HomeMy WebLinkAboutRS_Geotechnical_Report_180423_v1GEOTECHNICAL -REPO R,T
.:' Wang Short Plat -
110,016- 116th Avenue SE.
,�,-Rent6n, Washington
Projeet No.: T-7841
I
Terra Associates, Inc.
A A 41
Propared for:
Ms. Jane Wang
Pbrt Co 11 ins,. Colorado.
Fe'brud.ry 261 2018
TERRA ASSOCIATES, Inc.
Consultants in Geotechnical Engineering, Geology
and
Environmental Earth Sciences
February 26, 2018
Project No. T-7841
Ms. Jane Wang
2601 S. Lemay Avenue, #7417
Fort Collins, Colorado 80525
Subject: Geotechnical Report
Wang Short Plat
19016 — 116th Avenue SE
Renton, Washington
Dear Ms. Wang:
As requested; we conducted a geotechnical engineering study for the subject project. The attached report presents
our findings and recommendations for the geotechnical aspects of project design and construction.
Our field exploration indicates the site is underlain at relatively shallow depths by till -like soils consisting of
medium dense to dense silty sand with gravel. We observed light to moderate perched groundwater seepage in all
seven of the test pits.
In our opinion, there are no geotechnical conditions that would preclude the planned residential development.
Residences can be supported on conventional spread footings bearing on competent native soils underlying the
surface organic soils or on structural fill placed on the competent native soils. Floor slabs and pavements can be
similarly supported.
Detailed recommendations addressing these issues and other geotechnical design considerations are presented in
the attached report., We trust the information presented is sufficient for your current needs. If you have any
questions or require additional information, please call.
Sincerely yours,
TERRA ASSOCIATES, INC.
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John C
Geologist
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s;j�NAI,
12220 113th Avenue NE, Ste. 130, Kirkland, Washington 98034
Phone (425) 821-7777 • Fax (425) 821-4334
TABLE OF CONTENTS
Pa-Ze No.
1.0
Project Description........................................................... ::........ ::..................................... l
2.0
Scope of Work.......................................................................................................:.........1
3.0
Site Conditions.. ...............................................................................................................
2
3.1 Surface................................................................................................................
2
3.2 Soils....................................................................................................................2
3.3 Groundwater.......................................................................................................2
3.4 Geologic Hazards................................................................................................3
3.4.1 Erosion Hazard Areas...............................................................................
3
3.4.2 Steep Slope Hazard Areas.........................................................................3
3.4.3 Landslide Hazard Areas............................................................................4
3.4.3 Seismic Hazard Areas...............................................................................
4
3.4.4 Coal Mine Hazard Areas.........................................................................4
3.5 Seismic Design Parameters ........ ........................
............................................... v. 5
4.0
Discussion and Recommendations ......... .....
.............................................................. .:.... 5
4.1 General.........................................................................................................
5
4.2 Site Preparation and Grading
.......................................................................6
4.3 Excavations..................................................................................................7
4.4 Foundations..................................................................................................7
4.5 Slab -on -Grade Floors...................................................................................
8
4.6 Infiltration Feasibility..................................................................................
8
4.7 Stormwater Detention Vault......
.................................................................. 9
4.8 Drainage.......................................................................................................9
4.9 Utilities................................,.........................................
--......................10
4.10 Pavements...................................:............................................................10
5.0
Additional Services..........................................
..........................................................11
6.0
Limitations.......................................................
......... ..........................................11
Figures
VicinityMap.........................................................................................................................Figure 1
ExplorationLocation Plan.....................................................................................................Figure 2
TypicalWall Drainage Detail...............................................................................................Figure 3
Appendix
Field Exploration and Laboratory Testing.......................................................................Appendix A
Geotechnical Report
Wang Short Plat
19016 —116th Avenue SE
Renton, Washington
1.0 PROJECT DESCRIPTION
The proposed project is a 7 -lot residential development with associated infrastructure improvements. An undated
preliminary grading and utilities plan, prepared by Core Design, Inc. shows site grading will be relatively minor
with maximum cuts and fills of about one to two feet. Building plans are currently not available; however, we
expect that the residences would be two-story, wood -frame structures, with their main floors constructed at grade
or framed over a crawl space. Foundation loads should be relatively light, in the range of 2 to 3 kips per foot for
bearing walls and 25 to 50 kips for isolated columns.
Site stormwater will be routed to a buried detention vault located in the eastern 65 to 70 feet of the site. The vault
footprint shown on the plan indicates that the vault will be about 116 feet long and about 45 feet wide. Vault
elevations are not given on the plans; however, plan notes suggest that the vault bottom will be at least nine to ten
feet below ground surface.
The recommendations contained in the following sections of this report are preliminary and based on our
understanding of the above design features. We should review design drawings as they become available to
verify that our recommendations have been properly interpreted and incorporated into project design and to
amend or supplement our recommendations, if required.
2.0 SCOPE OF WORK
We explored subsurface conditions at the site by observing conditions in seven test pits excavated to maximum
depths of about five to nine feet below existing surface grades using a track -mounted excavator. Using the results
of our field study and laboratory testing, analyses were undertaken to develop geotechnical recommendations for
project design and construction. Specifically, this report addresses the following:
• Soil and groundwater conditions
• Geologic hazards per the City of Renton Municipal Code
• Seismic design parameters per the current International Building Code (IBC)
• Site preparation and grading
■ Excavations
■ Foundations
• Slab -on -grade floors
• Stormwater infiltration feasibility
■ Stormwater detention vault
• Subsurface drainage
• Utilities
• Pavements
February 26, 2018
Project No. T-7841
It should be noted that recommendations outlined in this report regarding drainage are associated with soil
strength, design earth pressures, erosion, and stability. Design and performance issues with respect to moisture as
it relates to the structure environment (i.e., humidity, mildew, mold) is beyond Terra Associates' purview. A
building envelope specialist or contactor should be consulted to address these issues, as needed.
