HomeMy WebLinkAbout03202 - Technical Information Report - Drainage � • �'�t..� �# 3��o�
STURi��i DRAIi�tAGE REPQRT
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' 't�VIL�YNSUN 5H017'T PLA'1'
��45 SHAT�'UG� A'tTE S
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FILE NO. 860-001-041 ;
PREPARED BY
TOUMA ENGINEERS
6632 SOUTH 191ST PLACE, SUITE E-102
KENT, WA 98032
(425) 251-0665
July 10, 2004
Revised August 18, 2004
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TABLE OF CONTENTS
I. PROJECT OVERVIEW
1t. COIVDITIONS & REQUIRMENT SUSMNIARY
I11. OFF-SITE ANALYSIS
[V. FLOW CONTROL ANU WATER QUALITY
V. ('ONVEYANCE SYS"I'EM ANALYSIS
V[. SPECIAL REPORT AND STLIDIES
VII. BAS[N AND OTHER COMMUNlTY AREAS
VIII. OTHER PERMITS ;
IY. EROSION/SEDMINTATION CONTROL DESIGN ,
i
X. BOND QUANITIES WORKSHEET i
' I!
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1. PROJECT OVERVIEW
This project consists of one parcel of land approximately 0.94 acres. The project is situated in the
south end of the City of Renton. It is located within southeast quarter of Section 19 Township 23
North, Range 5 East W. M. The proposal is to create 5 lots, "Fill in" subdivision. Access to the
proposed project will be via from Shattuck Avenue S situated east of the property. There exists a
house on the property, which the owners are planning to remove.
The soils on the site are composed of Beausite (BeC) gravelly and fine sand per the King County
Soil Survey. This type of soil is designated by King County as Type A soil.
The site is flat with an average slope of 2 to 4 percent for the easterly portion, which is referred to
as the East Sub-basin. There are steeper slopes present along the southwest portion of the site.
There are two sub-basins delineated on the site. The easterly sub-basin orients the sheet flow to
the east and north toward Shattuck Avenue South, and the westerly sub-basin orie�ts the sheet
flow south and west toward the steep area toward State Road SR 167..
The proposed development of the property is considered as "infill" project and it is located in a
Single Family(SF) zone. The City of Renton designates the site with the SF zoning. The SF
zoning allows for a minimum lot size of 4500 square feet and a maximum density of 8 units per
acre. The setbacks allowed in the SF zone are as follows: front yard setbacks, 20 feet; rear yard
setbacks, 20 feet, and side yards setbacks, 5 feet on all interior lots, and 15 feet on corner lots.
The City of Renton Comprehensive Plan designates the site as Single-Family Residence, which is
consistent with present zoning.
The density for the developed portion calculates at 6.2 dwelling unit per net developed acre. The
proposed lots will be served from private access easements.
Majority of trees will be retained except for those which fal( within the building envelope of the '
proposed new lots.
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11. PRELIMINARY COND[TIONS SUMMARY
CORE REQUIREMENTS 1-5
CORE REQUIREMENT #1: DISCHARGE AT NATURAL LOCAT[ON
The allowable outflows from the site will be discharged to its natural locations. The level I flow
control is calculated to match the developed peak discharge rate to the existing site condition
peak discharge for the 2-year and 10-year.
CORE REQUIREMENT #2: OFF-SITE ANALYSIS
A level one downstream analysis is a portion of this report. See the analysis later in this report
below.
CORE REQUIREMENT #3: RUNOFF CONTROL
The increased peak runof�; due to any change in pervious conditions and increase of impervious
areas, will be attenuated using peak rate runoffcontrol in accordance with KCSWM , 1990
Edition. Initial calculation indicates there is no detention facility required. Please refer to attached
calculation.
CORE REQUIREMENT #4: CONVEYANCE SYSTEM
Conveyance system is minimal for this proposed short plat. Storm runoff from private road and
new homes will be directed into either the storm drain system exists on Shattuck Avenue S for the
east sub-basin or southwest into a proposed spreader to sheet flow across the steep slope into the
valley floor
I
CORE REQUIREMENT #5: EROSION/SEDIMENTATION CONTROL PLAN
The erosion control plan will be designed using the King County storm manual. Silt fences will be
used to intercept silt laden material resulting from clearing and/or construction of short plat
improvements. These facilities will be adhered to minimize any impact to downstream or offsite
areas.
CORE REQUIREMENT #6 —MAINTENANCE AND OPERATION
N/A
CORE REQUIREMENT #7— FINANCIAL QUARANTEES AND LIABILITY
N/A
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CORL REQUIREMENT #8 - WATER QUALITY
The impervious surfaces subject to vehicular traffic within this proposed short plat is less than
5000 square feet, thus exempted from design and installation of water quality tank or pond or
similar faci(ities.
SPECIAL REQUIREMENTS: THOSE APPLICABLE TO PROJECT
1. Critical Drainage Area - N/A
2. Compliance with existing Master Drainage Plan - N/A
3. Conditions Requiring Master Drainage Plan -N/A
I
4. Adopted Basin or Community Plans - N/A
5. Special Water Quality Controls-N/A
6. Coalescing Plate Oil/Water Separators-N/A
7. Closed Depressions - N/A
8. Use of Lakes, Wetlands, or Depressions for Detention - N/A
9. Delineation of l00 Year Flood Plain - N/A
10. Flood Protection for Type 1 and 2 Streams - N/A
11. Geotechnical Analysis and Report - Preliminary Report prepared by Geo Engineers, dated
June 2, 2000.
