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Geotechnical Engineering Report
Walker Renton Auto Dealership
3400 East Valley Road
Renton, Washington
P/N 302305-9067
April 27, 2018
prepared for:
HHJ Architects, PLLC
Attention: Roger Hansen
601 St Helens
Tacoma Washington 98402
prepared by:
Migizi Group, Inc.
PO Box 44840
Tacoma, Washington 98448
(253) 537-9400
MGI Project P1238-T18
RECEIVED
04/23/2019
amorganroth
PLANNING DIVISION
i
TABLE OF CONTENTS
Page No.
1.0 SITE AND PROJECT DESCRIPTION .............................................................................................. 1
2.0 EXPLORATORY METHODS ............................................................................................................ 2
2.1 Auger Boring Procedures...................................................................................................... 3
3.0 SITE CONDITIONS ............................................................................................................................ 3
3.1 Surface Conditions ................................................................................................................. 3
3.2 Soil Conditions ....................................................................................................................... 4
3.3 Groundwater Conditions ...................................................................................................... 5
3.4 Seismic Conditions ................................................................................................................. 5
3.5 Liquefaction Potential ........................................................................................................... 5
3.6 Infiltration Conditions ........................................................................................................... 6
4.0 CONCLUSIONS AND RECOMMENDATIONS............................................................................ 6
4.1 Site Preparation ...................................................................................................................... 7
4.2 Augercast Piles ....................................................................................................................... 9
4.3 Slab-On-Grade-Floors .......................................................................................................... 11
4.4 Drainage Systems ................................................................................................................. 11
4.5 Asphalt Pavement ................................................................................................................ 12
4.6 Structural Fill ........................................................................................................................ 13
5.0 RECOMMENDED ADDITIONAL SERVICES ............................................................................. 14
6.0 CLOSURE ........................................................................................................................................... 15
List of Tables
Table 1. Approximate Locations and Depth of Explorations .............................................................................. 2
Table 2. Recommended Allowable Pile Capacities ............................................................................................... 9
List of Figures
Figure 1. Topographic and Location Map
Figure 2. Site and Exploration Plan
APPENDIX A
Soil Classification Chart and Key to Test Data .................................................................................................. A-1
Log of Auger Borings B-1 and B-2 ............................................................................................................. A-2…A-3
Page 1 of 15
MIGIZI GROUP, INC.
PO Box 44840 PHONE (253) 537-9400
Tacoma, Washington 98448 FAX (253) 537-9401
April 17, 2018
HHJ Architects, PLLC
601 St Helens
Tacoma, Washington 98402
Attention: Roger Hansen
Subject: Geotechnical Engineering Report
Walker Renton Auto Dealership
3400 East Valley Road
Renton, Washington
P/N 302305-9067
MGI Project P1238-T18
Dear Mr. Hansen:
Migizi Group, Inc. (MGI) is pleased to submit this report describing the results of our geotechnical
engineering evaluation of the proposed Walker Renton Auto Dealership development at 3400
East Valley Road in Renton, Washington.
This report has been prepared for the exclusive use of HHJ Architects, PLLC, and their
consultants, for specific application to this project, in accordance with generally accepted
geotechnical engineering practice.
1.0 SITE AND PROJECT DESCRIPTION
The project site consists of an irregularly-shaped, 5.65-acre, commercially-zoned parcel located
towards the south end of the city limits of Renton, Washington, as shown on the enclosed
Topographic and Location Map (Figure 1). The subject property is situated between East Valley
Road and SR-167 in a heavily developed commercial area. The project site has previously been
developed, being utilized as an auto junk yard. A 4,000-sf warehouse building and 1,160 sf
radiator shop are still present towards the northwest corner of the project area, as well as paved
parking facilities. The remainder of the site had been graded to accommodate storage of vehicles
and auto parts, though these have previously been removed from the site. During this initial
development of the site, the wetland area along the west side of SR-167 had been severely
damaged, and in many cases had been filled.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 2 of 15
It is our understanding that improvement plans involve the demolition of existing site features
and the repurposing of the property as an auto dealership. This will involve the construction of
a new 50,000 to 60,000 sf two-story, wood-framed structure towards the center of the property, in
addition to extensive pavements surrounding this facility. The new structure will contain a
showroom/sales area, business offices, parts sales and storage, service and shop regions, storage
and other supportive areas. In addition to the aforementioned improvements, the subject
development will also entail the restoration of the existing wetland which was damaged by
previous actions at the east side of the site along SR-167, and the establishment and maintaining
of a new 75-foot buffer area from this wetland. Environmental cleanup will be performed in
conjunction with new construction as appropriate, based on recommendations provided by Kane
Environmental.