3.0 SITE CONDITIONS
3.1 Surface
The project site is a 2.04 -acre residential parcel located east of and adjacent to 116th Avenue SE, approximately
475 feet to 625 feet north of the intersection with SE 192nd Street in Renton, Washington. A single-family
residence and detached garage currently occupy the western portion of the site.
Site topography is relatively flat. Site vegetation consists of grass lawn and landscape trees and shrubs in the
vicinity of the residence. The central and eastern portions of the site are generally vegetated with deciduous trees
and brush.
We observed soft, wet surface conditions including several localized areas of surface water in the central portion
of the site. The surface water appears to be an accumulation of direct precipitation in subtle depressions. We did
not observe indications of drainage being conveyed onto the site from adjacent properties.
3.2 Soils
The site soils generally consist of about 8 to 18 inches of duff and topsoil overlying till -like glacial deposits
consisting primarily of silty sand with varying amounts of gravel. The upper approximately one to seven feet of
the glacial deposits have typically weathered to a medium dense to dense condition, and are generally moist to
wet and mottled, with scattered weakly cemented zones. The underlying unweathered deposits appear till -like
and are typically dense, moist, and weakly to moderately cemented.
The Geologic map of the Renton quadrangle, King County, Washington by D.R. Mullineaux (1965) shows the site
mapped as ground moraine deposits (Qgt) consisting primarily of lodgment till. The dense, weakly to moderately
cemented silty sand with gravel that we observed in the test pits is generally consistent with this geologic map
unit.
Detailed descriptions of the subsurface conditions we observed in the test pits are presented on the Test Pit Logs
in Appendix A. The approximate locations of the test pits are shown on Figure 2.
3.3 Groundwater
We observed light to moderate seepage of perched groundwater in all seven test pits. The seepage generally
occurred within the upper weathered zone of the till -like soils between depths of about one and five feet.
Page No. 2
February 26, 2018
Project No. T-7841
The occurrence of shallow perched groundwater is typical for sites underlain by till and till -like soils. We expect
that perched groundwater levels and flow rates will fluctuate seasonally and will typically reach their highest
levels during and shortly following the wet winter months (October through May). Considering that our field
work was conducted in February, we expect that the observed groundwater levels are representative of seasonal
high levels.
3.4 Geologic Hazards
We evaluated potential geologic -related hazards at the subject site as defined in Section 4-3-050G5 (Geologically
Hazardous Areas Defined) of the Renton Municipal Code (RMC). Geologic hazards are defined by the RMC as
"Areas which may be prone to one or more of the following conditions: erosion, flooding, landslides, coal mine
hazards, or seismic activity."
3.4.1 Erosion Hazard Areas
Section 4-3-050G5c of the RMC defines erosion hazards as follows:
i. Low Erosion Hazard (EL): Areas with soils characterized by the Natural Resource Conservation Service
(formerly U.S. Soil Conservation Service) as having slight or moderate erosion potential, and a slope less
than 15 percent.
ii. High Erosion Hazard (EH): Areas with soils characterized by the Natural Resource Conservation Service
(formerly U.S. Soil Conservation Service) as having severe or very severe erosion potential, and a slope
more than 15 percent.
The NRCS has mapped the site soils as Alderwood gravelly sandy loam, 0 to 8 percent slopes (AgB), which are
described as having a slight erosion hazard. Based on the above criteria, the site is categorized as having a low
erosion hazard (EL).
In our opinion, no significant erosion hazard exists at the site, and the erosion potential of site soils would be
adequately mitigated with proper implementation and maintenance of Best Management Practices (BMPs) for
erosion prevention and sedimentation control. All BMPs for erosion prevention and sedimentation control will
need to be in place prior to and during site grading activity, and should conform to City of Renton requirements.
3.4.2 Steep Slope Hazard Areas
Section 4-3-050G5a of the RMC defines steep slopes as follows:
L Sensitive Slopes: A hillside, or portion thereof, characterized by: (a) an average slope of 25 percent to less
than 40 percent as identified in the City of Renton Steep Slope Atlas or in a method approved by the City;
or (b) an average slope of 40 percent or greater with a vertical rise of less than 15 percent as identified in
the City of Renton Steep Slope Atlas or in a method approved by the City; (c) abutting an average slope
of 25 percent to 40 percent as identified in the City of Renton Steep Slope Atlas or in a method approved
by the City. This definition excludes engineered retaining walls.
ii. Protected Slopes: A hillside, or portion thereof, characterized by an average slope of 40 percent or greater
grade and having a minimum vertical rise of 15 percent as identified in the City of Renton Steep Slope
Atlas or in a method approved by the City.
Slope areas meeting the above criteria do not exist at the site.
Page No. 3
February 26, 2018
Project No. T-7841
3.4.3 Landslide Hazard Areas
Section 4-3-050G5b of the RMC defines landslide hazards as follows:
Low Landslide Hazard (LL): Areas with slopes less than 15 percent.
ii. Medium Landslide Hazard (LM): Areas with slopes between 15 percent and 40 percent and underlain by
soils that consist largely of sand, gravel, or glacial till.
iii. High Landslide Hazards (LH): Areas with slopes greater than 40 percent, and areas with slopes between
15 percent and 40 percent and underlain by soils consisting largely of silt and clay.
iv. Very High Landslide Hazards (LV): Areas of known mapped or identified landslide deposits.
