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GEOTECHNICAL REPORT
Windsor Court
19411 and 19505 108th Avenue Southeast
Renton, Washington
Project No. T-8955
Prepared for:
DeDonato Group, LLC
Kirkland, Washington
July 23, 2024
Revised December 30, 2024
TABLE OF CONTENTS
Page No.
1.0 Project Description .......................................................................................................... 1
2.0 Scope of Work ................................................................................................................. 1
3.0 Site Conditions ................................................................................................................ 2
3.1 Surface ................................................................................................................ 2
3.2 Soils .................................................................................................................... 2
3.3 Groundwater ....................................................................................................... 3
3.4 Geologic Hazards ............................................................................................... 3
3.4.1 Erosion Hazard Areas ............................................................................... 3
3.4.2 Steep Slope Hazard Areas ........................................................................ 4
3.4.3 Landslide Hazard Areas ............................................................................ 4
3.4.4 Seismic Hazard Areas ............................................................................... 5
3.4.5 Coal Mine Hazard Areas .......................................................................... 5
3.5 Seismic Site Class ............................................................................................... 5
4.0 Discussion and Recommendations .................................................................................. 6
4.1 General ...................................................................................................................... 6
4.2 Site Preparation and Grading .................................................................................... 6
4.3 Excavations ............................................................................................................... 7
4.4 Foundations .............................................................................................................. 8
4.5 Slab-on-Grade Floors ................................................................................................ 8
4.6 Lateral Earth Pressure for Retaining Walls .............................................................. 9
4.7 Infiltration Feasibility ............................................................................................... 9
4.8 Drainage .................................................................................................................. 10
4.9 Utilities ................................................................................................................... 10
4.10 Pavements ............................................................................................................. 10
5.0 Additional Services ........................................................................................................ 11
6.0 Limitations ..................................................................................................................... 11
Figures
Vicinity Map ......................................................................................................................... Figure 1
Exploration Location Plan .................................................................................................... Figure 2
Typical Wall Drainage Detail ............................................................................................... Figure 3
Appendix
Field Exploration and Laboratory Testing ....................................................................... Appendix A
Geotechnical Report
Windsor Court
19411 and 19505 – 108th Avenue Southeast
Renton, Washington
1.0 PROJECT DESCRIPTION
The proposed project consists of developing the site with 20 residential building lots, stormwater facilities, and
associated access and utilities. We were provided with a concept plan by ESM Consulting Engineers, LLC, dated
October 31, 2024, that shows the 20 residential building lots, that are accessed by a road off 108th Avenue
Southeast, and the location of the proposed stormwater facility. Grading plans were not available at the time of this
report. Based on existing topography grading is expected to be minimal with cuts and fills from one to six feet.
We expect that the residences would be two- to three- 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
4 kips per foot for bearing walls and 25 to 75 kips for isolated columns.
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
On October 9, 2023, we explored subsurface conditions at the site in seven test pits excavated to maximum depths
of about seven to eight 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 site class per the current International Building Code (IBC).
Site preparation and grading.
Excavations.
Foundations.
Slab-on-grade floors.
Lateral earth pressures for retaining walls.
Infiltration Feasibility.
July 23, 2024
Revised December 30, 2024
Project No. T-8955
Page No. 2
Drainage.
Utilities.
Pavements.
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 are 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 2.52-acre assemblage of two residential parcels located west of and adjacent to 108th Avenue
Southeast, approximately 180 to 580 feet north of the intersection with Southeast 196th Street in Renton,
Washington. The site location is shown on Figure 1.
Two residences and a detached garage/shop building currently occupy the northern parcel. The southern parcel is
occupied by a residence and detached garage/shop. Site vegetation generally consists of grass lawn, landscape trees
and shrubs, and mature coniferous trees.
Site topography is relatively flat except for the northeastern site corner, which slopes gently to moderately down to
the northeast. Elevation contours shown on the City of Renton COR Maps interactive map website (COR Maps)
(https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps) shows slope gradients in this area are
generally between about 14 and 16 percent, with localized 4- to 6-foot-high slope areas between 20 and 40 percent.
The southern approximately 320 feet of the eastern site margin is adjacent to the top of a 5- to 6-foot-high rockery
constructed along the western side of the 108th Avenue Southeast right-of-way (ROW). We did not observe any
indications of instability, groundwater seepage, or erosion on the site slopes. The ROW rockery appeared sound
with no indications of soil loss or seepage through the rocks.