2.0 EXPLORATORY METHODS
We explored surface and subsurface conditions at the project site on March 16, 2018. Our
exploration and evaluation program comprised the following elements:
• Surface reconnaissance of the site;
• Two auger boring explorations (designated B-1 and B-2), advanced on March 16,
2016; and
• 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 AND DEPTHS OF EXPLORATIONS
Exploration Functional Location
Termination
Depth
(feet)
B-1
B-2
East end of proposed building footprint
Southwest corner of proposed building footprint
61½
51½
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 eva luation 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.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 3 of 15
2.1 Auger Boring Procedures
Our exploratory borings were advanced through the soil with a hollow-stem auger, using a truck-
mounted drill rig, operated by an independent drilling firm working under subcontract to MGI.
An engineering geologist from our firm continuously observed the boring, logged the subsurface
conditions, and collected representative soil samples. All samples were stored in watertight
containers and later transported to a laboratory for further visual examination and testing. After
the borings were completed, the borehole was backfilled in accordance with state requirements.
Throughout the drilling operation, soil samples were obtained at 2½ to 5-foot depth intervals by
means of the Standard Penetration Test (SPT) per American Society for Testing and Materials
(ASTM:D-1586), or using a large split-spoon sampler. This testing and sampling procedure
consists of driving a standard 2-inch-outside-diameter steel split-spoon sampler 18 inches into
the soil with a 140-pound hammer free-falling 30 inches. The number of blows struck during the
final 12 inches is recorded on the boring log. If a total of 50 blows are struck within any 6 -inch
interval, the driving is stopped, and the blow count is recorded as 50 blows for the actual
penetration distance. The resulting blow count values indicate the relative density of granular
soils and the relative consistency of cohesive soils. The soils were classified visually in general
accordance with the system described in Figure A-1, which includes a key to our exploration logs.
Summary logs of our explorations are included as Figures A-2 and A-3.
The enclosed boring logs describe the vertical sequence of soils and materials encountered in the
borings, based primarily on our field classifications and supported by our subsequent laboratory
examination and testing. Where a soil contact was observed to be gradational, our logs indicate
the average contact depth. Where a soil type changed between sample intervals, we inferred the
contact depth. Our logs also graphically indicate the blow count, sample type, sample number,
and approximate depth of each soil sample obtained from the boring, as well as any laboratory
tests performed on these soil samples. If any groundwater was encountered in the borehole, the
approximate groundwater depth is depicted on the boring logs. Groundwater depth estimates
are typically based on the moisture content of soil samples, the wetted height on the drilling rods,
and the water level measured in the borehole after the auger has been extracted.
3.0 SITE CONDITIONS
The following sections present our observations, measurements, findings, and interpretations
regarding, surface, soil, groundwater, and infiltration conditions.
3.1 Surface Conditions
As previously indicated, the project site consists of an irregularly-shaped, 5.65-acre,
commercially-zoned parcel, located towards the south end of the city limits of Renton, between
East Valley Road and SR-167 in a heavily developed commercial area. The site is bound on the
north by East Valley RV & Boat Storage, on the south by the Brickman Group storage yard, on
the west by East Valley Road, and on the east by SR 167. The northwest corner of the project area
retains an existing 4,000-sf warehouse building, a 1,160-sf radiator shop, and asphalt pavement
parking facilities. The aforementioned structures were originally constructed in 1996. The
remainder of the site had been graded and resurfaced with recycled concrete, to serve as a storage
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 4 of 15
location for automobile parts and debris. The entirety of the site is enclosed by a chain-link fence,
with access to the interior being gained through a locked gate.
