HomeMy WebLinkAboutRS_Geotechnical_Report_190712_v1.pdf
February 12, 2019
WCSD 16950 116th Ave SE LLC
107 Spring Street, Suite 4025
Seattle, WA 98104
Attn: Ms. Anna Johnson
(503) 970-7487
Geotechnical Engineering Report
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
Doc ID: WCSD.CascadeVillage.RG
INTRODUCTION
This geotechnical engineering report summarizes our site observations, subsurface
explorations, and engineering analyses and provides geotechnical recommendations and design
criteria for the proposed improvements for the existing building located at 17060 – 116th Avenue
Southeast in Renton, Washington. The site location is shown on the Site Location Map, included as
Figure 1.
Our understanding of the project is based on our conversations with Ms. Anna Johnson with
Washington Charter School Development, our review of the conceptual Site Plan by Miller Hayashi
Architects, LLC dated April 30, 2018, our review of the Survey Plan by Puget Sound Surveying, Inc.
dated October 31, 2017, our December 14, 2018 site visit and subsurface explorations, and our
experience in the City of Renton area.
We understand that the development will consist of a structural retrofit of the existing
building and improvements to the parking area. You are proposing to convert the old grocery store
into a new school building that will include a new awning area in the front and a playground area in
the existing parking area. You are currently planning to keep the existing building and its
foundations. Portions of the current parking area that is impervious will be converted to landscape
areas. We anticipate that any new additions or structures will be a one or two-story, steel framed
structure likely founded on conventional shallow foundations, similar to the existing building. The
site is currently developed with a storm system and all other typical utilities and you are not
proposing to increase the impervious area at the site and are actually decreasing it.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 2
SCOPE
The purpose of our services is to evaluate the surface and subsurface conditions across the
site as a basis for providing geotechnical recommendations and design criteria for the proposed
development. Specifically, the scope of services for this project will include the following:
1. Reviewing the available geologic, hydrogeologic, and geotechnical data for the site area;
2. Exploring surface and subsurface conditions by reconnoitering the site and performing 2
borings at select locations across the site;
3. Describing surface and subsurface conditions, including soil type, depth to groundwater,
depth to impermeable soil strata, and an estimate of seasonal high groundwater levels;
4. Providing geotechnical conclusions and recommendations regarding seismic site class and
design coefficients, seismic hazard analyzes, site grading activities, including site preparation,
subgrade preparation, fill placement criteria, suitability of on-site soils for use as structural
fill, temporary and permanent cut slopes and drainage and erosion control measures;
5. Providing conclusions regarding shallow foundations and floor slab support and design
criteria, including bearing capacity and subgrade modulus as appropriate;
6. Providing our opinion about the feasibility of onsite infiltration in accordance with the 2017
City of Renton Surface Water Design Manual (2017 CORSWDM), including a preliminary
design infiltration rate based on grain size analysis, as applicable;
7. Providing recommendations for erosion and sediment control during wet weather grading
and construction; and,
8. Preparing a written Geotechnical Engineering Report summarizing our site observations and
conclusions, and our geotechnical recommendations and design criteria, along with the
supporting data.
The above scope of work was summarized in our Proposal for Geotechnical Engineering Services
dated November 13, 2018. We received authorization to proceed from you on December 3, 2018.
SITE CONDITIONS
Surface Conditions
The site is located 17060 – 116th Avenue Southeast in Renton, Washington, also known as
Cascade Village, within an area of existing residential and commercial development. Based on King
County iMap website, the site is irregular in shape, measures approximately 700 to 960 feet wide
(north to south) by approximately 1,185 feet deep (east to west) and encompasses about 13.63
acres. The site is bounded by the CVAC baseball complex and Southeast 168th Street to the north, by
existing residential development to the east, by existing commercial and residential development to
the south, and by 116th Avenue Southeast to the west.
Based on the topographic information obtained from the King County iMap website, the site
is generally flat, and gently slopes down from the Northwest to the Southeast at approximately 2
percent. Total topographic relief across the site is on the order of 35 feet. The site was originally
developed as a retail grocery store. The western and the southwestern portion of the site is
currently developed with multiple commercial buildings, while the northern and northeastern
portion of the site is currently developed with a paved parking lot. The existing site topography is
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 3
shown on the Site Topography Map, included as Figure 2a, while the existing and proposed site
development is shown on the Site and Exploration Plan, included as Figure 2b.
Site Soils
The USDA Natural Resource Conservation Service (NRCS) Web Soil Survey maps the subject
site as being underlain by Arents, Alderwood material (AmC) soils. A small portion, southeastern
portion of the site, is mapped as being underlain by Alderwood gravelly sandy loam (AgC) soils; and
a small portion, western portion of the site, is mapped as being underlain by Alderwood gravelly
sandy loam (AgB) soils. The Arents (AmC) soils are derived from Basal till, from on slopes of 6 to 15
percent, are listed as having a “moderate” erosion hazard, and are included in hydrologic soils group
C. The Alderwood (AgB & AgC) soils are derived from glacial drift and/or glacial outwash over dense
glaciomarine deposits, and are included in hydrologic soils group B. The Alderwood (AgB) soils form
on slopes of 0 to 8 percent and are listed as having a “slight” erosion hazard. The Alderwood (AgC)
soils form on slopes of 8 to 15 percent and are listed as having a “slight” to “moderate” erosion
hazard. A copy of the NRCS Soil Survey Map is included as Figure 3.
Site Geology
The Geologic Map of the Renton Quadrangle, King County, Washington (D. R. Mullineaux) maps
the site and adjacent areas as being underlain by glacial till (Qgt). The glacial till soils were generally
deposited during the Vashon Stade of the Fraser Glaciation, approximately 12,000 to 15,000 years ago.
