22-102038-SF Original Geotech Report 7-20-22Limited Geotechnical Investigation
Proposed Single -Family Residence
30016 - 20th Place SW
Federal Way, Washington
October 30, 2019
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
Table of Contents
i.o INTRODUCTION.............................................................................................................
2
2.0 PROJECT DESCRIPTION..............................................................................................
2
3.0 SITE DESCRIPTION.......................................................................................................
2
4.o FIELD INVESTIGATION...............................................................................................
2
4.1.1 Site Investigation Program...................................................................................
3
5.0 SOIL AND GROUNDWATER CONDITIONS..............................................................
3
5.1.1 Area Geology........................................................................................................
3
5.1.2 Groundwater........................................................................................................
4
6.o GEOLOGIC HAZARDS...................................................................................................
4
6.1 Steep Slope Hazard..............................................................................................
4
6.2 Erosion Hazard....................................................................................................
6
6.3 Seismic Hazard....................................................................................................
6
7.o DISCUSSION...................................................................................................................
7
7.1.1 General.................................................................................................................7
8.o RECOMMENDATIONS.................................................................................................. 7
8.1.1
Site Preparation...................................................................................................
8
8.1.2
Temporary Excavations & Temporary Shoring .....................................................
8
8.1.3
Erosion and Sediment Control..............................................................................
9
8.1.4
Foundation Design...............................................................................................
9
8.1.5
Retaining Walls...................................................................................................10
8.1.6
Stormwater Management....................................................................................11
8.1.7
Slab-on-Grade.....................................................................................................11
8.1.8
Groundwater Influence on Construction.............................................................12
9.o CONSTRUCTION FIELD REVIEWS...........................................................................12
io.o CLOSURE...................................................................................................................13
LIST OF APPENDICES
Appendix A — Statement of General Conditions
Appendix B — Figure
Appendix C — Exploration Logs
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
1. o Introduction
In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a limited
geotechnical investigation for the proposed single-family residence located at 3oo16 — 20th Place SW in
Federal Way, Washington (Figure 1).
The purpose of the geotechnical investigation was to identify subsurface conditions and to provide
geotechnical recommendations for foundation design, stormwater management, earthwork, soil
compaction, and suitability of the on -site soils for use as fill.
The scope of work for the geotechnical evaluation consisted of a site investigation followed by engineering
analyses to prepare this report. Recommendations presented herein pertain to various geotechnical
aspects of the proposed development, including foundation support of the building, geologic hazards, and
erosion control.
2.0 Project Description
The project includes construction of a new multi -story residence in the area of an existing residence.
Additional development may include driveway and drainage improvements.
Anticipated building loads are expected to be site and site grading may include cuts on the order of 3 feet
or less. We should be provided with the final plans so that we may review them for consistency with our
recommendations.
3.0 Site Description
The site is located at 30016 — 20th Place SW in Federal Way, Washington. The site consists of one
irregularly shaped parcel (No. 0121039113) with a total area of about 171,000 square feet. The property is
very long and narrow, with only the extreme north end developed with residential structures and
driveways. For the purpose of this report, the site consists of the area shown in Figure 2 excluding the
structure north of the delineated slope area.
As noted, the northern quarter of the property is developed with a single-family residence and driveway.
There is another structure (30020) located north of the subject residence. This residence is located along
Puget Sound below a short slope area. The remainder of the property is undeveloped and vegetated with
grasses, ferns, ivy, blackberry vines and variable diameter trees.
The site slopes gently to moderately downward from south to north at magnitudes of 5 to over 50 percent.
The total relief is about 230 feet over the entire length of the property. Within the localized development
area, the site has relief of about 55 feet. There is a moderately steep to steep slope located about 50 feet
north of the subject residence. This slope has relief of 20 to 25 feet and magnitudes of 30 to 8o percent.
The slope area is developed with a concrete walkway, modular block walls, and vegetated/landscaped
areas. The slope has been modified through prior grading.
2
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
The property is bordered to the east, west, and south by residential properties, and to the north by Puget
Sound.
4. o Field Investigation
4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on October 15 and 22, 2019 and included
excavating one test pit with a mini excavator and two hand borings to confirm the soil conditions.
The soils encountered were logged in the field and are described in accordance with the Unified Soil
Classification System (USCS).