3.2 Soils
The native soils observed in the test pits are till-like glacial deposits consisting of silty sand with gravel. The upper
approximately 1.5 to 2.5 feet of soil are generally in a medium dense and moist condition. We observed exceptions
to this in Test Pit TP-2, where the upper approximately 5 feet of soil was medium dense and dry, and in Test Pit
TP-3, where the upper 2.5 feet of soil was moist to wet. The soils observed below these depths are generally dense
to very dense, weakly to strongly cemented, and moist.
We observed approximately 1.5 feet of medium dense, moist, silty sand with gravel fill overlying the native deposits
in Test Pit TP-1. The fill at this location appears to be related to grading of the yard area on the eastern side of the
residence. The site soils are typically mantled by about two to six inches of sod and topsoil.
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Revised December 30, 2024
Project No. T-8955
Page No. 3
The Geologic map of the Renton quadrangle, King County, Washington by D.R. Mullineaux (1965) shows the site
mapped as Vashon ground moraine deposits (Qgt). The dense to very dense, 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 did not observe groundwater seepage in the test pits. However, we observed mottling in the soils between
depths of about one and one-half and four feet, indicating that they have been impacted by perched groundwater at
times. The occurrence of shallow perched groundwater is typical for sites underlain by relatively impermeable till
and till-like soils. Perched groundwater levels and seepage flow rates will typically fluctuate on a seasonal basis
with the highest levels developing during the wet winter months.
3.4 Geologic Hazards
We evaluated potential geologic-related hazards at the subject site as defined in Title IV, Chapter 3, Section 4-3-
050G (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
RMC Title IV, Chapter 3, Section 4-3-050G5c 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, 8 to 15 percent slopes (AgC), which are
described as having a moderate erosion hazard. Based on this erosion hazard rating, per the above criteria, the vast
majority of the site is categorized as having a low erosion hazard (EL). However, as discussed, localized slope
areas exist in the northeastern portion of the site with slope inclinations steeper than 15 percent that would be better
classified as Alderwood gravelly sandy loam, 15 to 30 percent slopes (AgD), which is described as having a severe
erosion hazard. Accordingly, the slope areas in the northeastern portion of the site that are steeper than 15 percent
meet the criteria defining a high erosion hazard area (EH).
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Revised December 30, 2024
Project No. T-8955
Page No. 4
As discussed, we did not observe any indications of erosion on the site slopes. Additionally, COR Maps shows no
erosion hazard areas mapped onsite. In our opinion, the erosion potential of the 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 activities and should conform to City of Renton requirements.
3.4.2 Steep Slope Hazard Areas
RMC Title IV, Chapter 3, Section 4-3-050G5a defines steep slopes as follows:
i. 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 feet 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 feet as identified in the City of Renton Steep Slope Atlas
or in a method approved by the City.
Based on our field observations, and site slope gradients shown on COR Maps, portions of the northeast-facing
slope in the northeastern portion of the site, and the ROW rockery adjacent to the eastern site margin meet the above
geometric criteria defining a sensitive steep slope hazard area. However, COR Maps does not show any sensitive
slopes (<15’ height, >25 percent to <40 percent inclination) onsite.
Based on our observation of stable slope conditions and considering that the slope areas are underlain by inherently-
stable, dense to very dense, cemented, till-like soils at relatively shallow depths, it is our opinion that any potential
hazard associated with the site slopes would be adequately mitigated by the geotechnical recommendations
presented herein.
3.4.3 Landslide Hazard Areas
RMC Title IV, Chapter 3, Section 4-3-050G5b defines landslide hazards as follows:
i. 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.
July 23, 2024
Revised December 30, 2024
Project No. T-8955
Page No. 5
Based on the above criteria, the site slopes would generally be categorized as having a low landslide hazard (LL)
or medium landslide hazard (LM).
In our opinion, there is no landslide hazard associated with the site slopes. The slopes are underlain by dense to
very dense, inherently stable till or till-like glacial deposits with no evidence of groundwater seepage or significant
erosion.
3.4.4 Seismic Hazard Areas
RMC Title IV, Chapter 3, Section 4-3-050G5d 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 to very dense till and till-like soils. As discussed in Section 3.5, 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.
3.4.5 Coal Mine Hazard Areas
RMC Title IV, Chapter 3, Section 4-3-050G5e defines coal mine hazards as follows:
i. 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.
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 Site Class
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.
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Revised December 30, 2024
Project No. T-8955
Page No. 6
4.0 DISCUSSION AND RECOMMENDATIONS
4.1 General
Based on our study, there are no geotechnical conditions that would preclude development of the site as currently
planned. Residences can be supported on conventional spread footings bearing on competent native soils
underlying organic topsoil and existing fill materials or on structural fill placed on a competent native soil subgrade.
