
Emergency Generator Facilities
Cover Measures
March 2006
Stormwater Drainage Report
All exposed and unworked soils shall be stabilized by application of effective Bl'v!Ps that
protect the soil from the erosive forces of raindrop impact, flowing water and wind.
From October 1 through April 30, no soils shall remain exposed and unworked for more
than two days. From May 1 to September 30, no soils shall remain exposed and unworked
for more than seven days. This condition applies to all soils on site, whether at final grade or
not. These time limits may be adjusted by the local permitting authority if it can be shown
that ,he average time between stom1 events justifies a different standard.
Soil stabilization measures should be appropriate for the time of year, site conditions,
estimated duration of use, and potential water quality impacts that stabilization agents may
have on downstream waters or ground water. Applicable practices include, but are not
lin1ited to, temporary and permanent seeding; sodding; mulching; plastic covering; erosion
control fabrics and matting; soil application of polyacrylamide (P/\i'v[); the early application
of gravel base on areas to be paved; and dust control.
Soil stockpiles must be stabilized from erosion, protected with sediment trapping measures,
and when possible, be located away from .storm drain 111lcts, watcrv.-avs and drainage
channels.
Soils shall be stabilized at the end of the shift before a holiday or weekend if needed based
on the weather forecast.
Applicable BMPs:
Bl'v!P C120: Temporary and Permanent Seeding
BMP C121: Mulching
BMP C 123: Plastic Covering
BMP C 125: Topsoiling
BMP C130: Surface Roughening
BMP C140: Dust Control
Perimeter Protection
Silt fence will be used to provide perimeter protection for the project site. Additionally, the
silt fence may be used to identify the clearing limits if applicable and properly maintained by
the contractor.
Applicable BMP
BMP C233: Silt Fence
Traffic Area Stabilizat10n
Due to the small project sizes, installation of construction entrances is not warranted. The
existing paven1ent areas around the project site will serve as construction roads and parking
for construction access and it is not anticipated vehicles will be entering/ exiting the unpaved
poruons of the site regularly. Dust control was addressed the Cover Measures Section
above.
1121 noo(, 1.07.24 PM ;;: \data\ R] •'.N \ 1 ll:i-!l71)\SI onTI 1:t 'IR\ Tn:hrnc:d T nfunnatmn llcporl.doc
North Talbot and Mt. Olivet
Emergency Generator Facilities
Sediment Retention
March 2006
Stormwater Drainage Report
Due to the small project sites, they do not warrant installing sediment traps/ponds.
Surface Water Control
Again, due to the small project sites, they do not warrant installing formal surface water
ccmtrol facilities.
Wet Season Requirements
The minimum requirements for work in the wet season have been identified in the Cover
l'vkasurcs Section above.
Critical Areas Restrictions
Critical areas are not known to exist on the project sites and therefore this requirement does
not apply.
Stormwater Pollution Prevention and Spill (SWPPS) Plan (Part B)
The proposed project ,.,,-il] be a public works project and the City does not want to limit the
contractor's option to provide competitive bid prices. Therefore, the Storage and Handling
of Liquids; Storage and Stockpiling of Construction Materials and Wastes; Fueling; and
Maintenance, Repairs, and Storage of V chicles and Equipment will be written as a
perfom1ance specification so as not to limit the contractor's ability to provide competitive
bidding using his/her desired means and methods.
Storage and Handling of Liquids
Cover, containment and protection from vandalism shall be provided for all chemicals,
liquid products, petroleum products, and non-inert wastes present on the site (sec WAC I 71-
.304 for the definition of inert waste).
Storavc and Stockpiling of Construction Materials and \'vastes
All pollutants, including waste materials and demolition debris, that occur onsite shall be
handled and disposed of in a manner that does not cause contamination of stomnvater.
Woody debris may be chopped and spread on site.
Fueling
Fueling will likely occur using truck-mounted tanks and using spill containment methods.
On-site fueling tanks shall include secondary containment.
Maintenance, Repairs, and Storage of Vehicles and Equipment
Maintenance and repair of heavy equipment and vehicles involving oil changes, hydraulic
system drain down, solvent and de-greasing cleaning operations, fuel tank drain down and
removal, and other activities which may result in discharge or spillage of pollutants to the
ground or into stormwater runoff must be conducted using spill prevention measures, such
as drip pans. Contaminated surfaces shall be cleaned immediately following any discharge or
spill incident. Emergency repairs may be performed on-site using temporary plastic placed
beneath and, if raining, over the vehicle.
l'agc 6 of 7
',/21 /200(, 1 IJ7:24 P:\-1 S: \Jala\RI :.N\ 1()5-()79\Sumn \"J'TR\'l'cchn1c:tl lnf!mnall!lll Rcp<1rr.d(H.:
North Talbot and Mt. Olivet
Emergency Generator Facilities
Concrete Saw Cutting Slurry and Wash Water Disposal
March 2006
Sturmwater I)_rainage Report
Wnen concrete or asphalt saw cutting occurs, the contractor shall vacuum the slurry behind
the saw to prevent it from entering surface waters. The slurry shall be disposed in a manner
that complies with all local, state and federal requirements.
Wheel wash or tire bath wastewater shall be discharged to a separate on-site treatment
systen1 or to the sanitary sewer. The contractor shall obtain written permission prior to
discharging to the sanitary sewer.
Applicable BMP:
Bl\11' C152: Sawcutting and Surfacing Pollution Prevention
Handling of pH Elevated Water
BMPs shall be used to prevent or treat contamination of stormwater runoff by pH
modifying sources. These sources include, but are not limited to, bulk cLment, cement kiln
dust, fly ash, new concrete washing and curing waters, waste streams generated from
concrete grinding and sawing, exposed aggregate processes, and concrete pumping and
mixer washout waters. Stomnvater discharges shall not cause or contribute to a violation of
the water ljUality standard for pH in the receiving water.
Construction sites with significant concrete work shall adjust the pH of stormwater if
necessary to prevent violations of water quality standards.
Applicable EMP:
RMP C151: Concrete Handling
Application of Chemicals including Pesticides and Fertilizers
,\pplicarion of agricultural chemicals, including fertilizers and pesticides, shall be conducted
in a manner and at application rates that will not result in loss of chemical to stormwater
runoff. Manufacturers' recommendations for application rates and procedures shall be
followed.
Section 9 -Bond Quantities, Facility Summaries, and Declaration of Covenant
'fhe proposed project will be constructed, owned and operated by the City of Renton and
there will be minimal stormwatcr facilities (surface infiltration trenches); therefore, the bond
quantities, facility summaries and Declaration of Covenant are not required for this project.
Section 10 -Operations and Maintenance Manual
Because there are no flow control or water quality systems proposed as part of this project,
the operations and niaintenance n1anual has been omitted.
1,/21 noori 1:117.24 Pl\.t S: \dal:1\RFN\ ! 05-07')\St1mn \TIR\To.:chntcal fnformauon Reporuloc
Appendices
Appendix A
KlNG COU'ITY. WASHINGTON. SURFACE WATER DESIGN MANUAL
I REPORT (TIR) WORKSHEET
16~~~R~~~~~~:.~~:ATI~.~.~~~ ~ /vi;,. ]
I Project Name_ .,·,.,/:;.-i,.; ~~-· ... · ~ ,-.~.,_ . .,,,?'~::G:.-:l~
I DOES Permit# '
0
"',.;" • •• ·• i• ;; I
I Location Township __ -;;.._";.JJ ____ · I
I
Range
_J I Site Address
I
Part 4 OTHER REVIEWS AND PERMITS
0
I~
!
I
lo ]o
1D
DFWHPA
COE 404
DOE Dam Safety
FEMA Floodplain
COE Wetlands
Other __ _
D Shoreline
Management
0 Structural
RockeryNault/_ ___ _
0 ESA Section 7
I
i .
