21-101917-Geotechnical (Soils) Report-05-18-2012-V1 E RA
3
Geotechnical Engineering Report
Steel Lake Residential Plat
South 304th Street at 20th Avenue South
Federal Way, Washington
Submitted to:
Attn: Bill Classon
KPR Hospitality Investment, LP
2200 6th Avenue, Suite 520
Seattle, Washington 98121
Submitted by:
E3RA, Inc.
PO Box 44840
Tacoma, Washington 98448
December 6, 2012
Project No. T12081
TABLE OF CONTENTS
Page No.
1.0 SITE AND PROJECT DESCRIPTION...................................................................................1
2.0 EXPLORATORY METHODS.................................................................................................1
2.1 Test Pit Procedures....................................................................................................2
3.0 SITE CONDITIONS ...............................................................................................................2
3.1 Surface Conditions.....................................................................................................2
3.2 Soil Conditions...........................................................................................................3
3.3 Groundwater Conditions ............................................................................................4
3.4 Seismic Conditions.....................................................................................................4
3.5 Liquefaction Potential.................................................................................................4
3.6 Infiltration Conditions..................................................................................................4
4.0 CONCLUSIONS AND RECOMMENDATIONS......................................................................5
4.1 Site Preparation .........................................................................................................6
4.2 Spread Footings.........................................................................................................8
4.3 Slab on Grade Floors.................................................................................................9
4.4 Possible Additional Erosion Control Measures ..........................................................9
4.5 Asphalt Pavement....................................................................................................10
4.6 Structural Fill............................................................................................................11
5.0 RECOMMENDED ADDITIONAL SERVICES ......................................................................12
6.0 CLOSURE............................................................................................................................13
List of Tables
Table 1. Approximate Locations, Elevations and Depths of Explorations....................................................2
Table 2. Laboratory Test Results for Non-Organic Onsite Soils .................................................................5
List of Figures
Figure 1. Topographic and Location Map
Figure 2. Site and Exploration Plan
APPENDIX A
Soils Classification Chart and Key to Test Data.........................................................................................A-1
Logs of Test Pits TP-1 through TP-6................................................................................................ A-2…A-7
APPENDIX B
Laboratory Testing Results
PO Box 44840
Tacoma, WA 98448
253-537-9400
253-537-9401 Fax
E3RA
December 6, 2012
T12081
KPR Hospitality Investment, LP
2200 6th Avenue, Suite 520
Seattle, Washington 98121
Attention: Bill Classon
Subject: Geotechnical Engineering Report
Steel Lake Residential Plat
South 304th Street at 20th Avenue South
Federal Way, Washington
Dear Mr. Classon:
E3RA is pleased to submit this report describing the results of our geotechnical engineering evaluation for the
residential plat planned at the southwest corner of South 304th Street and 20th Avenue South in Federal Way,
Washington.
This report has been prepared for the exclusive use of KPR Hospitality Investment, LP, and their consultants,
for specific application to this project, in accordance with generally accepted geotechnical engineering
practice.
1.0 SITE AND PROJECT DESCRIPTION
The Steel Lake Residential Plat is located at the northwest corner of Steel Lake in Federal Way, Washington,
as shown on the enclosed Topographic and Location Map (Figure 1). It is a roughly triangular parcel that
encompasses 5.71 acres and is bounded on the west, southwest, south and southeast by Redondo Creek and
Steel Lake. Development will not occur next to the environmentally sensitive Steel Lake and Redondo Creek.
Consequently, development will only be on the northeast 2.69 acres of the site, where 13 residential lots and a
storm water tract will be located.
An existing sewer line crosses the part of the site that will be developed.
Because of the sloped nature of the site, we anticipate that significant grading will occur to provide road
access and level building areas.
2.0 EXPLORATORY METHODS
Our exploration and evaluation programs comprised the following elements:
• Surface reconnaissance of the site;
• Six test pits (designated TP-1 through TP-6), advanced on November 7, 2012;
• Two Grain Size Analyses, conducted on samples collected from the storm water tract; and
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T12081 / Steel Lake Residential Plat Geotechnical Report
• A review of published geologic and seismologic maps and literature.
Table 1 summarizes the approximate functional locations and termination depths of our subsurface
explorations, and Figure 2 depicts their approximate relative locations. The following sections describe the
procedures used for excavation of test pits.
TABLE 1
APPROXIMATE LOCATIONS, ELEVATIONS AND DEPTHS OF EXPLORATIONS
Exploration Functional Location
Termination
Depth
(feet)
TP-1
TP-2
TP-3
TP-4
TP-5
TP-6
Top of hill, vicinity of planned lot 2
Side of hill, vicinity of planned lot 3
Side of hill, vicinity of planned lot 13
Relatively level area, vicinity of planned lot 11
East part Storm Tract B
West part Storm Tract B
8
8
7
9
8
8
The specific number and locations of our explorations were selected in relation to the existing site features,
under the constraints of surface access, underground utility conflicts, and budget considerations.