A Cobalt Geosciences field representative conducted the exploration, collected disturbed soil samples,
classified the encountered soils, kept a detailed log of the exploration, and observed and recorded
pertinent site features.
The results of the soil sampling are presented in Appendix C.
5.0 Soil and Groundwater Conditions
5-m Area Geology
The site lies within the Puget Lowland. The lowland is part of a regional north -south trending trough that
extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington,
this lowland is glacially carved, with a depositional and erosional history including at least four separate
glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to
the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of
interbedded gravel, sand, silt, till, and peat lenses.
The Geologic Map of Washington — Northwest Quadrant indicates that the site is underlain by several
geologic units. From south to north, these include Vashon Glacial Till, Vashon Recessional Outwash,
Vashon Advance Outwash, and Pre -Olympia Coarse Grained and Pre -Olympia Deposits. Beach deposits
are present near the other residence (30020).
In the area of the proposed residence, the soils are mapped as Pre -Olympia Deposits with an overprint of
mass -wastage materials. The mass wastage deposits appear to be upslope geologic units that have been
eroded and deposited in the large area near Puget Sound. These materials would have been deposited
following the most recent glacial retreat.
Explorations
As part of our evaluation, we excavated one test pit and two hand auger borings to determine the shallow
soil and groundwater conditions, where accessible.
3
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
The test pit and hand borings encountered approximately 6 inches of topsoil and grass underlain by
approximately 3.5 to 4.5 feet of loose to medium dense, silty fine to medium grained sand with gravel to
gravel with sand, silt, and cobbles. These materials were underlain by medium dense to dense, silty fine
to medium grained sand with gravel, which continued to the termination depths of the test pit and hand
borings.
5.1.2 Groundwater
Groundwater was not encountered in the explorations. We anticipate that perched groundwater may be
present at shallow depths during late winter and spring months between about 3 and 8 feet below grade.
Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors
that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability.
Water levels at the time of the field investigation may be different from those encountered during the
construction phase of the project.
6.o Geologic Hazards
6.1 Steep Slope Hazard & Critical Area Information
The City of Federal Way Critical Areas Map (2012) indicates that the northern margin of the site is located
within an erosion hazard area. There are mapped landslide hazard areas east and west of the property
along the bluffs/slopes above Puget Sound.
Based on our observations, it appears that there are local steep slope/landslide hazard areas north of the
residence. In this area, the overall topography includes slopes of 20 to 25 feet in height at magnitudes of
30 to 80 percent. There are locally steeper areas that were previously graded to create a pathway down to
the residence at 30020 loth Place SW.
During our field assessment, we observed the accessible portions of the slope. Overall, the steep slope
area north of the residence is stable at this time with no evidence of severe erosion, exposed soils,
hummocky terrain, or other signs of recent landslide activity. The slope is developed with short modular
block walls, a concrete pathway, and is otherwise vegetated with grasses. The steep slope area is mapped
as intermediate according to the Coastal Zone Atlas. Adjacent areas are mapped as stable slopes.
Some of the geologic maps indicate that this area is underlain by mass -wastage deposits overlying
glacially consolidated soils. In general, the soils encountered in our explorations consisted of silty -sand
with gravel as fill, possible colluvium, and weathered glacial deposits underlain by relatively dense silty -
sand with gravel.
It is our opinion that this slope is stable and not likely to experience landslide activity provided it remains
vegetated and drainage is not directed toward the slope system. We understand that the new residence
will be situated about 50 feet from the slope area. We recommend a minimum setback of 40 feet from the
top of the slope. Additional foundation embedment may be utilized to create an effective setback of 40
feet, if required. No additional buffer is recommended since the slope is fully developed and/or vegetated
with grasses.
4
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
19.145.o8o Critical area report.
(1) Unless waived or modified by the director in accordance with subsection (4) of this section, an
applicant proposing activities where impacts or alteration of a critical area or its associated buffer and/or
setback shall submit a critical areas report that adequately evaluates the proposal and probable impacts.
We are not aware of any proposed alterations to the critical area of buffer zone as part of this project. The
project includes a new residence in the area of the existing residence, approximately 50 feet from the top
of the localized steep slope in the northern portion of the property. It appears that previous grading and
construction have already taken place within the steep slope/landslide hazard area. Provided earthwork
and grading activities are periodically monitored by the geotechnical engineer, all runoff collection
devices are connected to the tightline extending to Puget Sound, and all erosion control measures are in
place during construction, there should be minimal effect on the critical areas or adjacent properties.