Site Improvement Plan (Engr. Plans) __ ,--..,
I 1 Type (circle one):
_ 1
1
I Date (include revision -I I dates):
_l_J_g_ate of Final:
'.Full· / Modified /
Small Site
' --~
~~--·-··"·--------~
:application / Experimental/ Blanket
I
J /1/05
·-----------------------------
TECHNICAL INFORMATIOI\
'
! Part 1 PROJECT OWNER AND
i PROJECT ENGINEER
-------
! Project Owner -~:~J-~-1 or R£.0 h~·'/·-._ ' '-'",,, ...Y__.L>.'2CU-''-'-l_
. Phone
I Address ___ _
I
·1 Project Engin!'..er !?,,-~. ':. ji'._,, c, u De., r-
Company 'K ,-/ / E ,.,., r ,' -· ,. · ,,_,:_
I Phone 1..../?<. C~::-'· ! ,;.:, .. _,r.-rD. (·
i Part 3 TYPE OF PERMIT APPLICATION
I
O Landuse Services
Subdivison I Short Subd. I UPD
I O Building Services I M/F I Commerical / SFR
I O Clearing and Grading
I O Right-of-Way Use
L El Other 8., C.1 ,.. ' )' ~------
[ Part 5 PLAN AND REPORT INFORMATION
I
Technical Inform!!~~ Report
1
Type of Drainage Review (E_~!_I...,;/ Targeted
i (circle): Large Site
I Date (include revision
I
dates):
, Date of Final:
r------------· ----
' ~ Part 6 ADJUSTMENT APPROVALS
I Type (circle one): (St;;~dari'7-~;mplex--;-;
I Description; (include ~nditi;;~s in TIR Section 2)
1--
1 -----
' Date of Ap_p31val:__ "------~~---__ ·
:2005 Surface \Vatcr Design rvfonual
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN 'Vl•\NUAL
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Part 15 EASEMENTS/TRACTS
0 Drainage Easement
D Access Easement
D Native Growth Protection Covenant
1
D Tract
; Part 16 STRUCTURAL ANALYSIS
I
0 Cast in P!ace Vault
D Retaining Wall
0 Rockery> 4' High
i
i
I D Other ___ ~
0 Structural on Steep Slope
D Other ___J
i Part 17 SIGNATURE OF PROFESSIONAL ENGINEER
11, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were
1
incorporated into this worksheet and the attached Technical Information Report To the best of my
1 know~~ the infor~.IL";' provided here is accurate.
I r', 1 : l~-<--?~C._ ... _--~--'""-~-~-"' _,, ____ ::-:>
---~ .. ---~----~--------_J
2005 Surface Water De:,;ign Manual 111.{)5
5
AppendixB
Flow Frequency Analysis
Time series File:predev.tsf
Project Location:Sea-Tac
---Annual Peak Flow Rates---
Flow Rate Rank Time of Peak
(CFS)
0.000 1 11/20/00 14:00
0.000 2 12/23/01 22:00
0.000 3 12/08/02 16: 00
0.000 8 10/01/03 0:00
0.000 4 12/03/04 12:00
0.000 5 12/02/05 20:00
0.000 6 11/23/06 21:00
0.000 7 10/09/07 12:00
Computed Peaks
Predev.pks
-----Flow Frequency Analysis-------
--Peaks --Rank Return Prob
A-EF~ Period
(.O.OOQ..._, 1 100.00 0.990 1r:·ooo 2 25.00 0.960
0.000 3 10.00 0.900
0.000 4 5.00 0.800
0.000 5 3.00 0.667
0.000 6 2.00 o. 500
0.000 7 1. 30 o. 231
0.000 8 1.10 0.091
0.000 50.00 0.980
Page 1, Monday, March 20, 2006, 1:53:19 PM
Flow Frequency Analysis
Time series File:dev.tsf
Project Location:sea-Tac
---Annual Peak Flow Rates---
Flow Rate Rank Ti me of Peak
(CFS)
0.019 7 2/09/01 2:00
0.017 8 1/05/02 16:00
0.023 3 12/08/02 18:00
0.020 6 8/26/04 2:00
0.023 4 10/28/04 16:00
0.020 5 1/18/06 16:00
0.028 2 10/26/06 0:00
0.037 1 1/09/08 6:00
computed Peaks
Dev. pks
-----Flow Frequency Analysis-------
--Peaks Rank Return Prob
µ;;; .. ~ Period
eg.037 1 100.00 0.990 ·~nzS" 2 25. 00 0.960
0.023 3 10.00 0.900
0.023 4 5. 00 0.800
0.020 5 3.00 0.667
0.020 6 2.00 o. 500
0.019 7 1. 30 0.231
0.017 8 1.10 0.091
0.034 50.00 0.980
..
. ,:.,,)"°',,~;. f ...... ~....rv: ~J'-,_.--'-.:_$
Page 1, Monday, March 20, 2006, 1:53:34 PM
AppendixC
4.1 Source Control BMPs
BMP C101: Preserving Natural Vegetation
Purpose
Conditions of Use
Design and
Installation
Specifications
4-2
The purpose of preserving natural vegetation is to reduce erosion wherever
practicable. Limiting site disturbance is tile single most effective metilod
for reducing erosion. For example, conifers can hold up to about 50
percent of all rain that falls during a storm. Up to 20-30 percent of this rain
may never reach the ground but is taken up by the tree or evaporates.
Another benefit is that the rain held in the tree can be released slowly to the
ground after the storm.
• Natural vegetation should be preserved on steep slopes, near
perennial and intermittent watercourses or swales, and on building
sites in wooded areas.
• As required by local governments.
Natural vegetation can be preserved in natural clumps or as individual
trees, shrubs and vines.
The preservation of individual plants is more difficult because heavy
equipment is generally used to remove unwanted vegetation. The points
to remember when attempting to save individual plants are:
• ls the plant worth saving? Consider the location, species, size, age,
vigor, and the work involved. Local governments may also have
ordinances to save natural vegetation and trees.
• Fence or clearly mark areas around trees that are to be saved. It is
preferable to keep ground disturbance away from the trees at least as
far out as the driplinc.
Plants need protection from three kinds of injuries:
• Construction Equipment -This injury can be above or below the
ground level. Damage results from scarring, cutting of roots, and
compaction of the soil. Placing a fenced buffer zone around plants to
be saved prior to construction can prevent construction equipment
injuries.
• Grade Changes -Changing the natural ground level will alter grades,
which affects the plant's ability to obtain the necessary air, water, and
minerals. Minor fills usually do not cause problems although
sensitivity between species does vary and should be checked. Trees
can tolerate fill of 6 inches or less. For shrubs and other plants. the fill
should be less.
When there arc major changes in grade, it may become necessary to
supply air to the roots of plants. This can be done by placing a layer or
gravel and a tile system over the roots before the fill is made. A tile
Volume II -Construction Stormwater Pollution Prevention February 2005
February 2005
system protects a tree from a raised grade. The tile system should be
laid out on the original grade leading from a dry well around the tree
trunk. The system should then be covered with small stones to allow
air to circulate over the root area.
Lowering the natural ground level can seriously damage trees and
shrubs. The highest percentage of the plant roots are in the upper 12
inches of the soil and cuts of only 2-3 inches can cause serious injury.
To protect the roots it may be necessary to terrace the immediate area
around the plants to be saved. If roots are exposed, construction of
retaining walls may be needed to keep the soil in place. Plants can
also be preserved by leaving them on an undisturbed, gently sloping
mound. To increase the chances for survival, it is best to limit grade
changes and other soil disturbances to areas outside the dripline of the
plant.
• Excavations -Protect trees and other plants when excavating for
drainticlds, power, waler, and sewer lines. Where possible, the
trenches should be routed around trees and large shrubs. When this is
not possible, it is best to tunnel under them. This can be done with
hand tools or with power augers. If it is not possible to route the
trench around plants to be saved, then the following should be
observed:
Cut as few roots as possible. When you have to cut, cut clean. Paint
cut root ends with a wood dressing like asphalt base paint.
Backfill the trench as soon as possible.
Tunnel beneath root systems as close to the center of the main trunk lo
preserve most of the important feeder roots.
Some problems that can be encountered with a few specific trees are:
• Maple, Dogwood, Red alder, Western hemlock, Western red cedar,
and Douglas fir do not readily adjust to changes in environment and
special care should be taken to protect these trees.
• The windthrow hazard of Pacific silver fir and madronna is high, while
that of Western hemlock is moderate. The danger of windthrow
increases where dense stands have been thinned. Other species (unless
they are on shallow, wet soils less than 20 inches deep) have a low
windthrow hazard.
• Cottonwoods, maples, and willows have water-seeking roots. These
can cause trouble in sewer lines and infiltration fields. On the other
hand, they thrive in high moisture conditions that other trees would
not.
• Thinning operations in pure or mixed stands of Grand fir, Pacific silver
fir, Noble fir, Sitka spruce, Western red cedar, Western hemlock,
Volume II -Construction Stormwater Pollution Prevention 4-3
Maintenance
Standards
4-4
Pacific dogwood, and Red alder can cause serious disease problems.
Disease can become established through damaged limbs, trunks, roots,
and freshly cut stumps. Diseased and weakened trees are also
susceptible to insect attack.
• Inspect flagged and/or fenced areas regularly to make sure flagging or
fencing has not been removed or damaged. If the flagging or fencing
has been damaged or visibility reduced, it shall be repaired or
replaced immediately and visibility restored.
• 1 r tree roots have been exposed or injured, "prune·• cleanly with an
appropriate pruning saw or lopers directly above the damaged roots
and recover with native soils. Treatment of sap flowing trees (fir,
hemlock, pine. soft maples) is not advised as sap forms a natural
healing barrier.