It should be realized that the explorations performed and utilized for this evaluation reveal subsurface
conditions only at discrete locations across the project site and that actual conditions in other areas could vary.
Furthermore, the nature and extent of any such variations would not become evident until additional
explorations are performed or until construction activities have begun. If significant variations are observed
at that time, we may need to modify our conclusions and recommendations contained in this report to reflect
the actual site conditions.
2.1 Test Pit Procedures
Our test pits were excavated with a rubber-tired backhoe by an owner-operator under contract to E3RA. An
engineering geologist from our firm observed the test pit excavations and logged the subsurface conditions.
The enclosed test pit logs indicate the vertical sequence of soils and materials encountered in each test pit,
based on our field classifications. Where a soil contact was observed to be gradational or undulating, our logs
indicate the average contact depth. We estimated the relative density and consistency of the in-situ soils by
means of the excavation characteristics and the stability of the test pit sidewalls. Our logs also indicate the
approximate depths of any sidewall caving or groundwater seepage observed in the test pits. The soils were
classified visually in general accordance with the system described in Figure A-1, which includes a key to the
exploration logs. Summary logs of the explorations are included as Figures A-2 through A-7.
3.0 SITE CONDITIONS
The following sections present our observations, measurements, findings, and interpretations regarding,
surface, soil, groundwater, seismic, liquefaction, and infiltration conditions.
3.1 Surface Conditions
As previously mentioned, the project site is bounded by the valley of Redondo Creek to the west and
southwest, by Steel Lake to the south and southeast, by residential properties to the east, and by South 304th
Street to the north. An existing, graveled access roadway extends south from South 304th Street for 250 to
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300 feet, then turns east and extends 100 feet or so to the east. The access roadway alignment is completely
contained within the planned area of development, but is relatively close to the boundary between the area
that will be developed and the environmentally sensitive area where no development will occur.
An existing sanitary sewer main alignment, marked by several manholes, passes across the south part of the
area of planned development; two spur sewers extend north from the main sanitary sewer along the alignment
of the access roadway to South 304th Street.
The northeast corner of the site is a relatively level upland with slopes that radiate downward to the west and
southwest toward Redondo Creek; and south and southeast toward Steel Lake. Slopes that descend to the
west average about 25 percent, but are over-steepened in the vicinity of the access roadway, where grading
cuts for the roadway have a maximum height of 10 to 12 feet with grades of 50 percent or more. Slopes that
descend to the southwest measure 25 percent or less, then moderate to 10 percent or less to the south of the
access roadway; grades that descend to the south and east measure 25 percent or less, and moderate to 10
percent or less at or near the access roadway.
The site is heavily timbered with fairly large fir, cedar, and madrona trees and with smaller alder, cottonwood,
and other hardwood trees. Under story vegetation consists of salal, evergreen huckleberries, sword fens, and
other brush. Bits of trash were observed in isolated areas across the site.
No indicators of unstable slopes, such as scarps, tension cracks, or hummocky terrain, were observed.
Grading cuts for the existing access roadway, though fairly steep in areas, are stable.
No signs of surface hydrology, other than Redondo Creek at the west and southwest site margins of the site
and Steel Lake, at the south and southeast site boundaries, were observed.
No seeps, springs, or other surface expressions of groundwater were observed on site.
3.2 Soil Conditions
Our onsite explorations indicate that soil conditions are fairly uniform across the site, but the thickness of the
observed soil layers vary somewhat. In general, we observed a surface covering of duff and topsoil overlying
a mantle of loose to medium dense, silty, gravelly sand. Beneath the loose to medium dense silty, gravelly
sand layer, we observed very dense and over-consolidated, unweathered, lodgement (also termed basal)
glacial till, also comprised of silty, gravelly sand.
Specifically, in test pit TP-1, the duff/ topsoil layer measured less than 6 inches thick and very dense glacial
till was encountered at a depth of 3 feet. In test pit TP-2, the duff/topsoil layer measured 1½ feet thick and
glacial till was encountered at a depth of 4 feet. In test pit TP-3, located on the south side of the hill, we
observed 6 inches of duff overlying 1 foot of fill comprised of loose to medium dense silty gravelly sand with
small chunks of concrete; very dense glacial till was encountered at a depth of 3½ feet.
Test pit TP-4 was conducted on gently sloping terrain south of the access road on the southeast part of the site
where development is planned. There we encountered 6 inches of duff and topsoil overlying, to a depth of
6 feet, loose to medium dense silty gravelly sand. Dense to very dense glacial till was observed at a depth to
6 feet and extended down to the termination of the test pit at a depth of 9 feet.
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Test pit TP-5 was excavated in the east part of Storm Tract B. There, we observed 6 inches of duff overlying,
to a depth of 6 feet, loose to medium dense silty gravelly sand. Very dense glacial till was observed below
depth of 6 feet.