The glacial till consists of a heterogeneous mixture of clay, silt, sand and gravel that was deposited at
the base of the continental ice mass and is typically encountered in a very dense condition. The till was
overridden by the ice mass, and as such is considered overconsolidated, and exhibits high strength and
low compressibility characteristics where undisturbed. An excerpt of the geologic map for the site
area is included as Figure 4.
No evidence of deep-seated instability or other active landslide activity was observed at the
time of our site visit. No areas of landslide deposits or mass wasting are noted on the referenced
map within the immediate vicinity of the site. Because the site is flat the risk of slope movement is
low.
Subsurface Explorations
On December 14, 2018, a field representative from GeoResources, LLC (GeoResources)
visited the site and monitored the drilling of two hollow stem auger boring to depths of 21½ feet
below existing ground surface. The boring was drilled by a licensed driller operating a track
mounted drill working under contract to GeoResources, LLC and utilities were located both with the
public locate system and a private locator using ground penetrating radar.
The specific number, locations, and depths of our explorations were selected with you based
on our understanding of the proposed development and were adjusted in the field based on
consideration for underground utilities, existing site conditions, site access limitations and
encountered stratigraphy. A field representative from our office continuously monitored the
explorations, maintained logs of the subsurface conditions encountered, obtained representative
soil samples, and observed pertinent site features. Representative soil samples obtained from the
explorations were placed in sealed plastic bags and taken to a laboratory for further examination
and testing as deemed necessary. The borings were then abandoned per Washington State
Department of Ecology requirements.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 4
During drilling, soil samples were obtained at 2½-foot and 5-foot depth intervals in
accordance with Standard Penetration Test (SPT) as per the test method outlined by ASTM: D-1586.
The SPT method consists of driving a standard 2-inch-diameter split-spoon sampler 18-inches into
the soil with a 140-pound hammer. The number of blows required to drive the sampler through
each 6-inch interval is counted, and the total number of blows struck during the final 12 inches is
recorded as the Standard Penetration Resistance, or “SPT blow count”. The resulting Standard
Penetration Resistance values indicate the relative density of granular soils and the relative
consistency of cohesive soils.
The subsurface exploration drilled as part of this evaluation indicate the subsurface
condition at specific location only, as actual subsurface conditions can vary across the site.
Furthermore, the nature and extent of such variation would not become evident until additional
explorations are performed or until construction activities have begun. Based on our experience in
the area and extent of prior explorations in the area, it is our opinion that the soils encountered in
the exploration are generally representative of the soils at the site. The soils encountered were
visually classified in accordance with the Unified Soil Classification System (USCS) and ASTM D: 2488.
The USCS is included in Appendix A as Figure A-1. The approximate locations of our borings are
shown on the attached Site and Exploration Plan, Figure 2b, while the descriptive logs of our boring
are included in Appendix A.
Subsurface Conditions
The subsurface conditions encountered in our borings generally confirmed the mapped
stratigraphy. Our borings encountered approximately 1½ to 2 inches of hot mix asphalt pavement with
no crushed rock below it. Underlying the asphalt pavement in boring B-2 we observed about 1 foot of
grey gravelly sand with silt in a dense, moist condition that appeared to be previously placed fill. We
have assumed this material is likely undocumented fill from the construction of the original building.
Both of our borings encountered several feet of brown and orange silty sand with some gravel in a
loose to medium dense, moist condition that appeared to be consistent with weathered glacial till.
Underlying these weathered and iron oxide stained soils, our borings encountered grey and orange
silty sand with some gravel in a medium dense to dense becomes very dense, moist to wet condition
that was encountered to the full depth explored. We interpret these deeper soils to be consistent with
native, undisturbed glacial till.
Laboratory Testing
Geotechnical laboratory testing was performed on select samples retrieved from the boring
explorations to determine soil index and engineering properties encountered. Laboratory testing
included visual soil classification per ASTM D: 2488, moisture content determinations per ASTM D:
2216, and grain size analyses per ASTM D: 422 standard procedures. The results of the laboratory
tests are included in Appendix B.
Groundwater Conditions
Groundwater seepage was observed in our boring B-1 at approximately 7 feet below the
existing ground surface at the time of drilling. Iron-oxide staining, consistent with mottling was also
observed in all our borings from about 2½ to 10 feet below the existing ground surface. Mottling can
be indicative of a seasonal high perched groundwater table that typically develops when the vertical
infiltration of precipitation through a more permeable soil is slowed at depth by a deeper, denser,
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 5
less permeable soil type, such as the deeper glacial till soils. We expect that perched groundwater
may develop seasonally atop the dense glacial till soils encountered across the site. We anticipate
fluctuations in the local groundwater levels will occur in response to precipitation patterns, off-site
construction activities, and site utilization.
ENGINEERING CONCLUSIONS AND RECOMMENDATIONS
Based on the results of our data review, site reconnaissance, subsurface explorations, slope
stability analyses and our experience in the area, it is our opinion that the proposed improvements
are feasible from a geotechnical standpoint. Pertinent conclusions and geotechnical recommen-
dations regarding the design and construction of the proposed development are presented below.
Seismic Site Class
Based on our observations and the subsurface units mapped at the site, we interpret the
structural site conditions to correspond to a seismic Site Class “C” for the onsite soils in accordance
with the 2015 IBC (International Building Code) documents and ASCE 7-10 Chapter 20 Table 20.3-1.
For design of seismic structures using the IBC 2015, mapped short-period and 1-second
period spectral accelerations, SS and S1, respectively, are required. The U.S. Geological Survey
(USGS) completed probabilistic seismic hazard analyses (PSHA) for the entire country in November
1996, which were updated and republished in 2002 and 2008. The PSHA ground motion results can
be obtained from the USGS website. The results of the updated USGS PSHA were referenced to
determine SS and S1 for this site. The results are summarized in the following table with the relevant
parameters necessary for IBC 2015 design.