(2) The critical area report shall be prepared by a qualified professional, incorporate best available
science, and include the following items:
(a) The name and contact information of the applicant, a description of the proposal, and identification of
the type of approval (use process, subdivision, building permit) requested;
Viral Shah, wo16 loth Place SW, Federal Way. The proposed development includes drainage
improvements, new driveway construction/modifications, and construction of a new single-family
residence in the area of the existing residence through building permits.
(b) Vicinity map;
Figure 1.
(c) The dates, names, and qualifications of the persons preparing the report and documentation of any
reconnaissance on site;
Phil Haberman has over 21 years of experience conducting geotechnical investigations in the Puget Sound
area. He is a licensed professional engineer, geologist, and engineering geologist. He visited the site on
two occasions between October 15 and October 28, 2019 to conduct exploration work, reconnaissance,
and evaluation of existing features. Other details are includes in this report.
(d) A scaled site plan depicting critical areas, buffers, setbacks, and proposed improvements;
Figure 2.
(e) Photographs of the site and critical areas;
Included at the end of this report.
(f) Identification and characterization of all critical areas adjacent to the proposed improvements;
Northern edge of the site is mapped as an erosion hazard area with landslide/steep slope areas mapped
east and west of the property_.
(g) A description of efforts made to apply mitigation sequencing pursuant to FWRC 1 .1145.13o to avoid,
minimize, and mitigate impacts to critical areas;
5
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
In general, the primary method to mitigate impacts to steep slope and erosion hazard areas is to control
all surface water, implement all erosion control devices, and permanently_ manage runoff through tightline
devices. Additional items do not appear warranted at this time.
19-145.24o Erosion and seismic hazard areas protection measures.
(1) Erosion hazard areas and seismic hazard areas do not contain standard buffers.
(2) All proposed improvements within an erosion hazard area or seismic hazard area shall follow the
recommendations within the critical area report to ensure the improvements will not adversely affect
geologic hazards and the improvements are at minimal risk by the geologic hazard as stated by a
geotechnical engineer or engineering geologist licensed in the state, as designed under anticipated
conditions.
No seismic hazards at this site (low liquefaction potential). Erosion hazards can be minimized through
implementation of temporary and permanent erosion control measures with permanent vegetation
following construction.
(3) Proposed improvements within an erosion hazard area shall also demonstrate all of the following via
the critical area report:
(a) The improvement will not increase surface water discharge or sedimentation to adjacent properties
and/or stormwater systems beyond predevelopment conditions;
(b) The improvement will not decrease slope stability on adjacent properties; and
(c) The improvement will not adversely impact other critical areas.
All of the above items can and will be true provided all work is performed in accordance with the
approved plans and permit requirements. This should include periodic geotechnical oversight during
construction to verify erosion control, soil bearing, drainage and final erosion control
6.2 Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site is
underlain by Alderwood gravelly sandy loam. These soils have a moderate to severe erosion potential in
slope areas with magnitudes greater than 15 percent.
It is our opinion that soil erosion potential at this project site can be reduced through landscaping and
surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of
rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt
fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with
regard to site grading, is from October 31st to April ist. Erosion control measures should be in place before
the onset of wet weather.
6.3 Seismic Hazard
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015
International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to
very dense soils within the upper loo feet.
6
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values
for Ss, Si, Fa, and F,. The USGS website includes the most updated published data on seismic conditions.
The site specific seismic design parameters and adjusted maximum spectral response acceleration
parameters are as follows:
PGA (Peak Ground Acceleration, in percent of g)
Ss 132.10% of g
S, 50.70% of g
FA 1.00
Fv 1.50
Additional seismic considerations include liquefaction potential and amplification of ground motions by
soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table.
The dense soils that underlie the site have a low potential for liquefaction.
7. o DISCUSSION
7.1.1 General
The proposed residence may be supported on a shallow foundation system bearing on medium dense or
firmer native soils, or on structural fill placed and compacted on the suitable native soils. The steep slope
area north of the residence is stable at this time and should remain stable provided all runoff is fully
controlled and not allowed to flow toward the slope. We recommend a minimum building setback of 40
feet from the top of the steep slope area.
