Report (4) 08/13/97 WED 10:22 FAX 503 684 0954 CARLSON TESTING t1' q 7 02911/, Q1001
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Construction Inspection, & Related Tests
Carson Testing, g Inc. Geotechntcat consulting
P.O. Box 23814
Tigard, Oregon 97281
Phone (503) 684-3460
August 14, 1997 FAX (503) 6840954
#97 -G1287
Car Toys
1833 Auburn Way N. - Suite Q
Auburn, WA 98002
Attn: Mr. Jeff Picolo
FOUNDATION INVESTIGATION
CAR TOYS RETAIL STORE
TIGARD, OREGON
This report presents the results of our foundation investigation for the proposed Car Toys retail
store located in Tigard, Oregon. Our report was prepared in general accordance to our
proposal letter (P561), dated July 15, 1997.
BACKGROUND
Project Information
Location - South of Pacific Highway (Hwy 99 West) between Highway 217 and SW
Dartmouth St., in Tigard, Oregon. (Thomas Bros. Map 655, Column F -G:Row 3, 1997
ed.)
Client - Car Toys - Address above.
Civil Engineer - Christensen Engineering 7150 SW Hampton Street - Suite 226 -
Portland, Oregon 97223.
Jurisdiction - City of Tigard
Proposed Construction
Based on our previous work with Car Toys and conversation with the Architect, we
understand that the proposed building will be a wood - framed structure founded on shallow
spread footing with a concrete slab -on- grade. Paved parking spaces and associated driveways
are included in the project. Typical underground utilities (water, sewer, power, telephone,
etc.) are anticipated.
Site Description
At the time of our explorations, the site was being cleared by Cummings Construction. The
existing building and pavement were completely removed.. CTI observed the removal of the
existing material. The site is currently comprised of a gravel parking lot and an approximately
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50 x 30 feet, by 8 -foot deep pit from the removal of the old half basement.
GEOLOGIC CONDITIONS
General
Based on our review of the Beaverton Quadrangle Geologic Map (USGS Open -File Report 0 -90-
2), the site is underlain by Quaternary flood deposits (Qff). The these soils are generally
comprised of silts, fine sands, clays. From our field explorations, the soils encountered at the
site comform to the geologic descriptions. We anticipate the depth to basement rock to be
approximately 450 feet.
EARTHQUAKE SOURCES AND SEISMIC RISK
Local
Based on review of the Beaverton quadrangle geologic map from Earthquake Hazard Geology
Maps of the Portland Metropolitan Area (USGS Open -File Report 0- 90 -2), an inferred fault has
been mapped approximately 2,000 feet northeast of the site.
Regional
In Western Washington and Oregon, the most likely sources of earthquakes appear to be 1)
shallow, moderate intensity earthquakes within the North American Plate; 2) somewhat
deeper, moderately large earthquakes in the subducting Juan de Fuca Plate, and 3) potentially
great earthquakes along the Cascadia Subduction Zone (the contact . between the two plates).
Couch and Deacon (1972) inferred a maximum credible earthquake of magnitude (M) 6' for
the North American Plate of western Oregon, with a recurrence frequency of about 130 years.
Deeper intraplate earthquakes (deeper than 30 km) have mostly occurred where the Juan de
Fuca Plate is bending; either where the dip steepens, or where the plate is buckled (Rogers,
1983). Gravity data (Dehlinger et. al, 1970) indicates the bend occurs beneath the east flank
of the Coast Range. Thus the most likely location for a deep intraplate earthquake is along
the western edge of the Willamette Valley. The recommended design earthquake for this
event would have a magnitude of 71/2.
The geologic evidence available is interpreted as indicative that the great earthquakes
(magnitude 9'/z) have occurred in recent prehistoric times. Such events appear to have
occurred infrequently, about every 600 years on the average. The best estimates indicate that
the last great earthquake occurred 300 years ago (Yamaguchi et. al., 1989).
Other studies indicate that the most likely seismic risks in western Oregon and Washington
over the next 100 years are from moderate sized, shallow earthquakes in the North American
plate. These studies indicate that earthquakes similar to historic events in the Puget Sound
area (magnitude 61/2 to 7 1) could occur nearly anywhere in western Oregon or Washington.
Such an event may have recurrence intervals on the order of 100 to 200 years.