12. Soils Analysis and Report - See Item 11
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1[I. OFF-SITE ANALYSIS
CORE REQUIREMENT #2: OFF-S1TE ANALYSIS
LEVEL l ANALYSIS
A: UPSTREAM ANALYSIS
The proposed short plat does not have appreciable of�=site runoff entering the site. There are two
sub-basins delineated on the site. The easterly sub-basin orients the sheet flow to the east and
north toward Shattuck Avenue South, and the westerly sub-basin orients the sheet flow south and
west toward the steep area toward State Road SR 167
B: DOWNSTREAM ANALYSIS �I
�
�� The downstream anal��sis is presented for each sub-hasin:
East Sub-basin
Sheet flow generated ti�on� Lut � ancl a porti�n ui�Lot �1 tl��ti�� east to an existii�� I .
Storm system in Shattuck Avenue S. This conveyance system flows north toward
South 19`h Street, approximately quarter of a mile downstream of the site. There are
no obvious problems noted during the field visit for the downstream system.
West Sub-basin
The sheet flow generated from this sub-basin is oriented to the south and west across
fairly steep slope for approximately 600 to 700 feet to an existing wetland situated east of
SR 167. The steep slope is fairly wooded. There was evidence of problem downstream
from the site.
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IV. FLOW CONTROL AND WATER QUALITY
The foilowing calculations conclude that the 100-year peak flow difference between the
pre-developed flow and post-developed flow is less than 0.5 cfs. In accordance with 1990
King Count Surface Water Drainage Manual detention is not required.
The following calculations include:
Basin areas
Time of Concentration
Isopulvial graphs for 2, 10 and 100 year storms
Pre-developed and Post-developed hydrographs
In discussing this matter with the owner, we ascertained that the size of homes proposed
on the vacant lots will average about 2000 square feet plus 400 feet allowed for
driveways.
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I
.
WILKINSON SHORT PLAT
BASIN AREA ANALYSIS
TOTAL AREA 0.94 ACRES
SOIL TYPE BeD- Beausite
Type "C"
EXISTING CONDITIONS -EAST SUB-BASIN
IMPERVIOUS
HOUSE & SHED 1111
DRIVEWAY 1120
2231 0.05 ACRES CN 98
PERVIOUS
GRASS 0.25 ACRES CN 85
0.3
POST CONDITIONS-EAST SUB-BASIN
IMPERVIOUS
2 HOUSES 4000
2 DRWYS 800
ROAD 2300
7100 0.16 ACRES CN 98
PERVIOUS
LANDSCAPING 0.14 ACRES CN 86
0.3
EXISTING CONDITIONS -WEST SUB-BASIN
I
IMPERVIOUS
SHOP 732
DRIVEWAY 420
1152 0.03 ACRES C�! 98
PERVIOUS
GRASS 0.61 ACRES CN 85
POST CONDITIONS-WEST SUB-BASIN
IMPERVIOUS
3 HOUSES 6000
3 DRINYS 6300
ROAD 3320
13500 0.31 ACRES CN 98
PERVIOUS
LANDSCAPING 0.33 ACRES CN 86
. �
TIME OF CONCENTRATION-PRE DEVELOPMENT-FOR 10-YEAR, 24-HOUR STORM DESIGN
EAST SUB-BASIN
SHEET FLOW- OFFSITE
MANNING-n 0.15
OVERLAND-L 130 FEET
PRECIPITATION-P 2.9 INCH
SLOPE- S U.03 FT/FT
T1 = 10.80 MINUTES
WEST SUB-BASIN
SHEET FLOW- OFFSITE
MANNING-n 0.15
OVERLAND-L 215 FEET
PRECIPITATION-P 2.9 INCH
TIME OF CONCENTRATION -POST DEVELOPMENT-FOR 10-YEAR, 24-HOUR STORM DESIGN
SINCE THERE WILL BE A SHORT SHEET FLOW RUNOFF ANTICIPATED
THE TIME OF CONCENTRATION USED IS THE MINIMUM OF 6.3 MINUTES
,
SBUIi/SCS METHOD FOR COMPUTING RUNOFF HYDROGRAPH
S`I'ORM OPTIONS :
1 - S .C. S . TYPE-lA G -ri'S�-' �QSi�'!
2 - 7-DAY DESIGN STORM
3 - STORM DATA FILE
SPECIFY STORM OPTION:
1
S .C. S . TYPE-lA RAINFALL DISTRIBUTION
ENTER: FREQ (YEAR) , DURATION(HOUR) , PRECIP (INCHES)
i100, 24, 3 . 9
******************** S .C. S . TYPE-lA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3 . 90" TOTAL PRECIP . *********
- - ---- ------------------------------------- ----- - - - - -- -
ENTER: A (PFRV) , CN(PFRV) , A(IMPERV) , CN (IMPERV) , TC FOR BASIN NO. 1
0 . 61, 86, 0 . 03 , 98, 6 . 8
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES)
A CN A CN
• 6 . 6 86 . 0 . 0 98 . 0 6 . 8
PEAK-Q (CFS) T-PEAK (HRS) VOL {CU-FT) �
. 42 7 . 83 5836
i
SPECIFY : C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP I
C ;
- -------- - - --------- ----------------- ------- -------- ------------ - -- - - - �
ENTER: A(PERV) , CN (PERV) , A(IMPERV) , CN(IMPERV) , TC FOR BASIN NO. 2 ;
0 . 33 , 85, 0 . 31, 98, 6 . 3 �
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC (MINUTES) j
A CN A CN '�
. 6 _ 3 85 . 0 . 3 98 . 0 6 . 3 ,
PEAK-Q(CFS) T-PEAK (HRS) VOL (CU-FT) I
. 50 7 . 67 6961
ENTER [d: ] [path] filename [ .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
SBUH/SCS METHOD FOR COMPUTING RUNOFF HYDROGRAPH
STORM OPTIONS :
1 - S .C. S _ TYPE-lA
2 - 7-DAY DESIGN S'i'ORM I��e
3 - S'I'ORM DATA F I L E S.� ��i S/�7
SPECIFY STORM OPTION
1
S .C . S . 'I'Y�'F- lA P�AINFALI, DISTRIBUTIOr1
ENTER: FREQ (YEAR) , DURATION (HOUR) , PRECIP ( INCHES)
100, 24, 3 . 9
- - - - - ---- - - - -- - --- -- --- - --- - --------- - ------- - - - - - - -- - - - - - - - - - - - -- - - - - -
******************** S .C. S . TYPE-lA DISTRIBUTION ********************
********* 100-YEAR 24-HOUR STORM **** 3 . 90" TOTAL PRECIP. I*********
- --- -------- ------- - - --------- ----- ---- ---- --- --- ---- --- -- -- -- - -- - - - - -
ENTER: A(PERV) , CN(PERV) , A(IMPERV) , CN(IMPERV) , TC FOR BASIN NO. 1
0 . 25, 86, 0 . 05, 98, 10 . 80
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC (MINUTES)
A CN A CN
_ 3 . 3 86 . 0 . 1 98 . 0 10 . 8
PEAK-Q (CFS) T-PEAK(HRS) VOL (CU-FT)
. 20 7 . 83 2891
ENTER [d: ] [path] filename ( .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
VAN1
FILE ALREADY EXIST; OVERWRITE (Y or N) ?