Topographically, the site is relatively level with minimal grade changes being observed over its
extent. Purportedly, the eastern margin of the site is a historic wetland region, which had been
infringed upon by past developments. This portion of the site served as part of the larger basin
for Panther Creek, which travels north and south of the project area along the east side of SR 167.
No vegetation was observed on site outside of scattered weeds which have taken root within the
existing gravel surfacing. No hydrological features were observed on site, such as seeps, springs,
ponds and streams. Scattered ponding was observed across the southwest portion of the site at
the time of our site visit. We believe that this is seasonal in nature, and not indicative of
hydrogeological conditions onsite.
3.2 Soil Conditions
We observed subsurface conditions through the advancement of two geotechnical borings within
the footprint of the 50,000 to 60,000-sf structure proposed towards the interior of the site. These
explorations extended 51½ to 61½ feet below existing grade, respectively, encountering relatively
consistent subgrade conditions. Approximately 5 feet of fill soils were observed at surface
elevations in both of our explorations, ranging in composition from recycled concrete to gravelly
silty sand. Both of these material types were encountered in a medium dense to dense in situ
condition. Underlying these fill soils, we encountered native, alluvial deposits. With depth,
alluvial deposits encountered onsite exhibited alternating layers of poorly consolidated
fine-grained soils, and moderately consolidated granular soils; highlighting the shifting, localized
depositional environment across the project area. The uppermost fine-grained zone, observed
immediately below existing fill material, contained a significant organic component. Both of our
explorations encountered, below a depth of ± 31 feet, through their respective termination depths,
moderately dense, fine silty sand.
In the Geologic Map of the Tacoma 1:100,000-scale Quadrangle, as prepared by the Washington State
Department of Natural Resources Division of Geology and Earth Resources (WSDNR) (2015), the
project site is mapped as containing Qp, or peat, which directly overlies Qa, or Quaternary
Alluvium. Peat, as per this publication, is described as loose, locally very soft and wet, organic
and organic rich sediment, including muck, silt and clay. Alluvium, as it pertains to the
geographic setting of the project area, refers to sedimentary deposits associated with the flood
plains of the Duwamish/Green Rivers, and are typically comprised of loose, stratified to
massively bedded fluvial silt, sand, and gravel that is typically, well rounded and moderately to
well sorted and locally includes sandy to silty estuarine deposits. Our field observations and
subsurface explorations generally conform with the geologic classification of the site performed
by the WSDNR.
The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata
encountered in our subsurface explorations.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 5 of 15
3.3 Groundwater Conditions
At the time of our reconnaissance and subsurface explorations (March 16, 2018), we encountered
groundwater seepage at a depth which ranged from 8 to 15 feet below existing grade .
Groundwater levels were generally higher towards the southwest corner of the project area,
where significant surficial ponding was observed at the time of our site visit. Given the fact that
our explorations were performed within what is generally considered the rainy season (October
1st through April 30th), we do not anticipate that groundwater will rise much higher than that
which we observed. Groundwater levels will fluctuate with localized geology and precipitation.
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 generally correspond with site class E, as defined by
Table 20.3-1 in ASCE 7, per the 2015 International Building Code (IBC).
Using 2015 IBC information on the USGS Design Summary Report website, Risk Category I/II/III
seismic parameters for the site are as follows:
Ss = 1.418 g SMS = 1.276 g SDS = 0.851 g
S1 = 0.528 g SM1 = 1.266 g SD1 = 0.844 g
Using the 2015 IBC information, MCER Response Spectrum Graph on the USGS Design Summary
Report website, Risk Category I/II/III, Sa at a period of 0.2 seconds is 1.28 g and Sa at a period of
1.0 seconds is 1.27 g.
The Design Response Spectrum Graph from the same website, using the same IBC information
and Risk Category, Sa at a period of 0.2 seconds is 0.85 g and Sa at a period of 1.0 seconds is 0.84 g.
