HomeMy WebLinkAboutRS_4th_Dim_CIVIL_TIR_PRELIM_190925_v3
SNOHOMISH COUNTY
125 E Main Street, Suite 104
Monroe, Washington 98272
tel: 360. 794.7811 | fax: 360.805.9732
ISLAND COUNTY
840 SE 8th Avenue, Suite 102
Oak Harbor, Washington 98277
tel: 360. 675.5973 | fax: 360.675.7255
www.HarmsenInc.com
SKAGIT COUNTY
603 South First Street
Mount Vernon, Washington 98273
tel: 360. 336.9199 | fax: 360.982.2637
PRELIMINARY
TECHNICAL INFORMATION REPORT
FOR THE
4TH DIMENSION MIXED USE BUILDING
RENTON, WASHINGTON
JUNE 11, 2019
REV SEPTEMBER 11, 2019
9/11/19
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I. PROJECT OVERVIEW
PROJECT DESCRIPTION
This technical information report and has been prepared for the 4 th Dimension Mixed Use
Building project located at the City of Renton. The developer is proposing to construct a 12,370
sf mixed use building on the 0.55 acre site. After dedication of right-of-way the site contains
0.44 acres. The site currently contains a single family home. The property is surrounded by
multi family housing to the north and west, across Bremerton Avenue NE and by commercial
uses to the east and south, across NE 4th Street.
PREDEVELOPED CONDITIONS
The site located at the northeast corner of the intersection of NE 4th Street and Bremerton
Avenue NE. The property currently contains a 1,053 sf single family residence, 665 sf walkways
and concrete on the southern end of the site. The remainder of the southern portion of the
site is lawn, 0.16 ac, and the northern 0.24 ac is trees and brush. There is a 24” storm pipe that
crosses through the center of the site. It is proposed as part of this project to remove that pipe
and provide for an open flow path under the building.
A review of King County iMAP and City of Renton mapping show the site is does not contain
any critical areas or lay within a hazard area, see mapping in Appendix C.
DEVELOPED CONDITIONS
4th Dimension Mixed Use project has proposed to construct a 12,370 sf mixed use building,
and underground parking access drive on the site. The building will include 3 stories of
residential units with a portion of the first floor being commercial and parking. An additional
level of parking lays below that. A stormwater detention vault will be constructed below the
lowest parking level and discharge to the storm system that existing on the site.
SITE AREA: 0.44 acres
PROJECT SITE AREA: 0.44 acres
SIZE OF IMPROVEMENTS
Roof Area: 0.28 ac
Walks: 0.02
Asphalt: 0.07 ac
Landscaping: 0.07 ac
DISPOSITION OF RUNOFF BEFORE/AFTER DEVELOPMENT
Runoff currently leaves the site in the 24” storm system that flows through the site. After
development detained and treated runoff will flow to the 24” storm system that flows through
the site.
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DIFFICULT SITE PARAMETERS
The project geoptechnical engineer observed 2.5-9 feet of fill soils on the site and groundwater
at a depths of 5-7 feet in two test pits.
NATURAL DRAINAGE SYSTEM
The topography of the site shows that most of the rainfall that falls on the site flows to a catch
basin located centrally along the western edge of the site.
ADJACENT PROPERTY DRAINAGE
The site and surrounding properties are developed and do not contribute significant surface
runoff to the site. There is an existing 24” storm main that crosses through the site. It will be
relocated as part of the site work. A discussion of the upstream basin is included in Section III
of this report.
BYPASS FLOWS
There is no portion of the proposed improvements that can not be collected and conveyed to
the detention facility.
DRAINAGE BASIN ANALYSIS
The entire site drains to the north and is within one drainage basin. The 0.76 acres basin used
for calculations consists of the street frontages and the 0.44 acre property.
SOIL DESCRIPTION
GEO Group Northwest explored the soil and groundwater conditions at the subject parcel by
excavating five test pits by mini-excavator at the site. The test pits were excavated to depths of
up to 9-feet 8-inches below ground surface. Soils observed at the test pits generally consist of
variable density silty soils and fills which include brick, wood, organic material, concrete and
asphalt debris overlying apparent dense and competent till and gravelly SAND and sandy
GRAVEL. The depth of fills ranged from 2.5-feet at the test pit TP-5 to around 9-feet at the test
pit TP-3. Groundwater seepage was not encountered at the test pits TP-1 through TP-3.
Significant groundwater seepage was encountered at a depth of 7-feet bgs at the test pit TP-4
and 5-feet bgs at the test pit TP-5. Based on the fill soils revealed in the geotechnical report, Till
soil parameters have been used in WWHM2012. See GEO Group Northwest report in Section
IV, Special Reports and Studies on page 18, for additional information.
The observed underlying soils appear to match the USDA SCS soil description for Alderwood
gravelly sandy loam (AgC) at the uplands and Everett very gravelly sandy loam (EvB) at the
depression as shown the SCS Soil Map, see Appendix A.
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II. PRELIMINARY CONDITIONS SUMMARY
CONDITIONS OF APPROVAL
There are currently no project specific conditions of approval for the project other than those
from the 2017 RSWDM as adopted by the City of Renton. The site is subject to Full Drainage
Review, including all 9 Core Requirements and all 6 Special Requirements.
SURFACE WATER MANAGEMENT DESIGN MANUAL CORE REQUIREMENTS:
1. Discharge at Natural Location: Runoff currently leaves the site in the 24” storm system that
flows through the site. After development, that pipe will be removed and a new extension
made to the storm system in Bremerton using 30” pipe. An open channel will be extended
under the building. Site runoff, after it is detained and treated, will flow to a new 30” pipe
to replace the existing 24” storm system.
2. Off-site Analysis: This is covered in Section III of this report.
3. Flow Control: The site will use a detention vault to meet the Flow Control Duration –
Forested Condition Area requirement. See Section IV of this report.
4. Conveyance System: A backwater analysis will be prepared for the project and will be
included in the final TIR in Section V of that report.
5. Erosion and Sedimentation Control: The project will construct a series of sedimentation
controls to address the specific site conditions. See Section IX of this report.
6. Maintenance and Operations: The proposed onsite storm system will be owned, operated
and maintained by the owner. A Maintenance and Operation Manual will be included in the
final TIR in Section X of that report.
7. Financial Guarantees and Liability: The owner and contractor will obtain all necessary
permits prior to beginning of construction. The owner will be responsible for any bonds.
8. Water Quality: The site is located within a Basic Water Quality Treatment Area. Though
being a commercial project it is required to meet the Enhanced Basic Water Quality
standard. The Filterra Treatment System has been chosen to meet the runoff treatment
requirements for the site runoff. See Section IV of this report for the design of the
treatment systems.
9. Onsite BMPs: In addition to the required detention facility, the RSWDM also requires the
implementation of Flow Control BMPs to a portion of the impervious surfaces on the site.
The intent of this requirement is to lessen the impact of the runoff from the proposed
impervious surfaces. For this 19,166 sf site, a Rain Garden is proposed, see page 12 for
more information.