Topography of the site and adjacent properties is relatively flat. In our opinion, there is no landslide hazard at the
site. Based on the above criteria, the site would be categorized as having a low landslide hazard (LL).
3.4.4 Seismic Hazard Areas
Section 4-3-050G5d of the RMC defines seismic hazards as follows:
i. Low Seismic Hazard (SL): Areas underlain by dense soils or bedrock. These soils generally have site
classifications of A through D, as defined in the International Building Code, 2012.
ii. High Seismic Hazard (SH): Areas underlain by soft or loose, saturated soils. These soils generally have
site classifications E or F, as defined in the International Building Code, 2012.
The site is underlain by dense till -like soils. In our opinion, the site soil classification "C" would apply to the
subject site. This site classification meets the above criteria defining a low seismic hazard (SL).
In our opinion, unusual seismic hazards requiring mitigation do not exist at the site, and design in accordance with
local building codes for determining seismic forces would adequately mitigate potential impacts associated with
ground shaking.
3.4. S Coal Mine Hazard Areas
Section 4-3-050G5e of the RMC defines coal mine hazards as follows:
Low Coal Mine Hazards (CL): Areas with no known mine workings and no predicted subsidence. While
no mines are known in these areas, undocumented mining is known to have occurred.
ii. Medium Coal Mine Hazards (CM): Areas where mine workings are deeper than 200 feet for steeply
dipping seams, or deeper than 15 times the thickness of the seam or workings for gently dipping seams.
These areas may be affected by subsidence.
iii. High Coal Mine Hazard (CH): Areas with abandoned and improperly sealed mine openings and areas
underlain by mine workings shallower than 200 feet in depth for steeply dipping seams, or shallower than
15 times the thickness of the seam or workings for gently dipping seams. These areas may be affected by
collapse or other subsidence.
Page No. 4
February 26, 2018
Project No. T-7841
Coal mining activities have not occurred on or extended below the subject site. In our opinion, no coal mine
hazard exists at the site.
3.5 Seismic Design Parameters
Based on the site soil conditions and our knowledge of the area geology, per the current International Building
Code (IBC), site class "C" should be used in structural design. Based on this site class, in accordance with the
IBC, the following parameters should be used in computing seismic forces:
Seismic Design Parameters
Spectral response acceleration Short Period), SMS
1.368
Spectral response acceleration (1 — Second Period), SMI
0.664
Five percent damped .2 second period, SDS
0.912
Five percent dam ed 1.0 second period, SDI
0.443
The above values were determined for Latitude 47.431672°N and Longitude -122.185197°W using the USGS
Ground Motion Parameter Calculator web site accessed February 22, 2018 at the web site
hqp://earthquake.usgs.s.gov/designmAps/ugAMl icatign.phh -
4.0 DISCUSSION AND RECOMMENDATIONS
4.1 General
Based on our study, there are no geotechnical conditions that would preclude the planned development.
Residences can be supported on conventional spread footings bearing on competent native soils underlying
organic topsoil or on structural fill placed on the competent native soils. Floor slabs and pavements can be
similarly supported.
The site soils contain a sufficient amount of fines (silt- and clay -sized particles) such that they will be difficult to
compact as structural fill when too wet or too dry. Accordingly, the ability to use the soils from site excavations
as structural fill will depend on their moisture content and the prevailing weather conditions at the time of
construction. If grading activities will take place during the winter season, the owner should be prepared to
import free -draining granular material for use as structural fill and backfill.
Based on our observations, it appears that a moderate perched groundwater condition exists beneath the site that
may persist throughout much of the year. Considering this, it would be prudent for the contractor to anticipate the
need for some initial construction drainage and soil moisture conditioning efforts to facilitate site grading.
Detailed recommendations regarding these issues and other geotechnical design considerations are provided in the
following sections of this report. These recommendations should be incorporated into the final design drawings
and construction specifications.
Page No. 5
February 26, 2018
Project No. T-7841
4.2 Site Preparation and Grading
To prepare the site for construction, all vegetation, organic surface soils, and other deleterious materials should be
stripped and removed from the site. We expect surface stripping depths of about 8 to 18 inches will be required to
remove the organic surficial soils in the planned development areas. Stripped vegetation debris should be
removed from the site. Organic soils will not be suitable for use as structural fill, but may be used for limited
depths in nonstructural areas or for landscaping purposes.
In the developed portions of the site, demolition of existing structures should include removal of existing
foundations and abandonment of underground septic systems and other buried utilities. Abandoned utility pipes
that fall outside of new building areas can be left in place provided they are sealed to prevent intrusion of
groundwater seepage and soil.
Once clearing and grubbing operations are complete, cut and fill operations to establish desired building grades
can be initiated. A representative of Terra Associates, Inc. should examine all bearing surfaces to verify that
conditions encountered are as anticipated and are suitable for placement of structural fill or direct support of
building and pavement elements. Our representative may request proofrolling exposed surfaces with a heavy
rubber -tired vehicle to determine if any isolated soft and yielding areas are present. If unstable yielding areas are
observed, they should be cut to firm bearing soil and filled to grade with structural fill. If the depth of excavation
to remove unstable soils is excessive, use of geotextile fabric such as Mirafi 500X or equivalent in conjunction
with structural fill can be considered in order to limit the depth of removal. In general, our experience has shown
that a minimum of 18 inches of clean, granular structural fill over the geotextile fabric should establish a stable
bearing surface.