Volume II -Construction Stonnwater Pollution Prevention February 2005
BMP C103: High Visibility Plastic or Metal Fence
Purpose
Conditions of Use
Design and
Installation
Specifications
Maintenance
Standards
4-6
Fencing is intended to: (1) restrict clearing to approved limits; (2) prevent
disturbance of sensitive areas, their buffers, and other areas required to be
letl undisturbed: (3) limit construction traffic to designated construction
entrances or roads: and, ( 4) protect areas where marking with survey tape
may not provide adequate protection.
To establish clearing limits, plastic or metal fence may be used:
, At the boundary of sensitive areas, their buffers, and other areas
required to be left uncleared.
• As necessary to control vehicle access to and on the site.
• High visibility plastic fence shall be composed of a high-density
polyethylene material and shall be at least four feet in height. Posts
for the fencing shall be steel or wood and placed every 6 feet on
center (maximum) or as needed to ensure rigidity. The fencing shall
be fastened to the post every six inches with a polyethylene tie. On
long continuous lengths of fencing, a tension wire or rope shall be
used as a top stringer to prevent sagging between posts. The fence
color shall be high visibility orange. The fence tensile strength shall
be 360 lbs./ft. using the ASTM 04595 testing method.
• Metal fences shall be designed and installed according to the
manufacturer's specifications.
• Metal fences shall be at least 3 feet high and must be highly visible.
• Fences shall not be wired or stapled to trees.
, If the fence has been damaged or visibility reduced, it shall be
repaired or replaced immediately and visibility restored.
Volume II -Construction Stormwater Pollution Prevention February 2005
BMP C104: Stake and Wire Fence
Purpose Fencing is intended to: (1) restrict clearing to approved limits; (2) prevent
disturbance of sensitive areas, their buffers, and other areas required to be
left undisturbed; (3) limit construction traffic to designated construction
entrances or roads: and, (4) protect any areas where marking with survey
tape may not provide adequate protection.
Conditions of Use To establish clearing limits, stake or wire fence may be used:
Design and
Installation
Specifications
]'rfaintenance
Standards
February 2005
• At the boundary of sensitive areas, their buffers, and other areas
required to be left uncleared.
• As necessary, to control vehicle access to and on the site.
• See Figure 4.1 for details.
• More substantial fencing shall be used if the fence does not prevent
encroachment into those areas that are not to be disturbed.
• If the fence has been damaged or visibility reduced, it shall be
repaired or replaced immediately and visibility restored.
Survey Flagging Baling Wire Do Not Nail or Staple
Wire to Trees
-,~~~~
3' MIN.
lf----10·-20·-----'' J Metal
Fence Post
-n=nti=m=rn=rn=rrr=!hr-:=rn=rn=rr=nil=n-=
Figure 4.1 -Stake and Wire Fence
Volume II -Construction Stonnwater Pollution Prevention 4-7
BMP C120: Temporary and Permanent Seeding
Purpose
Conditions of Use
Design and
Installation
Specifications
February 2005
Seeding is intended to reduce erosion by stabilizing exposed soils. A
well-established vegetative cover is one of the most effective methods of
reducing erosion.
• Seeding may be used throughout the project on disturbed areas that
have reached final grade or that will remain unworked for more than
30 days.
• Channels that will be vegetated should be installed before major
earthwork and hydroseeded with a Bonded Fiber Matrix. The
vegetation should be well established (i.e., 75 percent cover) before
water is allowed to flow in the ditch. With channels that will have
high flows, erosion control blankets should be installed over the
hydroseed. If vegetation cannot be established from seed before water
is allowed in the ditch, sod should be installed in the bottom of the
ditch over hydromulch and blankets.
• Retention/detention ponds should be seeded as required.
• Mulch is required at all times because it protects seeds from heat,
moisture loss, and transport due to runoff.
• All disturbed areas shall be reviewed in late August to early September
and all seeding should be completed by the end of September.
Otherwise, vegetation will not establish itself enough to provide more
than average pwtection.
• At final site stabilization, all disturbed areas not otherwise vegetated or
stabilized shall be seeded and mulched. Final stabilization means the
completion of all soil disturbing activities at the site and the
establishment of a permanent vegetative cover, or equivalent
permanent stabilization measures (such as pavement, riprap, gabions
or geotextiles) which will prevent erosion.
• Seeding should be done during those seasons most conducive to
growth and will vary with the climate conditions of the region.
Local experience should be used to determine the appropriate
seeding periods.
• The optimum seeding windows for western Washington are April 1
through June 30 and September 1 through October 1. Seeding that
occurs between July I and August 30 will require irrigation until 75
percent grass cover is established. Seeding that occurs between
October I and March 30 will require a mulch or plastic cover until
75 percent grass cover is established.
• To prevent seed from being washed away, confirm that all required
surface water control measures have been installed.
Volume II -Construction Stormwater Pollution Prevention 4-13
4-14
• The seedbed should be firm and rough. All soil should be roughened
no matter what the slope. If compaction is required for engineering
purposes, slopes must be track walked before seeding. Backblading or
smoothing of slopes greater than 4: 1 is not allowed if they are to be
seeded.
• New and more effective restoration-based landscape practices rely on
deeper incorporation than that provided by a simple single-pass
rototilling treatment. Wherever practical the subgrade should be
initially ripped to improve long-term permeability, infiltration, and
water inflow qualities. At a minimum, permanent areas shall use soil
amendments to achieve organic matter and permeability performance
defined in engineered soil/landscape systems. For systems that are
deeper than 8 inches the rototilling process should be done in multiple
lifts, or the prepared soil system shall be prepared properly and then
placed to achieve the specified depth.
• Organic matter is the most appropriate form of "fertilizer" because it
provides nutrients (including nitrogen, phosphorus, and potassium) in
the least water-soluble form. A natural system typically releases 2-10
percent of its nutrients annually. Chemical fertilizers have since been
formulated to simulate what organic matter does naturally.
• In general, 10-4-6 N-P-K (nitrogen-phosphorus-potassium) fertilizer
can be used at a rate of90 pounds per acre. Slow-release fertilizers
should always be used because they are more efficient and have fewer
environmental impacts. It is recommended that areas being seeded for
final landscaping conduct soil tests to determine the exact type and
quantity of fertilizer needed. This will prevent the over-application of
fertilizer. Fertilizer should not be added to the hydromulch machine
and agitated more than 20 minutes before it is to be used. If agitated
too much, the slow-release coating is destroyed.
• There are numerous products available on the market that take the
place of chemical fertilizers. These include several with seaweed
extracts that are beneficial to soil microbes and organisms. If 100
percent cottonseed meal is used as the mulch in hydroseed, chemical
fertilizer may not be necessary. Cottonseed meal is a good source of
long-term, slow-release, available nitrogen.
• Hydroseed applications shall include a minimum of 1,500 pounds per
acre of mulch with 3 percent tackifier. Mulch may be made up of 100
percent: cottonseed meal; fibers made of wood, recycled cellulose,
hemp, and kenaf; compost; or blends of these. Tackifier shall be plant-
based, such as guar or alpha plantago, or chemical-based such as
polyacrylamide or polymers. Any mulch or tackifier product used
shall be installed per manufacturer's instructions. Generally, mulches
come in 40-50 pound bags. Seed and fertilizer are added at time of
application.
Volume II -Construction Stormwater Pollution Prevention February 2005
February 2005
• Mulch is always required for seeding. Mulch can be applied on top of
the seed or simultaneously by hydroseeding.
• On steep slopes, Bonded Fiber Matrix (BFM) or \;[echanically Bonded
Fiber Matrix (MBFM) products should be used. BFYVMBFM
products are applied at a minimum rate of 3,000 pounds per acre of
mulch with approximately 10 percent tackifier. Application is made
so that a minimum of 95 percent soil coverage is achieved. Numerous
products are available commercially and should be installed per
manufacturer's instructions. Most products require 24-36 hours to
cure before a rainfall and cannot be installed on wet or saturated soils.
Generally, these products come in 40-50 pound bags and include all
necessary ingredients except for seed and fertilizer.
BFMs and MBFMs have some advantages over blankets:
• No surface preparation required;
• Can be installed via helicopter in remote areas;
• On slopes steeper than 2.5: 1, blanket installers may need to be roped
and harnessed for safety;
• They are at least $1,000 per acre cheaper installed.
In most cases, the shear strength of blankets is not a factor when used on
slopes, only when used in channels. BFMs and MBFMs are good
alternatives to blankets in most situations where vegetation establishment
is the goal.
• When installing seed via hydroseeding operations, only about 1/3 of
the seed actually ends up in contact with the soil surface. This reduces
the ability to establish a good stand of grass quickly. One way to
overcome this is to increase seed quantities by up to 50 percent.