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SPECIAL REQUIRMENTS
1. Other Adopted Area-Specific Requirements:
a. Critical Drainage Areas: The site is not within a CDA.
b. Master Drainage Plans: The site is not within a MDP.
c. Basin Plan: The site is not within a BP.
d. Salmon Conservation Plan: The site is not within a SCP.
e. Stormwater Compliance Plan: The site is not within a SWCP.
f. Lake Management Plan: The site is not within a LMP.
g. Flood Hazard Reduction Plan Update: The site is not within a FHRP.
h. Shared Facility Drainage Plan: The site is not within a SFDP.
2. Flood Hazard Area Delineation: This site not within the 100-year flood plain per the City of
Renton Flood Hazard Area Map in Appendix C. The site has been determined to have an
infrastructure induced floodplain to elevation 394.87. The 24” culvert exiting the site is not
adequate to pass runoff through the site with out a backwater effect, both inlet controlled
and a backwater from the downstream storm system. This was originally determined in a
report by Coughlin-Porter-Lundeen (CPL) and the City has mandataed that compensatory
storage be provided for any reduction in storage volume. See page 7 for additional
information.
3. Flood Protection Facilities: The site does not rely on an existing flood protection facility or
modify or construct a new flood protection facility.
4. Source Control: The project is considered a commercial project and therefore is required to
have source control measures, see Section IV of this report for more information.
5. Oil Control: The site does not meet the criteria for a high-use site, see Section IV of this
report for more information.
6. Aquifer Protection: The site is not within a critical aquifer recharge area and is shown in
the low groundwater contamination suseptability area, see maps in Appendic C.
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III: OFFSITE ANALYSIS
Task 1: Study Area Definition and Maps
See Figures 3, 3a, 3b and 4 in Appendix A for sub basins and downstream analysis map.
Task 2: Resource Review
a. Adopted Drainage Plans – The site is within the Maplewood Drainage Basin per the
City of Renton Drainage Basin Map in Appendix C. There is no specific adopted
drainage plan for this area.
b. FEMA maps – the site is not within Flood Hazard Zone per the City of Renton Flood
Hazard Area Map in Appendix C.
c. Sensitive Areas Folio – the site is not within a shoreline, erosion hazard, coal mine
hazard, channel migration hazard or seismic hazard area. see King County SAO and
CAO Basin Condition Maps in Appendix C.
d. DNRP Drainage Complaints and studies – see Appendix B.
e. Wetland Inventory Maps – none per the City of Renton online mapping, see Appendix
C.
f. Community Plan – None
g. Review of the 303d Listings – see Appendix B and Task 4, below.
Task 3: Field Investigation
Runoff leaves the site in the 24” storm piping that conveys flow through the site. This piped
storm system continues to the west across Bremerton Avenue, through the apartment complex
and into the shopping center. At a distance of about 850 feet from the site the storm system
bends to the south, across NE 4th Street, then back to the west, outfalling at the headwater for
the west fork of Maplewood Creek. The creek meanders in a southerly direction through
wooded tracts of several residential developments. At a distance of 3,160 feet from the site a
culvert conveys the stream under Bremerton Place NE. The stream continues to a distance of
about 5,760 feet from the site where it joins the east fork of Maplewood Creek. At this point a
deep gulley has formed where the creek drops down to the valley floor. At about 8,000 feet
from the site the creek enters the Maplewood Golf Course. The channel continues through
both open and wooded stretches of the golf course, bending to the west, then back to the
south where it crosses under Highway 169 and into the Cedar River about 9,860 feet from the
site. Cedar creek flows westerly an additional 3.3 miles to Lake Washington.
Task 4: Drainage System Description and Problem Descriptions
There are no known drainage problems along the downstream drainage route. Beyond the
initial 1000 feet an opened unnamed stream.
A review of the drainage complaint data revealed the closest two drainage complaints, both
over 30 years old and located about 2,650 feet from the site. One was related to water on the
adjacent roadway and is unrelated to the stream system. The second was regarding diversion
of the stream onto filled land. There are no other known drainage issues along the
downstream flowpath within the area of study.
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The City’s Lower Cedar River Habitat Study list several restoration projects within Reach 3 of
the river see Herrera figure in Appendix B. These projects are to be implemented by the City
using grant and other public funding.
Review of the 303d Listings
The Washington State Department of Ecology Water Quality Atlas Map has been reviewed for
any 303d listings associated with the downstream drainage system. A portion of the
Maplewood Creek, at a distance of about 7,000 feet from the site, is listed as Category 5 for
Bioassesment. Further downstream the Cedar River is listed as Category 5 for pH, temperature
and dissolved oxygen levels. See figures in Appendix B.
Task 5: Mitigation of Existing or Potential Problems
Downstream Flooding Issues
To be included with full TIR
Conclusions
To be included with full TIR
UPSTREAM BASIN
The 24" culvert that crosses the property collects a significant upstream drainage basin. The
culvert needs to be rerouted to clear the proposed building construction. Based on City of
Renton GIS for storm systems and Lidar contours, a basin boundary has been estimated that
encompasses 140 acres of mostly developed property, see Figure in Appendix B.
Complicating a calculation of upstream runoff rates are the approximately 11 existing
detention systems that have been constructed in the basin over the years and designed over
several drainage manual iterations. In working with City Staff, Rohini Nair, Engineer for the City
of Renton provided the following guidance:
“The City of Renton will allow a modified Level 2 Offsite Analysis to be conducted to
meet the City's requirements, which include the following:
a. Perform a hydrologic analysis comparing the existing condition and developed
condition at the 25-year and 100-year peak flow per Core requirement #4. The
intent of the comparison is to show that the developed conditions will not adversely
impact the existing and future conveyance system.
i. This analysis will compare the existing and developed condition under the
following tailwater conditions:
1. The downstream pipe at the point of compliance is empty
2. The downstream pipe at the point of compliance is full
3. The downstream pipe at the point of compliance is 1/2 full
4. The downstream pipe at the point of compliance is near full with 1" of
freeboard from top of pipe
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*Note: The POC of the site is the catch basin with the Facility ID No. 131550 per
COR maps
ii. To determine the upstream land-use designation the City of Renton will allow the
following options:
1. All upstream land-use shall be existing unless shown to have a detention
facility. If there is a detention facility the associated land-use can be designated
as pasture.
2. All upstream land-use shall be existing unless shown to have a detention
facility with an available recorded TIR.
*Note: The upstream flows shall be modeled assuming that there are no
upstream conveyance systems beyond the property line.”
Based on this guidance the upstream basin has been analyzed using the following land uses:
Impervious roads, parking, and roofs 22.01 ac
Pasture (sites with existing detention) 75.67 ac
Lawn and landscaping 19.25 ac
Native vegetation areas 25.67 ac
The upstream basin has the following current 25 year and 100 year flow frequency runoff
rates, see Appendix D:
Storm Event Rate
25 Year 26.50 cfs
100 Year 35.52 cfs
The project site itself has the following existing and mitigated 25 year and 100 year flow
frequency runoff rates:
Storm Event Existing Mitigated
25 Year 0.14 cfs 0.08 cfs
100 Year 0.17 cfs 0.10 cfs
Note: The tailwater conditions in the original requirements no longer apply as a larger study of
the downstream has been performed to obtain tailwater conditions per the floodplain
calculations below.