Infiltration and dispersion devices are not recommended. We recommend full collection of runoff from
new/existing impervious surfaces into existing or new solid pipes extending downward to the level
spreader at Puget Sound.
8.o Recommendations
8.1.1 Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil and
fill. Based on observations from the site investigation program, it is anticipated that the stripping depth
will be 6 to 12 inches. Deeper excavations will be necessary below large trees and in areas where fill is
present.
The near -surface soils consist of silty -sand with gravel. Soils with less than 35 percent fines (passing the
No. 200 sieve) may be used as structural fill provided they achieve compaction requirements and are
within 3 percent of the optimum moisture. These soils may only be suitable for use as fill during the
7
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
summer months, as they will be above the optimum moisture levels in their natural state. These soils are
variably moisture sensitive and may degrade during periods of wet weather and under equipment traffic.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches
and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). Structural fill should be
placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of
the modified proctor maximum dry density, as determined by the ASTM D 1557 test method.
8.1.2 Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts on the
order of approximately 3 feet or less for foundation placement.
Any deeper excavations should be sloped no steeper than 1.511:1V (Horizontal:Vertical) in loose native
soils and 111:1V in medium dense to dense native soils. If an excavation is subject to heavy vibration or
surcharge loads, we recommend that the excavations be sloped no steeper than 2H:1V, where room
permits.
Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N,
Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a qualified
person during construction activities and the inspections should be documented in daily reports. The
contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope
erosion during construction.
Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the
slopes should be closely monitored until the permanent retaining systems or slope configurations are
complete. Materials should not be stored or equipment operated within 10 feet of the top of any
temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation work
exposes the soil. Typically, as excavation work progresses the maximum inclination of temporary slopes
will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be
made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be
adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines
can be met.
If any variations or undesirable conditions are encountered during construction, we should be notified so
that supplemental recommendations can be made. If room constraints or groundwater conditions do not
permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring
systems may be required. The contractor should be responsible for developing temporary shoring
systems, if needed. We recommend that Cobalt Geosciences and the project structural engineer review
temporary shoring designs prior to installation, to verify the suitability of the proposed systems.
8
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
8.1.3 Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands,
streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures
should be implemented and these measures should be in general accordance with local regulations. At a
minimum, the following basic recommendations should be incorporated into the design of the erosion
and sediment control features for the site:
• Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the
site soils, to take place during the dry season (generally May through September). However, provided
precautions are taken using Best Management Practices (BMP's), grading activities can be completed
during the wet season (generally October through April).
• All site work should be completed and stabilized as quickly as possible.
• Additional perimeter erosion and sediment control features may be required to reduce the possibility
of sediment entering the surface water. This may include additional silt fences, silt fences with a
higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems.
• Any runoff generated by dewatering discharge should be treated through construction of a sediment
trap if there is sufficient space. If space is limited other filtration methods will need to be
incorporated.
8.1.4 Foundation Design
The proposed residential structure may be supported on a shallow spread footing foundation system
bearing on undisturbed medium dense or firmer native soils or on properly compacted structural fill
placed on the suitable native soils. If structural fill is used to support foundations, then the zone of
structural fill should extend beyond the faces of the footing a lateral distance at least equal to the
thickness of the structural fill. If overexcavation becomes necessary and is excessively deep, driven pipe
piles may be used in lieu of the overexcavation and backfill for foundation support. We can provide
specific pile recommendations upon request.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively, for
continuous wall and isolated column footings supporting the proposed structure. Provided that the
footings are supported as recommended above, a net allowable bearing pressure of 2,000 pounds per
square foot (psf) may be used for design.
A 1/3 increase in the above value may be used for short duration loads, such as those imposed by wind
and seismic events. Structural fill placed on bearing, native subgrade should be compacted to at least 95
percent of the maximum dry density based on ASTM Test Method D1557. Footing excavations should be
inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or adjacent
exterior grade, whichever is lower. Interior footings should have a minimum depth of 12 inches below pad
subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25-foot exterior wall footing, or between adjoining column footings,
9
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most settlement is
expected to occur during construction, as the loads are applied. However, additional post -construction
settlement may occur if the foundation soils are flooded or saturated. All footing excavations should be
observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of 0.40
acting between the base of foundations and the supporting subgrades. Lateral resistance for footings can
also be developed using an allowable equivalent fluid passive pressure of 250 pounds per cubic foot (pcf)
acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior
areas).