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. Subsurface explorations performed for this project indicate that the
site is underlain by poorly consolidated alluvial soils, ranging in composition from a fine sand
with silt to sandy silt; with intermittent layers or lenses of peat. Given the geologic/hydrogeolgic
conditions of the project area, we interpret this site as having a moderate susceptibility to
liquefaction. In Section 4.2 of this report, we provide recommendations for the preparation of the
foundation subgrade which would help mitigate much of this risk, however, during a large-scale
seismic event, some degree of liquefaction and related post-construction settlement should be
anticipated. We recommend that the structure be designed to prevent catastrophic collapse
during a seismic event.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 6 of 15
3.6 Infiltration Conditions
As indicated in the Soil and Groundwater Conditions sections of this report, the site is underlain
by fill material and poorly-drained, slowly permeable, alluvial soils, with a shallow groundwater
table. As such, we do not interpret infiltration as being feasible for this project, and recommend
that site produced stormwater be diverted to an existing storm system, managed through
detention, or other appropriate means.
4.0 CONCLUSIONS AND RECOMMENDATIONS
It is our understanding that improvement plans involve the demolition of existing site features
and the repurposing of the property as an auto dealership. This will involve the constructi on of
a new 50,000 to 60,000 sf two-story, wood-framed structure towards the center of the property, in
addition to extensive pavements surrounding this facility. The new structure will contain a
showroom/sales area, business offices, parts sales and storage, service and shop regions, storage
and other supportive areas. In addition to the aforementioned improvements, the subject
development will also entail the restoration of the existing wetland which was damaged by
previous actions at the east side of the site along SR-167, and the establishment and maintaining
of a new 75-foot buffer area from this wetland. Environmental cleanup will be performed in
conjunction with new construction as appropriate, based on recommendations provided by Kane
Environmental. We offer the following recommendations:
• Feasibility: Based on our field explorations, research, and evaluations, the
proposed structures and pavements appear feasible from a geotechnical
standpoint.
• Foundation Options: In order to address soil and liquefaction conditions within
the proposed expansion area and limit settlement of the addition and new
settlement of the existing structure, we recommend that all foundation elements
be supported by augercast piles. Recommendations for augercast pile
foundations are presented in Section 4.2.
• Floor Options: In our opinion, soil-supported slab-on-grade floors can be used if
the subgrades are properly prepared. However, there is a potential that
liquefaction settlement of the underlying site soils could cause cracking and
damage to soil-supported slab-on-grade floors during the design earthquake. If
the potential for damage is not acceptable, we recommend that floor slabs be
structurally supported.
If used, soil supported floor sections should bear on medium dense or denser
native soils or on properly compacted structural fill that extends down to medium
dense or denser native soil. Recommendations for slab-on-grade floors are
included in Section 4.3. Fill underlying floor slabs should be compacted to
95 percent (ASTM:D-1557).
• Pavement: We recommend a conventional pavement section comprised of asphalt
concrete over crushed rock base course over properly prepared subgrade. Because
soft soils immediately underlie proposed pavements, subgrade preparation
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 7 of 15
generally should consist of an over-excavation of two feet, compaction of exposed
subgrade soils, then replacement with a suitable structural fill. Compaction
should be done in accordance with the structural fill recommendations presented
in Section 4.6.
All soil subgrades below 24 inches 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 an additional maximum depth of 12 inches and replaced with a
suitable structural fill material.
The following sections of this report present our specific geotechnical conclusions and
recommendations concerning site preparation, spread footings, slab-on-grade floors, 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, excavations, cutting, subgrade compaction, and filling.
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.
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.
Clearing and Stripping: After surface and near-surface water sources have been controlled, sod,
topsoil, and root-rich soil should be stripped from the site. Our explorations and field
observations indicate that no significant organic horizon is observed at surface elevations across
the project area.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 8 of 15
Site Excavations: Based on our explorations, we expect that excavations will encounter
moderately consolidated fill soils at shallow elevations, and poorly consolidated alluvial soils
immediately beneath this material, both of which can be readily excavated using standard
excavation equipment.
Dewatering: Groundwater seepage was encountered in both of our subsurface explorations at a
depth of 8 to 15 feet below existing grade, with higher groundwater tables being observed across
regions exhibiting surficial ponding. If groundwater is encountered in excavations above the
water table, or slightly below, we anticipate that an internal system of ditches, sumpholes, and
pumps will be adequate to temporarily dewater shallow excavations. For excavations
significantly below the water table, we anticipate that expensive dewatering equipment, such as
well points, will be required to temporarily dewater 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 1½H:1V and should conform to Washington Industrial Safety and
Health Act (WISHA) regulations.