FLOODPLAIN CALCULATIONS
The site has been determined to have an infrastructure induced floodplain. The 24” culvert is
not adequate to pass runoff through the site with out a backwater effect, both inlet controlled
and a backwater from the downstream storm system. This was originally determined in a
report by Coughlin-Porter-Lundeen (CPL) and the City has mandataed that compensatory
storage be provided for any reduction in storage volume. Therefore, to account for the
development, it is proposed to have the flow system run under the building with the building
constructed on columns, leaving space for water storage.
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CPL has assumed that the downstream system had no backwater onto the site. Harmsen has
determined that there is a backwater. Since the elevation of the backwater has a direct impact
through storage on how much water flows through the culvert and the amount of flow through
the culvert determines the elevation of the backwater, an iterative calculation needed to be
made that matched those two elevations. The intial step is to determine the flood plain stage
storage. This was calculated by creating a surface model in Autocad Civil 3D using the following
three topographic sources. First the topographic survey for the project has been used,
modified to include the proposed compensatory storage under the building. The second source
of information is the proposed flood plain compensatory storage plan by Coughlin-Porter-
Lundeen for the Renton Highlands project. The proposed grading contours shown on this plan
have been used to create a ground surface model. It was found that this would create a more
accurate map than using available lidar as the area is heavily vegetated. Lastly, available lidar
topographic information, obtained from the Lidar Consortium, has been used to fill between
the two primary flood plain sources.
Using Autocad Civil 3D, the flood plain storage volume was calculated in one foot increments to
prepare a stage – storage – discharge table for input into WWHM2012. The table is as follows:
Elevation (ft) Area (ac) Volume (ac-ft) Discharge (cfs)
388 0.00 0.00 varies
389 0.15 0.14
390 0.16 0.28
391 0.17 0.45
392 0.24 0.61
393 0.70 1.16
394 1.02 2.00
395 1.50 3.47
396 1.88 5.10
397 2.05 7.12
398 2.16 9.04
399 2.20 11.39
400 2.30 13.59
To get outflow rates for a variety of storage depths, the upstream runoff was routed through
the ‘pond’ with a spread of different orifice sizes to force the flow to reach various depths and
thus give various outflows. Then these flows would be used in the downstream backwater
calculations to determine backwater elevations. Once the two models match, that would be
the storage elevation.
The discharge values for 13 different orifice sizes (10”, 12”, 14”, 15”, 16”, 17”, 18”, 19”, 20”,
21”, 22”, 24” and 26”) were calculated using the orifice equation for elevations between 388
and 400. Each of these stage – storage – discharge tables were input into WWHM2012 to
calculate the 100 year discharge from the flood plain. The cooresponding 100 year ponding
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elevation could then be interpolated from the stage – storage – discharge table for each orifice
size creating a data point. These data points are as follows:
Orifice Size 100yr Discharge (cfs) 100yr Elevation (feet)
10” 8.52 397.73
12” 11.53 396.79
14” 14.62 395.56
15” 16.20 395.00
16” 17.38 394.23
17” 18.75 393.73
18” 20.20 393.27
19” 22.55 392.84
20” 23.33 392.64
21” 24.70 392.25
22” 25.90 391.89
24” 28.41 391.32
26” 29.86 390.67
These data points were then graphed:
DOWNSTREAM BACKWATER CALCULATIONS
Backwater calculations have been prepared for the removal of the culvert through the site. The
downstream system is based on the CPL report and the flows therein with the addition of the
upstream flow calculated above. The Hydraflow Storm Sewer Extension for Autodesk Civil 3D
was used perform the backwater calculations. A 30” smooth-walled pipe was proposed to
replace the 24” cmp. An open channel will be used under the building with a special inlet in the
west wall of the facility per plan detail. Below is a graph showing the elevation and flow rate
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where the backwater and upstream floodplain calcalutions match up. This is at 16.7 cfs and
elevation 394.82, see Hydraflow output in Appendix D.
This then indicates that the artificial floodplain is expected to have a 100 year elevation of
394.81 and compensatory calculations and building clearances are calculated from that
elevation.
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IV: RETENTION/DETENTION ANALYSIS AND DESIGN
PART A: EXISTING SITE HYDROLOGY
The site located at the northeast corner of the intersection of NE 4th Street and Bremerton
Avenue NE. The property currently contains a 1,053 sf single family residence, 665 sf walkways
and concrete on the southern end of the site. The remainder of the southern portion of the
site is lawn, 0.16 ac, and the northern 0.24 ac is trees and brush, see Figure 3A: Existing
Drainage Basin Map in Appendix A. There is a 24” storm pipe that crosses through the center of
the site. This pipe will be removed, replaced with an open channel under the building, and a
30” connection to the storm system in Bremerton.
Existing Basin
The basin for the existing condition contains 0.76 acres including 0.32 ac of existing and future
right or way. There is currently 0.15 acres of the right-of-way that is paved and is included in
the basin. The remainder of the basin, the portion being redeveloped has been modeled as
forest. Based on the geotechnical report the site is underlain with till soils so the appropriate
parameters have been used in WWHM2012.
The existing basin has the following land uses and areas:
Land Use Area(ac)
Impervious 0.15
Forest 0.61
The existing basin has the following flow frequency runoff rates:
2 Year 0.08
10 Year 0.12
50 Year 0.16
100 Year 0.17
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PART B: DEVELOPED SITE HYDROLOGY
Developed Basin
The 0.76 acre basin includes the frontage street, access drive, walkways and landscaping and
the proposed building, see Figure 3B: Developed Drainage Basin Map in Appendix A.
The runoff from the road, walkways, landscaping and access will be collected and treated, then
will be routed to the proposed detention vault to be located under the building. The building
roof will flow directly to the vault. The impervious areas have been calculated for the access,
parking areas, walkways and building using AutoCAD by constructing a polyline around the
specific area.
The developed basin has the following land uses and areas:
Land Use Area(ac)
Impervious (roof) 0.28
Impervious (paving) 0.39
Landscaping 0.09
The developed basin has the following flow frequency runoff rates:
2 Year 0.28
10 Year 0.42
50 Year 0.54
100 Year 0.60
ONSITE FLOW CONTROL BMP’S
In addition to the required detention facility, the RSWDM also requires the implementation of
Flow Control BMPs to a portion of the impervious surfaces on the site. The intent of this
requirement is to lessen the impact of the runoff from the proposed impervious surfaces. The
site area is 0.44 ac or 19,166 sf which is less than 22,000 sf so the Small Lot BMP Requirement
applies. The BMP must be applied to a minimum of 20% of the site area in this case an area of
atleast 3,850 sf. Per Section 1.2.9.2.1 Small Lot BMP Requirements, there are 12 possible ways
to meet this requirement. They are addressed below:
Full Dispersion
Full Dispersion is not a feasible Flow Control BMP for this project. Per Section C.2.1.1.1.3 in the
RSWDM:
“A native vegetated flowpath segment of at least 100 feet in length (25 feet for sheet
flow from a nonnative pervious surface) must be available along the flowpath”.