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Liquefaction
The greatest potential seismic hazard in the Tualatin Valley is from soil liquefaction.
Liquefaction typically depends on various factors which include soil type, the strength and
duration of local ground shaking, hydraulic conditions at the site, and the thickness and
seismic velocity profile of the sediment column at the site (Madin, 1990). If liquefaction
occurs during a seismic event, it can produce damaging secondary effects such as lateral
spreading, sand boils, ground rupture, and seismic - induced settlement. Liquefaction potential
and seismic design requirements are addressed in the Site Response section of this report.
FIELD EXPLORATION AND TESTING
Our field exploration consisted of three test pits excavated just outside the perimeter of the
proposed building. The test pits were excavated with a large trackhoe supplied by Site Tech,
Inc.
A general description of the field exploration procedures and the test pit logs are presented
in Appendix A. No plan was available for plotting of the test holes, however the contractor
was present during excavation.
SUBSURFACE CONDITIONS
The upper 8 inches of the site consisted of crushed aggregate ( " -0). Underlying the
aggregate, approximately 2 feet of a stiff, brown, sandy SILT with occasional crushed gravel
(old fill) was encountered. Below the fill at roughly 2 feet, the native soils consisted of a
stiff, brown, sandy SILT. The soils transitioned to silty SAND at approximately 6 feet depth.
The test pits ended in the silty SAND at the 10 feet depth. The soils ranged from moist and
wet between 3 and 10 feet.
Groundwater
No groundwater was encountered in any of our test pits.
CONCLUSIONS AND RECOMMENDATIONS
General
It is our opinion that the proposed structure is geotechnically feasible given the subsurface
conditions encountered in this investigation, provided the recommendations of this report are
incorporated into the design and construction of the project.
The contractor should be held contractually responsible for scheduling and frequency of the
proper inspections.
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Site Preparation
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All structural areas and areas to be graded should have been previously stripped of the existing
pavement and structure. The surface of the site is covered by about 8 inches of base rock.
The site also contains a pit from the old basement. The old foundation and debris (concrete,
wood, etc.) from the demolition and clearing was hauled off site. Any sloughing should be
removed from the pit immediately prior to filling.
Engineered Fill
We recommend the imported replacement fill for backfilling of the basement consist of 2 " -0
reject rock.
Alternative imported materials should be approved by the Soil Engineer prior to arrival on site.
All engineered fill should be observed and tested by the Soil Engineer's representative.
Engineered fill should be compacted in horizontal lifts not exceeding 8 inches using standard
compaction equipment. A minimum of 90 percent of the maximum dry density obtained from
the modified Proctor, ASTM D1557, or equivalent (AASHTO T -180) is recommended for
engineered fill placed for building pad construction. Field density testing should conform to
ASTM D 2922 and D 3017, or D 1556. Density tests should be conducted for every one
vertical foot of fill placed.
Foundations
Spread footing design and construction should conform to Chapter 18 of the 1994 Uniform
Building Code (UBC). The recommended minimum widths for continuous wall and pad
footings is 15 and 24 inches, respectively. We estimate that an allowable bearing capacity
of 2,000 lb/ft' may be used for spread footing design. The bearing pressure may be increased
by 33 percent for short duration loads, such as wind and seismic. The coefficient of friction
between the on -site soil and poured -in -place concrete should be taken as 0.30 for wet weather
construction; an increase of 33 percent maybe taken for dry weather construction. The
maximum anticipated total and differential footing movements (generally from soil expansion
and /or settlement) are 1 inch and h inch over a span of 20 feet, respectively.
Seismic Design
Deep explorations (auger borings) were not requested or performed as a part of this
investigation. As a result of the limited exploration depths, we cannot provide quantitative
estimations of the dynamic soil response to seismic activity. We also cannot provide definitive
predictions of site - specific earthquake hazards such as deep liquefaction and lateral spreading
potential. Based on our experience and familiarity with the on -site soils, the upper 20 feet of
the site is not considered to be potentially responsive and /or liquefiable. The potential for
damaging surface disruption is considered low. We estimate the differential settlement from
liquefaction to be negligible. The potential for lateral spreading and loss of foundation bearing
during a maximum probable event is also considered low. We consider the site soil profile is
best modelled as an S according to the Uniform Building Code (UBC) classification. The site
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is located in UBC Seismic Zone 3.