• Vegetation establishment can also be enhanced by dividing the
hydromulch operation into two phases:
1. Phase I-Install all seed and fertilizer with 25-30 percent mulch
and tackifier onto soil in the first lift;
2. Phase 2-Install the rest of the mulch and tackifier over the first lift.
An alternative is to install the mulch, seed, fertilizer, and tackifier in one
lift. Then, spread or blow straw over the top of the hydromulch at a rate of
about 800-1000 pounds per acre. Hold straw in place with a standard
tackifier. Both of these approaches will increase cost moderately but will
greatly improve and enhance vegetative establishment. The increased cost
may be offset by the reduced need for:
1. Irrigation
2. Reapplication of mulch
3. Repair of failed slope surfaces
Volume II -Construction Stonnwater Pollution Prevention 4-15
4-16
This technique works with standard hydromulch (1,500 pounds per acre
minimum) and BFM/MBFMs (3,000 pounds per acre minimum).
• Areas to be permanently landscaped shall provide a healthy topsoil
that reduces the need for fertilizers, improves overall topsoil quality,
provides for better vegetal health and vitality, improves hydrologic
characteristics, and reduces the need for irrigation. This can be
accomplished in a number of ways:
Recent research has shown that the best method to improve till soils is
to amend these soils with compost. The optimum mixture is
approximately two parts soil to one part compost. This equates to 4
inches of compost mixed to a depth of 12 inches in till soils. Increasing
the concentration of compost beyond this level can have negative
effects on vegetal health, while decreasing the concentrations can
reduce the benefits of amended soils. Please note: The compost should
meet specifications for Grade A quality compost in Ecology
Publication 94-038.
Other soils, such as gravel or cobble outwash soils, may require
different approaches. Organics and fines easily migrate through the
loose structure of these soils. Therefore, the importation of at least 6
inches of quality topsoil, underlain by some type of filter fabric to
prevent the migration of fines, may be more appropriate for these soils.
Areas that already have good topsoil, such as undisturbed areas, do not
require soil amendments.
, Areas that will be seeded only and not landscaped may need compost
or meal-based mulch included in the hydroseed in order to establish
vegetation. Native topsoil should be re-installed on the disturbed soil
surface before application.
• Seed that is installed as a temporary measure may be installed by hand
if it will be covered by straw, mulch, or topsoil. Seed that is installed
as a permanent measure may be installed by hand on small areas
(usually less than l acre) that will be covered with mulch, topsoil, or
erosion blankets. The seed mixes listed below include recommended
mixes for both temporary and permanent seeding. These mixes, with
the exception of the wetland mix, shall be applied at a rate of 120
pounds per acre. This rate can be reduced if soil amendments or slow-
release fertilizers are used. Local supp lien or the local conservation
district should be consulted for their recommendations because the
appropriate mix depends on a variety of factors, including location,
exposure, soil type, slope, and expected foot traffic. Alternative seed
mixes approved by the local authority may be used.
Volume II -Construction Stormwater Pollution Prevention February 2005
February 2005
Table 4.1 represents the standard mix for those areas where just a
temporary vegetative cover is required.
Table 4.1
Temporarv Erosion Control Seed Mix
% Weight %Puritv % Germination
Chewings or annual blue grass 40 98 90
Festuca rubra var. commutata or Paa anna ·-
Perennial rye -50 98 90
Loliurn eerenne
Redtop or colonial bentgrass 5 92 85
A2rostis alb~_9I Ag_rostis tenuis
White dutch clover 5 98 90
Tri(olium renens
Table 4.2 provides just one recommended possibility for landscaping seed.
Table 4.2
Landscaping Seed Mix
% \Veieht % Puritv % Germination
Perennial rye blend 70 98 90
Lolium oerenne
Chewings and red fescue blend 30 98 90
Festuca rubra var. commutata
or F estuca rubra
This turf seed mix in Table 4.3 is for dry situations where there is no need
for much water. The advantage is that this mix requires very little
maintenance.
Table 4.3
Low-Growina Turf Seed Mix
% \Vei!!'ht % Puritv % Germination
Dwarf tall fescue (several varieties) 45 98
Festuca arundinacea var.
Dwarf perennial rye (Barclay) 30 98
Lolium nerenne var. bare!,,.,.,,
Red fescue 20 98
F estuca rubra
Colonial bentgrass 5 98
Aerostis tenuis
Table 4.4 presents a mix recommended for bioswales and other
intermittently wet areas.
Table 4.4
Bioswale Seed Mix'
90
90
90
90
% Weh!ht % Puritv % Germination
Tall or meadow fescue 75-80 98 %
Festuca arundinacea or Festuca elatior --·
Seaside/Creeping bentgrass I 0-15 92 85
Acrrostis nalustris --------
Redtop bentgrass 5-10 90 80
Ar!rostis alba or A_grostis gfrwntea
• Modified Briargreen, Inc. Hydroseeding Guide Wetlands Seed .\.fix
I
I
Volume II -Construction Stormwater Pollution Prevention 4-17
11/aintenance
Standards
4-18
The seed mix shown in Table 4.5 is a recommended low-growing,
relatively non-invasive seed mix appropriate for very wet areas that are
not regulated wetlands. Other mixes may be appropriate, depending on
the soil type and hydrology of the area. Recent research suggests that
bentgrass (agrostis sp.) should be emphasized in wet-area seed mixes.
Apply this mixture at a rate of 60 pounds per acre.
Table 4.5
Wet Area Seed Mix'
%, \Veieht % Puritv % Germination
Tall or meadow fescue 60-70 98 90
Festuca arundinacea or
Festuca elatior
--·
Seaside/Creeping bentgrass 10-15 98 85
Agrostis palustris
Meadow foxtail 10-15 90 80
AleDocurus nratensis
Alsike dover 1-6 98 90
Trifo/ium hvbridum -·--
Redtop bentgrass 1-6 92 85
Azrostis alba
• Modified Briargreen, Inc. Hydroseeding Guide Wetlands Seed !>fix
The meadow seed mix in Table 4.6 is recommended for areas that will be
maintained infrequently or not at all and where colonization by native
plants is desirable. Likely applications include rural road and utility right-
of-way. Seeding should take place in September or very early October in
order to obtain adequate establishment prior to the winter months. The
appropriateness of clover in the mix may need to be considered, as this can
be a fairly invasive species. If the soil is amended, the addition of clover
may not be necessary.
Table 4.6
Meadow Seed Mix
% Wci2ht 0/o Puritv % Germination
Redtop or Oregon bentgrass 20 92 85
Agrostis alba or Agrostis orezonensis
Red fescue 70 98 90
Festuca rubra -----
White dutch clover 10 98 90
Trifolium reoens
• Any seeded areas that fail to establish at least 80 percent cover ( 100
percent cover for areas that receive sheet or concentrated flows) shall
be reseeded. If reseeding is ineffective, an alternate method, such as
sodding, mulching, or nets/blankets, shall be used. If winter weather
prevents adequate grass growth, this time limit may be relaxed at the
discretion of the local authority when sensitive areas would otherwise
be protected.
i
I
Volume II -Construction Stormwater Pollution Prevention February 2005
February 2005
• After adequate cover is achieved, any areas that experience erosion
shall be reseeded and protected by mulch, If the erosion problem is
drainage related, the problem shall be fixed and the eroded area
reseeded and protected by mulch.
• Seeded areas shall be supplied with adequate moisture, but not watered
to the extent that it causes runoff.
Volume II -Construction Storm water Pollution Prevention 4-19
BMP C121: Mulching
Purpose
Conditions of Use
Design and
Installation
Specifications
Maintenance
Standard.,
4-20
The purpose of mulching soils is to provide immediate temporary
protection from erosion. Mulch also enhances plant establishment by
conserving moisture, holding fertilizer, seed, and topsoil in place, and
moderating soil temperatures. There is an enormous variety of mulches
that can be used. Only the most common types are discussed in this
section.
As a temporary cover measure, mulch should be used:
• On disturbed areas that require cover measures for less than 30 days.
• As a cover for seed during the wet season and during the hot summer
months.
• During the wet season on slopes steeper than 3H: IV with more than I 0
feet of vertical relief.
• Mulch may be applied at any time of the year and must be refreshed
periodically.
For mulch materials, application rates, and specifications, see Table 4.7.
Note: Thicknesses may be increased for disturbed areas in or near
sensitive areas or other areas highly susceptible to erosion.
Mulch used within the ordinary high-water mark of surface waters should
be selected to minimize potential flotation of organic matter. Composted
organic materials have higher specific gravities (densities) than straw,
wood, or chipped material.
• The thickness of the cover must be maintained.