TABLE 4:
2015 IBC PARAMETERS FOR DESIGN OF SEISMIC STRUCTURES
Spectral Response Acceleration (SRA) and Site
Coefficients Short Period 1 Second Period
Mapped SRA Ss = 1.394 S1 = 0.519
Site Coefficients (Site Class C) Fa = 1.0 Fv = 1.3
Maximum Considered Earthquake SRA SMS = 1.394 SM1 = 0.675
Design SRA SDS = 0.929 SD1 = 0.450
Earthquake-induced Geologic Hazards
Earthquake-induced geologic hazards may include liquefaction, lateral spreading, slope
instability, and ground surface fault rupture. According to the Department of Natural Hazard Map
(Geologic Information Portal), the site is located south to the Seattle Fault Zone, as shown on Figure
5. Given the distance to the mapped fault zones and thickness of young, dense glacial sediments
underlying the site, we interpret the risk for ground fault surface rupture to be low. No evidence of
faulting was observed in our subsurface explorations.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 6
Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength
due to an increase in pore water pressure. The increase in pore water pressure is induced by
seismic vibrations. Liquefaction mainly affects geologically recent deposits of loose, fine-grained
sands that are below the groundwater table. Based on the density of the soils observed to underlie
the site, it is our opinion that the risk for liquefaction to occur at this site during the design
earthquake is negligible.
Because the site is relatively flat and the risk of liquefaction is low, it is also our opinion that
the risk of earthquake induced slope instability or lateral spreading is also low.
Foundation Support
As stated, we understand that you are keeping the existing building and its foundation. A
structural engineer should be consulted if any additional loads are added to the existing building
and its current foundation system. Based on the encountered subsurface soil conditions
encountered across the site, we recommend that any new structures be supported by conventional
spread and column footings, assuming the loads are relatively light. Footings should be founded on
the shallow medium dense weathered till, dense to very dense native glacial till encountered at
depth, or on structural fill that extends to suitable native soils.
The native soils at the base of the excavations should be disturbed as little as possible. All
loose, soft or unsuitable material should be removed or recompacted, as appropriate. A
geotechnical expert or representative of GeoResources LLC should observe the foundation subgrade
at the time of excavation to determine if suitable bearing surfaces have been prepared.
We recommend a minimum width of 30 inches for isolated footings and at least 24 inches
for continuous wall footings. All footing elements should be embedded at least 18 inches below
grade for frost protection. Footings founded on the shallow weathered till or properly placed and
compacted structural fill may be designed using an allowable soil bearing capacity of 2,500 psf
(pounds per square foot) for combined dead and long-term live loads. While footings founded on
the native, unweathered, and undisturbed glacial till encountered at depth may be designed using
an allowable soil bearing capacity of 3,500 psf. The allowable bearing value may be increased by
one-third for transient loads such as those induced by seismic events or wind loads.
Lateral loads may be resisted by friction on the base of footings and floor slabs and as
passive pressure on the sides of footings. We recommend that an allowable coefficient of friction of
0.35 be used to calculate friction between the concrete and the underlying native glacially
consolidated outwash soils. Passive pressure may be determined using an allowable equivalent
fluid density of 350 pcf (pounds per cubic foot) for structural fill. Factors of safety have been applied
to these values.
We estimate that settlements of footings designed and constructed as recommended will be
less than 1-inch, for the anticipated load conditions, with differential settlements between
comparably loaded footings of ½-inch or less across a 50–foot span. Most of the settlements should
occur essentially as loads are being applied. However, disturbance of the foundation subgrade
during construction could result in larger settlements than predicted.
We do not know the original design criteria used for the existing footings. As such if
additional loads are to be added to the existing foundation system the current elements should be
analyzed by a structural engineer to determine if adequate support is available or additional
separate elements added.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 7
Floor Slab Support
Slab-on-grade floors, if constructed, should be supported on the near surface medium dense
to dense glacial till soils or on structural fill prepared as described above. Areas of old fill material
should be evaluated during grading activity for suitability of structural support. Areas of significant
organic debris, fines, or other debris should be removed and replaced with appropriately prepared
structural fill.
We recommend that floor slabs be directly underlain by a capillary break of a minimum 4-
inch thick pea gravel or clean 5/8-inch crushed rock. This layer should be placed and compacted to
an unyielding condition and should contain less than 2 percent fines.
A synthetic vapor retarder is recommended to control moisture migration through the slabs.
This is of particular importance where the foundation elements are underlain by the silty soils in
heated and covered indoo spaces where moisture migration through the slab is an issue, such as
where adhesives are used to anchor carpet or tile to the slab.
A subgrade modulus of 350 kcf (kips per cubic foot) may be used for floor slab design. We
estimate that settlement of the floor slabs designed and constructed as recommended, will be ½-
inch or less over a span of 50 feet.
Subgrade and Below Grade Walls
We do not anticipate subgrade and below grade walls be utilized at the site; however, if
utilized, the lateral pressures acting on subgrade and retaining walls (such as basement walls) will
depend upon the nature and density of the soil behind the wall. It is also dependent upon the
presence or absence of hydrostatic pressure. If the walls are backfilled with granular well-drained soil,
we recommend using an allowable equivalent fluid pressures of 35 pcf for the active condition and 55
pcf for the at rest condition. This design value assumes a level backslope and drained conditions as
described below. Where required by code, a seismic surcharge of 10H is recommended for active
conditions, calculated using the Mononobe-Okabe method. This surcharge is in addition to the static
lateral earth pressure and should be assumed to have resultant at 0.6H and assumes the wall will be
backfilled with adequately compacted structural fill.