Our study indicates that the existing inorganic native soils contain a significant amount of fines (silt and clay
sized particles) that will make the soils difficult to compact as structural fill when too wet or too dry. Provided
these soils are near optimum moisture when excavated, and are placed during dry weather conditions, we
anticipate they will be suitable for direct use as structural fill. However, as observed in our site explorations,
much of the soil in the upper approximately three feet of the test pits appeared to be wet of optimum. In order to
use these wet of optimum materials for structural fill, drying the soils by aeration during dry weather conditions or
using soil amendments such as lime or Portland cement to reduce and stabilize the soil's moisture content will
need to be considered. If soil amendment products are used, additional Temporary Erosion and Sedimentation
Control (TESC) BMPs will need to be implemented to mitigate potential impacts to stormwater runoff associated
with possible elevated pH levels.
If grading activities are planned during the wet winter months, or if they extend into fall and winter, the owner
should be prepared to import wet weather structural fill. For this purpose, we recommend importing a granular
soil that meets the following grading requirements:
U.S. Sieve Size
Percent Passing
6inches
100
No. 4
75 maximum
No. 200
5 maximum*
*Based on the 3/4 -inch fraction.
Prior to use, Terra Associates, Inc. should examine and test all materials planned to be imported to the site for use
as structural fill.
Page No. 6
February 26, 2018
Project No. T-7841
Structural fill should be placed in uniform loose layers not exceeding 12 inches and compacted to a minimum of
95 percent of the soil's maximum dry density, as determined by American Society for Testing and Materials
(ASTM) Test Designation D-698 (Standard Proctor). The moisture content of the soil at the time of compaction
should be within two percent of its optimum, as determined by this ASTM standard. In nonstructural areas, the
degree of compaction may be reduced to 90 percent.
4.3 Excavations
All excavations at the site associated with confined spaces must be completed in accordance with local, state, and
federal requirements. Based on the Washington State Safety and Health Administration (WSHA) regulations, the
medium dense to dense, weathered soils would typically be classified as Type C soils. The dense, weakly- to
moderately -cemented till -like soils would typically be classified as Type A soil.
Accordingly, for temporary excavations of more than 4 feet and less than 20 feet in depth, the side slopes in Type
C soils should be laid back at a slope inclination of 1.5:1 (Horizontal:Vertical) or flatter. Temporary excavations
in Type A soils can be laid back at inclinations of 0.75:1 or flatter.
For temporary excavation slopes less than 8 feet in height in Type A soils, the lower 3.5 feet can be cut to a
vertical condition with a 0.75:1 slope graded above. For temporary excavation slopes greater than 8 feet in height
up to a maximum height of 12 feet, the slope above the 3.5 -foot high vertical portion should be laid back to an
inclination of 1:1 or flatter. No vertical cut with a backslope immediately above is allowed for excavation depths
that exceed 12 feet. In this case, a 4 -foot high vertical cut with an equivalent horizontal bench to the cut slope toe
is required. If there is insufficient room to complete the excavations in the manners discussed above, or if
excavations greater than 20 feet deep are planned, you may need to use temporary shoring to support the
excavations.
Seepage of perched groundwater should be anticipated within excavations extending to the surface of the dense
till -like soils. In our opinion, the volume of water and rate of flow into the excavation should be relatively minor
and would not be expected to impact the stability of the excavations when completed as described above.
Conventional sump pumping procedures along with a system of collection trenches, if necessary, should be
capable of maintaining a relatively dry excavation for construction purposes in these soils.
The above information is provided solely for the benefit of the owner and other design consultants, and should not
be construed to imply that Terra Associates, Inc. assumes responsibility for job site safety. It is understood that
job site safety is the sole responsibility of the project contractor.
4.4 Foundations
Residential structures may be supported on conventional spread footing foundations bearing on competent native
soils or on structural fill placed above these native soils. Foundation subgrades should be prepared, as
recommended in Section 4.2 of this report.
Page No. 7
February 26, 2018
Project No. T-7841
Perimeter foundations exposed to the weather should bear at a minimum depth of 1.5 feet below final exterior
grades for frost protection. Interior foundations can be constructed at any convenient depth. We recommend
designing foundations for a net allowable bearing capacity of 2,500 pounds per square foot (psf). For short-term
loads, such as wind and seismic, a one-third increase in this allowable capacity can be used in design. With the
anticipated loads and this bearing stress applied, building settlements should be less than one-half inch total and
one-fourth inch differential.
For designing foundations to resist lateral loads, a base friction coefficient of 0.35 can be used. Passive earth
pressure acting on the sides of the footings may also be considered. We recommend calculating this lateral
resistance using an equivalent fluid weight of 300 pounds per cubic foot (pcf). We recommend not including the
upper 12 inches of soil in this computation because they can be affected by weather or disturbed by future grading
activity. This value assumes the foundations will be constructed neat against competent native soil or the
excavations are backfilled with structural fill, as described in Section 4.2 of this report. The recommended
passive and friction values include a safety factor of 1.5.
4.5 Slab -on -Grade Floors
Slab -on -grade floors may be supported on a subgrade prepared as recommended in Section 4.2 of this report.
Immediately below the floor slab, we recommend placing a four -inch thick capillary break layer composed of
clean, coarse sand or fine gravel that has less than three percent passing the No. 200 sieve. This material will
reduce the potential for upward capillary movement of water through the underlying soil and subsequent wetting
of the floor slab.
The capillary break layer will not prevent moisture intrusion through the slab caused by water vapor transmission.
Where moisture by vapor transmission is undesirable, such as covered floor areas, a common practice is to place a
durable plastic membrane on the capillary break layer and then cover the membrane with a layer of clean sand or
fine gravel to protect it from damage during construction, and aid in uniform curing of the concrete slab. It
should be noted that if the sand or gravel layer overlying the membrane is saturated prior to pouring the slab, it
will be ineffective in assisting uniform curing of the slab and can actually serve as a water supply for moisture
seeping through the slab and affecting floor coverings. Therefore, in our opinion, covering the membrane with a
layer of sand or gravel should be avoided if floor slab construction occurs during the wet winter months and the
layer cannot be effectively drained.