Y
SPECIFY: C - CONTINUE, N - NEWSTORM, P - PRINT, S - STOP
C
- - - - -- - --- - - - - - - - --- - ---- ------------- --------- ----- - - - - - - - - - - - - - - - - - - -
EN'I'ER: A(PERV) , CN(PERV) , A(IMPERV) , CN(IMPERV) , TC FOR BASIN NO. 2
0 . 14 , 85, 0 . 16, 98, 6 . 3
DATA PRINT-OUT:
AREA(ACRES) PERVIOUS IMPERVIOUS TC (MINUTES)
A CN A CN
. 3 . l 85 . 0 .2 98 . 0 6 . 3
PEAK-Q (CFS) T-PEAK(HRS) VOL(CU-FT)
- 24 7 • 67 3332
ENTER [d: ) [path) filename [ .ext] FOR STORAGE OF COMPUTED HYDROGRAPH:
VAN2
V. CONVEYANCE SYSTEM ANALYSIS
The proposed storm system consists of one 12-inch pipe (double wall pipe) conveying
runoff from the site. The 100-year developed runoff from the site calculated at
0.55 cfs.
The 12-inch pipe proposed for this development is designed at a slope of 1.0
percent slope. The attached calculation determined that the proposed pipe at a
minimum of 1.0 percent slope carries 3.86 cfs, more than 0.55 cfs anticipated for
100-year flows during post development.
�
- �
VI. SPECIAL REPORT AND STUDIES
i
i
_ 9 I
I
.�i��..
�i/ ,
�if',O�L'II�;IIl(',�I'S
June 2, 2000
Consultin�Engincers
:111(I�itUSCtI'l1�ISIti
King Construction
P.O. Box 1351 (�ffices in wxshingtun,
Snoqualmie,Washington 98038 (irc'gon.and;11ask;�
Attention: Bud King
Report
Preliminary Geotechnical Engineering Services
Proposed Single Family Development
2200 Wilkinson Short Plat
Renton, Washington
File No. 700 I-O(11-�I
INTRODUCTION
This report presents the results of our preliminary geotechnical engincering scrvices for tl�c
proposed Wilkinson Short Plat located in Renton, Washington. Our services have been
completed in accordance with the scope of services presented in our proposal dated May 4, 2000.
Our services were formally authorized by you on May 8, 2000.
The Wilkinson Short Plat comprises Lot 72, Plat 2 Renton Cooperative Coal Company. The
site location is show on the Vicinity Map, Figure 1. We reviewed the Preliminary Short Plat for
King Construction prepared by Baima & Holmberg, Inc. dated October, 6 1998. The short plat
included division of the property into four lots. These lots were the focus�of our report titled
"Geotechnical Consultation — Coal Mine Hazard Evaluation, Wilkinson Short Plat, Renton
Washington", dated October 27, 1998. Three 7,500 square foot lots (Lots 1 through 3) are
located in the northeast corner of the Wilkinson Short Plat. 11�e remaining area of the original
sho�t plat(Lot 4), is the focus of this report, and occupies the area due south, southwest, and west
of the previously mentioned Lots 1 though 3. This azea will be subdivided into 5 lots designated
as Shado�v Lots I through 5 on the site plan provided to us prepared by Baima and Holmberg
dated January 25, 1999. The size of the shadow lots ranges from about 6,600 to 10,500 square
fcct.
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June 2, 2000
Page 2
SCOPE
The purpose of our preliminary geotechnical engineering services is to provide you with an
assessment of the critical azeas hazards at the property and respond to the requirements set forth by
d�e city of Renton. Specifically,our scope of services includes the following tasks:
1. Review available geologic information for the site and surrounding area from published reports
and maps, and our files. This includes reviewing our previous study regarding historical coal
mines in the project azea and our copies of the Critical Areas Inventory maps.
2. Complete a geologic reconnaissance of the site and immediately adjacent area to identify any
apparent signs of stability or erosion hazard.
3. Excavate five test pits using a rubber-tired backhoe to evaluate the general!subsurface soil and
groundwater conditions.
4. Provide guidelines for earthwork including sUipping, removal of soft, organic or other
unsuitable material and grading. This includes evaluating the effects of weather and
construction equipment on the site soils.
5. Provide recommendations for suitable building setback from adjacent steep slopes
(as defined by city of Renton).
6. Provide an update to our previous assessment of potential coal mine hazards areas which �vill
affect development of the property, if necessary.
7. Provide an assessment of potential landslide hazards and describe any evidence of existing or
past instability eviden[ during our site reconnaissance and explorations. This includes ,
recommendations for fill placement on or near slopes.
8. Provide an assessment of potential erosion hazards and describe any existing erosion problems '
evident during our site reconnaissance. This includes discussion of potential impacts and '
necessary mitigating techniques.