There is no native vegetation on the site, nor is there a vegetated flowpath longer than 5 feet
from the proposed improvements to a property line. Therefore Full Dispersion is not a feasible
for this site.
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Full infiltration
GEO Group Northwest explored the soil and groundwater conditions at the subject parcel and
documented their findings in their report titled Geotechnical Report With Infiltration
Evaluation, dated May 18, 2018. From pages 2 and 3 of their report:
“The soils observed at the uplands, overlying much of the site consist of silty soils which
are relatively impermeable. The soils observed at the depression (lowland) portion of
the site include where located below silty fills may have relatively high permeability,
however, the groundwater level at this area is also relatively high, thereby reducing the
effectiveness for an infiltration system. Groundwater seepage ranged in depth at test
pits TP-4 and TP-5 from 5 to 7-feet below ground surface. This level may vary
dependent upon the time of year, precipitation amounts and changed land use in the
area. Due to the presence of silty soils overlying much of the site and the relatively high
groundwater conditions we do not recommend attempting to infiltrate stormwater at
the subject site.”
Per Section C.2.2.2.1.a in the RSWDM:
“Existing soils must be coarse sands or cobbles or medium sands and cannot be
comprised of fill materials where the infiltration device will be located.”
The site soils are silty sands and fill, therefore full infiltration is not feasible on the site.
Limited infiltration
Limited Infiltration is similar to Full Infiltration except that per Section C.2.2.2.1.a in the
RSWDM:
“The minimum design requirements for limited infiltration are the same as those for full
infiltration, except infiltration depressions are excluded and existing soils in the location
of the infiltration device may be fine sands, loamy sands, sandy loams, or loams as
opposed to only medium sands or better.”
And:
“Silt and clay loams, and cemented till (hardpan) are not suitable for limited infiltration
systems.”
The GEO Group Northwest report titled Geotechnical Report With Infiltration Evaluation, dated
May 18, 2018. From pages 2 and 3 of their report:
“Soils observed at the test pits generally consist of variable density silty soils and fills which
include brick, wood, organic material, concrete and asphalt debris overlying apparent dense
and competent till and gravelly SAND and sandy GRAVEL.
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“Due to the presence of silty soils overlying much of the site and the relatively high
groundwater conditions we do not recommend attempting to infiltrate stormwater at
the subject site.”
The site soils are silty sands and fill with more pervious soils below, however groundwater
encroaches into the pervious soils, therefore limited infiltration is not feasible on the site.
Rain Garden
For this 19,166 sf site, a Rain Garden is proposed. With this BMP being applied to 20% of the
targeted surfaces, a raingarden is required for an area of about 3,850 sf. Per Appendix C,
Section C.2.1.2, rain gardens must have a minimum horizontal projected surface area below
the overflow that is at least 5% of the area draining to it. With a maximum ponding depth of 12
inches, the required raingarden would have an overflow area of atleast 193 sf. Two rain
gardens are proposed to be located in planting areas along the southern side of the building
with a combined area of about 200 sf.
For this preliminary TIR no Flow Control credits have been applied in the design of the
detention system based on the reduction in runoff that will result from use of the raingarden.
The final TIR will incorporate the reduction by modeling 50% of the roof surface area draining
to the raingarden as grass. With 3,850 sf, 0.088 ac, 0.044 ac will be modeled as grass and 0.044
ac will be modeled as impervious in the Developed Basin. This will be incorporated in the final
TIR and WWHM2012 design calculations.
Bioretention
With less than 5,000 sf of contributing area, a Rain Garden is proposed to meet the Flow
Control BMP requirement.
Permeable Pavement
Full Dispersion is not a feasible Flow Control BMP for this project. Per Section C.2.7.24 in the
RSWDM:
“24. Where appropriate field testing indicates soils have a measured (a.k.a., initial)
native soil saturated hydraulic conductivity less than 0.3 inches per hour.”
Initial unfactored infiltration rates were calculated by GEO Group Northwest. Page 4 of their
report states:
“From Darcy’s Law for permeability and using the D10 grain size correlation we
calculate that the initial infiltration rate for the soils at the TP-4 2’ and TP-5 2’-
8”sampled depths is 0.14 to 1.3 inches/hour. These rates are relatively low, highly
variable and subject to correction due to the potential for groundwater mounding and
possible groundwater movement. “
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“If stormwater infiltration is to occur for the subject development then we recommend
that the designer apply appropriate correction factors to account for site variability, the
test methodology and siltation/maintenance.”
The Washington State Department of Ecology Stormwater Management Manual, DOE Manual,
was reviewed to determine the appropriate correction factors to determine a design
infiltration rate. From Section 3.3.6 of the DOE Manual, Table 3.3.1, the following factors have
been applied:
Correction Factor for Site Variability, CFv = 0.40
Correction Factor for Testing Procedure, CFt = 0.40
Correction Factor for Influent Control, CFm = 0.90
Therefore CFT = CFv x CFt x CFm = 0.144. After applying the correction factor the resulting
design infiltration rates would be 0.02 to 0.18 inches per hour. This is significantly less than the
minimum rate of 0.3 inches per hour so the use of permeable pavement is not feasible.
Basic Dispersion
With a possible maximum vegetated flow length of 5 feet the use of Basic Dispersion is not
feasible for this site. The minimum required vegetated flow path for each dispersion device is
addesssed below:
USE OF SPLASH BLOCKS FOR BASIC DISPERSION
Per Section C.2.4.2.2 in the RSWDM:
“2. A “vegetated flowpath segment” of at least 50 feet in length must be available along
the flowpath that runoff would follow upon discharge from the splash block.”
USE OF ROCK PADS FOR BASIC DISPERSION
Per Section C.2.4.3.2 in the RSWDM:
“2. A “vegetated flowpath segment” of at least 50 feet in length as illustrated in Figure
C.2.4.C must be available along the flowpath that runoff would follow upon discharge
from the rock pad.”
C.2.4.4 USE OF GRAVEL FILLED TRENCHES FOR BASIC DISPERSION
Per Section C.2.4.4.2 in the RSWDM:
“2. A “vegetated flowpath segment” of at least 25 feet in length must be available along
the flowpath that runoff would follow upon discharge from a dispersion trench. This
length must be increased to 50 feet if the discharge is toward a steep slope hazard area
or a landslide hazard steeper than 15%. All or a portion of the vegetated flowpath
segment may be within the buffer for the steep slope hazard area or landslide hazard.”
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USE OF SHEET FLOW FOR BASIC DISPERSION
Per Section C.2.4.5.3 in the RSWDM:
“3. A “vegetated flowpath segment” of at least 10 feet in length must be available along
the flowpath that runoff would follow upon discharge from the strip of crushed rock.”
Reduced Impervious Surface Credit
As it is possible that a Rain Garden BMPs can be installed, it is not necessary to utilize this BMP
option.
Native Growth Retention Credit
There is no native growth areas on the site.
Tree Retention Credit
There are no significant trees on the site.
Soil Amendment
This BMP will be used in the planter beds and other landscaped areas on site and disturbed
areas adjacent to the site.
Perforated Stubout Connection
Other than the portion of the roof that will flow to the rain gardens, the roof drains for the
building will flow directly through the building and into the detention vault directly below the
building.