Adequate drainage behind retaining structures is imperative. Positive drainage which controls
the development of hydrostatic pressure can be accomplished by placing a zone of drainage behind
the walls. Granular drainage material should contain less than 2 percent fines and at least 30
percent greater than the US #4 sieve. A geocomposite drain mat may also be used instead of free
draining soils, provided it is installed in accordance with the manufacturer’s instructions. A soil
drainage zone should extend horizontally at least 18 inches from the back of the wall. The drainage
zone should also extend from the base of the wall to within 1 foot of the top of the wall. The soil
drainage zone should be compacted to approximately 90 percent of the MDD. Over-compaction
should be avoided as this can lead to excessive lateral pressures.
A minimum 4-inch diameter perforated or slotted PVC pipe should be placed in the drainage
zone along the base and behind the wall to provide an outlet for accumulated water and direct
accumulated water to an appropriate discharge location. We recommend that a nonwoven
geotextile filter fabric be placed between the soil drainage material and the remaining wall backfill to
reduce silt migration into the drainage zone. The infiltration of silt into the drainage zone can, with
time, reduce the permeability of the granular material. The filter fabric should be placed such that it
fully separates the drainage material and the backfill and should be extended over the top of the
drainage zone.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 8
Lateral loads may be resisted by friction on the base of footings and as passive pressure on
the sides of footings and the buried portion of the wall, as described in the “Foundation Support”
section. We recommend that an allowable coefficient of friction of 0.35 be used to calculate friction
between the concrete and the underlying soil. Passive pressure may be determined using an
allowable equivalent fluid density of 350 pcf (pounds per cubic foot). Factors of safety have been
applied to these values.
Temporary Excavations
All job site safety issues and precautions are the responsibility of the contractor. The
following cut/fill slope guidelines are provided for planning purposes only. Temporary cut slopes will
likely be necessary during grading operations or utility installation.
All excavations at the site associated with confined spaces, such as utility trenches and
retaining walls, must be completed in accordance with local, state, or federal requirements. Based
on current Washington State Safety and Health Administration (WSHA) regulations, the fill and the
weathered till encountered at the site would be classified as Type C soils. The deeper glacial till
encountered at the site would be classified as Type A soils.
According to WSHA, for temporary excavations of less than 20 feet in depth, the side slopes
in Type C soils should be sloped at a maximum inclination of 1½H:1V (Horizontal:Vertical); and the
side slopes in Type A soils should be sloped at a maximum inclination of ¾H:1V. All exposed slope
faces should be covered with a durable reinforced plastic membrane during construction to prevent
slope raveling and rutting during periods of precipitation. These guidelines assume that all surface
loads are kept at a minimum distance of at least one half the depth of the cut away from the top of
the slope and that significant seepage is not present on the slope face. Flatter cut slopes will be
necessary where significant raveling or seepage occurs, or if construction materials will be stockpiled
along the slope crest.
Where it is not feasible to slope the site soils back at these inclinations, a retaining structure
should be considered. Where retaining structures are greater than 4-feet in height (bottom of
footing to top of structure) or have slopes of greater than 15 percent above them, they should be
engineered per Washington Administrative Code (WAC 51-16-080 item 5). This information is
provided solely for the benefit of the owner and other design consultants, and should not be
construed to imply that GeoResources assumes responsibility for job site safety. It is understood
that job site safety is the sole responsibility of the project contractor.
Site Drainage
All ground surfaces, pavements and sidewalks at the site should be sloped away from the
structures. The site should also be carefully graded to ensure positive drainage away from all
structures and property lines. Surface water runoff should be controlled by a system of curbs,
berms, drainage swales, and or catch basins, and conveyed to an appropriate discharge point.
We recommend that footing drains are installed for new strip footings in accordance with IBC
1807.4.2, and basement walls (if utilized) have a wall drain as described above. The roof drain should
not be connected to the footing drain. Figure 6 shows typical wall drainage and backfilling details. We
do not know the current configuration or if the existing building was constructed with footing drains.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 9
Stormwater Infiltration
As stated, groundwater seepage was observed in boring B-1 at approximately 7 feet below the
existing ground surface at the time of drilling. Iron oxide staining consistent with mottling was
observed in all our borings at approximately 1½ feet below the existing ground surface. Mottling is
often evidence of seasonal perched groundwater table. We also encountered undocumented fill in
boring B-2 and the glacial till soils encountered in both borings at a shallow depth are recognized by
most jurisdictions as an indurate layer that DOES NOT support on site infiltration. Based on our
observations and subsurface explorations, it is our opinion that onsite infiltration is not feasible for this
project. We highly recommend that the City not require infiltration testing as it is a waste of resources
that could be invested into the school instead. Additionally, there appears to be a functional storm
system at site that any new or replaced impervious surface could be directed to this system provided it
is determined to be adequate by the project team. Because the amount of impervious area currently
at the site is proposed to be reduced it is our opinion that using the existing system should be the most
appropriate method of stormwater management and should result in the lowest impact to the
environment.
EARTHWORK RECOMMENDATIONS
Site Preparation
Because the site is currently developed, we do not anticipate significant stripping will be
required. The areas where new foundations or hardscaping are proposed will likely require the
removal of existing hardscaping and we recommend the underlying soils be observed by
GeoResources after the areas have been cleared. Areas of current landscaping could be assumed to
have topsoil thicknesses of 12 to 18 inches, however no subsurface explorations where completed in
these areas.
The previously placed, undocumented fill soils observed in the vicinity of boring B-2 at the
site should be removed where new work is proposed and settlement is a concern.
Recommendations regarding the potential reuse of the undocumented fill and native soils are
discussed in the “Suitability of On-Site Materials as Fill” section.