9. Prepare a written report presenting our conclusions and recommendations along w�th
supporting field data. �
SITE CONDITIONS
SURFACE CONDITIONS
General
The site is situated in the southw�est 1/4 of the southeast 1/4 of Section 19, Township
23 North, Range 5 East,Willamette Meridian. 'The Wilkinson Short Plat measures about 300 feet
east to west and 210 feet north to south. The site is bordered on the east bv Shattuck Avenue
South and on the south by the right-of-way extension for South 23`d Street. The site is also
bordered by residential property to the north and a steep slope with open space to the west. An
underground pipeline and an overhead po�ver line also border the site to the south. The project
site that this report focuses on includes the southern '/2 of the plat and tlie building lot located in
G e o E n g i n e e r s File No. 7001-001-0I-1130
}tin� C�,nst�uctiun
June 2, 2(�UO
Pagc 3
thc nortliwest portion of[hc plat. 7�hc sitc consists of Shado��� Lots 1 throu�h 5 sho���n on thc Sitc
Plan, Figure 2.
Site Reconnaissance
A reconnaissance «as completed on I�1a� 19, 2O00 to cJetcrmine slopc gcoemtries, obscrvc
areas of slope instability, and evtdence of potential erosion har,rds, if prescnt. The ground
surface topography is shown in Figure 2.
The ground surface slopes down to the south��est in t}�e «est portion of tl�c sitc (Shado«
Lots 1, 2 and 3). The ground surface in the west portion of the site slopes down to the southwest
from about Elevation 190 feet to about Elevation 160 feet near the southwest property corner.
The eastern portion of the site (Shadow Lots 4 and 5) is generally f
east portion of the site ranges from about Elevation 190 to 194 feet.
The north portion of Shadow Lot 1 has a small slope area �ti�,:� ��ti..„�.,��,.,�, r,u�b+��b �r;_��n
about 55 to 65 percent. The top portion of this slope appears to be the result of fill placement
during construction of 22"d Place South. The remaining slopes on Shadow Lots 1, 2, and 3 have
inclinations ranging from about 20 to 30 percent. An existing residence and out buildings are
located in the east portion of the site (Shadow Lots 4 and 5). Driveway access to the residence
enters the site from Shattuck Avenue South. Vegetation on the site includes scattered deciduous
trees, with a moderately dense understory inc(uding shrubs and areas of thick blackberries on the
sloping w�est portion of the site.
No signs of slope instability �vere observed at the site during our reconnaissance. Erosion
features such as incision, surface slumping or seepage were not evident at the site. Our
reconnaissance included observation on the slope west of the site to an apprdximate elevation of
50 feet. We observed no signs of slope instability on these slopes. Steep, near vertical access
road cuts oriented north to south were observed at hvo locations west of, and below, the site
during the reconnaissance. These steep cuts were up to 20 feet in height and located about
75 to 125 feet below the site elevation. These steep cuts showed no indication of instability.
SUBSURFACE CONDITIONS
We reviewed the Geologic Map of the Renton Quadrangle, King County, Washington, by
D.R. Mullincaux, datcd 1965, for information regarding site geology. Tt�e site and vicinity are
mapped as Tertiary Renton Formation, which generally consists of sandstone, mudstone, and
shale that has been folded and faulted. Coal was mined from seams within this formation beneath
a portion of the site.
Five test pits (TP-1 through TP-5) were excavated at the site on May 19, 2000, using a
rubber-tired backhoe. The test pits were excavated in the west portion of the site. The location of
the test pits are shown in Figure 2.
The exp(orations were continuously monitored by a geologist from our firm who visually
eYamined and classified the soils encountered, obtained representative soil samples, observed
ground water conditions, and prepared a detailed log of each exploration. Disturbed samples
were obtained from the test pits. Soils encountered were visually classified in general accordance
G e o E n g i n e e r s FileNo. 7001-001-01-1130
' � King Construction
June 2, 2000
Page 4
I
with the classification system described in Figure 3. The test pit logs are presented in Figures 4
through 6. The logs are based on our interpretation of the field data and indicate tl�e various types
of soiis encountered. They also indicate the depths at which the soils or their characteristics
change, although the change might actually be gradual. The densities noted on the test pit logs
are based on the difficulty of excavating, probing with a 1/2-inch-diameter hand probe, and our
experience and judgment of the conditions encountered. The test pits were backfilled with the
excavated material and compacted with the bucket of the backhoe upon completion.
In general, we encountered an upper layer of soft silt with sand from about 0.5 to 2.0 feet
thick in TP-1, TP-3 and TP-4 located in the sloping west portion of the site. In TP-2 located in
the west portion ofthe site, we encountered and upper layer of fil( about 3 feet thick consisting of
soft silt and medium dense silty sand with grave(. The fill layer overlies a fc�rmer topsoil layer
about 6 inches thick consisting of soft silt with roots. In all four of these test pits, we encountered
interbedded medium dense silty sand and medium stiff silt below the upper layers described
above and extending to depths of 3 to 8 feet. We encountered loose to medium dense silty sand
with gravel to a depth of about 2 feet in TP-5 overlying medium stiff silt to a depth of about
6 feet. Bedrock consisting of dense sandstone and hard siltstone was encountered in all five test
pits from depths of about 3 to 8 feet and extending to the bottom of the test pits at depths of about
8 to 10 feet.
No groundwater was encountered in any of the five test pit excavations. The mottled silts
noted in the descriptions indicate the possibility of seasonal groundwater fluctuations. Ground
water levels are expected to fluctuate as a function of season, precipitation and other factors. It is I
also possible, given the subsurface soil conditions that a shallow perched ground water table
could develop in the shallow near surface soil units.