Subgrade Compaction: Exposed subgrades for the foundations of the planned structures 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 over-excavated 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).
Onsite Soils: We offer the following evaluation of these onsite soils in relation to potential use as
structural fill:
• Surficial Organic Soil and Organic-Rich Topsoil: Where encountered, surficial
organic soils, like duff, topsoil, root-rich soil, and organic-rich fill soils are not
suitable for use as structural fill under any circumstances, due to high organic
content. Consequently, this material can be used only for non-structural purposes,
such as in landscaping areas.
• Existing Fill Material: As described in the Soil Conditions section of this report, the
uppermost 5-feet of soils encountered onsite are comprised of existing fill material
placed during the original site development. This material ranged in composition
between recycled concrete and gravelly silty sand. These materials should be
considered moderately sensitive, and reuse should be confined to periods of
extended dry weather.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
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• Alluvial Soils: Underlying existing fill material, we encountered native alluvial
soils exhibiting alternating layers of poorly consolidated fine -grained soils and
moderately consolidated granular soils. The uppermost fine -grained layer is the
only native soils which could be feasibly reused as structural fill throughout the
course of this project. This soil group contains a high organic content, is extremely
moisture sensitive, and is generally encountered in an over-saturated condition.
Reuse of this material should be limited to landscaping areas.
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 Augercast Piles
Based on the soil conditions discussed above, we recommend that the new building be supported
on augercast piles installed to a depth of 55 feet below the existing ground surface in the medium
dense silty sands. The following table provides estimated allowable design capacities for 14-inch,
16-inch, and 18-inch diameter augercast concrete pilings installed to the aforementioned
embedment depth:
TABLE 2
RECOMMENDED ALLOWABLE PILE CAPACITIES
14-INCH, 16-INCH AND 18-INCH DIAMETER AUGERCAST CONCRETE PILES
Pile Diameter
(inches)
Depth Below
Existing Ground
Surface
(feet)
Downward Capacity
(tons)
Uplift Capacity
(tons)
14 55 66 26
16 55 80 30
18 55 98 34
The allowable pile capacities presented above apply to all long-term live and dead loads and may
be increased by one-third when considering short-term loads such as wind or seismic influence.
The allowable pile capacities are based on the strength of the supporting soils for the penetrations
indicated and include a factor of safety of at least 2. The allowable uplift capacities indicated for
augercast piles may be used provided that a reinforcing bar is installed the entire length of the
pile. This bar should be centered in the pile.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
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Static pile settlements are expected to be essentially elastic in nature and occur as loads are applied.
Total static settlement of piles constructed as recommended are not expected to exceed 1 inch, while
differential static settlements between comparably loaded piles are not expected to exceed about
50 percent of this value.
The pile capacities provided above apply to single piles. If piles within groups are spaced at least
three pile diameters on center, no reduction for pile group action need be made. The structural
characteristics of the pile materials and allowable internal stresses may impose more stringent
limitations and should be evaluated by the structural engineer.
Lateral loadings due to wind or seismic forces can be resisted by uplift or lateral loading on the
piles, or lateral soil resistance of the pile cap. The manner in which these loads are transferred into
the piles will be a function of the design of the foundation system. Passive soil resistance of the pile
cap may be computed using an equivalent fluid density of 220 pcf (pounds per cubic foot) for a
level backfill surface, provided the backfill around the pile cap is compacted to at least 95 percent
of maximum dry density per American Society for Testing and Materials (ASTM) D-1557. This
value incorporates a factor of safety of about 1.5.
Lateral capacities for augercast piling are dependent upon the characteristics of the reinforcing
steel and the coefficient of subgrade reaction for the surrounding soils. We recommend that the
pile stiffness, T, be computed using the formula T = (EI/f)1/5 where E equals the pile modulus of
elasticity, I equals the pile moment of inertia, and f equals the soil coefficient of subgrade reaction.
A value of 6 tcf (tons per cubic foot) should be used for f. For the recommended penetration, the
maximum moment for piles fixed against rotation at the ground surface will occur at a depth
equal to about 1.8 T and the magnitude of this moment, M, can be computed using the formula
M = 0.25 PT where P is the lateral force applied at the ground surface. The moment will decrease
to zero at a depth of about 4.5 T. The maximum pile deflection at the ground surface can be
computed using the formula D = 0.93 (PT3/EI).