Where placement of fill material is required, the stripped/exposed subgrade areas should be
compacted to a firm and unyielding surface prior to placement of any fill. Excavations for debris
removal should be backfilled with structural fill compacted to the densities described in the
“Structural Fill” section of this report.
We recommend that a member of our staff evaluate the exposed subgrade conditions after
removal of vegetation and topsoil stripping is completed and prior to placement of structural fill.
The exposed subgrade soil should be proof-rolled with heavy rubber-tired equipment during dry
weather or probed with a ½-inch-diameter steel rod during wet weather conditions.
Soft, loose or otherwise unsuitable areas delineated during proofrolling or probing should
be recompacted, if practical, or over-excavated and replaced with structural fill. The depth and
extent of over-excavation should be evaluated by our field representative at the time of construction.
The areas of previously placed, undocumented fill material should be evaluated during grading
operations to determine if they need mitigation; recompaction or removal.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 10
Structural Fill
All material placed as fill associated with mass grading, as utility trench backfill, under
building areas, under retaining structures or under roadways should be placed as structural fill. The
structural fill should be placed in horizontal lifts of appropriate thickness to allow adequate and
uniform compaction of each lift. Structural fill should be compacted to at least 95 percent of MDD
(maximum dry density as determined in accordance with ASTM D-1557).
The appropriate lift thickness will depend on the structural fill characteristics and
compaction equipment used. We recommend that the appropriate lift thickness be evaluated by
our field representative during construction. We recommend that our representative be present
during site grading activities to observe the work and perform field density tests.
The suitability of material for use as structural fill will depend on the gradation and moisture
content of the soil. As the amount of fines (material passing US No. 200 sieve) increases, soil
becomes increasingly sensitive to small changes in moisture content and adequate compaction
becomes more difficult to achieve. During wet weather, we recommend use of well-graded sand
and gravel with less than 5 percent (by weight) passing the US No. 200 sieve based on that fraction
passing the 3/4-inch sieve, such as Gravel Backfill for Walls (WSDOT 9-03.12(2)). If prolonged dry
weather prevails during the earthwork and foundation installation phase of construction, higher
fines content (up to 10 to 12 percent) may be acceptable.
Material placed for structural fill should be free of debris, organic matter, trash and cobbles
greater than 6-inches in diameter. The moisture content of the fill material should be adjusted as
necessary for proper compaction.
Suitability of On-Site Materials as Fill
During dry weather construction, any nonorganic onsite soil may be considered for use as
structural fill; provided it meets the criteria described above in the “Structural Fill” section and can
be compacted as recommended. If the moisture content of the soil is over optimum when
excavated, it will be necessary to aerate or dry the soil prior to placement as structural fill.
The previously placed, undocumented fill encountered at shallow depths at the site
consisted of gravelly sand with silt. These soils could be suitable for reuse as structural fill but they
should be evaluated as exposed by GeoResources personnel. Removal and processing of the
undocumented fill soils should include excavating down to native soils, and an appropriate level of
processing to meet the specification for common borrow WSDOT 9-03.14(3). GeoResources
personnel should provide sufficient laboratory testing and monitoring to ensure the above
specification is met and the material is replaced as structural fill.
The shallow native weathered till and glacial till soils encountered at depths across the site
generally consisted of silty sand with variable amounts of gravel. These soils are generally
comparable to “common borrow” material and will be suitable for use as structural fill provided the
moisture content is maintained within 2 percent of the optimum moisture level. Because of the high
fines content, these soils are highly moisture sensitive, and will be difficult to impossible to compact
during wet weather conditions, or where seepage occurs, such as in B-. If these soils are excessively
moist to saturated, it will be necessary to aerate or dry the soil prior to placement as structural fill.
We recommend that completed graded-areas be restricted from traffic or protected prior to
wet weather conditions. The graded areas may be protected by paving, placing asphalt-treated
base, a layer of free-draining material such as pit run sand and gravel or clean crushed rock material
containing less than 5 percent fines, or some combination of the above.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 11
Erosion Control
Weathering, erosion and the resulting surficial sloughing and shallow land sliding are natural
processes. As noted, no evidence of surficial raveling or sloughing was observed at the site at the
time of our site visit. To manage and reduce the potential for these natural processes, temporary
and permanent erosion control measures should be installed and maintained during construction or
as soon as practical thereafter to limit the additional influx of water to exposed areas and protect
potential receiving waters.
As a minimum, we recommend implementing the erosion and sediment control Best
Management Practices (BMPs) prior to, during, and immediately after clearing and grading activities
at the site. Erosion hazards can be mitigated by applying Best Management Practices (BMP’s)
outlined in the Washington State Department of Ecology’s (DOE) Stormwater Management Manual
for Western Washington.
If the recommended erosion and sediment control BMPs are properly implemented and
maintained, it is our opinion that the planned development will not increase the potential for
erosion of the site. Similarly, it is our opinion that the planned development will not increase the
potential for slope instability at the site or adjacent properties resulting from erosion.
Wet Weather and Wet Condition Considerations
In the Puget Sound area, wet weather generally begins about mid-October and continues
through about May, although rainy periods could occur at any time of year. Therefore, it is strongly
encouraged that earthwork be scheduled during the dry weather months of June through
September. Most of the soil at the site contains sufficient fines to produce an unstable mixture
when wet. Such soil is highly susceptible to changes in water content and tends to become unstable
and impossible to proof-roll and compact if the moisture content exceeds the optimum.
In addition, during wet weather months, the groundwater levels could increase, resulting in
seepage into site excavations. Performing earthwork during dry weather would reduce these
problems and costs associated with rainwater, construction traffic, and handling of wet soil.
However, should wet weather/wet condition earthwork be unavoidable, the following
recommendations are provided:
• The ground surface in and surrounding the construction area should be sloped as much as
possible to promote runoff of precipitation away from work areas and to prevent ponding of
water.