GEOLOGIC HAZARD AREAS ,
General �
The City of Renton Critical Areas Ordinance(Ordinance No. 4835) identifies geologic hazard
areas that require special siudies and design considerations. The project site is in an area diat is
mapped as a landslide, erosion and coal mine hazard azea. Our report provides information
relatcd to evaluation of the potcntial haz.�rds.
Landslide Hazard
The project site contains slopes that indicate the potential for landslide hazards. The majority
of the west portion of the site contains slopes ranging from less that IS percent to about
30 percent. The subsurface soil conditions in this portion of the site consist of interbedded silt '
and silty sand overlying bedrock at depths of 3 to 8 feet below the ground surface. This portion
of the site is best classified as a "medium landslide hazard" area ("areas with slopes between
fifteen and forty percent (15-40%)and underlain by soils that consist largely of sand, gravel or
glacial till").
GeoEngi neers FileNo. 7001-001-0I-1130
�
, ,
King Construction
June 2, 2000
Page 5
A small portion of the site located along the north edge of Shadow Lot 1 in the northwest
corner of the site contains slopes ranging from about 40 to 65 percent (area identified in Figure
2). This portion of the site is best classified as a "high landslide hazard" area ("areas with slopes
greater than forty percent (40%), and areas w�th slopes between fifteen and forty percent
(15-40%)and underlain by soils consisting largely of silt and clay").
The east portion of the site is an area that contains slopes of less than 15 percent. This
portion of the site is therefore best classified as a"low landslide hazard area".
Erosion Hazard
The su�cial soil deposits at the site are identified in the "Soil Survey, King County Area,
Washington" by the Soil Conservation Service dated November 1973 as Beausite gravelly sandy
loam. The Beausite (BeC) soil classification is mapped on slopes of 6 to IS percent with a
moderate erosion hazard; the Beausite (BeD) soil classification is mapped on slopes of
15 to 30 percent with a severe erosion hazard; and the Beausite (BeE) soil classification is
mapped on slopes of 40 to 75 percent with a very severe erosion hazard.
The east, and much of the west, portions of the site where slopes are less than IS percent are
best classified as "low erosion hazard". However, slopes steeper than 15 percent are best
classified as"high erosion hazard".
Coal Mine Hazard
We reviewed a study of coal mining in the Renton area by Momson Knudsen titled! '
"Engineering Investigation for the Renton, Washington Area," dated January+ 1985. This study
provides information regarding the location and history of coal mining activity on and near the
site. We also revie�ved coal mine maps available from the Department of Natural Resources.
Based on information from these sources, we understand that portions of the workings of the
Sunbeam Mine are located below the site. The entrance to the workings is located to the west of
the site at about Elevation 49 feet. The workings that are shown in the reference studies dip down
to the north at about 60 degrees and extend to the east below the Wilkinson Short Plat on a '
heading of about S70E. The trace of the south edge of the workings passes below the northwest
corner of the site and extends below the middle of the east side of the site. The workings appear
to rise slightly to the east and are located at about Elevation 68 feet approximately 1,000 feet east
of the site. The coal seams mined in the workings dip steeply to the north-northeast at about
60 degrees following the trend of the bedrock structure. The w�orkings described above are the
south extent of this portion of the Sunbeam Mine.
The Sunbeam Mine began operation in about 1925 and closed in 1929 as a result of a mine
fire. We did not observe any sinkholes, spoil piles or other features that appear to be related to
coal mine activity on the site. We understand from maps of features observed by others that some
of these features may be present on undeveloped land south of the site.
G e o E n g i n e e r s File Na. 7001-OOI-OI-I 130
. •
King Construction
June 2, 2000
Page 6
Based on the geometry and location of the old mine workings, the site is best classified as a
high coal mine hazard.
CONCLUSIONS AND RECOMMENDATIONS
GENERAL
Our research and field observations indicate that the site is properly delineated as a coa( mine,
landslide, and erosion hazard. However, in our opinion, the site may be developed as proposed
contingent on following our recommendations related to building set back and site preparation
presented in this report. We also recommend that development of Shadow Lots 1 and 2 include
additional geotechnical services to review development plans and construction activities to reduce
the risk of adverse impacts associated with building on the sloping portions of�he site.
Our research indicates that the workings of the Sunbeam Mine are located below the extreme
north portion of Shadow Lot 1. However, we did not observe any indication of coal mine
workings or surface features on the site or adjacent properties. The presence of workings of the
Sunbeam Mine about 140 feet below portions of the site present a low risk to development of the
site that in our opinion can be mitigated through appropriate construction techniques. Additional
discussion and our specific recommendations to mitigate the potential risk are presented below.
LANDSLIDE HAZARDS
The City of Renton defines high landslide hazard areas as slopes inclined at more than
40 percent. According to the City of Renton Critical Areas Ordinance, development is prohibited
on slopes greater than 40 percent. This will prohibit development of the north portion of Shadow I
Lot 1 where slopes are in excess of 40 percent. In addition, we recommend a combined buffer and '
setback of at least 25 feet be maintained from the sides of the 40 percent or steeper slope azea
located on Shadow Lot 1. This will consist of a 10-foot buffer where the existing vegetation is �
maintained plus a IS-foot setback to the building. We also recommend that development plans for
Shadow Lots 1 and 2 include geotechnical review and analysis of all walls and fill slopes proposed
on the lots.
EROSION HAZARDS
Based on our review of the "Soil Survey, King County Area, Washington" by the Soil
Conservation Service dated November 1973 the sloping west portion of the site is best classified
as a high erosion hazard and the east portion is a low erosion hazard, in accordance with the City
of Renton Critical Areas Ordinance. In our opinion, the potential risks of erosion and off-site
sedimentation in the west portion of the site can be reduced through proper implementation of
Best Management Practices for erosion control during construction.