Pile Installation: Augercast (cast-in-place) concrete piles should be installed using a continuous-
flight, hollow-stem auger. As is common practice, the pile grout would be pumped under
pressure through the hollow-stem as the auger is withdrawn. Reinforcing steel for bending and
uplift would be placed in the fresh grout column immediately after withdrawal of the auger.
No direct information regarding the capacity of augercast piles (e.g., driving resistance data) is
obtained while this type of pile is being installed. Therefore, it is particularly important that the
installation of augercast piles be carefully monitored by a qualified individual working under the
direct supervision of a geotechnical engineer.
It should be noted that the recommended pile penetration and allowable capacities presented above
assumed uniform soil conditions. There may be unexpected variations in the depth and
characteristics of the supporting soils across the site.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
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Accordingly, we recommend that pile installation be monitored by a member of our staff who
will observe installation procedures and evaluate the adequacy of individual pile installations.
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used if the subgrades are properly
prepared. However, there is a potential that liquefaction settlement of the underlying site soils
could cause cracking and damage to soil-supported slab-on-grade floors during the design
earthquake. If the potential for damage is not acceptable, we recommend that floor slabs be
structurally supported.
We offer the following comments and recommendations concerning soil-supported slab-on-
grade floors.
Floor Subbase: We recommend over-excavation of slab-on-grade floor subgrades to a minimum
depth of 2 feet, then placement of properly compacted structural fill as a floor subbase. If floor
construction occurs during wet conditions, it is likely that a geotextile fabric, placed between the
structural fill floor subbase and native soils, will be necessary.
All subbase fill 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.
Vertical Deflections: Due to elastic compression of subgrades, soil-supported slab-on-grade
floors can deflect downwards when vertical loads are applied. In our opinion, a subgrade
reaction modulus of 140 pounds per cubic inch can be used to estimate such deflections.
4.4 Drainage Systems
In our opinion, the proposed expansion area should be provided with a permanent drainage
system to reduce the risk of future moisture problems. We offer the following recommendations
and comments for drainage design and construction purposes.
Perimeter Drains: We recommend that the structure be encircled with a perimeter drain system
to collect seepage water. This drain should consist of a 4-inch-diameter perforated pipe within
an envelope of pea gravel or washed rock, extending at least 6 inches on all sides of the pipe, and
the gravel envelope should be wrapped with filter fabric to reduce the migration of fines from
the surrounding soils. Ideally, the drain invert would be installed no more than 8 inches above
the base of the perimeter footings.
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 12 of 15
Subfloor Drains: We recommend that subfloor drains be included beneath the new building.
These subfloor drains should consist of 4-inch-diameter perforated pipes surrounded by at least
6 inches of pea gravel and enveloped with filter fabric. A pattern of parallel pipes spaced no more
than 20 feet apart and having inverts located about 12 inches below the capillary break layer
would be appropriate, in our opinion.
Discharge Considerations: If possible, all perimeter drains should discharge to a sewer system
or other suitable location by gravity flow. Check valves should be installed along any drainpipes
that discharge to a sewer system to prevent sewage backflow into the drain system. If gravity
flow is not feasible, a pump system is recommended to discharge any water that enters the
drainage system.
Runoff Water: Roof-runoff and surface-runoff water should not discharge into the perimeter
drain system. Instead, these sources should discharge into separate tightline pipes and be routed
away from the building to a storm drain or other appropriate location.
Grading and Capping: Final site grades should slope downward away from the buildings so that
runoff water will flow by gravity to suitable collection points, rather than ponding near the
building. Ideally, the area surrounding the building would be capped with concrete, asphalt, or
low-permeability (silty) soils to minimize or preclude surface-water infiltration.
4.5 Asphalt Pavement
Since asphalt pavements will be expanded during the course of the proposed development, we
offer the following comments and recommendations for pavement design and construction.