• Work areas or slopes should be covered with plastic. The use of sloping, ditching, sumps,
dewatering, and other measures should be employed as necessary to permit proper
completion of the work.
• Earthwork should be accomplished in small sections to minimize exposure to wet conditions.
That is, each section should be small enough so that the removal of unsuitable soils and
placement and compaction of clean structural fill could be accomplished on the same day.
The size of construction equipment may have to be limited to prevent soil disturbance. It
may be necessary to excavate soils with a backhoe, or equivalent, and locate them so that
equipment does not pass over the excavated area. Thus, subgrade disturbance caused by
equipment traffic would be minimized.
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 12
• Fill material should consist of clean, well-graded, sand and gravel, of which not more than 5
percent fines by dry weight passes the No. 200 mesh sieve, based on wet-sieving the fraction
passing the ¾-inch mesh sieve. The gravel content should range from between 20 and 50
percent retained on a No. 4 mesh sieve. The fines should be non-plastic.
• No exposed soil should be left uncompacted and exposed to moisture. A smooth-drum
vibratory roller, or equivalent, should roll the surface to seal out as much water as possible.
• In-place soil or fill soil that becomes wet and unstable and/or too wet to suitably compact
should be removed and replaced with clean, granular soil (see gradation requirements
above).
• Excavation and placement of structural fill material should be observed on a full-time basis
by a geotechnical engineer (or representative) experienced in wet weather/wet condition
earthwork to determine that all work is being accomplished in accordance with the project
specifications and our recommendations.
• Grading and earthwork should not be accomplished during periods of heavy, continuous
rainfall.
We recommend that the above requirements for wet weather/wet condition earthwork be
incorporated into the contract specifications as necessary.
Additional Services
Prior to construction we are available for plan review, project coordination and consulting if
necessary. Geotechnically related RFI’s or materials submittals can be reviewed at your request and
the required time for all our tasks after the preparation of this report would be billed on a time and
materials basis, We recommend that GeoResources, LLC be retained to observe the geotechnical
aspects of construction including stripping, processing of the undocumented fill, foundation
subgrade preparation, fill placement and compaction, drainage activities and othe geotechnical
portions of the construction. This observation would allow us to verify the subsurface conditions as
they are exposed during construction and to determine that work is accomplished in accordance
with our recommendations and potentially avoid unnecessary costs and delays. If conditions
encountered during construction differ from those anticipated, we can provide recommendations
for the conditions actually encountered to mitigate potential problems.
LIMITATIONS
We have prepared this report for use by Ms. Anna Johnson, Washington Charter School
Development and other members of the design team, for use in the design of a portion of this
project. The data used in preparing this report and this report should be provided to prospective
contractors for their bidding or estimating purposes only. Our report, conclusions and interpreta-
tions are based on our subsurface explorations, data from others and limited site reconnaissance,
and should not be construed as a warranty of the subsurface conditions.
Variations in subsurface conditions are possible between the explorations and may also occur
with time. A contingency for unanticipated conditions should be included in the budget and schedule.
Sufficient monitoring, testing and consultation should be provided by our firm during construction to
confirm that the conditions encountered are consistent with those indicated by the explorations, to
provide recommendations for design changes should the conditions revealed during the work differ
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 13
from those anticipated, and to evaluate whether earthwork and foundation installation activities
comply with contract plans and specifications.
The scope of our services does not include services related to environmental remediation and
construction safety precautions. Our recommendations are not intended to direct the contractor's
methods, techniques, sequences or procedures, except as specifically described in our report for
consideration in design.
If there are any changes in the loads, grades, locations, configurations or type of facilities to be
constructed, the conclusions and recommendations presented in this report may not be fully
applicable. If such changes are made, we should be given the opportunity to review our
recommendations and provide written modifications or verifications, as appropriate
WCSD.CascadeVillage.RG.doc
February 12, 2019
page | 14
We have appreciated the opportunity to be of service to you on this project. If you have any
questions or comments, please do not hesitate to call at your earliest convenience.
Respectfully submitted,
GeoResources, LLC
Kyle Billingsley, PE
Project Geotechnical Engineer
Dana C. Biggerstaff, PE Keith S. Schembs, LEG
Senior Geotechnical Engineer Principal
CC:KSS:DCB/cc
Doc ID: WCSD.CascadeVillage.RG
Attachments: Figure 1: Site Location Map
Figure 2a: Site Topography Map
Figure 2b: Site & Exploration Plan
Figure 3: Geologic Map
Figure 4: Washington DNR Natural Hazard Map
Figure 5: Fault Hazard Map
Figure 6: Typical Wall Drainage and Backfill Detail
Appendix A – Subsurface Explorations
Appendix B – Laboratory Test Results
Approximate Site Location
(map created from King County Public GIS https://gismaps.kingcounty.gov/iMap/)
Not to Scale
Site Location Map
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure 1
Approximate Site Location
(map created from King County Public GIS https://gismaps.kingcounty.gov/iMap/))
Not to Scale
Topographic Map
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure 2a
Not to scale
Legend
Map created from Site Plan by Puget Sound Surveying, Inc. dated October 31, 2017
Site and Exploration Plan
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F2b February, 2019 Figure 2b
B-2
B-1
Number and approximate location of borings
Approximate Site Location
Map created from Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx)
Soil
Type Soil Name Parent Material Slopes Erosion Hazard Hydrologic
Soils Group
AgB Alderwood gravelly sandy loam Glacial drift and/or glacial
outwash over dense
glaciomarine deposits
0 to 8 Slight B
AgC Alderwood gravelly sandy loam 8 to 15 Slight to Moderate B
AmC Arents, Alderwood gravelly
sandy loam Basal till 6 to 15 Moderate C
Not to Scale
NRCS Soils Map
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure 3
Approximate Site Location
(An excerpt from the Geologic Map of the Renton Quadrangle, King County, Washington by D. R. Mullineaux (1965))
Qgt Glacial Till
Not to Scale
USGS Geologic Map
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure 4
Approximate Site Location
Map created from Washington DNR Geologic Information Portal (https://geologyportal.dnr.wa.gov/)
Not to Scale
Fault Hazard Map
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure 5
Seattle Fault
Zone
Notes
Typical Wall Drainage and Backfill Detail
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: AllSeasonsIneVillage.F February, 2019 Figure 6
1. Washed pea gravel/crushed rock beneath floor slab could be
hydraulically connected to perimeter/subdrain pipe. Use of 1”
diameter weep holes as shown is one applicable method. Crushed
gravel should consist of 3/4” minus. Washed pea gravel should consist
of 3/8” to No. 8 standard sieve.