The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of
safety of 1.5. The frictional and passive resistance of the soil may be combined without reduction in
determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction. Any
extremely wet or dry materials, or any loose or disturbed materials at the bottom of the footing
excavations, should be removed prior to placing concrete. The potential for wetting or drying of the
bearing materials can be reduced by pouring concrete as soon as possible after completing the footing
excavation and evaluating the bearing surface by the geotechnical engineer or his representative.
8.1.5 Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall system
is used. This has been included for any concrete walls that may be constructed.
Wall Design Criteria
"At -rest" Conditions (Lateral Earth Pressure —
55 pcf (Equivalent Fluid Density)
EFD+)
"Active" Conditions (Lateral Earth Pressure —
35 pcf (Equivalent Fluid Density)
EFD+)
Seismic Increase for "At -rest" Conditions
11H* (Uniform Distribution)
(Lateral Earth Pressure)
Seismic Increase for "Active" Conditions
6H* (Uniform Distribution)
(Lateral Earth Pressure)
Passive Earth Pressure on Low Side of Wall
Neglect upper 2 feet, then 250 pcf EFD+
(Allowable, includes F.S. = 1.5)
Soil -Footing Coefficient of Sliding Friction
0.40
(Allowable; includes F.S. = 1.5)
'H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in 50 years),
*EFD — Equivalent Fluid Density
10
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by water
accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest pressures on the
retaining walls from vertical surcharges behind the wall may be calculated using active and at -rest lateral
earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight of 125 pcf may be used to
calculate vertical earth surcharges. The parameters assume level backfill conditions. If sloping
backslopes are anticipated, we recommend increasing the values 0.75 pcf per degree of slope.
To reduce the potential for the buildup of water pressure against the walls, continuous footing drains
(with cleanouts) should be provided at the bases of the walls. The footing drains should consist of a
minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed down and enveloped
by a minimum 6 inches of pea gravel in all directions.
The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should consist of
free -draining granular material. All free draining backfill should contain less than 3 percent fines
(passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S. Standard No. 4 Sieve
with at least 30 percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary
purpose of the free -draining material is the reduction of hydrostatic pressure. Some potential for the
moisture to contact the back face of the wall may exist, even with treatment, which may require that more
extensive waterproofing be specified for walls, which require interior moisture sensitive finishes.
We recommend that the backfill be compacted to at least go percent of the maximum dry density based
on ASTM Test Method D1557• In place density tests should be performed to verify adequate compaction.
Soil compactors place transient surcharges on the backfill. Consequently, only light hand operated
equipment is recommended within 3 feet of walls so that excessive stress is not imposed on the walls.
8.1.6 Stormwater Management
The site is located within a steep slope and landslide hazard area. We do not recommend utilizing
dispersion or infiltration devices for stormwater management. We understand that portions of the
driveway runoff flow to the north and through the subject property. Some portion of this runoff is cutoff
by a shallow interceptor trench and collection pipe that diverts runoff to a seasonal stream near the west
property line. This trench is located in the northern quarter of the property approximately shown in
Figure 2.
The remainder of new/existing runoff north of this area will be collected and routed through an existing 8
inch diameter tightline to the beach near Puget Sound and north of the property. We concur with the
proposed system. We can provide additional recommendations as needed.
8.1.7 Slab on Grade
We recommend that the upper 12 inches of the existing native soils within slab areas be re -compacted to
at least 95 percent of the modified proctor (ASTM D1557 Test Method). Overexcavation of poor quality
soils will be required to variable depths if they are present. The geotechnical engineer should observe slab
excavations to determine the depth of overexcavation.
11
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could
result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the
usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of
the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers.
The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-04 Guide for
Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor
slab detailing. A 4 to 6 inch thick capillary break consisting of 5/8 inch clean angular rock or pea gravel
should be placed over the prepared subgrades.
Slabs on grade may be designed using a coefficient of subgrade reaction of 210 pounds per cubic inch (pci)
assuming the slab -on -grade base course is underlain by structural fill placed and compacted as outlined in
Section 8.1.
A perimeter drainage system is recommended unless interior slab areas are elevated a minimum of 12
inches above adjacent exterior grades. If installed, a perimeter drainage system should consist of a 4 inch
diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped in a non -woven
geosynthetic filter fabric to reduce migration of soil particles into the drainage system. The perimeter
drainage system should discharge by gravity flow to a suitable stormwater system.
Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface
water flow away from the building and preferably with a relatively impermeable surface cover
immediately adjacent to the building.
8.1.8 Groundwater Influence on Construction
Groundwater was not encountered in the explorations. We anticipate that perched groundwater may be
present at shallow depths during late winter and spring months between about 3 and 8 feet below grade.
If groundwater is encountered perched below weathered native soils, we anticipate that typical sump
excavations and pumps will adequately de -water the areas for short term work.
g.o Construction Field Reviews
Cobalt Geosciences should be retained to provide part time field review during construction in order to
verify that the soil conditions encountered are consistent with our design assumptions and that the intent
of our recommendations is being met. This will require field and engineering review to:
■ Monitor and test structural fill placement and soil compaction
■ Observe foundation subgrade conditions
■ Observe drainage placement
■ Observe slab -on -grade preparation
Geotechnical design services should also be anticipated during the subsequent final design phase to
support the structural design and address specific issues arising during this phase. Field and engineering
review services will also be required during the construction phase in order to provide a Final Letter for
the project.
12
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
LIMITED GEOTECHNICAL INVESTIGATION
FEDERAL WAY, WASHINGTON
October 30, 2019
lo.o Closure
This report was prepared for the exclusive use of Viral Shah and his appointed consultants. Any use of this
report or the material contained herein by third parties, or for other than the intended purpose, should
first be approved in writing by Cobalt Geosciences, LLC.
The recommendations contained in this report are based on assumed continuity of soils with those of our
test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural
and civil drawings when they become available in order that we may review our design recommendations
and advise of any revisions, if necessary. Our recommendations pertain to the proposed residence only
and not slope stability with regard to the lower building at 30020 20th Place SW or the remainder of the
subject property.
Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the
responsibility of Viral Shah who is identified as "the Client" within the Statement of General Conditions,
and its agents to review the conditions and to notify Cobalt Geosciences should any of these not be
satisfied.
Respectfully submitted,
Cobalt Geosciences, LLC
Original signed by:
Exp. 6/26/2020
Phil Haberman, PE, LG, LEG
Principal
PH/sc
13
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
FIVIJ-DOWIMIMM
Statement of General Conditions
Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent and
may not be used by any third party without the express written consent of Cobalt Geosciences and the
Client. Any use which a third party makes of this report is the responsibility of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are
in accordance with Cobalt Geosciences present understanding of the site specific project as described by
the Client. The applicability of these is restricted to the site conditions encountered at the time of the
investigation or study. If the proposed site specific project differs or is modified from what is described in
this report or if the site conditions are altered, this report is no longer valid unless Cobalt Geosciences is
requested by the Client to review and revise the report to reflect the differing or modified project specifics
and/or the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the specific
professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and
statements regarding their condition, made in this report are based on site conditions encountered by
Cobalt Geosciences at the time of the work and at the specific testing and/or sampling locations.
Classifications and statements of condition have been made in accordance with normally accepted
practices which are judgmental in nature; no specific description should be considered exact, but rather
reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to
some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock
and groundwater conditions as influenced by geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test locations,
Cobalt Geosciences must be notified immediately to assess if the varying or unexpected conditions are
substantial and if reassessments of the report conclusions or recommendations are required. Cobalt
Geosciences will not be responsible to any party for damages incurred as a result of failing to notify Cobalt
Geosciences that differing site or sub -surface conditions are present upon becoming aware of such
conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications
should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next project stage (property
acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated
project specifics and that the contents of this report have been properly interpreted. Specialty quality
assurance services (field observations and testing) during construction are a necessary part of the
evaluation of sub -subsurface conditions and site preparation works. Site work relating to the
recommendations included in this report should only be carried out in the presence of a qualified
geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being
present.
APPENDIX B
Figure: Site Plan
10.2
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
2o6-331-1097
O
3" 7 CUL KR
E- E
3"JPVC CUL4E1T—
IE 5.98
CB—.
RIM 11 78
'IE 9.63 {N,5)
RIM 11.28
HB-
FggUND i/2'
APROXIMATE RE9AR & CAP
LOCATION OF
SEPTIC SYSTEM �PLS 44857
PER RECORD OF
SURVEY No.