• Any areas that experience erosion shall be remulched and/or protected
with a net or blanket If the erosion problem is drainage related, then
the problem shall be fixed and the eroded area remulched.
Volume II -Construction Stonnwater Pollution Prevention February 2005
Mulch
Material
Straw
Hydromukh
Composted
Mulch and
Compost
Chipped Site
Vegetation
Wood-hascd
Ylukh
Oualitv Standards
Air-dried: free from
undesirable seed and
coarse material.
:'Jo growth
inhibiting factors.
No visible water or
dust during
handling. Must be
purchased from
suppher w1lh Solid
Waste I tandling
Permit (unless
exempt).
Average size shal!
be several inches.
Gradations from
lines to 6 inches in
length for texture,
variation. and
interlocking
properties.
>lo visible water or
dust during
handling. Must be
purcha,;;;ed from a
supplicr with a Solid
Waste l !andling
Pennit or one
exempt from solid
waste regulcitions.
Table4.7
Mulch Standards and Guidelines
Application
Rates
2"-3" thick; 5
bales per 1000 sf
or 2-3 tom. per
acre
Approx. 25-30
lbs per 1000 sf
or 1500 -2000
lhs per acre
2" thick min.;
approx. I 00 tons
per acre (approx.
800 lbs per yard)
2" minimum
thickness
2" thick: approx.
I 00 tons per ar.;re
(approx. 800 lbs.
per cubic yard)
Remarks
Cost-effective protection when applied with adequate
thickness. I-land-application generally requires greater
thickness than blown straw. The thickness of straw may he
reduced by half when used in conjunction with seeding. In
\\.·indy areas straw must be held in place hy crimping, using a
tackifier, or cmering with netting. 8\own straw always has
to be held in place with a Lackifier a.;, even light winds will
hlow· it away. Straw, however, has several deficiencies that
should be considered when selecting mulch materials. It
otlen introduces and/or encourages the propagation of weed
species and it hITT:i no significant long-tenn benefits. Straw
should be used only if mulches with long-term benefits are
unavailable locally. lt should also not be used within the
ordinary high-water elevation of surface w,1ters (due to
flotation).
Shall be applied with hydromulcher. Shall not be used
without seed and tackifier unless the application rate is at
ka.st doubled. Fibers longer than about J/4-I inch dog
hydromulch equipment. Fibers should be kept to less than 3/.i
inch.
More effective control can he obtained by increasing
thickness to 3". Excellent mulch for protecting final grades
until landscaping because il can be diredly seeded or tilled
into soil a,;;; an amendment. Composted mulch has a coarser
size gradation than compost. It is more stabk: and practical
to use in wet area) and during rainy weather conditions.
This is a cost-effective way to dispose of debris from
clearing and grubbing. and it eliminates the prohlems
associated with burning. Cicncrnlly, it should not be used on
slopes above approx. 10% because of its tendency to be
transported by runoff~ It is not recommended within 200
feet of surface waters. If seeding is expected shortly after
mulch, the Jccomposition of the chipped vegetation may tie
up nutrients important to grass estahlishmcnt.
This material is often called "hog or hogged fuel."' [tis
usable as a material for Stabilized Construction Entrances
(BMP CI05) and as a mulch. The use or mulch ultimately
improves the organic matter in the soil. Special <;aution is
advised regarding the source and composition of woud-
based mulches. lts preparation typically does not provide
any weed seed control. so evidence of residual vegetation in
its composition or known inclusion ofwet:t.l plants or seeds
should be monitored and prevented (or minim.izcd).
I I
February 2005 Volume II -Construction Stormwater Pollution Prevention 4-21
BMP C123: Plastic Covering
Purpose
Conditions of
Use
4-26
Plastic covering provides immediate, short-term erosion protection to
slopes and disturbed areas.
, Plastic covering may be used on disturbed areas that require cover
measures for less than 30 days, except as stated below.
, Plastic is particularly useful for protecting cut and lill slopes and
stockpiles. Note: The relatively rapid breakdown of most polyethylene
sheeting makes it unsuitable for long-term (greater than six months)
applications.
, Clear plastic sheeting can be used over newly-seeded areas to create a
greenhouse effect and encourage grass growth if the hydroseed was
installed too late in the season to establish 75 percent grass cover, or if
the wet season started earlier than normal. Clear plastic should not be
used for this purpose during the summer months because the resulting
high temperatures can kill the grass.
, Due to rapid runoff caused by plastic sheeting, this method shall not be
used upslope of areas that might be adversely impacted by
concentrated runoff. Such areas include steep and/or unstable slopes.
, While plastic is inexpensive to purchase, the added cost of installation.
maintenance. removal, and disposal make this an expensive material,
up to$ l .50-2.00 per square yard.
, Whenever plastic is used to protect slopes, water collection measures
must be installed at the base of the slope. These measures include
plastic-covered berms, channels, and pipes used to covey clean
rainwater away from bare soil and disturbed areas. At no time is clean
runoff from a plastic covered slope to be mixed with dirty runoff from
a project.
, Other uses for plastic include:
I. Temporary ditch liner;
2. Pond liner in temporary sediment pond;
3. Liner for bermed temporary fuel storage area if plastic is not
reactive to the type of fuel being stored;
4. Emergency slope protection during heavy rains; and.
5. Temporary drninpipe ("elephant trunk") used to direct water.
Volume II -Construction Stormwater Pollution Prevention February 2005
Design and
Installation
Specifications
Maintenance
Standards
, Plastic slope cover must be installed as follows:
I. Run plastic up and down slope, not across slope;
2. Plastic may be installed perpendicular to a slope if the slope length
is less than 10 feet;
3. Minimum of 8-inch overlap at seams;
4. On long or wide slopes, or slopes subject to wind, all seams should
be taped;
5. Place plastic into a small (12-inch wide by 6-inch deep) slot trench
at the top of the slope and backfill with soil to keep water from
flowing underneath;
6. Place sand filled burlap or geotextilc bags every 3 to 6 feet along
seams and pound a wooden stake through each to hold them in
place:
7. Inspect plastic for rips, tears, and open scams regularly and repair
immediately. This prevents high velocity runoff from contacting
bare soil which causes extreme erosion;
8. Sandbags may be lowered into place tied to ropes. However, all
sandbags must be staked in place.
• Plastic sheeting shall have a minimum thickness of 0.06 millimeters.
, If erosion at the toe of a slope is likely, a gravel berm, riprap, or other
suitable protection shall be installed at the toe of the slope in order to
reduce the velocity of runoff.
, Torn sheets must be replaced and open scams repaired.
, If the plastic begins to deteriorate due to ultraviolet radiation, it must
be completely removed and replaced.
, When the plastic is no longer needed, it shall be completely removed.
, Dispose of old tires appropriately.
---------------------------·· -------
February 2005 Volume II -Construction Sto,mwater Pollution Prevention 4-27
BMP C125: Topsoiling
Purpose
Conditions of
Use
Design and
Installation
Specifications
February 2005
To provide a suitable growth medium for final site stabilization with
vegetation. While not a permanent cover practice in itself, topsoiling is an
integral component of providing permanent cover in those areas where
there is an unsuitable soil surface for plant growth. Native soils and
disturbed soils that have been organically amended not only retain much
more stormwater, but they also serve as effective biofilters for urban
pollutants and, by supporting more vigorous plant growth, reduce the
water, fertilizer and pesticides needed to support installed landscapes.
Topsoil docs not include any subsoils but only the material from the top
several inches including organic debris.
• Native soils should be !ell undisturbed to the maximum extent
practicable. Native soils disturbed during clearing and grading should
be restored, to the maximum extent practicable, to a condition where
moisture-holding capacity is equal to or better than the original site
conditions. This criterion can be met by using on-site native topsoil.
incorporating amendments into on-site soil, or importing blended
topsoil.
• Topsoiling is a required procedure when establishing vegetation on
shallow soils, and soils of critically low pH (high acid) levels.
• Stripping of existing, properly functioning soil system and vegetation
for the purpose of topsoiling during construction is not acceptable. If
an existing soil system is functioning properly it shall be preserved in
its undisturbed and uncompacted condition.
• Depending on where the topsoil comes from, or what vegetation was
on site before disturbance, invasive plant seeds may be included and
could cause problems for establishing native plants, landscaped areas,
or grasses.
• Topsoil from the site will contain mycorrhizal bacteria that are
necessary for healthy root growth and nutrient transfer. These native
mycorrhiza arc acclimated to the site and will provide optimum
conditions for establishing grasses. Commercially available
mycorrhiza products should be used when topsoil is brought in from
off-site.