4.6 Infiltration Feasibili
Based on the conditions observed in our test pits, it is our opinion that on-site infiltration is not a viable option for
management of site stormwater. It is also our opinion that the site conditions will generally not be suitable for
applying low impact development (LID) natural drainage practices (NDPs).
Page No. 8
February 26, 2018
Project No. T-7841
4.7 Stormwater Detention Vault
Based on our study, we expect that dense to very dense, till -like silty sand with gravel will be exposed throughout
the bottom of the vault excavation. Vault foundations supported by these dense to very dense native soils may be
designed for an allowable bearing capacity of 6,000 pounds per square foot (psf) provided the bottom -of -footing
elevation is at least 8 feet below finished grade. For short-term loads, such as seismic, a one-third increase in this
allowable capacity can be used. Friction at the base of foundations and passive earth pressure will provide
resistance to these lateral loads. For designing foundations to resist lateral loads, a base friction coefficient of
0.35 can be used. Passive earth pressure acting on the sides of the vault footings may also be considered. We
recommend calculating this lateral resistance using an equivalent fluid weight of 350 pounds per cubic foot (pcf).
Lateral Earth Pressures
The magnitude of earth pressures developing on the vault walls will depend in part on the quality and compaction
of the wall backfill. We recommend placing and compacting wall backfill as structural fill as recommended in
Section 4.2.
To prevent development of hydrostatic pressure and uplift on the vault, wall drainage must be installed. A typical
recommended wall drainage detail is shown on Figure 3. If it is not possible to discharge collected water at the
footing invert elevation, we recommend setting the invert elevation of the wall drainpipe equivalent to the outfall
invert and connecting the drain to the outfall pipe for discharge.
With the recommended wall backfill and drainage, we recommend designing the vault walls for an earth pressure
imposed by an equivalent fluid weighing 50 pcf. For any portion of the wall that falls below the invert elevation
of the wall drain, an earth pressure equivalent to a fluid weighing 85 pcf should be used. For evaluating walls
under seismic loading, an additional uniform earth pressure equivalent to 8H psf, where H is the height of the
below -grade wall in feet, can be used. These values assume a horizontal backfill condition. If necessary, a
uniform horizontal traffic surcharge value of 75 psf should be included in design of vault walls.
Uplift Resistance
The vault will be subject to uplift pressures if drainage is not provided the full depth of the structure. The weight
of the structure and the weight of the backfill soil above its foundation will provide resistance to uplift. A soil
unit weight of 125 pcf can be used for the vault backfill provided the backfill is placed and compacted as
structural fill as recommended in Section 4.2.
4,8 Drainage
Surface
Final exterior grades should promote free and positive drainage away from the building areas. We recommend
providing a positive drainage gradient away from the building perimeter. If a positive gradient cannot be
provided, provisions for collection and disposal of surface water adjacent to the structure should be provided.
Page No. 9
February 26, 2018
Project No. T-7841
Subsurface
We recommend installing a continuous drain along the outside lower edge of the perimeter building foundations.
The drains can be laid to grade at an invert elevation equivalent to the bottom of footing grade. The drains can
consist of four -inch diameter perforated PVC pipe that is enveloped in washed 1/2- to 3/4 -inch gravel -sized drainage
aggregate. The aggregate should extend six inches above and to the sides of the pipe. The foundation drains and
roof downspouts should be tightlined separately to an approved point of controlled discharge. All drains should
be provided with cleanouts at easily accessible locations. These cleanouts should be serviced at least once each
year.
4.9 Utilities
Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) or
local jurisdictional requirements. At minimum, trench backfill should be placed and compacted as structural fill
as described in Section 4.2 of this report. As noted, soils excavated on-site should generally be suitable for use as
backfill material provided they are near optimum moisture when excavated, and are placed during dry weather
conditions. However, the site soils are fine grained and moisture sensitive, and much of the soil in the upper
approximately three feet of the test pits appeared to be wet of optimum; therefore, moisture conditioning may be
necessary to facilitate proper compaction. If utility construction takes place during the winter, it may be
necessary to import suitable wet weather fill for utility trench backfilling.
4.10 Pavements
Pavements should be constructed on subgrades prepared as recommended in Section 4.2 of this report.
Regardless of the degree of relative compaction achieved, the subgrade must be firm and relatively unyielding
before paving. Proofrolling the subgrade with heavy construction equipment should be completed to verify this
condition.
The pavement design section is dependent upon the supporting capability of the subgrade soils and the traffic
conditions to which it will be subjected. For traffic consisting mainly of light passenger vehicles with only
occasional heavy traffic, and with a stable subgrade prepared as recommended, we recommend the following
pavement sections:
• Two inches of hot mix asphalt (HMA) over six inches of crushed rock base (CRB)
• 4 inches full depth HMA over prepared subgrade
The paving materials used should conform to the Washington State Department of Transportation (WSDOT)
specifications for '/2 -inch class HMA and CRB.
Long-term pavement performance will depend on surface drainage. A poorly -drained pavement section will be
subject to premature failure as a result of surface water infiltrating into the subgrade soils and reducing their
supporting capability. For optimum pavement performance, we recommend surface drainage gradients of at least
two percent. Some degree of longitudinal and transverse cracking of the pavement surface should be expected
over time. Regular maintenance should be planned to seal cracks when they occur.