200D07269000 DS FD IS 1 2"
EASEMENT AREA REEA & CAPS
PER SETTLEMENT D,44' E ts--
AGREEMEN PliS 11657
PER ROS
2D000726900005
h177'16'E '
5.66' •mow
FEE TITLE TRAMVSFER
PER SETTLEMENT
AGREEMEN
PER RDS
20000726900005 (`
NM741'31' W
2526'
POINT 'A'
REAR & CAP
NOT
Y1SITED THIS
SURVEY
APFQXIM ATE LOCATION QF
CENTERLINE WADER LINE
EASEMENT PER KING COUNTY
CGUR CAUSE
N.C. 97 2 32I82 4 KNT
FOUND 1/2.
RAR &CAP
RIM A5.89 �
011
4D3 ,
T' ti4JLVERY
IE 8.95
r� )" CULVERT
E'er$ 8 CULVERT
IE 98
6 _ ART Steep Slo e & Erosion Hazard
2
`
- VC T
, .MR CULVERAreas
u V
CUL1
sXy �
I f M � / f ✓
Figure from provided topographic survey.
Note that this is the northern quarter to third
of the property which is the portion where there
will be proposed construction.
PVC
I� TP-1
CB SOLID LID
RIM 4tOh
IE(N,S) 3b.46
8, PVC
CB
TRIM 43.fi5
IE(W) 41.7a
E(N) 41.63
,]
tIM 43.42
-(W) 42.13 6" PVC
E(E) 42.1.0 8" PVC
Z
Area with e fisting catchbasin
and diversion pipe to seasonal stream
r' FOUND 1/2"
REBAR & CAP
OAB' W 8n
PLS 2295
'' /
HB-1
Approximate Test Pit and
Hand Boring Location
TP-1
199=100'
Proposed Residence
30016 - 2oth Place SW
Federal Way, Washington
SITE PLAN
FIGURE 2
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Looking north at walkway, modular block wall
and residence at 30020.
Looking west at steep slope area,
walkway, and adjacent property.
Looking west at top of steep slope area (and septic work)
Proposed Residence
30016 - 2oth Place SW
Federal Way, Washington
SITE
PHOTOS
FIGURE 3
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(2o6) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
APPENDIX C
Exploration Logs
Unified Soil Classification System (USCS)
MAJOR DIVISIONS
YMBOL
TYPICAL DESCRIPTION
Clean Gravels
Gw Well -graded gravels, gravels, gravel -sand mixtures, little or no fines
Gravels
(less than 5%P
(more than 50%
fines)
1
Poorly graded gravels, gravel -sand mixtures, little or no fines
fraction
of coarse GM
retained on No. 4
Gravels with
.:.
Silty gravels, gravel -sand -silt mixtures
COARSE
sieve)
Fines
GRAINED
(more than 12%
fines)
GC
Clayey gravels, gravel -sand -clay mixtures
SOILS
,
(more than 50%
SW
retained on
Clean Sands
Well -graded sands, gravelly sands, little or no fines
No. zoo sieve)
Sands
(less than 5%
r.a4
„<
sP
(50% or more
fines)
Poorly graded sand, gravelly sands, little or no fines
of coarse fraction
passes the No. 4
sieve)
Sands with
sM
Silty sands, sand -silt mixtures
Fines
(more than 12%
sc
fines)
i
Clayey sands, sand -clay mixtures
ML
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
or clayey silts with slight plasticity
Silts and Clays
(liquid limit less
Inorganic
CL
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays
FINE GRAINED
than 50)
silty clays, lean clays
Organic
OL
Organic silts and organic silty clays of low plasticity
SOILS
(50% or more
passes the
MH
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
No. 200 sieve)
elastic silt
Silts and Clays
(liquid limit 50 or
InorganicUCH
Inorganic clays of medium to high plasticity, sandy fat clay,
more)
or gravelly fat clay
Organic
OH
Organic clays of medium to high plasticity, organic silts
HIGHLY ORGANIC
Primarily organic matter, dark in color,
PT
Peat, humus, swamp soils with high organic content (ASTM D4427)
SOILS
and organic odor
1 Classification of Soil Constituents 1
MAJOR constituents compose more than 50 percent,
by weight, of the soil. Major constituents are capitalized
(i.e., SAND).
Minor constituents compose 12 to 50 percent of the soil
and precede the major constituents (i.e., silty SAND).