If topsoiling is to be done, the following items should be considered:
• Maximize the depth of the topsoil wherever possible to provide the
maximum possible infiltration capacity and beneficial growth
medium. Topsoil depth shall be at least 8 inches with a minimum
organic content of l O percent dry weight and pH between 6.0 and 8.0
or matching the pH of the undisturbed soil. This can be accomplished
either by returning native topsoil to the site and/or incorporating
organic amendments. Organic amendments should be incorporated to
a minimum 8-inch depth except where tree roots or other natural
Volume II -Construction Stormwater Pollution Prevention 4-29
features limit the depth of incorporation. Subsoils below the 12-inch
depth should be scarified at least 2 inches lo avoid stratified layers,
where feasible. The decision to either layer topsoil over a subgrade or
incorporate topsoil into the underlying layer may vary depending on
the planting specified.
• lfblended topsoil is imported, then fines should be limited to 25
percent passing through a 200 sieve.
• The final composition and construction of the soil system will result in
a natural selection or favoring of certain plant species over time. For
example, recent practices have shown that incorporation of topsoil
may favor grasses, while layering with mildly acidic. high-carbon
amendments may favor more woody vegetation.
• Locate the topsoil stockpile so that it meets specifications and does not
interfere with work on the site. It may be possible to locate more than
one pile in proximity to areas where topsoil will be used.
• Allow sufficient time in scheduling for topsoil to be spread prior to
seeding, sodding, or planting.
• Care must be taken not to apply to subsoil if the two soils have
contrasting textures. Sandy topsoil over clayey subsoil is a
particularly poor combination. as water creeps along the junction
between the soil layers and causes the topsoil to slough.
• lftopsoil and subsoil are not properly bonded, water will not infiltrate
the soil profile evenly and it will be difficult to establish vegetation.
The best method to prevent a lack of bonding is to actually work the
topsoil into the layer below for a depth of at least 6 inches.
• Ripping or re-structuring the subgrade may also provide additional
benefits regarding the overall infiltration and interflow dynamics of
the soil system.
• Field exploration of the site shall be made lo determine if there is
surface soil of sufficient quantity and quality to justify stripping.
Topsoil shall be friable and loamy (loam, sandy loam, silt loam, sandy
clay loam, clay loam). Areas of natural ground water recharge should
be avoided.
• Stripping shall be confined to the immediate construction area. A 4-to
6-inch stripping depth is common, but depth may vary depending on
the particular soil. All surface runoff control structures shall be in
place prior to stripping.
Stockpiling of topsoil shall occur in the following manner:
• Side slopes of the stockpile shall not exceed 2: 1.
• An interceptor dike with gravel outlet and silt fence shall surround all
topsoil stockpiles between October 1 and April 30. Between May 1
---------------------------------------
4-30 Volume II -Constrnction Stormwater Pollution Prevention February 2005
Maintenance
Standards
February 2005
and September 30, an interceptor dike with gravel outlet and silt fence
shall be installed if the stockpile will remain in place for a longer
period of time than active construction grading.
• Erosion control seeding or covering with clear plastic or other
mulching materials of stockpiles shall be completed within 2 days
(October I through April 30) or 7 days (May I through September 30)
of the formation orthe stockpile. Native topsoil stockpiles shall not be
covered with plastic.
• Topsoil shall not be placed while in a frozen or muddy condition,
when the subgrade is excessively wet, or when conditions exist that
may otherwise be detrimental to proper grading or proposed sodding
or seeding.
• Previously established grades on the areas to be topsoiled shall be
maintained according to the approved plan.
• When native topsoil is to be stockpiled and reused the following
should apply to ensure that the mycorrhizal bacterial, earthworms, and
other beneficial organisms will not be destroyed:
I. Topsoil is to be re-installed within 4 to 6 weeks;
2. Topsoil is not to become saturated with water;
3. Plastic cover is not allowed.
• Inspect stockpiles regularly, especially after large storm events.
Stabilize any areas that have eroded.
·-------·----
Volume II -Construction Stormwater Pollution Prevention 4-31
BMP C130: Surface Roughening
Purpose
Conditions for
Use
Design and
Installation
Specifications
ll4aintenance
Standards
4-36
Surface roughening aids in the establishment of vegetative cover, reduces
runoff velocity, increases infiltration, and provides for sediment trapping
through the provision of a rough soi I surface. Horizontal depressions are
created by operating a tiller or other suitable equipment on the contour or
by leaving slopes in a roughened condition by not fine grading them.
• All slopes steeper than 3: I and greater than 5 vertical feet require
surface roughening.
• Areas with grades steeper than 3: 1 should be roughened to a depth of 2
to 4 inches prior to seeding.
• Areas that will not be stabilized immediately may be roughened to
reduce runoff velocity until seeding takes place.
• Slopes with a stable rock face do not require roughening.
, Slopes where mowing is planned should not be excessively roughened.
There are different methods for achieving a roughened soil surface on a
slope, and the selection of an appropriate method depends upon the type of
slope. Roughening methods include stair-step grading, grooving, contour
furrows, and tracking. See Figure 4.6 for tracking and contour furrows.
Factors to be considered in choosing a method are slope steepness, mowing
requirements, and whether the slope is formed by cutting or filling.
, Disturbed areas that will not require mowing may be stair-step graded,
grooved, or left rough after filling.
, Stair-step grading is particularly appropriate in soils containing large
amounts of soft rock. Each "step" catches material that sloughs from
above, and provides a level site where vegetation can become
established. Stairs should be wide enough to work with standard earth
moving equipment. Stair steps must be on contour or gullies will form
on the slope.
, Areas that will be mowed (these areas should have slopes less steep
than 3:1) may have small furrows left by disking, harrowing, raking, or
seed-planting machinery operated on the contour.
, Graded areas with slopes greater than 3: l but less than 2: I should be
roughened before seeding. This can be accomplished in a variety of
ways, including "track walking," or driving a crawler tractor up and
down the slope, leaving a pattern of cleat imprints parallel to slope
contours.
, Tracking is done by operating equipment up and down the slope to
leave horizontal depressions in the soil.
, Areas that are graded in this manner should be seeded as quickly as
possible.
, Regular inspections should he made of the area. !frills appear, they
should be re-graded and re-seeded immediately.
Volume II -Construction Stormwater Pollution Prevention February 2005
Tracking
'TRACKING' with machinery up and down
the slope provides grooves that will catch
seed, rainfall and reduce runoff.
Contour Furrows
"-50'
(15m) ,'
Grooves Will Catch Seed,
Fertilizer, Mulch, Rainfall
and Decrease Runoff.
6" min __ J
(150mm)
3
Maximum I' 1
I
/,,": / ,_
Figure 4.6 -Surface Roughening by Tracking and Contour Furrows
-----------------------
February 2005 Volume II -Construction Stormwater Pollution Prevention 4-37
BMP C140: Dust Control
Purpos·e
Conditions of Use
Design and
Installation
Specifications
Dust control prevents wind transport of dust from disturbed soil surfaces
onto roadways, drainage ways, and surface waters.
• In areas (including roadways) subject to surface and air movement of
dust where on-site and off~site impacts to roadways, drainage ways, or
surface waters arc likely.
• Vegetate or mulch areas that will not receive vehicle tratlic. In areas
where planting, mulching, or paving is impractical, apply gravel or
landscaping rock.
• Limit dust generation by clearing only those areas where immediate
activity will take place, leaving the remaining area(s) in the original
condition, if stable. Maintain the original ground cover as long as
practical.
• Construct natural or artificial windbreaks or windscreens. These may
be designed as enclosures for small dust sources.
• Sprinkle the site with water until surface is wet. Repeat as needed. To
prevent carryout of mud onto street, refer lo Stabilized Construction
Entrance (BMP CI 05).
• irrigation water can be used for dust control. Irrigation systems should
be installed as a first step on sites where dust control is a concern.
• Spray exposed soil areas with a dust palliative, following the
manufacturer's instructions and cautions regarding handling and
application. Used oil is prohibited from use as a dust suppressant.
Local governments may approve other dust palliatives such as calcium
chloride or PAM.
• PAM (BMP C 126) added to water at a rate of 0.5 lbs. per 1,000
gallons of water per acre and applied from a waler truck is more
effective than water alone. This is due to the increased infiltration of
water into the soil and reduced evaporation. In addition, small soil
particles arc bonded together and are not as easily transported by wind.
Adding PAM may actually reduce the quantity of water needed for
dust control, especially in eastern Washington. Since the wholesale
cost or Pi\M is about$ 4.00 per pound, this is an extremely cost-
effective dust control method.
Techniques that can b~ used for unpaved roads and lots include:
• Lower speed limits. High vehicle speed increases the amount of dust
stirred up from unpaved roads and lots.
• Upgrade the road surface strength by improving particle size, shape,
and mineral types that make up the surface and base materials.