Page No. 10
February 26, 2018
Project No. T-7841
5.0 ADDITIONAL SERVICES
Terra Associates, Inc. should review the final designs and specifications in order to verify that earthwork and
foundation recommendations have been properly interpreted and implemented in project design. We should also
provide geotechnical services during construction in order to observe compliance with our design concepts,
specifications, and recommendations. This will allow for design changes if subsurface conditions differ from
those anticipated prior to the start of construction.
6.0 LIMITATIONS
We prepared this report in accordance with generally accepted geotechnical engineering practices. No other
warranty, expressed or implied, is made. This report is the copyrighted property of Terra Associates, Inc. and is
intended for specific application to the Wang Short Plat project. This report is for the exclusive use of Ms. Jane
Wang and her authorized representatives. No other warranty, expressed or implied, is made.
The analyses and recommendations presented in this report are based on data obtained from our on-site test pits.
Variations in soil conditions can occur, the nature and extent of which may not become evident until construction.
If variations appear evident, Terra Associates, Inc. should be requested to reevaluate the recommendations in this
report prior to proceeding with construction.
Page No. 11
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Consultants in Geotechnical Engineering RENTON, WASHINGTON
Geology and
Environmental Earth Sciences
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DESIGN OR CONSTRUCTION PURPOSES.
REFERENCE:
CORE DESIGN, INC.
;,Tt14 81,040-0441
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Consultants in Geotechnical Engineering
Geology and
Environmental Earth Sciences
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EXPLORATION LOCATION PLAN
WANG SHORT PLAT
RENTON, WASHINGTON
Proj. No.T-7841 Date FEB 2018 Figure 2
SLOPE TO DRAIN
4" DIAMETER PERFORATED PVC PIPE
NOTE:
12" OVER PIPE
3" BELOW PIPE
NOT TO SCALE
EXCAVATED SLOPE
(SEE REPORT TEXT
FOR APPROPRIATE
INCLINATIONS)
MIRADRAIN G100N PREFABRICATED DRAINAGE PANELS OR SIMILAR
PRODUCT CAN BE SUBSTITUTED FOR THE 12 -INCH WIDE GRAVEL
DRAIN BEHIND WALL. DRAINAGE PANELS SHOULD EXTEND A MINIMUM
OF SIX INCHES INTO 12 -INCH THICK DRAINAGE GRAVEL LAYER
OVER PERFORATED DRAIN PIPE.
Terra Associates, Inc.
Consultants in Geotechnical Engineering
Geology and
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TYPICAL WALL DRAINAGE DETAIL
WANG SHORT PLAT
RENTON, WASHINGTON
Proj. No.T-7841 I Date FEB 2018 1 Figure 3
APPENDIX A
FIELD EXPLORATION AND LABORATORY TESTING
Wang Short Plat
Renton, Washington
On February 8, 2018, we explored subsurface conditions at the site by excavating 7 test pits to maximum depths
of about 5 to 9 feet below existing surface grades using a track -mounted excavator. The test pit locations are
shown on Figure 2. The test pit locations were approximately determined in the field by sighting relative to
existing surface features. The Test Pit Logs are presented on Figures A-2 through A-8.
An engineering geologist from our office maintained a log of each test pit as it was excavated, classified the soil
conditions encountered, and obtained representative soil samples. All soil samples were visually classified in the
field in accordance with the Unified Soil Classification System. A copy of this classification is presented as
Figure A-1.
Representative soil samples obtained from the test pits were placed in sealed plastic bags and taken to our
laboratory for further examination and testing. The moisture content of each sample was measured and is
reported on the Test Pit Logs. Grain size analyses were performed on three of the soil samples. The results are
shown on Figure A-9.
Project No. T-7841
HIGHLY ORGANIC SOILS I PT
Peat.
DEFINITION OF TERMS AND SYMBOLS
CO)
MAJOR DIVISIONS
W
LETTER
SYMBOL
TYPICAL DESCRIPTION
J
Clean
GW
Well -graded gravels, gravel -sand mixtures, little or no fines.
W
Loose
4-10
Gravels (less
Medium Dense
10-30
O
`
GRAVELS
than 5%
Very Dense
>50
J
More than 50%
fines)
GP
Poorly -graded gravels, gravel -sand mixtures, little or no fines.
O
rn
co N of
coarse fraction
U)
To (n
is larger than No.
GM
Silty gravels, gravel -sand -silt mixtures, non -plastic fines.
0
= a)
4 sieve
Gravels with
GC
Clayey gravels, gravel -sand -clay mixtures, plastic fines -
LU E .Cn
fines
C)
C7
N
LO
Clean Sands
SW
Well -graded sands, sands with gravel, little or no fines.
LU N
o
C Z
SANDS
(less than
Q-
t
More than 50%
5% fines)
SP
Poorly -graded sands, sands with gravel, little or no fines.
O
�'
of coarse fraction
SM
Silty sands, sand -silt mixtures, non -plastic fines.
V o
is smaller than
No. 4 sieve
Sands with
SC
Clayey sands, sand -clay mixtures, plastic fines -
fines
A2
ML
Inorganic silts, rock flour, clayey silts with slight plasticity.
CO)
CU a)
E .N
SILTS AND
CLAYS
CL
Inorganic clays of low to medium plasticity. (Lean clay)
O
Liquid Limit is less than 50%
OL
Organic silts and organic clays of low plasticity.
D m '(n
Z
Eo
C NMH
Inorganic silts, elastic.
� o
O
C: Z
SILTS AND
CLAYS
LU
Z
ca r -CH
t
Liquid Limit is greater than 50%
Inorganic clays of high plasticity. (Fat clay)
o
OH
Organic clays of high plasticity.