Test pit TP-6 was excavated in the west part of Storm Tract B. There we observed a thin cover of duff
overlying, to a depth of 3 feet, medium dense fill (likely generated during the building of the access roadway)
comprised of silty gravelly sand. In situ soil native soil, similar in consistency and density to the
above-described fill, was then observed. At a depth of 6 feet, we encountered very dense glacial till.
The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata encountered in
our subsurface explorations.
3.3 Groundwater Conditions
At the time of our reconnaissance and subsurface explorations (November 7, 2012), we did not observe
groundwater in any of our test pits, which extended down to depths of up to 9 feet, nor did we observe soil
mottling.
3.4 Seismic Conditions
Based on our analysis of subsurface exploration logs and our review of published geologic maps, we interpret
the onsite soil conditions to correspond with site class C, as defined by Table 1613.5.2 of the 2012
International Building Code (IBC).
3.5 Liquefaction Potential
Liquefaction is a sudden increase in pore water pressure and a sudden loss of soil shear strength caused by
shear strains, as could result from an earthquake. Research has shown that saturated, loose, fine to medium
sands with a fines (silt and clay) content less than about 20 percent are most susceptible to liquefaction. Our
onsite subsurface explorations did not reveal saturated (or potentially saturated), loose, silty sand layers or
lenses.
3.6 Infiltration Conditions
Our two test pit explorations in Storm Tract B indicate that very dense, relatively impermeable, glacial till
underlies that area at a depth of about 6 feet. A storm water facility with an invert based within the glacial till
layer would likely have to be a detention-dispersal system.
Moderately permeable, loose to medium dense, native, in situ silty, gravelly sand overlies the glacial till layer.
Our test pit explorations indicate that the moderately permeable layer is 6 feet thick on the east side of the
Storm Tract B, and 3 feet thick on the west side of Storm Tract B, where a surface layer of 3 feet of fill was
observed. The thinness of this layer will make it difficult or impossible for use in a larger infiltration facility.
Nonetheless, we performed Grain Size Analyses on soil samples collected from the moderately permeable
layer, in case a facility can be designed that can utilize this layer. Our Grain Size Analyses indicate that it
consists of silty, gravelly sand. The results of our Grain Size Analyses are presented in Table 2 and the
attached Soil Gradation Graph (Appendix B) displays the grain-size distribution of the tested samples.
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TABLE 2
LABORATORY TEST RESULTS FOR NON-ORGANIC ONSITE SOILS
Soil Sample, Depth % Coarse
Gravel
% Fine
Gravel
% Coarse
Sand
% Medium
Sand
% Fine
Sand % Fines D10
TP-5, S-1, 3½ -4 feet
TP-6, S-1, 3½ -4 feet
9
11
23
24
27
26
9
7
14
12
18
19
N/A
N/A
Using the U.S.D.A. Textural Triangle, the native silty, gravelly sand that overlies the impermeable glacial till
is Sandy Loam. Using tables modeled on State DOE criteria, Sandy Loam has a Short Term Infiltration Rate
of 2 inches per hour. And, using the suggested Correction Factor of 4 for this material, we estimate the
Design Infiltration Rate to be 0.25 inches per hour.
Although unlikely, if a storm water system can be designed that employs the thin, moderately permeable layer
as an infiltration medium, further testing, in the form of falling head tests conducted at facility invert elevation
during the rainy season, and additional test pits to determine if seasonally high groundwater is a factor, will
need to be performed. In order to calculate a Design Infiltration Rate from the falling head test that is in
accordance with City of Federal Way storm water management criteria, the shape of infiltration facility would
have to be known.
4.0 CONCLUSIONS AND RECOMMENDATIONS
Plans call for the residential development of the site. Development will occur only on the northeast 2.69 acres
of the site, where 13 residential lots and a storm water tract will be located.
• Feasibility: Based on our field explorations, research, and analyses, the proposed structures
and pavements appear feasible from a geotechnical standpoint.
• Foundation Options: We recommend conventional spread footings that bear on the medium
dense or denser native soils, which typically can be found within three feet of the surface.
• Floor Options: Existing site soils will adequately support slab-on-grade floors. Surface
compaction of floor subgrades is recommended.
• Pavement Sections: We recommend a conventional pavement section comprised of an
asphalt concrete pavement over a crushed rock base course over a properly prepared
(compacted) subgrade. Existing site soils provide an adequate pavement subgrade.
All soil subgrades should be thoroughly compacted, then proof-rolled with a loaded dump
truck or heavy compactor. Any localized zones of yielding subgrade disclosed during this
proof-rolling operation should be overexcavated to a depth of 12 inches and replaced with a
suitable structural fill material.
• Critical Areas-Erosion Hazards: Existing site soils on site generally consist of a surface
layer of silty, gravelly sand underlain by glacial till that also consists of silty, gravelly sand.
Both soil types will be prone to silty runoff during rainy periods, especially when soils are
disturbed. Silt fences, ditches, and berms will likely control erosion during relatively dry
periods of the year, but erosion control materials, such as plastic sheeting and straw bales for
temporary cover and quarry spalls for check dams, should be stockpiled on site to insure
rapid response to rainy weather. We provide recommendations for silt fencing is Section
4.1. A Stormwater Pollution Prevention Plan (SWPPP) might be required by the City of
Federal Way for this project, due to the proximity of Steel Lake and Redondo Creek.