2. Wall backfill should meet WSDOT Gravel Backfill for walls Specification
9-03-12(2).
3. Drainage sand and gravel backfill within 18” of wall should be
compacted with hand-operated equipment. Heavy equipment should
not be used for backfill, as such equipment operated near the wall
could increase lateral earth pressures and possibly damage the wall.
The table below presents the drainage sand and gravel gradation.
4. All wall back fill should be placed in layers not exceeding 4” loose
thickness for light equipment and 8” for heavy equipment and should
be densely compacted. Beneath paved or sidewalk areas, compact to
at least 95% Modified Proctor maximum density (ASTM: 01557-70
Method C). In landscaping areas, compact to 90% minimum.
5. Drainage sand and gravel may be replaced with a geocomposite core
sheet drain placed against the wall and connected to the subdrain
pipe. The geocomposite core sheet should have a minimum
transmissivity of 3.0 gallons/minute/foot when tested under a gradient
of 1.0 according to ASTM 04716.
6. The subdrain should consist of 4” diameter (minimum),
slotted or perforated plastic pipe meeting the requirements
of AASHTO M 304; 1/8-inch maximum slot width; 3/16- to 3/8-
inch perforated pipe holes in the lower half of pipe, with
lower third segment unperforated for water flow; tight joints;
sloped at a minimum of 6”/100’ to drain; cleanouts to be
provided at regular intervals.
7. Surround subdrain pipe with 8 inches (minimum) of washed
pea gravel (2” below pipe” or 5/8” minus clean crushed gravel.
Washed pea gravel to be graded from 3/8-inch to No.8
standard sieve.
8. See text for floor slab subgrade preparation.
Materials
Drainage Sand and Gravel ¾” Minus Crushed Gravel
Sieve Size % Passing by
Weight
Sieve Size % Passing by
Weight
¾” 100 ¾” 100
No 4 28 – 56 ½” 75 – 100
No 8 20 – 50 ¼” 0 – 25
No 50 3 – 12 No 100 0 – 2
No 100 0 – 2 (by wet
sieving)
(non-plastic)
Appendix A
Subsurface Explorations
SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS
GROUP
SYMBOL
GROUP NAME
COARSE
GRAINED
SOILS
More than 50%
Retained on
No. 200 Sieve
GRAVEL
More than 50%
Of Coarse Fraction
Retained on
No. 4 Sieve
CLEAN
GRAVEL
GW
WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL
GP
POORLY-GRADED GRAVEL
GRAVEL
WITH FINES
GM
SILTY GRAVEL
GC
CLAYEY GRAVEL
SAND
More than 50%
Of Coarse Fraction
Passes
No. 4 Sieve
CLEAN SAND
SW
WELL-GRADED SAND, FINE TO COARSE SAND
SP
POORLY-GRADED SAND
SAND
WITH FINES
SM
SILTY SAND
SC
CLAYEY SAND
FINE
GRAINED
SOILS
More than 50%
Passes
No. 200 Sieve
SILT AND CLAY
Liquid Limit
Less than 50
INORGANIC
ML
SILT
CL
CLAY
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
Liquid Limit
50 or more
INORGANIC
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES: SOIL MOISTURE MODIFIERS:
1. Field classification is based on visual examination of soil Dry- Absence of moisture, dry to the touch
in general accordance with ASTM D2488-90.
Moist- Damp, but no visible water
2. Soil classification using laboratory tests is based on
ASTM D2487-90. Wet- Visible free water or saturated, usually soil is
obtained from below water table
3. Description of soil density or consistency are based on
interpretation of blow count data, visual appearance of
soils, and or test data.