PART C: PERFORMANCE STANDARDS
1. The drainage design for the site was prepared using the requirements of the 2009 City
of Renton Surface Water Design Manual, (RSWDM Manual). WWHM2012, by the
Department of Ecology, has been used to calculate basin runoff and for
retention/detention facility sizing.
2. The Flow Control Duration Standard – Forested Conditions, has been applied for sizing
of the detention facilities.
3. The site is subject to Flow Control BMPs per Section 5.2.1.3 Large Lot High Impervious
BMP Requirements.
4. The conveyance system is subject to the design standards applicable to “Conveyance
Requirements for New Systems”.
5. The Enhanced Basic Water Quality Treatment standard has been used for design of the
treatment facilities.
6. The site has been evaluated in respect to the Enhanced Basic Treatment requirement,
see Part E of this Section.
7. The site has been evaluated in respect to Special Requirement #4 – Source Controls and
Special Requirement #5 – Oil Controls, see Part E of this Section.
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PART D: FLOW CONTROL SYSTEM
Flow Control Design
The site is located within the Flow Control Duration Standard – Forested Condition Area. Storm
water detention will be accomplished through the use of an “L” shaped detention vault located
along the north and east sides of the building. WWHM2012 has been used to size the
detention pond. The required vault contains a ‘live’ storage volume of 8,780 cf, see Appendix
D.
The proposed “L” shaped detention vault has the following dimensions:
Vault Length 58 feet
Vault Width 18 feet
“L” Vault Length 50 feet
“L” Vault Width 8 feet
Bottom of ‘Live’ Elevation 100.00 feet (assumed)
Top of ‘Live’ Elevation 106.10 feet
Release from the vault will be controlled by a three orifice flow control riser with the following
dimensions:
Riser Diameter 12
Outlet Elevation 100.00 feet
1st Orifice Elevation 98.00 feet
1st Orifice Diameter 0.88”
2nd Orifice Elevation 103.80 feet
2nd Orifice Diameter 0.88”
3rd Orifice Elevation 105.00 feet
3rd Orifice Diameter 0.80”
Top of Riser Elevation 106.10 feet
The existing and mitigated flow frequency runoff rates are:
Storm Event Existing Mitigated
2 Year 0.08 cfs 0.04 cfs
10 Year 0.12 cfs 0.06 cfs
50 Year 0.16 cfs 0.09 cfs
100 Year 0.17 cfs 0.10 cfs
An emergency overflow will be installed along the west edge of the vault to allow for a safe
discharge of rogue storm events.
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PART E: WATER QUALITY SYSTEM
Water Quality Facility
The site is located within the Basic Water Quality Treatment Area. As runoff to the facility is
from roadway and the development is considered a commercial land use, it is necessary to
meet the Enhanced Basic Water Quality Treatment standards. The Filterra Treatment System
has been chosen to meet the runoff treatment requirements for the site runoff. The system
has Department of Ecology GULD and City of Renton approval for enhanced treatment. The
treatment basin includes the roadway, planters and sidewalks as well as the rain garden runoff.
FILTERRA OPERATION
Stormwater runoff enters the Filterra bioretention system through a curb-inlet opening and
flows through a specially designed filter media mixture contained in a landscaped concrete
container. The filter media captures and immobilizes pollutants; those pollutants are then
decomposed, volatilized and incorporated into the biomass of the Filterra system’s
micro/macro fauna and flora. Stormwater runoff flows through the media and into an
underdrain system at the bottom of the container, where the treated water is discharged.
FILTERRA SIZING
Treatment facility has been performed using WWHM2012 with the parameters based on
Contech’s approved media, see output in Appendix B. For this site a Filterra Facility with a 4’x6’
filter bed will provide treatment of 92.3% of the treatment basin runoff. This exceeds the
required treatment level of 91% of the total runoff record. The facility will be located in the
northwest corner of the site near the access drive and will drain to the detention vault. See
WWHM2012 report for sizing calculations in Appendix D.
ROOF AREA
The building roof is not subject to any vehicular traffic and therefore does not require
treatment.
Special Requirement #4 – Source Controls
Structural Source Controls
Structural source controls will include covered garbage and recycle areas. Additionally, vehicle
washing will not be permitted on the site.
Non-Structural Source Controls
See Section VIII of this report for the Construction Stormwater Pollution Prevention Plan. This
includes Erosion and Sedimentation Control Plan design where the minimum requirements of
Core Requirement #5: Temporary Erosion and Sedimentation Control are discussed. The
section also includes discussion of the Construction Stormwater Pollution Prevention where
the BMPs contained in Volume II of the current DOE Manual will be used to control pollution
from sources other than sedimentation.
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Special Requirement #5 – Oil Controls
The site does not qualify as a high use site as it is does not meet the three criteria of a high use
site per Section 1.3.5 of the RSWDM. The site is does not contain an open parking lot as the
parking is under the structure and will discharge any runoff to the sewer. Petroleum storage or
transfer will not occur on the site. The site is not subject to use, storage or maintenance of 25
or more 10 ton diesel vehicles.
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V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Site conveyance calculations to be included with full TIR.
The upstream storm system re-routing conveyance calculations are included in Section III of
this report.
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VI. SPECIAL REPORTS AND STUDIES
The geotechnical report prepared by GEO Group Northwest for the project follows.
No other special reports or studies have been prepared at this time.
13705 Bel-Red Rd – Bellevue, WA 98005
Phone: 425/649-8757 – Fax: 425/649-8758
GEO Geotechnical Engineers, Geologists, &
Environmental Scientists Group Northwest, Inc.
May 14, 2018 G-4661
Hari Ghadia
12505 Bel-Red Rd, Suite 212
Bellevue, WA 98005-2510
Send via: ghadia_hari@hotmail.com
Subject: GEOTECHNICAL REPORT WITH INFILTRATION EVALUATION
PROPOSED DEVELOPMENT
4502 NE 4TH ST
RENTON, WASHINGTON
Dear Hari Ghadia:
In accordance with our March 8, 2018 contract with you we have investigated the soil and
groundwater conditions at the subject property and prepared the following geotechnical report
for the proposed commercial/residential development.
SITE AND PROJECT DESCRIPTION
The subject site consists of a developed parcel containing one building which is located at the
south side of the lot as shown on the attached Plate 2 – Topographic Map. The existing
building is a 1-story building with daylight basement which daylights toward the north. The
property has an approximate area of 0.55 acres. There is an ecology block retaining wall which
is located at the west side of the lot or at the adjacent west right-of-way which has an estimated
height of around 12-feet and retains Bremerton Ave NE. The site topography includes a relative
depression at the north-central portion of the lot with moderate to steep relatively minor slopes at
the north and south sides of the lot.
Based upon plans provided by Kaul Design Associates the subject site is proposed to be
developed with a mixed-use building containing parking as shown on the attached Plate 2 – Site
Plan. We understand that the development will consist of the following:
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GEO Group Northwest, Inc.
1. The building and garage lowest level is roughly at the existing grade at the depression near
the center of the lot.
2. At the "6600 sf commercial" portion of the building there will be main floor level (above the
garage) which roughly matches the adjacent existing grades at the south side of the lot and then
will have 3-stories of apartments above this level.