Erosion protection for the project should be in accordance with the minimum requirements of
City of Renton. The design should include the follo�ving considerations to reduce the risk of
erosion and off-site sedimentation:
G e o E n g i n e e r s FileNo. 7001-001-0]-1130
� � King Construction
June 2, 2000
Page 7 �
1. Plan development to fit the topography of the site.
2. Reduce the extent of exposed soil as much as possible and the length of time the soils are
exposed before vegetation is reestablished.
3. Protect disturbed areas as soon as possible using straw mulch or other methods until
vegetation is reestablished.
4. Avoid concentrating runoff and discharging over the slopes.
5. Use drainage swales and straw bales to keep runoff velocities low.
6. Retain sediment on site by effective use of silt fences and other methods.
In addition, we recommend that specific measures be taken in the steep slope portions of
Shadow Lots 1 and 2 to reduce the risk of post construction erosion, as described in the following
paragraphs. We recommend that the disturbed area on the steep slopes be minimized to reduce
the risk of erosion, and that the ground surface be regraded to conform to the original slope
configuration. The final graded slope surface should be roughened prior to hydroseeding.
Surface erosion protection including maintenance of the slopes until full ground cover is
established is the single most important element to the long term protection of the slopes. We
recommend that all areas be hydroseeded following final grading, except where slopes are steeper
than 3H:1 V (horizontal to vertical) where we recommend that the slopes should also be protected
using erosion mats designed for that purpose. Some of these mats are available with the seed,
fertilizer and mulch included. The mats should be anchored to the slope to provide increased
protection on steep areas.
We recommend as a minimum that two applications of hydroseed be included in the project '�
plans and specifications for Shadow Lots 1 and 2. The disturbed portions ofl'the lots should be I
hydroseeded immediately after work on the slopes is complete and again in the fall to provide full
coverage of the slopes. Maintenance should include regrading and corrective measures, as
necessary.
We also recommend that a 10-foot wide undisturbed native vegetative buffer be left in place ,
along the west side of Shadow Lots 1 and 2, and along the south side of Shadow Lot 2. If this
area is disturbed during construction it should be replanted using appropriate native species.
COAL MINE HAZARDS I
Based on our review of published literature and our site observations, it is our opinion that
coal mining activity in the vicinity of the site is limited to workings of the Sunbeam Mine.
Documented su�cial features related to this mine are situated east and west of the site. We did '
not observe any. mine-related features on or nearby the property that would impact the
development of the site.
Based on our review of available information, underground mines exist approYimately
140 feet below the ground surface at the northwest portion of the site. The generally accepted
depth or thickness of overburden for "safe" surface development over underground mines is
200 feet. Case history studies in underground mining areas in Colorado have shown that the
G e o E n g i n e e r s File No. 7001-0O1-01-1130
� � King Construction
June 2, 2000
Page 8
200-foot-depth represents a risk of less than 1 percent for the possible occurrence of a sinkhole
(subsidence feature). Our research in King County indicates that no known sinkholes have been
observed where the depth to the workings is in excess of about 150 feet. It is therefore our
opinion that the risk of sinkhole occurrence is small for the workings underlying the site.
It is also our opinion that the site is at risk regarding regional downwarping of the ground
surface caused by collapse of the abandoned underground workings. The damage to structwes as
a result of regional downwarping may be manifested by jammed doorways or windows, or cracks
in drywall or masonry finishing. The structural damage could be similar to that expected for most
new; wood-framed construction as a result of ground adjustments to the home foundation loads.
We do not characterize this type of damage as threatening to the structure.
There is an inherently higher risk of potential damage to a structure built o��the subject lots as
compared to a similar home built in an area with no underground mines. This is because of the
possibility of undocumented underground workings and prospects, and the unknown nature by
which regiona) downwarping is manifested in this area. The area mine workings have been
abandoned for several decades, and for this reason, a substantial amount of the collapse in the
deep workings should have already occurred.
In order to minimize the risk of structural damage because of potential regional
dowmvarping, we recommend that the use of extra rigid foundations supporting a flexible
superstructure be considered in the general design. A small, square or a nearly square-shaped
building pad is favored over a large, irregularly shaped building pad. Consideration in design of
the superstructure should include the ability to relevel the building and any gravity-dependent
utilities (sanitary sewers, for example). Utility couplings should be flexible enough to anticipate
some post-construction ground movement. ,
We recommend that we be present at the site when the site preparation is initiated. The �
purpose of our site observations during grading would be to observe the underlying soils to see
whether any mining related features (subsidence, sinkholes, adits, air shafts, portals, etc.) are
exposed and to recommend appropriate corrective action.
SITE PREPARATION AND EARTHWORK ;
The following recommendations apply to all earth�vork and structural fill placement and �I
compaction for roadway construction and will generally apply to development of individual lots.
• We recommend that the existing sod and vegetation be stripped to a depth of about 4 to
6 inches. i
• We recommend that the topsoil and any loose soil underlying the sod and vegetation be '
removed to expose a firm base. We expect that the total stripping depth will be about 6 to
12 inches or more below the existing ground surface.
• The exposed subgrade should be compacted to a firm, nonyielding condition prior to placing
structural fill or formwork.
G e o E n g i n e e r s FileNo. 7001-001-01-1130
� King Construction
lune 2, 2000
Page 9
• Imported structural fill should consist of sand and gravel containing less than 5 percent fines by
weight relative to the fraction of the material passing the 3/4-inch sieve. Structural fill should
be free of rock fragments (arger than 6 inches, debris and organic materials.
• Structural fill placed within 2 feet of pavement subgrade in proposed roadway areas must be
compacted to at least 95 percent of MDD (maximum dry density)as determined in accordance
with the ASTM-D1557 test standard. This will also app(y to all struciural fill for foundation
and floor slab support. Structural fill placed below a depth of 2 feet to pavement subgrade
should be compacted to at least 90 percent MDD.
• During periods of wet weather, subgrade areas being prepazed should be limited in size such
that the exposure of the subgrade to wet weather is minimized.