Minor constituents preceded by "slightly" compose
5 to 12 percent of the soil (i.e., slightly silty SAND).
Trace constituents compose 0 to 5 percent of the soil
(i.e., slightly silty SAND, trace gravel).
Relative Density
(Coarse Grained Soils)
Consistency
(Fine Grained Soils)
N, SPT,
Relative
N, SPT,
Relative
Blows/FT
Density
Blows/17 T
Consistency
0-4
Very loose
Under 2
Very soft
4 -10
Loose
2-4
Soft
10 - 30
Medium dense
4-8
Medium stiff
30 - 5o
Dense
8 -15
Stiff
Over 50
Very dense
15 - 3o
Very stiff
Over 30
Hard
Grain Size Definitions
Description
Sieve Number and/or Size
Fines
<#zoo (0.08 mm)
Sand
-Fine
#200 to #40 (o.o8 to 0.4 mm)
-Medium
#40 to #10 (0.4 to 2 mm)
-Coarse
#10 to #4 (2 to 5 mm)
Gravel
-Fine
#4 to 3/4 inch (5 to 19 mm)
-Coarse
3/4 to 3 inches (19 to 76 mm)
Cobbles
3 to 12 inches (75 to 305 mm)
Boulders
>12 inches (305 mm)
Moisture Content Definitions 1
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Cobalt Geosciences, LLC
P.O. Box 82243
a Kenmore, WA 98028
(2o6) 331-1097 Soil Classification Chart Figure Cs
www.cobaltgeo.com
cobaltgeo (&gmail.com
Log of Test Pit TP-1
Date: October 22, 2019
Depth: 7'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
o
o
Moisture Content (�o)
Plastic 1 Liquid
U
_ >
U
-
U
E
V
Limit �� Limit
t
o
t
Material Description
o
m
�
o
SPT N-Value
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
---
SM
--------------------------------------------
Loose to medium dense, silty -fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist.
2
-Locally mottled
3
:',--
4—
----
--
SM
--------------------------------------------
Medium dense to dense, silty -fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist to very moist.
5
' •
6
,
1CA e
;
At
End of Test Pit 7'
8
9
10
Cobalt Geosciences, LLC
Proposed Residence
Test Pit
P.O. Box 82243
Kenmore, WA 98028
30016 - 2oth Place SW
Log
(2o6) 331-1097
Federal Way, Washington
www.cobaltgeo.com
cobaltgeo@gmail.com
Log of Hand Boring HB-1
Date: October 22, 2019
Depth: 8'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
o
o
Moisture Content (�o)
Plastic 1 Liquid
��
U
t
_ >
U
-
U
t
E
V
Limit Limit
o
Material Description
o
m
�
o
SPT N-Value
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
---
SM
------------------------------------------
Loose to medium dense, silty fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist.
2
a.a.;
-Locally mottled
3
AY
;
,
4
ka
as
.
�—
----
---
---
SM
--------------------------------------------
Medium dense to dense, silty -fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist to very moist.
b
k a
a.
7
ky
1
aR
End of Hand Boring 8'
9
10
Cobalt Geosciences, LLC
Proposed Residence
Hand
P.O. Box 82243
Kenmore, WA 98028
30016 - 2oth Place SW
Boring
(zo6) 33i-io97
Federal Way, Washington
Log
www.cobaltgeo.com
cobaltgeo@gmail.com
Log of Hand Boring HB-2
Date: October 22, 2019
Depth: 8'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
o
o
Moisture Content (�o)
Plastic 1 Liquid
��
U
t
_ >
U
-
U
t
E
V
Limit Limit
o
Material Description
o
m
�
o
SPT N-Value
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
---
SM
--------------------------------------------
Loose to medium dense, silty fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist.
2
k
-Locally mottled
�E
1
3
-4
----
---*
---
SM
---------------------------------------------
Medium dense to dense, silty -fine to medium grained sand with
gravel, yellowish brown to grayish brown, moist to very moist.
5
b
ky
7
1.
9 F
T1
End of Hand Boring 8'
9
10
Cobalt Geosciences, LLC
Proposed Residence
Hand
P.O. Box 82243
Kenmore, WA 98028
30016 - 2oth Place SW
Boring
(2o6) 33i-io97
Federal Way, Washington
Log
www.cobaltgeo.com
cobaltgeo@gmail.com