3. At the "structured garage" section we understand that the top of the garage roughly matches
existing grade at the adjacent Bremerton Ave NE (also main floor level for the "6600 sf
commercial" building). A ramp goes down to the lower garage level at the east side of the
garage building. A detention or infiltration system may be constructed below the lowest level in
the garage although depth/elevation information for this structure has not been provided.
GEOLOGIC CONDITIONS
The USGS geologic map 1 for the site vicinity indicates that the soils at the subject lot consist of
Quaternary-age Ground Moraine deposits. These soils consist of ablation till overlying
lodgement till. Till soils are generally described as a mixture of silt, sand and gravel which was
both deposited and overridden by glacial ice at least 14,000 years ago.
SUBSURFACE CONDITIONS
On April 18, 2018 GEO Group Northwest explored the soil and groundwater conditions at the
subject parcel by excavating five test pits by mini-excavator at the Test Pit locations noted on the
attached Plate 2 – Site Plan. The test pits were excavated to depths of up to 9-feet 8-inches
below ground surface (bgs).
Soils observed at the test pits generally consist of variable density silty soils and fills which
include brick, wood, organic material, concrete and asphalt debris overlying apparent dense and
competent till and gravelly SAND and sandy GRAVEL. The depth of fills ranged from 2.5-feet
at the test pit TP-5 to around 9-feet at the test pit TP-3.
The observed underlying soils appear to match the USDA SCS soil description for Alderwood
gravelly sandy loam (AgC) at the uplands and Everett very gravelly sandy loam (EvB) at the
depression as shown on the attached Plate 4 – SCS Soil Map.
1 “Geologic Map of the Renton Quadrangle, King County, Washington”, USGS, D.R. Mullineaux, 1965.
May 14, 2018 G-4661
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GEO Group Northwest, Inc.
Groundwater seepage was not encountered at the test pits TP-1 through TP-3. Significant
groundwater seepage was encountered at a depth of 7-feet bgs at the test pit TP-4 and 5-feet bgs
at the test pit TP-5.
The results of our subsurface investigation are shown on the attached Appendix A - Test Pit
Logs and USCS Soil Legend.
GRADATIONAL ANALYSES
We performed gradational analyses for soil samples collected at a depth of 2-feet at the test pit
TP-4 and a depth of 2-feet 8-inches at the test pit TP-5. These analyses confirmed the soil
descriptions at these levels as being gravelly silty SAND. The results of these analyses are
attached as Plates 5 and 6 – Gradational Analysis. Extrapolation of the gradation curves
indicates that the D10 value for the sampled soils at TP-4 and TP-5 to be 0.01 mm and 0.03 mm,
respectively.
INFILTRATION EVALUATION
The USDA NRCS maps the site soils as Alderwood gravelly sandy loam AgC) and Everett very
gravelly sandy loam (EvB) which appear to match the observed conditions at our test pits. The
USDA NRCS online data indicates that for the AgC soil unit the capacity of the most limiting
layer to transmit water (Ksat): very low to moderately low. Per NRCS for the EvB soil unit the
capacity of the most limiting layer to transmit water (Ksat): is high.
The soils observed at the uplands, overlying much of the site consist of silty soils which are
relatively impermeable. The soils observed at the depression (lowland) portion of the site
include where located below silty fills may have relatively high permeability, however, the
groundwater level at this area is also relatively high, thereby reducing the effectiveness for an
infiltration system. Groundwater seepage ranged in depth at test pits TP-4 and TP-5 from 5 to 7-
feet below ground surface. This level may vary dependent upon the time of year, precipitation
amounts and changed land use in the area. Due to the presence of silty soils overlying much of
the site and the relatively high groundwater conditions we do not recommend attempting to
infiltrate stormwater at the subject site. If site stormwater must be infiltrated then we
recommend that infiltration rate testing be performed at the infiltration location and it may also
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GEO Group Northwest, Inc.
be necessary to install a monitoring well to determine the groundwater level conditions
throughout the year.
INFILTRATION RATE
From Darcy’s Law for permeability and using the D10 grain size correlation we calculate that the
initial infiltration rate for the soils at the TP-4 2’ and TP-5 2’-8”sampled depths is 0.14 to 1.3
inches/hour. These rates are relatively low, highly variable and subject to correction due to the
potential for groundwater mounding and possible groundwater movement. As noted above we
recommend performing infiltration rate testing at the infiltration rate location and depth if it is
determined that infiltration must occur at the site.
If stormwater infiltration is to occur for the subject development then we recommend that the
designer apply appropriate correction factors to account for site variability, the test methodology
and siltation/maintenance. Catchbasins with sump pits should be installed between the
stormwater collection system and the infiltration system(s) and periodic maintenance should be
required to clean-out the catchbasin sump pit(s). The infiltration system may eventually clog due
to siltation and require replacement construction. This reason may also make it difficult to
maintain an infiltration system below the proposed structure as currently proposed.
SEISMIC DESIGN CRITERIA
Based upon our subsurface investigation the project site has Site Class D soil (Stiff Soil) per the
IBC based upon the observed subsurface soil conditions and provided that the buildings are
constructed to bear on the competent soils as described herein.
CONCLUSIONS AND RECOMMENDATIONS
General
Based upon the results of our study, it is our professional opinion that the site is geotechnically
suitable for the proposed development. The primary geotechnical concern with regard to the
design for the proposed building is the presence of loose and unacceptable fill soils which
present risks of damage due to soil settlement, if not properly over-excavated and filled with
compacted structural fill. It is our opinion that these risks can be mitigated by implementing a
building pad improvement program. Building foundations should not be constructed to bear
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Geotechnical Report – 4502 NE 4th St, Renton, Washington Page 5
GEO Group Northwest, Inc.
directly on top of the loose fill soils or fill debris. An alternative to the building pad
improvement program would be to construct the building on top of a pile foundation system. In
the final recommendations section of this report we provide recommendations for an augercast
pile foundation which may be used as an alternative to the recommended building pad
improvement. It may also be an option for a portion of the building to be supported on a spread
footing foundation, such as at the south side of the property, with the remainder of the building
supported on a pile foundation, provided that competent soils are encountered at the foundation
subgrades at the proposed spread footing areas.
Site Preparation and General Earthwork
The building pad areas should be stripped and cleared of surface vegetation and organic soils
(forest duff).
Silt fences should be installed around areas disturbed by construction activity to prevent
sediment-laden surface runoff from being discharged off-site. Exposed soils that are subject to
erosion should be compacted and covered with plastic sheeting.
Temporary Excavation Slopes
Under no circumstances should temporary excavation slopes be greater than the limits specified
in local, state and national government safety regulations. Temporary cuts greater than four feet
in height should be sloped at an inclination no steeper than 1H:1V (Horizontal:Vertical) in the
overlying loose site soils. If seepage is encountered at the excavation slopes should have
inclinations of no steeper than 2H:1V for the temporary construction time period. If excavations
with the aforementioned slope inclinations encroach upon the adjacent properties or remove
support for the existing ecology block wall at the west side of the site than shoring may be
required.