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Subsequent to the preparation of the SWPPP and before construction commences, an Erosion
and Sediment Control (ESC) plan should be prepared in accordance with the SWPPP. We
provide recommendations for an ESC for your consideration in Section 4.4.
• Infiltration Conditions: The site storm tract is overlain by a layer of moderately permeable
silty, gravelly sand that will be difficult or impossible to reuse because it is only 3 to 6 feet
thick. A storm water facility with an invert based within the glacial till layer would likely
have to consist of a detention-dispersal system.
In the unlikely case that an infiltration system is attempted in the thin gravelly sand layer, we
have tentatively determined that this layer has a Design Infiltration Rate of 0.25 inches per
hour. If the gravelly sand layer is used for infiltration, further testing, in the form of falling
head tests and test pit excavations (for determining if seasonal groundwater is a factor) will
have to be conducted during the rainy season.
The following sections of this report present our specific geotechnical conclusions and recommendations
concerning site preparation, spread footings, slab-on-grade floors, possible additional erosion control
measures, asphalt pavement, and structural fill. The Washington State Department of Transportation
(WSDOT) Standard Specifications and Standard Plans cited herein refer to WSDOT publications M41-10,
Standard Specifications for Road, Bridge, and Municipal Construction, and M21-01, Standard Plans for
Road, Bridge, and Municipal Construction, respectively.
4.1 Site Preparation
Preparation of the project site should involve erosion control, temporary drainage, clearing, stripping, cutting,
filling, excavations, and subgrade compaction.
Erosion Control: Before new construction begins, an appropriate erosion control system should be installed.
This system should collect and filter all surface water runoff through silt fencing. We anticipate a system of
berms and drainage ditches around construction areas will provide an adequate collection system. Silt
fencing fabric should meet the requirements of WSDOT Standard Specification 9-33.2 Table 3. In addition,
silt fencing should embed a minimum of 6 inches below existing grade. An erosion control system requires
occasional observation and maintenance. Specifically, holes in the filter and areas where the filter has shifted
above ground surface should be replaced or repaired as soon as they are identified.
Also, erosion control materials, such as plastic sheeting and straw bales for temporary cover and quarry spalls
for check dams, should be stockpiled on site to insure rapid response to rainy weather.
Temporary Drainage: We recommend intercepting and diverting any potential sources of surface or
near-surface water within the construction zones before stripping begins. Because the selection of an
appropriate drainage system will depend on the water quantity, season, weather conditions, construction
sequence, and contractor's methods, final decisions regarding drainage systems are best made in the field at
the time of construction. Based on our current understanding of the construction plans, surface and
subsurface conditions, we anticipate that curbs, berms, or ditches placed around the work areas will
adequately intercept surface water runoff.
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Clearing and Stripping: After surface and near-surface water sources have been controlled, sod and topsoil,
and root-rich soil should be stripped from the building site. The duff/topsoil layer varies in thickness from a
few inches to 1½ feet.
Site Excavations: Based on our explorations, we expect that excavations will encounter dense to very glacial
till which can be difficult to quickly excavate at times. “Tiger Teeth” or other special equipment might be
necessary for rapid excavation, in some cases, although standard excavation equipment will be adequate for
most site excavations.
Dewatering: Our site explorations did not encounter groundwater and we do not anticipate that groundwater
will be encountered during site excavations.
Temporary Cut Slopes: All temporary soil slopes associated with site cutting or excavations should be
adequately inclined to prevent sloughing and collapse. Temporary cut slopes in site soils should be no steeper
than 1¼ H:1V, and should conform to Washington Industrial Safety and Health Act (WISHA) regulations.
Subgrade Compaction: Exposed subgrades for the retaining wall should be compacted to a firm, unyielding
state before new concrete or fill soils are placed. Any localized zones of looser granular soils observed within
a subgrade should be compacted to a density commensurate with the surrounding soils. In contrast, any
organic, soft, or pumping soils observed within a subgrade should be overexcavated and replaced with a
suitable structural fill material.
Site Filling: Our conclusions regarding the reuse of onsite soils and our comments regarding wet-weather
filling are presented subsequently. Regardless of soil type, all fill should be placed and compacted according
to our recommendations presented in the Structural Fill section of this report. Specifically, building pad fill
soil should be compacted to a uniform density of at least 95 percent (based on ASTM:D-1557). Fill areas on
slopes 20 percent or greater should be “benched” prior to fill placement. Benches should be at least 8 feet
wide and should grade very slightly back into the slope face.
Onsite Soils: We offer the following evaluation of these on-site soils in relation to potential use as structural
fill:
• Surficial Organic Soils: Sod and topsoil are not suitable for use as structural fill under any
circumstances, due to their high organic content. Consequently, these materials can be used
only for non-structural purposes, such as in landscaping areas.