• No fill should be placed on Shadow Lots 1 and 2 without additional geotechnical assessment of
the proposed development plans.
LIMITATIONS
We have prepared this report for use by King Construction and other members of the design
team in the design of a portion of this project. The data and report should be provided to '
prospective contractors for bidding or estimating purposes. However, our report, conclusions ar� i Ij
interpretations should not be construed as a warranty of the subsurface conditions. 71,
conclusions and recommendations in this report should be applied in their entirety.
The project was in the preliminary planning stage at the time this report w�as completed. We
therefore eYpect that modifications to the proposed plat layout may be made as plans are,
finalized. We also expect the geotechnical design studies will be completed fbr development of
Shadow Lots 1 and 2,as recommended above. If there are any changes in the locations or type of
improvements to be constructed, the conclusions and recommendations presented in this report
might not be fully applicable. If any such changes are made, we should be retained to review our
conclusions and recommendations and to provide written modifications or verifications of these
recommendations.
Within the limitations of scope, schedule and budget, our services have been eYecuted in
accordance with generally accepted geotechnical practices in tllis area at the time the report was
prepared. No warrantv or other conditions, express or implied, should be understood.
. o ► -
(i e o E n g i n e e r s FileNo. 7001-001-0i-1130
, •
King Construction
June 2, 2000
Page 10
We appreciate the opportunity to be of service to you on this project. Please call if you have
any questions regarding this report or require additional information.
Respectfully submitted,
3, McF,q i
�,4� � WAS1y�d gineers, n
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SLOPES �TEEPER � �� k'�r,�
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,� Note: The locations of all features shown SITE PLAN
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� PUBUC USE UPON RECOR(LNG � �N � � � ' �
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-�_ ' 234.00' ' � � Q SEWER MANHO�E
ASPHALT 81.34'
THAN 40 PERCENT : _ _ _ ,�E�y,�K . RAMP Y 221�CD `P!_A.CE S. 152.66'• EDGE OT�.^ -
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i"�T PUBUC USE FOR RC�� - � r� �l'�
I PURPOSES UPON RECORDir:� ��; ;
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-- _ _,,,�. s� •:'1. sEc ��-�,-� ,
,� No:e: Tne locat;ons o` all teatures shown are app-oximcte. r�, SITE PLAN
3 Reference: Drawing entitlec "Wilkinson Short Piai, Snadow Plat" da:ed 01/25/99 by 9iamc & Holmbe�rg Inc. Vc�=`�es�En 'neers
� �� � FIGURE 2
N
� ... ._. .
SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS GROUP GROUP NAME
SYMBOL
aW WELI-GAADED ORAVEI,FINE TO COARSE GRAVEL
GRAVEL CLEAN GRAVEL
COARSE GP POORLY-GRADED GRAVEI
GRAINED More Than 50%
SOILS of Coarse Fractian GM SILTY GRAVEL
GRAVEL
Retained
on No.4 Sieve W�TH FINES
GC CLAYEY GRAVEL
SW WELL-GRADED SAND.FINE TO COARSE SAND
SAND CLEAN SAND
More Than 50% SP POORLY-GRADED SAND
Retained on
More Than 50%
No.200 Sieve SM SILTY SAND
of Coarse F�action SAND
Passes WITH FINES ��
SC CLAYEY SAND
No.4 Sieve
ML SILT
FINE SILT AND CLAY INORGANIC
GRAINED CL CLAY
SOILS
Liquid limit
ORGANIC OL ORGANIC SIIT,ORGANIC CLAY
Less Than 50
MH SIIT OF HIGH PLASTICITY,ELASTIC SIIT
More Than 50% SILT AND CLAY INORGANIC
Passes CH CLAY OF HIGH PLASTICITY,FAT CLAY
No. 200 Sieve
Liquid Limit
ORGANIC OH ORGANIC CLAY,ORGANIC SILT
50 or More
HIGHLY ORGANIC SOIIS PT PEAT
NOTES: SOIL MOSTURE MODIFIERS:
1. Field ciassification is based on visual examination oi soil in Dry- Absence of moistu�e,dusty,dry to the touch
general accordance with ASTM D2488-90.
Moist- Damp,but no visible water
2. Soil ctassification using leboratory tests is in general
accordance with ASTM 02487-90. Wet- Visible iree water or saturated,usually sail is obtained from below
3. Descriptions of soil density or co�sistency are based on water table
interpretation of blaw count data, visuai appearence of soils,
andlortest data.
��(� SOIL CLASSIFICATION SYSTEM
Geo��Engineers FIGURE 3
r�.,�,�i�-i.����
w 4
Project Job Number Location
Wilkinson Short Plat 7001-001-01 Renton, WA
LOG OF TEST PIT TP-1
Date Excavated: S�ly����� Logged by: SJ�'�
Equipment: Rubber-tired bacl:lioc Surface Elevation (ft): 187
F- p � � 7 v E aqi F-
W z � � �- °;� Other Tests W
LL. N d c) C�� U— u� m�
z E n � Q � T Material Description ;� a o ;a And Z
x ,� � � "� � v�- � Notes x
� �
a � m � � $ o LL a
�� Dark brown silt�vith trace tine to cuarse sand(sott. a Oo
1 moist)
SM Orangish brown silty tine to medium sand with trace
2 coarse sand(medium dense,moist)
h1L Brown gray silt with trace line to coarse gravel(medium
stiQ moist)
5 3 5
��
�.f y,,} RX Light bruwn tine to coarse sand with tine to coarse gracel -
4 � � (sandstone)(dense,moist) I
'�y �ri
r y..2lC1C.1 �I
�� Test pit completed at 10.0 lcet on 05%I'h�0 �� '
No ground�vater secpaee observed
No cavine observed
15
Nofes:The depths of the test pit logs are based on an average of ineasurements across the test pit and should be considered accurate to 0.5 foot.