Structural Fill
All fill material used to achieve design site elevations below the building areas and below non-
structurally supported slabs, parking lots, sidewalks, driveways, and patios, should meet the
requirements for structural fill. During wet weather conditions, material to be used as structural
fill should have the following specifications:
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GEO Group Northwest, Inc.
1. Be free draining, granular material containing no more than five (5) percent fines (silt and
clay-size particles passing the No. 200 mesh sieve);
2. Be free of organic material and other deleterious substances, such as construction debris
and garbage;
3. Have a maximum size of three (3) inches in diameter.
All fill material should be placed at or near the optimum moisture content. The optimum
moisture content is the water content in soil that enables the soil to be compacted to the highest
dry density for a given compaction effort.
Based upon our subsurface investigation the overlying apparent fill site soils consist of sandy
SILT and silty SAND with some organic soils and debris. These soils are relatively silty and
may be difficult to compact to meet the minimum structural fill compaction requirements. If
work occurs during a period of wet weather it is likely that the native site soils may become too
wet to achieve the compaction criteria. We recommend that the contractor take measures to
protect the site soils from wet weather impacts such as using plastic sheeting to cover stockpiles.
Additionally all unsuitable debris such as concrete, wood, organic soil, plastic sheeting and other
deleterious materials must be removed from the site soils if they are to be used as structural fill.
An imported granular fill material may provide more uniformity and be easier to compact to the
required structural fill specification, especially if work occurs during periods of wet weather.
Structural fill should be placed in thin horizontal lifts not exceeding ten inches in loose thickness.
Structural fill under building areas (including foundation and slab areas), should be compacted to
at least 95 percent of the maximum dry density, as determined by ASTM Test Designation D-
1557-91 (Modified Proctor).
Structural fill under driveways, parking lots and sidewalks should be compacted to at least 90
percent maximum dry density, as determined by ASTM Test Designation D-1557-91 (Modified
Proctor). Fill placed within 12-inches of finish grade should meet the 95% requirement.
We recommend that GEO Group Northwest, Inc., be retained to evaluate the suitability of
structural fill material and to monitor the compaction work during construction for quality
assurance of the earthwork.
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GEO Group Northwest, Inc.
Building Pad Improvement
The observed overlying loose soils and fills observed at the test pits present risks of soil
settlement related damage to proposed building if the building is constructed to bear on top of
these soils. Consequently we recommend that the building pad areas on which foundations and
any building slabs are to be constructed be over-excavated to remove loose fills, organic soils
and deleterious debris to a depth where the underlying medium dense to dense soils are
encountered and then replaced with compacted structural fill as required, dependent upon the
proposed building/floor elevations. These fills and unacceptable loose soils were observed at
depths as deep as 9-feet below the existing ground surface at the test pit TP-3. Following
removal of the unsuitable loose soils and debris fills the base of the excavation should be
compacted by vibratory equipment to a firm and unyielding condition, approved by GEO Group
Northwest and then backfilled with compacted structural fill placed in accordance with the
Structural Fill section of this report. The over-excavation and fill placement at the building pad
area is expected to require a significant amount of earthwork and may also present difficulties if
loose and unsuitable fills are encountered at the base of the existing block wall at the west side of
the site since this would necessitate shoring for the adjacent right-of-way. New building
foundations and slabs may be constructed to bear on the compacted structural fill which is in turn
placed on top of the competent medium dense to dense underlying soils.
Spread Footing Foundations
The proposed buildings can be supported on conventional spread footings bearing on top of an
improved building pad constructed per our recommendations noted above and which has been
approved by GEO Group Northwest, Inc., at the time of construction.
Individual spread footings may be used for supporting columns and strip footings for bearing
walls. Our recommended minimum design criteria for foundations bearing on improved building
pad or on compacted structural fill placed on top of the improved building pad are as follows:
- Allowable bearing pressure, including all dead and live loads
Medium dense to dense native soils (competent soils) = 2,000 psf
Compacted structural fill on top of competent soil (improved building pad) = 2,000 psf
- Minimum depth to bottom of perimeter footing below adjacent final exterior grade = 18
inches
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GEO Group Northwest, Inc.
- Minimum depth to bottom of interior footings below top of floor slab = 18 inches
- Minimum width of wall footings = 16 inches
- Minimum lateral dimension of column footings = 24 inches
- Estimated post-construction settlement = 1/4 inch
- Estimated post-construction differential settlement; across building width = 1/4 inch
A one-third increase in the above allowable bearing pressures can be used when considering
short-term transitory wind or seismic loads.
Lateral loads can also be resisted by friction between the foundation and the supporting
compacted fill subgrade or by passive earth pressure acting on the buried portions of the
foundations. For the latter, the foundations must be poured "neat" against the existing
undisturbed soil or be backfilled with a compacted fill meeting the requirements for structural
fill. Our recommended parameters are as follows:
- Passive Pressure (Lateral Resistance)
• 350 pcf equivalent fluid weight for compacted structural fill
• 350 pcf equivalent fluid weight for native dense soil.
- Coefficient of Friction (Friction Factor)
• 0.35 for compacted structural fill
• 0.35 for native dense soil
We recommend that footing drains be placed around all perimeter footings. More specific
details of perimeter foundation drains are provided below in the section titled: Subsurface
Drainage.
Conventional Retaining Walls and Basement Walls
Based upon the preliminary plans we understand that conventional concrete retaining walls are
proposed for the below-grade portions of the building and this may be at the lower level at the
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GEO Group Northwest, Inc.
south side of the site. These walls should be constructed on top of footings which bear on the
building pad improvement discussed above or on top of augercast concrete piles.
Permanent retaining walls restrained horizontally on top (such as basement walls) are considered
unyielding and should be designed for a lateral soil pressure under the at-rest condition; while
conventional reinforced concrete walls free to rotate on top should be designed for an active
lateral soil pressure.
Active Earth Pressure
Conventional reinforced concrete walls that are designed to yield an amount equal to
0.002 times the wall height, should be designed to resist the lateral earth pressure
imposed by an equivalent fluid with a unit weight of 35 pcf for level backfill;
At-Rest Earth Pressure
Walls supported horizontally by floor slabs are considered unyielding and should be
designed for lateral soil pressure under the at-rest condition. The design lateral soil
pressure should have an equivalent fluid pressure of 40 pcf for level backfill;
Seismic Surcharge
For the anticipated 100 year seismic event a horizontal surcharge load of 8H psf should
be applied;
Passive Earth Pressure
350 pcf equivalent fluid weight for compacted structural fill and native undisturbed soil;
Base Coefficient of Friction
0.35 for compacted structural fill and native undisturbed soil;
To prevent the buildup of hydrostatic pressure behind permanent concrete basement or
conventional retaining walls, we recommend that a vertical drain mat, such as Miradrain 6000 or
equivalent, be used to facilitate drainage behind such walls. The drain mat core should be placed
against the wall(s) with the filter fabric side facing the backfill. The drain mat should extend
from near the finished surface grade down to the footing drain system. Additionally all backfill
placed between the excavation slopes or temporary shoring and the new basement/retaining walls
should consist of free-draining fills having less than 5% passing the No. 200 sieve. Also, a
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GEO Group Northwest, Inc.
waterproofing layer should be placed between the drainage mat layer and the concrete wall, for
moisture protection at all basement wall locations.