• Loose to medium Dense Silty, Gravelly, Sand: The silty, gravelly, sand layer that overlies
the glacial till contains a significant amount of silt, so it will be moisture sensitive and
difficult to reuse when wet or during wet weather. It will be reusable as structural fill when
near optimum moisture content.
• Glacial Till: The glacial till that underlies the site contains a significant amount of silt, so it
will be moisture sensitive and difficult to reuse when wet or during wet weather. It will be
reusable as structural fill when near optimum moisture content.
• Fill: The fill observed in test pit TP-6, in Storm Tract B, is likely derived from glacial till, so
it is moisture sensitive and will be difficult to reuse when wet or during wet weather. It will
be reusable as structural fill when near optimum moisture content.
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Permanent Slopes: All permanent cut slopes and fill slopes should be adequately inclined to reduce long-term
raveling, sloughing, and erosion. We generally recommend that no permanent slopes be steeper than 2H:1V.
For all soil types, the use of flatter slopes (such as 2½H:1V) would further reduce long-term erosion and
facilitate revegetation.
Slope Protection: We recommend that a permanent berm, swale, or curb be constructed along the top edge of
all permanent slopes to intercept surface flow. Also, a hardy vegetative groundcover should be established as
soon as feasible, to further protect the slopes from runoff water erosion. Alternatively, permanent slopes
could be armored with quarry spalls or a geosynthetic erosion mat.
4.2 Spread Footings
In our opinion, conventional spread footings will provide adequate support for buildings on the site if the
subgrades are properly prepared.
Footing Depths and Widths: For frost and erosion protection, the bases of all exterior footings should bear at
least 18 inches below adjacent outside grades, whereas the bases of interior footings need bear only 12 inches
below the surrounding slab surface level. To reduce post-construction settlements, continuous (wall) and
isolated (column) footings should be at least 18 and 24 inches wide, respectively.
Bearing Subgrades: Footings should bear on medium dense or denser site soils or on properly compacted
structural fill which bears on medium dense or denser site soil.
In general, before footing concrete is placed, any localized zones of loose soils exposed across the footing
subgrades should be compacted to a firm, unyielding condition, and any localized zones of soft, organic, or
debris-laden soils should be overexcavated and replaced with suitable structural fill.
Lateral Overexcavations: Because foundation stresses are transferred outward as well as downward into the
bearing soils, all structural fill placed under footings, should extend horizontally outward from the edge of
each footing. This horizontal distance should be equal to the depth of placed fill. Therefore placed fill that
extends 24 inches below the footing base should also extend 24 inches outward from the footing edges.
Subgrade Observation: All footing subgrades should consist of firm, unyielding fill soils that currently cover
the site, firm native soils, or structural fill materials that have been compacted to a density of at least 95
percent (based on ASTM:D-1557). Footings should never be cast atop loose, soft, or frozen soil, slough,
debris, existing uncontrolled fill, or surfaces covered by standing water.
Bearing Pressures: In our opinion, for static loading, footings that bear on properly prepared subgrades can
be designed for a maximum allowable soil bearing pressure of 2,500 psf. This value is conservative, and may
be increased for specific footings if we are consulted. A one-third increase in allowable soil bearing capacity
may be used for short-term loads created by seismic or wind related activities.
Footing Settlements: Assuming that structural fill soils are compacted to a medium dense or denser state, we
estimate that total post-construction settlements of properly designed footings bearing on properly prepared
subgrades will not exceed 1 inch. Differential settlements for comparably loaded elements may approach
one-half of the actual total settlement over horizontal distances of approximately 50 feet.
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Footing Backfill: To provide erosion protection and lateral load resistance, we recommend that all footing
excavations be backfilled on both sides of the footings and stemwalls after the concrete has cured. Either
imported structural fill or non-organic onsite soils can be used for this purpose, contingent on suitable
moisture content at the time of placement. Regardless of soil type, all footing backfill soil should be
compacted to a density of at least 90 percent (based on ASTM:D-1557).
Lateral Resistance: Footings that have been properly backfilled as recommended above will resist lateral
movements by means of passive earth pressure and base friction. We recommend using an allowable passive
earth pressure of 250 psf for all soils on site and an allowable base friction coefficient of 0.35.
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in structures if the subgrades are properly
prepared. We offer the following comments and recommendations concerning slab-on-grade floors.
Floor Subbase: Generally, structural fill subbases do not appear to be needed under soil-supported
slab-on-grade floors. Surface compaction of slab subgrades is recommended.
If a subbase is required for some reason, it should be compacted to a density of at least 95 percent (based on
ASTM:D-1557).
Capillary Break and Vapor Barrier: To retard the upward wicking of moisture beneath the floor slab, we
recommend that a capillary break be placed over the subgrade. Ideally, this capillary break would consist of a
4-inch-thick layer of pea gravel or other clean, uniform, well-rounded gravel, such as “Gravel Backfill for
Drains” per WSDOT Standard Specification 9-03.12(4), but clean angular gravel can be used if it adequately
prevents capillary wicking. In addition, a layer of plastic sheeting (such as Crosstuff, Visqueen, or Moistop)
should be placed over the capillary break to serve as a vapor barrier. During subsequent casting of the
concrete slab, the contractor should exercise care to avoid puncturing this vapor barrier.