----------------~-----------------
4-40 Volume II -Construction Stormwater Pollution Prevention February 2005
I
Maintenance
Standards
February 2005
• i\dd surface gravel to reduce the source of dust emission. Limit the
amount of fine particles (those smaller than .075 mm) to IO to 20
percent.
• Use geotextile fabrics to increase the strength of new roads or roads
undergoing reconstruction.
• Encourage the use of alternate. paved routes. if available.
• Restrict use by tracked vehicles and heavy trucks to prevent damage to
road surface and base.
• Apply chemical dust suppressants using the admix method. blending
the product with the top few inches of surface material. Suppressants
may also be applied as surface treatments.
• Pave unpaved permanent roads and other trafficked areas.
• Use vacuum street sweepers.
• Remove mud and other dirt promptly so it does not dry and then turn
into dust.
• Limit dust-causing work on windy days.
• Contact your local Air Pollution Control Authority for guidance and
training on other dust control measures. Compliance with the local Air
Pollution Control Authority constitutes compliance with this UMP.
Respray area as necessary to keep dust to a minimum.
Volume II -Construction Stormwater Pollution Prevention 4-41
BMP C151: Concrete Handling
Purpose
Conditions of Use
Design and
Installation
Specifications
Maintenance
Standards
February 2005
Concrete work can generate process water and slurry that contain fine
particles and high pH, both of which can violate water quality standards in
the receiving water. This BMP is intended to minimize and eliminate
concrete process water and slurry from entering waters of the state.
Any time concrete is used, these management practices shall be utilized.
Concrete construction projects include, but are not limited to, the
following:
•
•
•
•
•
•
•
•
•
•
Curbs
Sidewalks
Roads
Bridges
Foundations
Floors
Runways
Concrete truck chutes. pumps, and internals shall be washed out only
into formed areas awaiting installation of concrete or asphalt.
Unused concrete remaining in the truck and pump shall be returned to
the originating batch plant for recycling.
Hand tools including, but not limited to, screeds, shovels. rakes. floats,
and trowels shall be washed off only into formed areas awaiting
installation of concrete or asphalt.
, Equipment that cannot be easily moved, such as concrete pavers, shall
only be washed in areas that do not directly drain to natural or
constructed stormwater conveyances.
• Washdown from areas such as concrete aggregate driveways shall not
drain directly to natural or constructed stormwater conveyances.
, When no formed areas are available, washwatcr and leftover product
shall be contained in a lined container. Contained concrete shall be
disposed of in a manner that does not violate groundwater or surface
water quality standards.
Containers shall be checked for holes in the liner daily during concrete
pours and repaired the same day.
Volume II -Construction Stormwa/er Pollution Prevention 4-43
BMP C152: Sawcutting and Surfacing Pollution Prevention
Purpose
Conditions of Use
Design und
Installation
Specifications
Maintenance
Standards
Sawcutting and surfacing operations generate slurry and process water
that contains fine particles and high pH ( concrete cutting), both of which
can violate the water quality standards in the receiving water. This BMP
is intended to minimize and eliminate process water and slurry from
entering waters or the State,
Anytime sawcutting or surfacing operations take place, these
management practices shall be utilized. Sawcutting and surfacing
operations include, but are not limited to, the following:
• Sawing
• Coring
• Grinding
• Roughening
• Hydro-demolition
• Bridge and road surfacing
• Slurry and cuttings shall be vacuumed during cutting and surfacing
operations.
• Slurry and cuttings shall not remain on permanent concrete or asphalt
pavement overnight.
• Slurry and cuttings shall not drain to any natural or constructed
drainage conveyance.
• Collected slurry and cuttings shall be disposed or in a manner that does
not violate groundwater or surface water quality standards.
• Process water that is generated during hydro-demolition, surface
roughening or similar operations shall not drain to any natural or
constructed drainage conveyance and shall be disposed of in a manner
that does not violate groundwater or surface water quality standards.
• Cleaning waste material and demolition debris shall be handled and
disposed of in a manner that does not cause contamination of water. If
the area is swept with a pick-up sweeper, the material must be hauled
out of the area to an appropriate disposal site.
Continually monitor operations to determine whether slurry, cuttings, or
process waler could enter waters or the state. If inspections show that a
violation of water quality standards could occur, stop operations and
immediately implement preventive measures such as berrns, barriers,
secondary containment, and vacuum trucks.
--. ---------------------------· February 2005 4-44 Volume II -Construction Stonnwater Pollution Prevention
Maintenance
Standards
4-98
, Any damage shall be repaired immediately.
• If concentrated flows arc evident uphill of the fence, they must be
intercepted and conveyed to a sediment pond.
• It is important to check the uphill side of the fence for signs of the
fence clogging and acting as a barrier to flow and then causing
channelization of flows parallel to the fence. If this occurs, replace the
fence or remove the trapped sediment.
, Sediment deposits shall either be removed when the deposit reaches
approximately one-third the height of the silt fence, or a second silt
fence shall be installed.
, If the filter fabric (geotextile) has deteriorated due to ultraviolet
breakdown, it shall be re laced.
Pondin9 h•ight
mSlll. 24~
Atta<cll faltrlc ta
up•tr.,._ •Ida of P°"t
FLOW
Drtra a,,. nclrl •Ide of
slit fence :1 to 4 •••• -d-c••-ng
60 p.s.l. a~ --•
100'% ,,_, ..... 4
POST SPACING:
7' 111a1t. on "P•n runs
POSTDE.PTH:
A• ,....,h llal.ow gTeund
••fabric....,,... g.-o!Mld
100% ca ... actlo"
-----····---·-· :of Fabric f
·1··
ATIAOMENI OfTAll.5:.
lllngonlll !!ltll.CtTile!tl --• .
.
• Gau-fabric. pom.. ',-,1ud.
• llllllre ttv99 llesper post. al wllttln top a~ of raa::.ic .
• Po,,,tiorl fll!lchtiedtagor,.tllly. ~ holm.~ly
.. ~cf1"npalt.
• H...ng-ch tie on a post.~ e11d llgt,larl -=-~-
Use cable Lies {5Ca:Hil or d -·
Roll of silt fence
Si'it Fence
~~?~1
·•· .. :, 0' ·-,,:,
afte
compacl!On
Vibratory plow iS not acceptable because of horizontal compaction
Figure 4.20 -Silt Fence Installation by Slicing Method
Volume II -Construction Storm water Pollution Prevention February 2005
BMP C233: Silt Fence
Purpose Use of a silt fence reduces the transport of coarse sediment from a
construction site by providing a temporary physical barrier to sediment
and reducing the runoff velocities of overland flow. See Figure 4.19 for
details on silt fence construction.
Conditions of Use Silt fence may be used downslope of all disturbed areas.
Design and
Installation
Specijicutions
4-94
, Silt lence is not intended to treat concentrated flows, nor is it intended
to treat substantial amounts of overland flow. Any concentrated flows
must be conveyed through the drainage system to a sediment pond.
The only circumstance in which overland flow can be treated solely by
a silt fence, rather than by a sediment pond, is when the area draining
to the fence is one acre or less and flow rates are less than 0.5 cfs.
, Silt fences should not be constructed in streams or used in Y-shaped
ditches. They are not an adequate method of silt control for anything
deeper than sheet or overland flow.
Joints !11 filter f;10r1c :-;hall be spliced at
posts. Use s:aples. wire r1nys ur :t'x2" b" 14 1~a. wire or
equivalent tc attach fa.br·c-. to post::; equivafent. 11 :otand,vc
~~+'f'Fcctts'=:c=:::-_-_-strength tab:"iC used
/
;:i11er rabnc ------·
6' max
?ost spacing rnay be increased
le 8' 1! wire backing is used
1,1
r-..,11nir1t,m 4"x4" uerch .,,, I
I !
\, BacK.fill trencr, with na1,ve soil
or 3.4"·1.5" wa.sned gravel
2"x2" wood oasis. steel 1ence
posls. or equivalent
Figure 4.19-Silt Fence
I
=
N
I
I
k.:---' j,"::'.Yl-,.S:
I
', ----:-1 -, E
' C\J ,-
• Drainage area oC l acre or less or in combination with sediment basin
in a larger site.
• Maximum slope steepness (normal (perpendicular) to fence line) I: I.
, Maximum sheet or overland flow path length to the fence of I 00 feet.
, No flows greater than 0.5 cfs.
, The geotextile used shall meet the following standards. All geotextile
properties listed below are minimum average roll values (i.e., the test
result for any sampled roll in a lot shall meet or exceed the values
shown in Table 4.10):
Volume II -Construction S/ormwaler Pollution Prevention February 2005
February 2005
r Table4.10 I
_ _____ Geotexti_le Standard!!_ ___ _ _ ___ J
Polymeric Mesh AOS r 0.60 mm maximum for slit film wovens (#30 sieve). 0.30
(ASTM !)4751) , mm maximum for all other geotextile types (#50 sieve).