The top 12 inches of backfill behind retaining or basement walls should consist of compacted
and relatively impermeable soil. This cap material can be separated from the underlying more
granular drainage material by a geotextile fabric, if desired. Alternatively, the surface can be
sealed with asphalt or concrete paving. Where possible the ground surface should be sloped to
drain away from the wall.
GEO Group Northwest, Inc., recommends that backfill material which will support structures or
improvements (such as patios, sidewalks, driveways, etc.) behind permanent concrete retaining
walls and basement walls be placed and compacted consistent with the structural fill
specifications in the Structural Fill section of this report.
Slab-on-Grade Concrete Floors
Slab-on-grade concrete floors may be constructed directly on top of the native medium dense to
dense site soils or on top of compacted structural fills placed on top of the medium dense to
dense site soils (building pad improvement). If structural fills are to be placed at these areas then
they should be compacted in accordance with the specifications in the section titled: Structural
Fill.
To avoid moisture build-up on the subgrade, slab-on-grade concrete floors should be placed on a
capillary break, which is in turn placed on the prepared subgrade. The capillary break should
consist of a minimum of a six (6) inch thick layer of free-draining crushed rock or gravel
containing no more than five (5) percent finer than the No. 4 sieve. A vapor barrier, such as a
10-mil plastic membrane, is recommended to be placed over the capillary break beneath the slab
to reduce water vapor transmission through the slab. Two to four inches of sand may be placed
over the barrier membrane for protection during construction.
Subsurface Drainage
We recommend that subsurface drains, footing drains, be installed around the perimeter of the
foundation footings. The drains should consist of a four (4) inch minimum diameter perforated
rigid drain pipe laid at or near the bottom of the footing with a gradient sufficient to generate
flow. The drain line should be bedded on, surrounded by, and covered with a free-draining rock,
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pea gravel, or other free-draining granular material. The drain rock and drain line should be
completely surrounded by a geotextile filter fabric, Mirafi 140N or equivalent. Once the drains
are installed, the excavation should be backfilled with a compacted fill material. The footing
drains should be tightlined to discharge to the stormwater collection system.
Under no circumstances should roof downspout drain lines be connected to the footing drainage
system. All roof downspouts must be separately tightlined to discharge into the stormwater
collection system. We recommend that sufficient cleanouts be installed at strategic locations to
allow for periodic maintenance of the footing drains and downspout tightline systems.
Augercast Concrete Pile Foundations
An alternative to implementing the building pad improvement noted above the new building may
be supported on augercast concrete piles that are embedded at least 10-feet into the underlying
dense native soils which are anticipated at a depth of around 10-feet below the ground surface.
Implementing this option may allow for less excavation at the site and may reduce the risk that
temporary shoring will be necessary. Concrete grade beams should be used to connect the pile
foundations and distribute the building loads. A structural concrete slab may be designed and
constructed to support the slab loads and transfer these loads to the piling. Based upon the depth
to competent soils at the test pits we estimate pile lengths may be on the order of 20-feet with 10-
foot embedment into the competent dense soil. The piles should be designed with a minimum
diameter of 14 inches. For concrete piles 14 to 18 inches in diameter embedded 10 feet into the
underlying dense soils, the following allowable bearing capacities may be used:
AUGERCAST CONCRETE PILE CAPACITIES
Pile Diameter
(Inches)
Pile Embedment
(Feet)
Allowable Bearing
(Tons)
Allowable Uplift
(Tons)
14 10 13 6.5
16 10 16 8
18 10 19 9.5
Note: Pile embedment length is based on the embedment depth below the top of the dense, native soil.
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No reduction in pile capacity is required if the pile spacing is at least three times the pile
diameter. A one-third increase in the above allowable pile capacities can be used when
considering short-term transitory wind or seismic loads.
Lateral forces can also be resisted by the passive earth pressures acting on the grade beams and
friction with the subgrade. To fully mobilize the passive pressure resistance, the grade beams
must be poured “neat” against compacted fill. Our recommended allowable passive soil pressure
for lateral resistance is 350 pcf (pounds per cubic foot) equivalent fluid weight. A coefficient of
friction of 0.35 may be used between the subgrade and the grade beams. We estimate that the
maximum total post-construction settlement should be one-half (1/2) inch or less, and the
differential settlement across building width should be one-quarter (1/4) inch or less.
The performance of piles depends on how and to what bearing stratum the piles are installed. It
is critical that judgement and experience be used as a basis for determining the embedment
length and acceptability of a pile. Therefore, we recommend that GEO Group Northwest, Inc.,
be retained to monitor the pile installation operation, collect and interpret installation data, and
verify suitable bearing stratum. We also suggest that the contractor’s equipment and installation
procedure be reviewed by GEO Group Northwest, Inc., prior to pile installation to help mitigate
problems which may delay work progress.
ADDITIONAL SERVICES
GEO Group Northwest, Inc., can provide additional exploration and testing services for the
project such as infiltration rate testing if it is determined to be necessary. We recommend that
GEO Group Northwest Inc. be retained to perform a general plan review of the final design and
specifications for the proposed development to verify that the earthwork and foundation
recommendations have been properly interpreted and implemented in the design and in the
construction documents. We also recommend that GEO Group Northwest Inc. be retained to
provide monitoring and testing services for geotechnically-related work during construction.
This is to observe compliance with the design concepts, specifications or recommendations and
to allow design changes in the event subsurface conditions differ from those anticipated prior to
the start of construction. We anticipate the following construction monitoring inspections may
be necessary:
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GEO Group Northwest, Inc.
1. Site clearing and grubbing;
2. Grading of temporary excavation slopes;
3. Preparation of building foundation subgrades;
4. Over-excavation and structural fill placement at building pad improvement areas, removal of
unsuitable fill soils;
5. Permanent subsurface drainage installation;
6. Installation of augercast piling, if implemented;
LIMITATIONS
This report has been prepared for the specific application to this site for the exclusive use of 4th
Creek Meadows LLC and their authorized representatives. Any use of this report by other
parties is solely at that party’s own risk. We recommend that this report be included in its
entirety in the project contract documents for reference during construction.
Our findings and recommendations stated herein are based on field observations, our experience
and judgement. The recommendations are our professional opinion derived in a manner
consistent with the level of care and skill ordinarily exercised by other members of the
profession currently practicing under similar conditions in this area and within the budget
constraint. No warranty is expressed or implied. In the event that soil conditions not anticipated
in this report are encountered during site development, GEO Group Northwest, Inc., should be
notified and the above recommendations should be re-evaluated.
If you have any questions, or if we may be of further service, please do not hesitate to contact us.
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Sincerely,
GEO GROUP NORTHWEST, INC.
Adam Gaston
Project Engineer
William Chang, P.E.
Principal
Attachments: Plate 1 – Vicinity Map
Plate 2 – Site Plan
Plate 3 – Topographic Map
Plate 4 – SCS Soil Map
Plates 5 – 6 – Gradational Analyses
Appendix A – USCS Soil Legend and Test Pit Logs
cc: Mr. Martin Reimer – Kaul Design Associates