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 upon their moisture content and the prevailing weather conditions at the time of
construction. If grading activities take place during the winter season, the owner should be prepared to import free-
draining granular material for use as structural fill and backfill.
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.
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 four to six inches will generally
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, slabs, and pavements, and abandonment of
drainfields and 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 pad and
roadway elevations can be initiated. A representative of Terra Associates, Inc. should examine all bearing surfaces
to verify that the 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 proof rolling 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.
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Revised December 30, 2024
Project No. T-8955
Page No. 7
Our study indicates that the site soils typically contain a significant percentage 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. Soils that are wet of optimum when excavated or become wet prior to use as structural
fill will require drying 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. 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
6 inches 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.
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 to very dense,
cemented, till and 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.
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Revised December 30, 2024
Project No. T-8955
Page No. 8
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 to
very dense till and till-like soils, particularly during the wet winter months. In our opinion, the volume of water
and rate of seepage flow into the excavation should be relatively minor and would not be expected to impact the
stability of the excavations that are sloped 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.
Perimeter foundations exposed to the weather should bear at a minimum depth of one and one-half 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 five 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.
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Revised December 30, 2024
Project No. T-8955
Page No. 9
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 Lateral Earth Pressures for Retaining Walls
The magnitude of earth pressures developing on below-grade walls will depend upon the quality and compaction
of the wall backfill. We recommend placing and compacting wall backfill as structural fill as described in Section
4.2 of this report. To prevent overstressing the walls during backfilling, heavy construction machinery should not
be operated within five feet of the wall. Wall backfill in this zone should be compacted with hand-operated
equipment. To prevent hydrostatic pressure development, wall drainage must also be installed. A typical wall
drainage detail is shown on Figure 3.
With wall backfill placed and compacted as recommended, and drainage properly installed, we recommend
designing unrestrained walls for an active earth pressure equivalent to a fluid weighing 35 pounds per cubic foot
(pcf). For restrained walls, an additional uniform load of 100 psf should be added to the 35 pcf. To account for
typical traffic surcharge loading, the walls can be designed for an additional imaginary height of two feet (two-foot
soil surcharge). For evaluation of wall performance under seismic loading, a uniform pressure equivalent to 8H
psf, where H is the height of the below-grade portion of the wall should be applied in addition to the static lateral
earth pressure. These values assume a horizontal backfill condition and that no other surcharge loading, sloping
embankments, or adjacent buildings will act on the wall. If such conditions exist, then the imposed loading must
be included in the wall design. Friction at the base of foundations and passive earth pressure will provide resistance
to these lateral loads. Values for these parameters are provided in Section 4.4 of this report.
4.7 Infiltration Feasibility
In our opinion, onsite infiltration of site stormwater will not be feasible due to the shallow presence of relatively
impermeable, dense to very dense, cemented, till and till-like soils beneath the site. It is also our opinion that soil
conditions are not favorable for the use of low impact development (LID) natural drainage practices (NDPs) due
the potential for the development of shallow, seasonal perched groundwater, as indicated by soil mottling within
one and one-half to two and one-half feet of the ground surface, and the fine-grained nature of the site soils, which
have a USDA textural classification of silt loam.
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Revised December 30, 2024
Project No. T-8955
Page No. 10
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.
Surface water from developed areas must not be allowed to flow in an uncontrolled and concentrated manner toward
or onto site slopes. Surface water should be directed away from the slopes to a point of collection and controlled
discharge. If site grades do not allow for directing surface water away from the slope, then the water should be
collected and tightlined to an approved point of controlled discharge.
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 onsite 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; 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.
Proof rolling 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 four inches of crushed rock base (CRB).
Three and one-half inches full depth HMA over prepared subgrade.
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Revised December 30, 2024
Project No. T-8955
Page No. 11
The paving materials used should conform to the Washington State Department of Transportation (WSDOT)
specifications for ½-inch class HMA and CRB.
Long-term pavement performance will depend upon 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.
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 Windsor Court project in Renton, Washington. This report is for the
exclusive use of DeDonato Group, LLC, and their 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.
REFERENCE: KING COUNTY IMAP
Environmental Earth Sciences
Terra Associates, Inc.
Consultants in Geotechnical Engineering
Geology and
NOT TO SCALE
Figure 1Date DEC 2024
VICINITY MAP
WINDSOR COURT
RENTON, WASHINGTON
Proj. No.T-8955
SITE
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TP-6 TP-7
Environmental Earth Sciences
Terra Associates, Inc.