4.4 Possible Additional Erosion Control Measures
The City of Federal Way might require a Stormwater Pollution Prevention Plan (SWPPP) for this project.
Subsequent to the preparation of the SWPPP and before construction commences, an Erosion and Sediment
Control (ESC) plan should be prepared in accordance with the SWPPP. We offer this example of an ESC for
your perusal:
The ESC plan consists of Best Management Practices (BMPs). As part of the geotechnical recommendations for
this project, as a minimum, the following BMPs should be adopted by the SWPPP and ESC plan:
BMP Description
Dry Weather Construction Reduces the potential for erosion by limiting significant grading operations to
“the dry months”, April through September.
Flagging of Clearing/Grading Limits Reduces the potential for erosion by providing a safeguard to unintended
clearing and grading operations
Phased Construction Reduces the potential for erosion by limiting the area of exposed soil for the
immediate grading operations
Filter Fabric Fence Reduces sedimentation by blocking the off-site transport of sediment
particles larger than fine silts
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Stabilized Construction Entrances Reduces the potential for off-site transport of sediment by removing mud
from construction equipment before leaving the site.
Street Sweeping Provides a safeguard for any soil tracking that should occur of public right-of-
ways.
Interceptor Ditches Directs sediment laden runoff to silt traps and ponds for treatment.
Rock Check Dams Traps sediment within the interceptor ditch
Sediment Traps and Ponds Removes sediment through Stoke's settling of particulate matter.
Slope Tracking Reduces erosion at the soil through soil compaction and elimination of
channelization.
Temporary Cover Measures Plastic Sheeting or Polyacrylamide will reduce erosion at the source.
Permanent Seeding Reduces erosion at the source through permanent stabilization.
BMP Maintenance Provides for optimal BMP efficiency throughout construction.
During general grading, all areas of bare, disturbed soils should be sealed at the end of each day by surface
compaction with a roller or other device. Also, erosion control materials, such as plastic sheeting and straw
bales for temporary cover and quarry spalls for check dams, should be stockpiled on site to insure rapid
response to rainy weather.
Reestablishment of vegetation may prove difficult, especially in cut areas, where unweathered, glacial till
soils will be exposed. Tilling of organic materials into the surface, before topsoil placement, should be
considered in order to provide an adequately thick growing layer.
4.5 Asphalt Pavement
Since asphalt pavements will be used for access roads, we offer the following comments and
recommendations for pavement design and construction.
Subgrade Preparation: All soil subgrades should be thoroughly compacted, then proof-rolled with a loaded
dump truck or heavy compactor. Any localized zones of yielding subgrade disclosed during this proof-rolling
operation should be over excavated to a maximum depth of 12 inches and replaced with a suitable structural
fill material. All structural fill should be compacted according to our recommendations given in the Structural
Fill section. Specifically, the upper 2 feet of soils underlying pavement section should be compacted to at
least 95 percent (based on ASTM D-1557), and all soils below 2 feet should be compacted to at least 90
percent.
Pavement Materials: For the base course, we recommend using imported crushed rock, such as "Crushed
Surfacing Top Course” per WSDOT Standard Specification 9-03.9(3). Although our explorations do not
indicate a need for a pavement subbase, if a subbase course is needed, we recommend using imported, clean,
well-graded sand and gravel such as “Ballast” or “Gravel Borrow” per WSDOT Standard Specifications
9-03.9(1) and 9-03.14, respectively.
Conventional Asphalt Sections: A conventional pavement section typically comprises an asphalt concrete
pavement over a crushed rock base course. We recommend using the following conventional pavement
sections:
10
December 6, 2012 E3RA, Inc.
T12081 / Steel Lake Residential Plat Geotechnical Report
Minimum Thickness
Pavement Course Driveway-Parking Areas Access-High Traffic Areas
Asphalt Concrete Pavement 2 inches 3 inches
Crushed Rock Base 4 inches 6 inches
Granular Fill Subbase (if needed) 6 inches 12 inches
Compaction and Observation: All subbase and base course material should be compacted to at least 95
percent of the Modified Proctor maximum dry density (ASTM D-1557), and all asphalt concrete should be
compacted to at least 92 percent of the Rice value (ASTM D-2041). We recommend that an E3RA
representative be retained to observe the compaction of each course before any overlying layer is placed. For
the subbase and pavement course, compaction is best observed by means of frequent density testing. For the
base course, methodology observations and hand-probing are more appropriate than density testing.