W:\AaWIP\Projects\SweetbriarHOA.132ndStE
Unified Soils Classification System
Proposed Building Improvements
17060 – 116th Avenue Southeast
Renton, Washington
PN: 2823059009
DocID: WCSD.CascadeVillage.F February, 2019 Figure A-1
0
5
10
15
20
25
30
535
530
525
520
515
510
Asphalt pavement (about 1.5 to 2" thick)
Brown silty SAND with occasional gravel
(loose, moist) (SM) (Topsoil)
Grey mottled silty SAND (dense, moist) (SM)
(Glacial Till)
Becomes wet
Grey silty SAND with gravel (dense, moist to
wet) (SM)
Becomes very dense
Bottom of Boring
Completed12/14/2018
1
2
3
4
5
6
7
8
120
80
120
4
4
4
25
27
21
10
13
17
12
16
24
11
20
29
35
50/5
19
33
47
50/5
TOTAL DEPTH:21.5'EXCAVATION METHOD:HSA LOGGED BY:KEB
TOP ELEVATION:538'EXCAVATION COMPANY:HAMMER TYPE:Cathead
LATITUDE:EQUIPMENT:Track-mounted Drill HAMMER WEIGHT:140lb
LONGITUDE:NOTES:Eastern parking lot
NOTES Proposed Building Improvements
1. Refer to log key for definition of symbols, abbreviations and codes
2. USCS designation is based on visual manual classification
and selected lab testing
3. Groundwater level, if indicated, is for the date shown and may vary
4. N.E. = Not Encountered LOG OF BORING B-1
JOB:WCSD.CascadeVillage Sheet 1 of
GeoResources, LLC FIG.DepthElevationSOIL DESCRIPTION Drilling
Notes
SampleSamplerSymbolTest Results
(new title)
TEST RESULTS
10 20 30 40 50
Penetration - (blow per foot)
% Fines (<0.075mm)
% Water Content
Plastic Limit Liquid Limit Blow
Count GroundWater1
0
5
10
15
20
25
30
535
530
525
520
515
510
Asphalt pavement (about 1.5 to 2" thick)
Grey angular gravelly SAND with silt (dense,
moist) (Fill)
Brown silty SAND with some gravel (loose to
medium dense, moist) (SM)
Grey mottled silty fine to medium SAND with
some gravel (medium dense to dense, moist
to wet) (SM) (Weathered Till)
Grey mottled silty SAND with some gravel
(dense, moist) (SM) (Glacial Till)
Bottom of Boring
Completed12/14/2018
1
2
3
4
5
6
7
8
89
100
300
40
23
12
5
4
5
11
9
13
11
11
15
16
19
25
16
39
50/6
50/6
45
50/2
TOTAL DEPTH:21.5'EXCAVATION METHOD:HSA LOGGED BY:KEB
TOP ELEVATION:538'EXCAVATION COMPANY:HAMMER TYPE:Cathead
LATITUDE:EQUIPMENT:Track-mounted Drill HAMMER WEIGHT:140lb
LONGITUDE:NOTES:Eastern corner of existing building
NOTES Proposed Building Improvements
1. Refer to log key for definition of symbols, abbreviations and codes
2. USCS designation is based on visual manual classification
and selected lab testing
3. Groundwater level, if indicated, is for the date shown and may vary
4. N.E. = Not Encountered LOG OF BORING B-2
JOB:WCSD.CascadeVillage Sheet 1 of
GeoResources, LLC FIG.DepthElevationSOIL DESCRIPTION Drilling
Notes
SampleSamplerSymbolTest Results
(new title)
TEST RESULTS
10 20 30 40 50
Penetration - (blow per foot)
% Fines (<0.075mm)
% Water Content
Plastic Limit Liquid Limit Blow
Count GroundWater1
Appendix B
Laboratory Results
These results are for the exclusive use of the client for whom they were obtained. They apply only to the samples tested and are not indicitive of apparently identical samples.Tested By: Checked By:
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 18.8 36.4 7.8 11.0 12.6 13.46 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM D 422 & ASTM D 1140)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Date Sampled:Location: B1
Sample Number: 1B Depth: 0-1
Client:
Project:
Project No:Figure
silty gravel with sand
1.25
1
.75
.5
0.375
#4
#10
#20
#40
#60
#100
#200
100.0
89.2
81.2
70.8
57.6
44.8
37.0
31.7
26.0
21.1
17.1
13.4
NP NV NV
GM A-1-a
25.8336 21.8401 10.0288
6.3040 0.6922 0.1009
12/14/2018 12/19/2018
AES
KEB
PM
12/14/2018
Washington Charter School Development
Proposed Building Improvements
WCSD.CascadeVillage
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)
GeoResources, LLC
Fife, WA B-1
These results are for the exclusive use of the client for whom they were obtained. They apply only to the samples tested and are not indicitive of apparently identical samples.Tested By: Checked By:
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 9.7 8.4 2.3 8.7 31.4 39.56 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM D 422 & ASTM D 1140)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Date Sampled:Location: B1
Sample Number: 4 Depth: 7.5
Client:
Project:
Project No:Figure
silty sand with gravel
1
.75
.5
0.375
#4
#10
#20
#40
#60
#100
#200
100.0
90.3
86.9
83.9
81.9
79.6
76.6
70.9
60.7
50.0
39.5
NP NV NV
SM A-4(0)
18.3278 10.6293 0.2416
0.1502
12/14/2018 12/19/2018
AES
KEB
PM
12/14/2018
Washington Charter School Development
Proposed Building Improvements
WCSD.CascadeVillage
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)
GeoResources, LLC
Fife, WA B-2
These results are for the exclusive use of the client for whom they were obtained. They apply only to the samples tested and are not indicitive of apparently identical samples.Tested By: Checked By:
Particle Size Distribution Report
PERCENT FINER0
10
20
30
40
50
60
70
80
90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3"Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 0.0 5.0 4.2 12.0 39.4 39.46 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Test Results (ASTM D 422 & ASTM D 1140)
Opening Percent Spec.*Pass?
Size Finer (Percent)(X=Fail)
Material Description
Atterberg Limits (ASTM D 4318)
Classification
Coefficients
Date Received:Date Tested:
Tested By:
Checked By:
Title:
Date Sampled:Location: B2
Sample Number: 6 Depth: 12.5
Client:
Project:
Project No:Figure
silty sand
.5
0.375
#4
#10
#20
#40
#60
#100
#200
100.0
98.4
95.0
90.8
86.5
78.8
65.3
52.0
39.4
NP NV NP
SM A-4(0)
1.7176 0.7414 0.2038
0.1341
12/14/2018 12/19/2018
AES
KEB
PM
12/14/2018
Washington Charter School Development
Proposed Building Improvements
WCSD.CascadeVillage
PL=LL=PI=
USCS (D 2487)=AASHTO (M 145)=
D90=D85=D60=
D50=D30=D15=
D10=Cu=Cc=
Remarks
*(no specification provided)
GeoResources, LLC
Fife, WA B-3