LOG OF TEST PIT TP-2
Date Excavated: �/19/00 Logged by: SJ�'�
Equipment: Rubber-tired bacl:hoe Surface Elevation (ft): 1�2
� �
W Z � "� � �o o a^ n r
Other Tests w
� �, v - � c�� u- u� �� �
z a �- ,�, a � T Material Description ;� a o ;a And� Z
� � o � � ��„� o I Yt � Notes F
n. m � � � o LL a
00 , Uurk brown s�lt with tinc to coiinc snnd(s��tt, muistl a 0�
(till)
SM Dark bro�en siltv tine to coarse sand(medium dense,
� moist)(till)
Mf, Dark bro�vn,%black silt�vith burned roots(medium stiti: _
SM � muist)(old[opsoil later) �
o 5 � �ar ro�vnis o'- r�i�tine to coarse san �v'itTi hne 5
� M� to coarse er i��el(medium dense.muist) /-
� Oranee ig it¢�av si t wrt Lne to coarse san an cra�e
� (stitl;moist)
^ �
� �;��;y; Light eray stratified weathered sandstone(dense,moist)
� ��r�A
a -'� �';,a
� -����:.t.�,
� �� "Cest pit completed at 10.0 teet on O�i 1y�00 ��
� No¢round water seepace observed
� No caving observed
w
��
a
c�
0
� 15
0
� Notes:The depths of the test pit logs are based on an average of ineasurements across the test pit and should be considered accurate to 0.5 foot.
a ���1� � LOG OF TEST PIT
N Geo,�Eng�neers FIGURE 4
w
�
,
Project Job Number Location
Wiikinson Short Plat 7001-001-01 Renton, WA
LOG OF TEST PIT TP-3
Date Excavated: 5/19/00 Logged by: SJW
Equipment: Rubbcr-tired bacl:hoe Surface Elevation (ft): 1 g�
F- � � 7 m � u
O O p F F-
w z � -� �o o e-- � Oth e r Tests W
LL � v � c�a �_ v'� �� And
Z � a � a � ;, Material Description ;� a o ;a z
? N � � � ��„�" o Yt � Notes F
a m � � � o LL a
00 a w
Dark bro�vn rooted silt�v�th lme to coarse sand(solt, Oo
ML � moist) �
� ig t¢ray orange si t�vit root structures(me ium st�f�
i ,
moist)
<<�� Lieht brown thinly laminated silUsiltstone(hard,moist)
2
5 5
„r'' lnterbedded light grayish brown tine to medium sand
3 w x'' (sandstone)and thin(y laminated siltstone(dense,
— ��;�,,r� moist)
1'est pit completed at 8.0 Ibet on OS/l9/00
No¢round water seepage observed
10 Nu cavine obsen�ed �Q
15
Notes:The depths of the test pit logs are based on an average of ineasurements across the test pit and should be considered accurate to 0.5 foot.
LOG OF TEST PIT TP-4
Date Excavated: 5/19/00 Logged by: SJW
Equipment: Rubber-tired backhoe Surface Elevation (ft): 132
~ � �p 0 j � E t F-
w z � � �o o �— N Other Tests W
li N Q U U' � U.-, m'� m� LL �
z a �, � a � T Material Description ;a a o ;a And Z
x � 3 � � �� � y~ � Notes =
a � m � � � g LL a
00 � l3rotvn oranee mottled silt�vidi trace tine[o coarse sand a 00
- (soft,moist) - '
ML U�uk brownish oranee silt with tine to coarse sand tivith I
uccasional Yine tu co�rse eravel(medium stitl; moist) �
I
0 5 5 I
o ML Lieht¢ray orange mottleJ silt(medium still;rnoist) �
0
0
n
0 1tX Lieht gray silty tine to medium sand with occasion:il line
� to coarse eravel(dense,moist)(sandstone)
�' 1'est prf comp ete at 9A leet on 0�l 19i00
0 10 No ground water seepaee observed ��
a No caving observed
0
U
W
�
�
U -
0
� 15
� Notes:The depths of the test pit logs are based on an average of ineasurements across the test pit and should be considered accurate to 0.5 foot.
n
c;
° ���� LOG OF TEST PIT
a lTe���Engineers FIGURE 5
�
W
W
�
Project Job Number Location
Wilkinson Short Plat 7001-001-01 Renton, WA
LOG OF TEST PIT TP-5
Date Excavated: 5/19/00 Logged by: SJW
Equipment: Rubber-tired backhoe Surface Elevation (ft): lgy
�
� o ` o � � g v �
w z � � �o o °� N Other Tests W
li y v U �� U-- m� uc
z n n ,E, a � �, Material Description �� a o ;a And Z
_ � � � � �� g o� y Notes �
F o
a N m � � f o LL a
w a W
�� . �llar brown silry Lne tu coarse sand(loose,moist) Oo
3M rown sir hne to coarse san wrt occasionaTtine to
coarse gravel(medium dense,moist)
ML i¢ t gray orange mott e siTt wrt occasiona t'�ne to
coarse gravel(medium still;moist)
5 5
��r RX Light¢rayish brown line to medium gravel(sandstone)
�" (dense,moist)
(��
'fest pit completed at 8.0 feet nn OSi I�)�00
No ground water seepaqe ubservecl -
�� No caving obserced 10
15
Notes:The depths of the test pit logs are based on an average o(measurements across the test pit and should be considered accurate to 0.5 foot.
b
0
0
0
r-
0
0
�
F-
O
1
�
n
U
W
f�
a
c�
0
0
r
c^
a �� LOG OF TEST PIT
w Geo��Engineers FIGURE 6
w
�,
. � .
IX EROSION & SEDIMENTATION CON7'ROL
The contractor shall install silt fences along the perimeter of the site as indicated on tiie
Temporary Erosion Sedimentation Control Plan, with construction entrance 12" thick
made of quarry spalls as indicated on the construction plans.
1 '
- 10