Pavement Life and Maintenance: No asphalt pavement is maintenance-free. The above described pavement
sections present our minimum recommendations for an average level of performance during a 20-year design
life; therefore, an average level of maintenance will likely be required. Furthermore, a 20-year pavement life
typically assumes that an overlay will be placed after about 10 years. Thicker asphalt and/or thicker base and
subbase courses would offer better long-term performance, but would cost more initially; thinner courses
would be more susceptible to “alligator” cracking and other failure modes. As such, pavement design can be
considered a compromise between a high initial cost and low maintenance costs versus a low initial cost and
higher maintenance costs.
4.6 Structural Fill
The term "structural fill" refers to any material placed under foundations, retaining walls, slab-on-grade
floors, sidewalks, pavements, and other structures. Our comments, conclusions, and recommendations
concerning structural fill are presented in the following paragraphs.
Materials: Typical structural fill materials include clean sand, gravel, pea gravel, washed rock, crushed rock,
well-graded mixtures of sand and gravel (commonly called "gravel borrow" or "pit-run"), and miscellaneous
mixtures of silt, sand, and gravel. Recycled asphalt, concrete, and glass, which are derived from pulverizing
the parent materials, are also potentially useful as structural fill in certain applications. Soils used for
structural fill should not contain any organic matter or debris, nor any individual particles greater than about
6 inches in diameter. Because pervious pavement may be planned, import fill should be granular and well
draining.
Fill Placement: Clean sand, gravel, crushed rock, soil mixtures, and recycled materials should be placed in
horizontal lifts not exceeding 8 inches in loose thickness, and each lift should be thoroughly compacted with a
mechanical compactor.
Compaction Criteria: Using the Modified Proctor test (ASTM:D-1557) as a standard, we recommend that
structural fill used for various onsite applications be compacted to the following minimum densities:
11
December 6, 2012 E3RA, Inc.
T12081 / Steel Lake Residential Plat Geotechnical Report
Fill Application Minimum
Compaction
Footing subgrade and bearing pad
Foundation backfill
Slab-on-grade floor subgrade and subbase
Asphalt pavement base
Asphalt pavement subgrade (upper 2 feet)
Asphalt pavement subgrade (below 2 feet)
95 percent
90 percent
95 percent
95 percent
95 percent
90 percent
Subgrade Observation and Compaction Testing: Regardless of material or location, all structural fill should
be placed over firm, unyielding subgrades prepared in accordance with the Site Preparation section of this
report. The condition of all subgrades should be observed by geotechnical personnel before filling or
construction begins. Also, fill soil compaction should be verified by means of in-place density tests
performed during fill placement so that adequacy of soil compaction efforts may be evaluated as earthwork
progresses.
Soil Moisture Considerations: The suitability of soils used for structural fill depends primarily on their
grain-size distribution and moisture content when they are placed. As the "fines" content (that soil fraction
passing the U.S. No. 200 Sieve) increases, soils become more sensitive to small changes in moisture content.
Soils containing more than about 5 percent fines (by weight) cannot be consistently compacted to a firm,
unyielding condition when the moisture content is more than 2 percentage points above or below optimum.
For fill placement during wet-weather site work, we recommend using "clean" fill, which refers to soils that
have a fines content of 5 percent or less (by weight) based on the soil fraction passing the U.S. No. 4 Sieve.
5.0 RECOMMENDED ADDITIONAL SERVICES
Because the future performance and integrity of the structural elements will depend largely on proper site
preparation, drainage, fill placement, and construction procedures, monitoring and testing by experienced
geotechnical personnel should be considered an integral part of the construction process. Consequently, we
recommend that E3RA be retained to provide the following post-report services:
• Review all construction plans and specifications to verify that the design criteria presented in
this report have been properly integrated into the design;
• Prepare a letter summarizing all review comments (if required by the City of Federal Way);
and
• Prepare a post-construction letter summarizing all field observations, inspections, and test
results (if required by the City of Federal Way).
12
STEEL LAKE RESIDENTIAL PLAT
TOPOGRAPHIC AND LOCATION MAP
FEDERAL WAY, WASHINGTON
FIGURE 1
T12081
APPROXIMATE SITE
LOCATION
E3RA, Inc.