2
HIGHLY ORGANIC SOILS I PT
Peat.
DEFINITION OF TERMS AND SYMBOLS
CO)
Standard Penetration
W
Density
Resistance in Blows/Foot
J
Z
O
Very Loose
0-4
W
Loose
4-10
W
Medium Dense
10-30
O
Dense
30-50
V
Very Dense
>50
W
US
W
2
O
V
Standard Penetration
Consistancy Resistance in Blows/Foot
Very Soft 0-2
Soft 2-4
Medium Stiff 4-8
Stiff 8-16
Very Stiff 16-32
Hard >32
Terra
Associates Inc.
Consultants in Geotechnical Ingineering
Geology and
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I2" OUTSIDE DIAMETER SPILT SPOON SAMPLER
2.4" INSIDE DIAMETER RING SAMPLER OR
SHELBY TUBE SAMPLER
1 WATER LEVEL (Date)
Tr TORVANE READINGS, tsf
Pp PENETROMETER READING, tsf
DD DRY DENSITY, pounds per cubic foot
LL LIQUID LIMIT, percent
PI PLASTIC INDEX
N STANDARD PENETRATION, blows per foot
UNIFIED SOIL CLASSIFICATION SYSTEM
WANG SHORT PLAT
RENTON, WASHINGTON
Proj. No.T-7841 Date FEB 2018 Figure A-1
LOG OF TEST PIT NO. 1 FIGURE A-2
PROJECT NAME: Wang Short Plat PROJ. NO: T-7841 LOGGED BY: JCS
LOCATION: Renton, Washington SURFACE CONDITIONS: Duff APPROX. ELEV: 518
DATE LOGGED: February 8, 2018 DEPTH TO GROUNDWATER: 5 ft DEPTH TO CAVING: NA
6
Z
r n Description
a E
a� m
18 inches Duff and Topsoil (Disturbed)
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist. (SM)
1
11111111111111itA
3E5
7
E
10
—1 2
Brown to gray -brown silty SAND to silty SAND with gravel, fine sand, fine to coarse
gravel, moist (locally wet at 5 feet), locallized weakly to moderately cemented zones
(SM) (Till like)
Test pit terminated at 6 feet.
Light groundwater seepage at 5 feet.
Consistency/ a
Relative Density Remarks
Medium Dense
Dense
20.3
11.2
.�� Terra
NOTE: This subsurface information pertains only to this test pit location and should not be ..• : Associates Inc.
interpreted as being indicative of other locations at the site. Cansultanis in Geotechnical Ingineering
Geology and
Environmental Earth Sciences
LOG OF TEST PIT NO. 2 FIGURE A-3
PROJECT NAME: Wang Short Plat PROJ. NO: T-7841 LOGGED BY: JCS
LOCATION: Renton Washin ton SURFACE CONDITIONS: Duff APPROX. ELEV: 513
DATE LOGGED: February 8,_.2018 DEPTH TO GROUNDWATER: 3 ft DEPTH TO CAVING: NA
6
Z
Q Description
s a
n E
m m
cn
0
1 8 inches Duff and Topsoil.
1
2
T 3
4
5
to
7
iK
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist to wet. (SM)
Gray to gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, wet,
mottled. (SM)
Brown to gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist,
weakly cemented. (SM) (Till like)
Test pit terminated at 7 feet.
Light groundwater seepage between 3 and 4 feet.
Consistency/ o
Relative Density Remarks
Medium Dense
Medium Dense
to Dense
Dense
., Terra
NOTE: This subsurface information pertains only to this test pit location and should not be Associates, Inc.
interpreted as being indicative of other locations at the site. Consultants in Geoteclinical Engineering
Geology and
Environmental Earth Sciences
LOG OF TEST PIT NO. 3 FIGURE A-4
PROJECT NAME: Wang Short Plat _ PROJ. NO: T-7841 LOGGED BY: JCS
LOCATION: Renton, Washington SURFACE CONDITIONS: Duff, surface water APPROX. ELEV: 509
DATE LOGGED: February 8, 2018 DEPTH TO GROUNDWATER: 1 ft DEPTH TO CAVING: NA
Z
Z Consistency/ o
s a Description Relative Density Remarks
n E
a) m
co
0-
12 inches Duff and Topsoil, wet.
Gray to gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, wet,
= 1 mottled. (SM)
2
3
7
10
1
Brown to gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist,
weakly cemented. (SM) (Till like)
Test pit terminated at 5 feet.
Moderate groundwater seepage at 1 foot.
NOTE: This subsurface information pertains only to this test pit location and should not be
interpreted as being indicative of other locations at the site.
Medium Dense
PA KI
Dense
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LOG OF TEST PIT NO.4 FIGURE A-5
PROJECT NAME: WN n Short Plat PROJ. NO: T-7841 LOGGED BY:JCS
LOCATION: Renton, Washington_ SURFACE CONDITIONS: Duff APPROX. ELEV: 508
DATE LOGGED: february 8_ 2018 DEPTH TO GROUNDWATER: 2 ft DEPTH TO CAVING: NA
Z Consistency/
(D
L 0. Description Relative Density Remarks
a E
a) as
0
10 inches Duff and Topsoil.
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist to wet. (SM) Medium Dense
1-
3F2
M7
6_� 1
7
=-I
Gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist (locally wet
between 2 and 3 feet), mottled. (SM)
Brown to gray -brown silty SAND to silty SAND with gravel, fine sand, fine to medium
gravel, moist, scattered weakly cemented zones. (SM)
Gray brown silty SAND to silty SAND with gravel, fine sand, fine to coarse gravel,
moist, weakly to moderately cemented. (SM) (Till like)
9— 2
Test pit terminated at 9 feet.