------
ater Pennittivity
STM D4491)
i rab Tensile Strength
Ii ( STM D4632)
h,rnb Tensile Strength
' (ASTM D4632)
! 0.15 mm minimum fora\] fabric types (#100 sieve).
I ---~
_10-02 s:_~ummum ··------
] &O lbs. Minimum for extra strength fabric.
l 00 lbs minimum for standard strength fabric.
i 30% max1;;u;,-:;---------·
'
------------
1 Ultraviolet Resistance
(ASTM D4355)
1 70% minimum
I
• Standard strength fabrics shall be supported with wire mesh, chicken
wire, 2-inch x 2-inch wire, safety fence, or jute mesh to increase the
strength of the fabric. Silt fence materials are available that have
synthetic mesh backing attached.
• Filter fabric material shall contain ultraviolet ray inhibitors and
stabilizers to provide a minimum of six months of expected usable
construction life at a temperature range of0°F. to 120°F.
• I 00 percent hiodegradable silt fence is available that is strong, long
lasting, and can be left in place after the project is completed, if
permitted by local regulations.
• Standard Notes for construction plans and specifications follow. Refer
to Figure 4.19 for standard silt fence details.
The contractor shall install and maintain temporary silt rences at the
locations shown in the Plans. The silt fences shall be constructed in
the areas of clearing, grading, or drainage prior to starting those
activities. A silt fence shall not be considered temporary if the silt
fence must function beyond the life of the contract. The silt fence
shall prevent soil carried by runoff water from going beneath, through,
or over the top of the silt fence, but shall allow the water to pass
through the fence.
The minimum height of the top of silt fence shall be 2 feet and the
maximum height shall be 2Y, feet above the original ground surface.
The gcotextile shall be sewn together at the point of manufacture, or at
an approved location as determined by the Engineer, to form geotcxtile
lengths as required. A II sewn seams shall be located at a support post.
Alternatively, two sections of silt fence can be overlapped, provided
the Contractor can demonstrate, to the satisfaction of the Engineer, that
the overlap is long enough and that the adjacent fence sections are
close enough together lo prevent silt laden water from escaping
through the fence at the overlap.
Volume II -Construction Stormwater Pollution Prevention 4-95
4-96
The geotexlile shall be attached on the up-slope side of the posts and
support system with staples, wire, or in accordance with the
manufacturer's recommendations. The geotextile shall be attached to
the posts in a manner that reduces the potential for geotextile tearing at
the staples, wire, or other connection device. Silt fence back-up
support for the geotextile in the form of a wire or plastic mesh is
dependent on the properties of the geotextile selected for use. If wire
or plastic back-up mesh is used, the mesh shall be fastened securely to
the up-slope of the posts with the geotexti le being up-slope of the
mesh back-up support.
The geotextile at the bottom of the fence shall be buried in a trench to
a minimum depth of 4 inches below the ground surface. The trench
shall be backfilled and the soil tamped in place over the buried portion
ofthe gcotextile, such that no flow can pass beneath the fence and
scouring can not occur. When wire or polymeric back-up support
mesh is used, the wire or polymeric mesh shall extend into the trench a
minimum of 3 inches.
The fence posts shall be placed or driven a minimum of 18 inches. A
minimum depth of 12 inches is allowed if topsoil or other soft
subgrade soil is not present and a minimum depth of 18 inches cannot
be reached. Fence post depths shall be increased by 6 inches if the
fence is located on slopes of 3: l or steeper and the slope is
perpendicular to the fence. If required post depths cannot be obtained,
the posts shall be adequately secured by bracing or guying to prevent
overturning of the fence due to sediment loading.
Silt fences shall be located on contour as much as possible, except at
the ends of the fence, where the fence shall be turned uphill such that
the silt fence captures the runoff waler and prevents water from
flowing armmd the end of the fence.
If the fence must cross contours, with the exception of the ends of the
fence, gravel check dams placed perpendicular to the back of the fence
shall be used to minimize concentrated flow and erosion along the
back of the fence. The gravel check dams shall be approximately ] -
foot deep at the back of the fence. It shall be continued perpendicular
to the fonce at the same elevation until the top of the check dam
intercepts the ground surface behind the fence. The gravel check dams
shall consist of crushed surfacing base course, gravel backfill for
walls, or shoulder ballast. The gravel check dams shall be located
every IO feet along the fence where the fence must cross contours.
The slope of the fence line where contours must be crossed shall not
he steeper than 3: l .
Wood, steel or equivalent posts shall be used. Wood posts shall have
minimum dimensions of2 inches by 2 inches by 3 feet minimum
length, and shall be free of defects such as knots, splits, or gouges.
Volume II -Construction Stormwater Pollution Prevention February 2005
February 2005
Steel posts shall consist of' either size No. 6 rebar or larger, ASTM A
120 steel pipe with a minimum diameter of I-inch, U, T, L, or C shape
steel posts with a minimum weight of 1.35 lbs./ft. or other steel posts
having equivalent strength and bending resistance to the post sizes
listed. The spacing of the support posts shall be a maximum of 6 feet.
Pence back-up support, if used, shall consist of steel wire with a
maximum mesh spacing of 2 inches, or a prefabricated polymeric
mesh. The strength of the wire or polymeric mesh shall be equivalent
to or greater than 180 lbs. grab tensile strength. The polymeric mesh
must be as resistant to ultraviolet radiation as the geotextile it supports.
• Silt fence installation using the slicing method specification details
follow. Refer to Figure 4.20 for slicing method details.
The base of both end posts must be at least 2 to 4 inches above the top
of the silt fence fabric on the middle posts for ditch checks to drain
properly. Use a hand level or string level, if necessary, to mark base
points before installation.
Install posts 3 to 4 feet apart in critical retention areas and 6 to 7 feet
apart in standard applications.
Install posts 24 inches deep on the downstream side of the silt fence,
and as close as possible to the fabric, enabling posts to support the
fabric from upstream water pressure.
Install posts with the nipples facing away from the silt fence fabric.
Attach the fabric to each post with three ties, all spaced within the top
8 inches of the fabric. Attach each tie diagonally 45 degrees through
the fabric, with each puncture at least I inch vertically apart. In
addition, each tie should be positioned to hang on a post nipple when
tightening to prevent sagging.
Wrap approximately 6 inches of fabric around the end posts and secure
with 3 ties.
No more than 24 inches of a 36-inch fabric is allowed above ground
level.
The rope lock system must be used in all ditch check applications.
The installation should be checked and corrected for any deviation
before compaction. Use a flat-bladed shovel to tuck fabric deeper into
the ground if necessary.
Compaction is vitally important for effective results. Compact the soil
immediately next to the silt fence fabric with the front wheel of the
tractor, skid steer, or roller exerting at least 60 pounds per square inch.
Compact the upstream side first and then each side twice for a total of
four trips.
Volume II -Construction Stormwater Pollution Prevention 4-97
Maintenance
Standard.,
4-98
• Any damage shall be repaired immediately.
•
•
•
If concentrated flows arc evident uphill of the fence, they must be
intercepted and conveyed to a sediment pond.
It is important to check the uphill side of the fence for signs of the
fence clogging and acting as a barrier to flow and then causing
channelization of !lows parallel to the fence. If this occurs, replace the
fence or remove the trapped sediment.
Sediment deposits shall either be removed when the deposit reaches
approximately one-third the height of the silt fence. or a second silt
fence shall be installed.
• If the filter fabric (geotextile) has deteriorated due to ultraviolet
breakdown, it shall be re laced.
~ondln9 h•i9h.t
mall. 24~
Anau, r..i,nc to
up~to,_ •ldo ol p-t
FLOW
POST SPACING:
r 111aK. on i,p•n !'\In•
4• rn••-an pooltn& •r•••
POST DEPTH:
A:a 111uch below ground
•• taante _,.,_ !I"•""'"'
-+-OpJ:!fation
·-·-----·:to(Fabfic 1
·1··
• G.,tha-fabric • posts. f needed.
• lltillle Uvee ties.per pest. al within top a· of ,aooc .
• Pooitlarl Bilich tie dill!JOl"llly. purict>..rio9 holm.wwtlca11y
" ...........,... or 1" npiirt.
• Hang 1tech tie on a p,:m nipple imd lighUln sea.ntly.
U. cable !Im {SOb,j or !NJft-.
Roll of silt fence
Silt Fence
afier
compaction
!ttJ;-
/:?,/;'
;(i"
Completed lmtala'llon
VitxatOf)' plow is not acceptable because of holizontal compaction
Figure 4.20 -Silt Fence Installation by Slicing Method
Volume JI -Construction Stormwater Pollution Prevention February 2005