P.O Box 44840
Tacoma, WA 98448
APPENDIX A
SOILS CLASSIFICATION CHART AND
KEY TO TEST DATA
LOG OF TEST PITS
CLAYEY GRAVELS, POORLY GRADED GRAVEL-SAND-CLAY
MIXTURES
SILTS AND CLAYSCOARSE GRAINED SOILSMore than Half > #200 sieveLIQUID LIMIT LESS THAN 50
LIQUID LIMIT GREATER THAN 50
CLEAN GRAVELS
WITH LITTLE OR
NO FINES
GRAVELS WITH
OVER 15% FINES
CLEAN SANDS
WITH LITTLE
OR NO FINES
MORE THAN HALF
COARSE FRACTION
IS SMALLER THAN
NO. 4 SIEVE
MORE THAN HALF
COARSE FRACTION
IS LARGER THAN
NO. 4 SIEVE
INORGANIC SILTS, MICACEOUS OR DIATOMACIOUS FINE
SANDY OR SILTY SOILS, ELASTIC SILTS
ORGANIC CLAYS AND ORGANIC SILTY CLAYS OF LOW
PLASTICITY
OH
INORGANIC SILTS AND VERY FINE SANDS, ROCK FLOUR,
SILTY OR CLAYEY FINE SANDS, OR CLAYEY SILTS WITH
SLIGHT PLASTICITY
CH
SILTY GRAVELS, POORLY GRADED GRAVEL-SAND-SILT
MIXTURES
SANDS
SILTS AND CLAYS
Figure A-1
INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY,
GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS,
LEAN CLAYS
E3RA
R-Value
Sieve Analysis
Swell Test
Cyclic Triaxial
Unconsolidated Undrained Triaxial
Torvane Shear
Unconfined Compression
(Shear Strength, ksf)
Wash Analysis
(with % Passing No. 200 Sieve)
Water Level at Time of Drilling
Water Level after Drilling(with date measured)
RV
SA
SW
TC
TX
TV
UC
(1.2)
WA
(20)
Modified California
Split Spoon
Pushed Shelby Tube
Auger Cuttings
Grab Sample
Sample Attempt with No Recovery
Chemical Analysis
Consolidation
Compaction
Direct Shear
Permeability
Pocket Penetrometer
CA
CN
CP
DS
PM
PP
PtHIGHLY ORGANIC SOILS
TYPICAL NAMES
GRAVELS
ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY,
ORGANIC SILTS
WELL GRADED GRAVELS, GRAVEL-SAND MIXTURES
MAJOR DIVISIONS
PEAT AND OTHER HIGHLY ORGANIC SOILS
WELL GRADED SANDS, GRAVELLY SANDS
POORLY GRADED SANDS, GRAVELLY SANDS
SILTY SANDS, POOORLY GRADED SAND-SILT MIXTURES
CLAYEY SANDS, POORLY GRADED SAND-CLAY MIXTURES
POORLY GRADED GRAVELS, GRAVEL-SAND MIXTURES
SOIL CLASSIFICATION CHART AND KEY TO TEST DATA
GW
GP
GM
GC
SW
SP
SM
SC
ML
FINE GRAINED SOILSMore than Half < #200 sieveLGD A NNNN02 GINT US LAB.GPJ 11/4/05INORGANIC CLAYS OF HIGH PLASTICITY, FAT CLAYS
CL
OL
MH
SANDS WITH
OVER 15% FINES
SM
SM
0.3
3.0
8.0
4 inches Forest Duff
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be considered
accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
TEST PIT NUMBER TP-1
PAGE 1 OF 1
Figure A-2
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SM
SM
1.5
4.0
8.0
Forest Duff overlying sandy Topsoil
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be considered
accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
TEST PIT NUMBER TP-2
PAGE 1 OF 1
Figure A-3
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SM
SM
SM
0.5
1.5
3.5
7.0
Forest Duff overlying sandy Topsoil
(SM) Light brown silty gravelly sand with some chunks of concrete (loose to medium dense, damp) (Fill)
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be considered
accurate to 0.5 foot.
Bottom of test pit at 7.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
TEST PIT NUMBER TP-3
PAGE 1 OF 1
Figure A-4
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SM
SM
0.5
6.0
9.0
Forest Duff overlying sandy Topsoil
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be considered
accurate to 0.5 foot.
Bottom of test pit at 9.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
TEST PIT NUMBER TP-4
PAGE 1 OF 1
Figure A-5
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
GB
S-1 6
SM
SM
0.5
6.0
8.0
Forest Duff overlying sandy Topsoil
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be
considered accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
TEST PIT NUMBER TP-5
PAGE 1 OF 1
Figure A-6
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.RECOVERY (in)(RQD)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
GB
S-1 6
SM
SM
SM
0.5
3.0
6.0
8.0
Forest Duff overlying sandy Topsoil
(SM) Light gray silty gravelly sand (loose to medium dense, damp) (Fill)
(SM) Light brown silty gravelly sand (loose to medium dense, damp)
(SM) Light gray silty gravelly sand with some cobbles (very dense, damp to moist) (Glacial Till)
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be
considered accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
GROUND ELEVATION
LOGGED BY FER
EXCAVATION METHOD Rubber-tired Backhoe
EXCAVATION CONTRACTOR Owner-Operator GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 11/7/12 COMPLETED 11/7/12
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZE
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
TEST PIT NUMBER TP-6
PAGE 1 OF 1
Figure A-7
CLIENT KPR Hospitality Investment, LP
PROJECT NUMBER T12081
PROJECT NAME Steel Lake Residential Plat
PROJECT LOCATION Federal Way, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/19/12 16:38 - \\TACOMA-SERVER\C\JOB FILES\2012 JOB FILES\T12081 KPR HOSPITALITY - STEELE LAKE RESIDENTIAL PLAT\T12081 TEST PITS.GPJE3RA, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
E3RA, Inc.RECOVERY (in)(RQD)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
APPENDIX B
LABORATORY TESTING RESULTS