Consultants in Geotechnical Engineering
Geology and
Proj. No.T-8955
EXPLORATION LOCATION PLAN
WINDSOR COURT
RENTON, WASHINGTON
Date DEC 2024 Figure 2
REFERENCE ONLY AND SHOULD NOT BE USED FOR
DESIGN OR CONSTRUCTION PURPOSES.
DIMENSIONS ARE APPROXIMATE. IT IS INTENDED FOR
NOTE:
THIS SITE PLAN IS SCHEMATIC. ALL LOCATIONS AND
REFERENCE:
SITE PLAN PROVIDED BY ESM.
LEGEND:
APPROXIMATE TEST PIT LOCATION
0 80 160
APPROXIMATE SCALE IN FEET
12"
COMPACTED
STRUCTURAL FILL
EXCAVATED SLOPE
(SEE REPORT TEXT
FOR APPROPRIATE
INCLINATIONS)
SLOPE TO DRAIN
12" MINIMUM 3/4"
MINUS WASHED
GRAVEL
3" BELOW PIPE
12" OVER PIPE
4" DIAMETER PERFORATED PVC PIPE
SEE NOTE
6"(MIN.)
NOT TO SCALE
NOTE:
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.
Environmental Earth Sciences
Terra Associates, Inc.
Consultants in Geotechnical Engineering
Geology and
Proj. No.T-8955
TYPICAL WALL DRAINAGE DETAIL
WINDSOR COURT
RENTON, WASHINGTON
Date DEC 2024 Figure 3
Project No. T-8955
APPENDIX A
FIELD EXPLORATION AND LABORATORY TESTING
Windsor Court
19411 and 19505 108th Avenue Southeast
Renton, Washington
On October 9, 2023, we explored subsurface conditions at the site in seven test pits excavated to maximum depths
of about seven to eight feet below existing surface grades using a track-mounted excavator. The test pit locations
were approximately determined in the field by sighting relative to existing surface features. The approximate test
pit locations are shown on Figure 2. 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 five select soil samples. The results are shown on Figures A-9
and A-10.
Environmental Earth Sciences
Terra
Associates, Inc.
Consultants in Geotechnical Engineering
Geology and
MAJOR DIVISIONS LETTER
SYMBOL TYPICAL DESCRIPTION
GRAVELS
More than 50%
of coarse fraction
is larger than No.
4 sieve
Clean
Gravels (less
than 5%
fines)
GW Well-graded gravels, gravel-sand mixtures, little or no fines.
GP Poorly-graded gravels, gravel-sand mixtures, little or no fines.
Gravels with
fines
GM Silty gravels, gravel-sand-silt mixtures, non-plastic fines.
GC Clayey gravels, gravel-sand-clay mixtures, plastic fines.
SANDS
More than 50%
of coarse fraction
is smaller than
No. 4 sieve
Clean Sands
(less than
5% fines)
SW Well-graded sands, sands with gravel, little or no fines.
SP Poorly-graded sands, sands with gravel, little or no fines.
Sands with
fines
SM Silty sands, sand-silt mixtures, non-plastic fines.
SC Clayey sands, sand-clay mixtures, plastic fines.
SILTS AND CLAYS
Liquid Limit is less than 50%
ML Inorganic silts, rock flour, clayey silts with slight plasticity.
CL Inorganic clays of low to medium plasticity. (Lean clay)
OL Organic silts and organic clays of low plasticity.
SILTS AND CLAYS
Liquid Limit is greater than 50%
MH Inorganic silts, elastic.
CH Inorganic clays of high plasticity. (Fat clay)
OH Organic clays of high plasticity.
HIGHLY ORGANIC SOILS PT
Peat.
CO
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DEFINITION OF TERMS AND SYMBOLS
CO
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S
I
O
N
L
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S
S
CO
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I
V
E
Standard Penetration
Density Resistance in Blows/Foot
Very Loose 0-4
Loose 4-10
Medium Dense 10-30
Dense 30-50
Very Dense >50
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
2" OUTSIDE DIAMETER SPILT SPOON SAMPLER
2.4" INSIDE DIAMETER RING SAMPLER OR
SHELBY TUBE SAMPLER
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
Proj. No.T-8955
UNIFIED SOIL CLASSIFICATION SYSTEM
WINDSOR COURT
RENTON, WASHINGTON
Date DEC 2024 Figure A-1