Slabs -on -Grade
The underslab base rock course should consist of 34"-0 crushed aggregate base. The total
thickness of this layer of crushed aggregate may be dependent on the subgrade conditions at
the time of construction and should be reviewed by proofrolling. The minimum required
thickness is estimated to be 6 inches during dry weather and 12 inches during the wet
weather season.
The use and placement of a vapor barrier should decided by the designer. Vapor barrier
materials beneath the office should be applicable for the intended use and products of
recognized long -term stability. The barrier should be located as near to the base of the slab
as feasible, without adversely affecting concrete slab curing or shrinkage cracking. The
building designer should use his experience for placement of any sand layer relative to the
vapor barrier to help offset potential slab curing problems.
Utilities
All deep excavations and shoring should conform to OSHA regulations (29 CFR Part 1926).
Based on our explorations, the on -site soils are considered to be OSHA "Type B" soils. The
trenches are expected to stand nearly vertical with only minor sloughing to a maximum depth
of 4 feet.
PVC pipe should be installed in accordance with ASTM D 2321 procedures. Initial backfill lift
thickness for a 3 4 " -0 crushed aggregate base may need to be as great as 4 feet to reduce the
risk of flattening flexible pipe. We recommend that structural trench backfill be compacted to
a minimum of 90 percent of the maximum dry density obtained from ASTM D1557 or
AASHTO T -180. Typically, density tests on granular backfills are taken at about every 4
vertical feet for every 200 lineal feet of trench backfill. Lift thicknesses should not exceed 12
inches, except if manufactured granular material is used for trench backfill. For granular
material the lift thicknesses when using large vibrating plate -type compaction equipment (e.g.
hoe compactor attachments) may be taken as great as 2 feet, provided proper compaction is
being achieved and tested at each lift, as feasible. Use of large reverberating compaction
equipment near existing structures should be carefully monitored for possibly harmful
vibrations.
Drainage
Surface water drainage should be directed away from the proposed structure. Roofdrain water
should be carried to the street if possible. Subdrains and wall drains should consist of a
schedule 40 perforated or slotted pipe (4 -inch diameter) enveloped in a minimum 4 ft /ft of
open graded gravel (drain rock) with a gradation range between 2" and Y, ". The drain rock
should be wrapped within an Amoco 4545 geofabric filter or equivalent filter fabric. A
minimum of one -half percent fall should be maintained throughout the drain and nonperforated
pipe outlet. Although shallow groundwater was not encountered during our explorations, we
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recommend the building have a perimeter footing drain to catch any temporarily perched near
surface water.
Soil Expansion Potential
The near - surface on -site soils are typical of Tualatin Basin SILT's and SAND's which generally
have a low to moderate expansion potential. No expansion index testing was performed as
a part of this study, nor is it considered nescessary.
Pavement Design
No pavement design was requested or performed as part of this investigation.
LIMITATIONS
We suggest that the owner incorporate the recommendations in this report into his agreement
with the general contractor. The earthwork should be performed to both the City of Tigard
standards and in general conformance with the site - specific geotechnical recommendations
in this report.
This report was prepared solely for the Owner and Engineer for the design of the project. We
encourage its review by bidders and /or the Contractor as it relates to factual data only (test
pits and laboratory data). The opinions and recommendations contained within the report are
not intended to be nor should they be construed to represent a warranty of subsurface
conditions or site performance but are forwarded to assist in the planning and design process.
Our reports pertain to the material tested /inspected only. Information contained herein is not
to be reproduced, except in full, without prior authorization from this office. Please feel free
to contact us for this work as well as for any questions • ••••ht have regarding this report.
c 5: c .O PRoF
Sincerely, �'� O 6 >o
CARLSON TESTING, INC. Q 14743 - r
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OREGON
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By - - ✓�v By: �£S 4� -
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Brian D. Leach, E.l. James I . mine, P.E.
Engineering Associate Geotechnical Engineer
•
Fqa 97% P.06
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APPENDIX A
FIELD INVESTIGATION
Three exploratory test holes to a maximum depth of 10.0 feet were excavated at the site with
a large trackhoe supplied by Site Tech, Inc. The logs of the test pits are presented as Figures
A -1 to A -3.
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