Light groundwater seepage between 2 and 3 feet.
10 .......................
_- Terra
NOTE: This subsurface information pertains only to this test pit location and should not beAss oc f ates Inc.
interpreted as being indicative of other locations at the site. -_:f� Consultants in Geotechnical Ingineering
Geology and
Environmental Earth Sciences
Medium Dense
to Dense
Dense
mialm
9.8
LOG OF TEST PIT NO. 5 FIGURE A-6
PROJECT NAME: Wang Short Plat PROJ. NO: T-7841._ _ LOGGED BY: JCS
LOCATION: Renton, Washington SURFACE CONDITIONS: Duff
DATE LOGGED:February 8_2018 DEPTH TO GROUNDWATER: 2 ft
r�
1
-V2
3
4
5
W
7
Description
APPROX. ELEV: 508
DEPTH TO CAVING: NA
Consistency/ o
Relative Density Remarks
10 inches Duff and Topsoil.
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist to wet. (SM) Medium Dense
Gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist (locally wet
between 2 and 3 feet), mottled. (SM)
Brown to gray -brown silty SAND to silty SAND with gravel, fine sand, fine to medium
gravel, moist, scattered weakly cemented zones. (SM)
Gray brown silty SAND to silty SAND with gravel, fine sand, fine to coarse gravel,
moist, weakly to moderately cemented. (SM) (Till like)
Test pit terminated at 9 feet.
Light to moderate groundwater seepage between 2 and 3 feet.
10 . ..... ...... ..
Medium Dense
to Dense
Dense
Terra
NOTE: This subsurface information pertains only to this test pit location and should not be Associates Inc.
interpreted as being indicative of other locations at the site. Consultants in Geotechriical Ingineering
Geology and
Environmental Earth Sciences
PROJECT NAME: Wang Short Plat
LOCATION: Renton, Washington
LOG OF TEST PIT NO. 6
SURFACE CONDITIONS:
FIGURE A-7
PROJ. NO: T-7841 LOGGED BY: JCS
APPROX. ELEV: 510
DATE LOGGED: February 8, 2018 DEPTH TO GROUNDWATER: 1.5 ft .................— DEPTH TO CAVING:NA
0
Z Consistency/ o
L a Description Relative Density Remarks
n E
0a
o U)
1
3E
2-
3-
4— 1
6._
12 inches Duff and Topsoil.
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist to wet. (SM)
Gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, moist to wet,
mottled. (SM)
Brown silty SAND to silty SAND with gravel, fine sand, fine to medium gravel, moist,
scattered weakly cemented zones. (SM)
Medium Dense I
Medium Dense
to Dense
Gray brown silty SAND to silty SAND with gravel, fine sand, fine to coarse gravel,
6._ moist, weakly to moderately cemented. (SM) (Till like) Dense
14.9
Test pit terminated at 6.5 feet.
7— Light groundwater seepage between 1.5 and 3.5 feet.
8-
9-
10 E ................................
Terra
NOTE: This subsurface information pertains only to this test pit location and should not be
interpreted as being indicative of other locations at the site. Associates � n Ci.
P g _ Consultants in Geotechnical engineering
Geology and
— Environmental Earth Sciences
LOG OF TEST PIT NO. 7 FIGURE A-8
PROJECT NAME: Wang Short Plat PROJ. NO: T-7841 LOGGED BY: JCS
LOCATION: Renton, Washington SURFACE CONDITIONS: Duff APPROX. ELEV: 512
DATE LOGGED: February 8, 2018 DEPTH TO GROUNDWATER: 2 ft DEPTH TO CAVING: NA
0
Z
s a Description
n E
(D m
rn
0
12 inches Duff and Topsoil.
Red -brown silty SAND with gravel, fine sand, fine to coarse gravel, wet. (SM)
1
S 2
3
4
�7
7
M
9
10 ....1 _i..
Gray -brown silty SAND with gravel, fine sand, fine to coarse gravel, wet, weakly
cemented, mottled. (SM)
Consistency/ o
Relative Density Remarks
Medium Dense
Brown silty SAND to silty SAND with gravel, fine sand, fine to medium gravel, moist, Medium Dense
scattered weakly cemented zones. (SM) to Dense
---------...... — — .... --- ------ ---- ---- ........... ...—
Gray brown silty SAND to silty SAND with gravel, fine sand, fine to coarse gravel,
moist, weakly to moderately cemented. (SM) (Till like) I Dense
Test pit terminated at 6 feet.
Light groundwater seepage between 2 and 3 feet.
NOTE: This subsurface information pertains only to this test pit location and should not be
interpreted as being indicative of other locations at the site.
Terra
Associates Inc.
Consultants in Geotechnical Ingineering
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Tested By: FQ
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0.0 1 8.4 3.9 18.2 39.8
29.7
❑
0.0
0.0 7.5 3.3 1 18.4 35.7
35.1
LL PL
Dgs DrO D D D15
DIO CrC
0
3.9251 0.9809 0.5375 0.2194
0
1.2172 0.2923 0.2179 0.0824
❑
1.0505 0.2592 0.1868
Material Description
USCS
AASHTO
o silty SAND
SM
o silty SAND
SM
,6 silty SAND
SM
Project No. T-7841
Client: Jane Wang
Remarks:
Project: Wang Short Plat
oTested February 9, 2018
❑Tested February 9, 2018
Location: TP-1
Depth: 2.5
❑Tested February 9, 2018
Location: TP-4
Depth: 6'
❑ Location: TP-4
Depth: 9'
Terra Associates, Inc.
Kirkland, WA
Figure A-9
Tested By: FQ