Subgrade Preparation: After removal of any organics underlying pavements, we recommend a
conventional pavement section comprised of an asphalt concrete pavement over a crushed rock
base course over a properly prepared (compacted) subgrade or a granular subbase. Given the
relative loose/soft soil conditions observed across the project area, we recommend the over-
excavation of 24 inches of the existing subgrade material underlying the new pavement sections,
and replacement with a suitable structural fill subbase. Given the extent of the proposed paving
operation and corresponding earthwork activities, we recommend limiting the subgrade
preparation to times of dry weather.
All soil subgrades below 24 inches 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 an additional 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). If a subbase
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 13 of 15
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:
Minimum Thickness
Pavement Course Automobile Parking
Area Driveways Areas Subject to
Frequent Truck Traffic
Asphalt Concrete Pavement 2 inches 3 inches 4 inches
Crushed Rock Base 4 inches 6 inches 6 inches
Granular Fill Subbase (if needed) 12 inches 24 inches 24 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 MGI 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 performan ce
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. Utilizing recycled content may require approval from the Tacoma
Pierce County Health Department for placement in an aquifer recharge area. Soils used for
HHJ Architects, PLLC – Walker Renton Auto Dealership, 3400 E Valley Rd, Renton, WA April 27, 2018
Geotechnical Engineering Report P1238-T18
Migizi Group, Inc. Page 14 of 15
structural fill should not contain any organic matter or debris, nor any individual particles greater
than about 6 inches in diameter.
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:
Fill Application Minimum Compaction
Slab-on-grade floor subgrade (upper 2 feet)
Slab-on-grade floor subgrade (below 2 feet)
Asphaltic pavement base and subbase
Asphaltic pavement subgrade (upper 2 feet)
Asphaltic pavement subgrade (below 2 feet)
95 percent
90 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 t o 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 MGI be retained to provide the
following post-report services:
• Review all construction plans and specifications to verify that our design criteria
presented in this report have been properly integrated into the design;
• Prepare a letter summarizing all review comments (if required);
APPROXIMATE SITE
LOCATION
P.O. Box 44840
Tacoma, WA 98448
Location Job Number Figure
DateTitle
3400 East Valley Road
Renton, WA
P/N 302305-9067
Topographic and Location Map
1
04/06/18
P1238-T18
APPENDIX A
SOIL CLASSIFICATION CHART AND
KEY TO TEST DATA
LOG OF AUGER BORINGS
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
Migizi Group, Inc.
SS
S-1
SS
S-2
SS
S-3
SS
S-4
SS
S-5
SS
S-6
SS
S-7
SS
S-8
6
6
12
12
12
18
18
18
11-19-12
(31)
6-3-6
(9)
3-3-4
(7)
1-2-2
(4)
2-1-2
(3)
2-5-6
(11)
2-3-12
(15)
4-1-1
(2)
SM
ML
SM
OH
SM
SP
ML
2.0
5.5
7.0
9.0
13.0
17.5
31.0
Recycled Concrete
(SM) Gray/brown silty sand with gravel (dense, damp) (Fill)
(ML) Gray/brown silt with some organics (stiff, moist) (Alluvium)
(SM) Gray silty sand with some gravel (loose, wet) (Alluvium)
(OH) Gray organic silt (soft, wet) (Alluvium)
(SM) Gray fine silty sand (very loose, wet) (Alluvium)
(SP) Black fine to coarse sand (medium dense, wet) (Alluvium)
With interbeds of silty sand
(ML) Gray silt (soft, wet) (Alluvium)
NOTES
LOGGED BY ZLL
DRILLING METHOD Truck Mounted Drill Rig
DRILLING CONTRACTOR Holocene GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 3/16/18 COMPLETED 3/16/18
AT TIME OF DRILLING 15.00 ft
AT END OF DRILLING ---
AFTER DRILLING ---
HOLE SIZE 4.25" HSAGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0
5
10
15
20
25
30
35
(Continued Next Page)
PAGE 1 OF 2
Figure A-2
BORING NUMBER B-1
CLIENT HHJ Architects, PLLC
PROJECT NUMBER P1238-T18
PROJECT NAME Walker Renton Auto Dealership Geotech Report
PROJECT LOCATION 3400 East Valley Road, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 4/5/18 15:42 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1238-T18\P1238-T18 BORING LOGS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
RECOVERY (in)(RQD)BLOWCOUNTS(N VALUE)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SS
S-9
SS
S-10
SS
S-11
SS
S-12
SS
S-13
SS
S-14
18
18
18
18
18
18
1-2-2
(4)
0-4-6
(10)
3-5-5
(10)
0-4-8
(12)
10-8-10
(18)
3-4-5
(9)
ML
SM
40.5
61.5
(ML) Gray silt (soft, wet) (Alluvium) (continued)
(SM) Gray fine silty sand (medium dense, wet) (Alluvium)
Grades to loose
Bottom of borehole at 61.5 feet.SAMPLE TYPENUMBERDEPTH(ft)35
40
45
50
55
60
PAGE 2 OF 2
Figure A-2
BORING NUMBER B-1
CLIENT HHJ Architects, PLLC
PROJECT NUMBER P1238-T18
PROJECT NAME Walker Renton Auto Dealership Geotech Report
PROJECT LOCATION 3400 East Valley Road, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 4/5/18 15:42 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1238-T18\P1238-T18 BORING LOGS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
RECOVERY (in)(RQD)BLOWCOUNTS(N VALUE)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SS
S-1
SS
S-2
SS
S-3
SS
S-4
SS
S-5
SS
S-6
SS
S-7
SS
S-8
6
12
18
12
18
18
18
18
6-2-2
(4)
1-0-0
(0)
5-5-8
(13)
5-3-3
(6)
7-8-10
(18)
2-3-7
(10)
1-0-1
(1)
1-1-1
(2)
SM
OH
SP-
SM
SP
SM
ML
SM
1.5
5.0
7.0
12.5
20.0
22.5
32.5
Recycled Concrete
(SM) Gray/brown fine silty sand (loose, moist) (Alluvium)
(OH) Gray/brown organic silt (very soft, wet) (Alluvium)
(SP-SM) Dark gray fine sand with silt and interbeds of silty sand (medium dense, wet) (Alluvium)
(SP) Dark gray fine to medium sand (medium dense, wet) (Alluvium)
(SM) Gray fine silty sand (medium dense, wet) (Alluvium)
(ML) Gray silt (very soft, wet) (Alluvium)
With shell debris
(SM) Gray fine silty sand with shell debris (medium dense, wet) (Alluvium)
NOTES
LOGGED BY ZLL
DRILLING METHOD Truck Mounted Drill Rig
DRILLING CONTRACTOR Holocene GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 3/16/18 COMPLETED 3/16/18
AT TIME OF DRILLING 17.50 ft
AT END OF DRILLING ---
AFTER DRILLING ---
HOLE SIZE 4.25" HSAGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0
5
10
15
20
25
30
35
(Continued Next Page)
PAGE 1 OF 2
Figure A-3
BORING NUMBER B-2
CLIENT HHJ Architects, PLLC
PROJECT NUMBER P1238-T18
PROJECT NAME Walker Renton Auto Dealership Geotech Report
PROJECT LOCATION 3400 East Valley Road, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 4/5/18 15:42 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1238-T18\P1238-T18 BORING LOGS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
RECOVERY (in)(RQD)BLOWCOUNTS(N VALUE)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
SS
S-9
SS
S-10
SS
S-11
SS
S-12
18
18
18
18
3-4-6
(10)
3-5-3
(8)
4-5-5
(10)
7-9-9
(18)
SM
SM
40.0
51.5
(SM) Gray fine silty sand with shell debris (medium dense, wet) (Alluvium) (continued)
(SM) Gray fine silty sand (loose, wet) (Alluvium)
Grades to medium dense
Bottom of borehole at 51.5 feet.SAMPLE TYPENUMBERDEPTH(ft)35
40
45
50
PAGE 2 OF 2
Figure A-3
BORING NUMBER B-2
CLIENT HHJ Architects, PLLC
PROJECT NUMBER P1238-T18
PROJECT NAME Walker Renton Auto Dealership Geotech Report
PROJECT LOCATION 3400 East Valley Road, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 4/5/18 15:42 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1238-T18\P1238-T18 BORING LOGS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
RECOVERY (in)(RQD)BLOWCOUNTS(N VALUE)U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION