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WCG/ West Coast Geotech, Inc.
GEOTECHNICAL CONSULTANTS
April 19, 1999 W -1431
The Keicher Company
19363 Willamette Drive, Suite 509
West Linn, Oregon 97068
Attn: Mr. Brian Keicher
Project Manager
GEOTECHNICAL/LIMITED LIQUEFACTION STUDY
BENNETT PORTER & ASSOCIATES OFFICE BUILDING
TIGARD, OREGON
Gentlemen:
• In general accordance with our proposal of March 22, 1999, and Mr. Porter's authorization of
March 30, West Coast Geotech, Inc., has completed a geotechnical/limited liquefaction study for
the above - referenced project that will be located west of SW 69th approximately north of SW
Gonzaga Street extension. This report provides a summary of our field and laboratory programs
and presents our recommendations for foundation design and grading operations. The results
from a limited Liquefaction Study are also included in this report.
This limited report was prepared for your use in the design of the subject facility and should be
made available to potential contractors for information on factual data only, i.e., field test boring
logs and samples, if any are taken. This report should not be used for contractual purposes as a
warranty of interpreted subsurface conditions such as those indicated by the formal test boring
Togs and /or discussion of subsurface conditions contained herein.
PROJECT AND SITE DESCRIPTIONS
One single -story, structural brick building (5,000 square feet, in plan area, with concrete slab -on-
grade floor) is planned for this property. The project will also include isolated, interior pipe
columns to support a trussed roof system. We anticipate that the loads will be relatively light,
say; 50 kips total for isolated columns, 2 to 4 kips per lineal foot for continuous wall type
footings.
P.O. Box 388 West Linn, Oregon 97068 503/655 -2347 FAX 503/655 -0642
Bennett Porter Office Building
April 19, 1999
Page 2
The building site will be located on the south side of the property in an area that contains small
deciduous trees and intermediate brush. The ground topography is relatively low at the building
site and appears to pond collected surface water.
FIELD EXPLORATIONS AND LABORATORY TESTING
The field exploration program consisted of two borings at their approximate locations shown on
the Boring Location Plan, Figure 1, which was taken from a preliminary site plan that you
provided for our use. The borings, designated B -1 and B -2, was drilled to auger refusal depths
that varied from 15 to 18 feet beneath current ground surface on March 29, 1999, using a trailer
mounted, 4 -inch solid stem auger (Versadrill 2400) provided by VanDeHey Soil Sampling of
Forest Grove, Oregon.
Samples were obtained at 2.5- to 5 -foot depth intervals at the boreholes. Sampling was
accomplished using a standard 2 -inch O.D. split -spoon sampler, and Standard Penetration testing
was performed in conjunction with the disturbed split -spoon sampling to measure in situ relative
density and consistency.
A West Coast Geotech, Inc., Technician, under the direct supervision of our Engineer, was
present on the site throughout the explorations to collect representative samples and prepare
descriptive field logs. Summary boring logs are presented in Figures 2 and 3. Soil descriptions
and interfaces on the logs are interpretive, and actual changes may be gradual.
All samples were visually examined in our laboratory to refine the field classifications. Based on
our experience in the vicinity, no strength or consolidation tests were considered to be necessary
for this project.
SUBSURFACE INTERPRETATION
The analyses, conclusions and recommendations contained in this report are based on site
conditions as they presently exist and assume the exploratory borings are representative of the
subsurface conditions throughout the site. If, during construction, subsurface conditions different
from those encountered in the exploratory borings are observed or appear to be present beneath
excavations, we should be advised at once so that we may review these conditions and reconsider
our recommendations where necessary.
Bennett Porter Office Building
April 19, 1999
Page 3
The field exploration disclosed native subsurface soils at the borings that generally consist of a
medium stiff becoming more stiff with depth, brown mottled, moist clayey silt containning a trace
of fine sand of low to medium plasticity to an approximate depth of 11.5 to 15 feet below current
ground surface. Standard Penetration Test (SPT) blowcounts generally vary from 6 to 13 blows
per foot (bpf).
As evidenced by Boring B -2, the clayey silt layer generally overlies a 6 -1/2 -foot thick layer of stiff
to very stiff, brown to brown mottled, moist sandy clay of medium plasticity. The bottom of this
sandy clay layer appears to terminate on rock.
These soils, as described, would probably be classified as belonging to the geological formation
that is known as "Undifferentiated Valley Alluvium ". At an approximate depth of 15 to 18 feet
beneath current ground surface, the subsurface condition consists of stiff' to hard, weathered
basalt that we believe should possibly be classified as "Boring Lava Formation ". This basalt
appeared to be fairly decomposed; although, auger refusal was encountered in both borings.
The topsoil layer is anticipated to be about 18 inches thick, more or less. In the south area where
the building site will be located and where surface water appears to pond, we believe that there
will be a soft, saturated clayey silt that underlies the topsoil layer that will vary, in thickness, from
8 to 12 inches thick, more or less. It is possible that this soft, saturated clay layer is present
beneath pavement areas as well.
This underlying saturated clayey silt will not sustain a truck in proof - rolling in wet times of the
year. In summer, if the layer is allowed time to dry and recompact or "crust over ", it might be
possible to allow this soft clayey silt layer to remain in place provided the layer has successfully
passed a proof -roll test with a loaded dump truck. In all liklihood, the soft clayey silt layer will
probably be removed from building and pavement areas.
Groundwater seepage was observed in the borings on March 29, 1999, at an approximate depth
of 4 feet below current ground surface. Groundwater will vary with time and season, and should
be anticipated at the highest level during winter and early spring when intense rainstorms are
common and at the lowest level in late summer or early fall when such rainstorms are more
infrequent.
Bennett Porter Office Building
April 19, 1999
Page 4
GEOTECHNICAL DESIGN RECOMMENDATIONS
Foundation Design
General. Based on our understanding of the project and the results of the field exploration and
laboratory testing program, it is our opinion that the proposed structures can be supported by
spread footings located on the native clayey silt layers that are present beneath existing topsoils,
soft, saturated clayey silt just under the topsoil layer, existing fill and existing pavement sections,
if any are observed to be present after the footing excavations are completed.
For footings founded on the native inorganic firm clayey silt soil layer as observed during our field
exploration program and as described in the previous paragraph, we recommend an allowable
bearing pressure of 1,500 pounds per square foot (psf).
When sizing footings for seismic considerations, the allowable bearing pressure may be increased
by 30 percent. Based on our review of the 1996 Uniform Building Code, the building site is
currently in Zone 3. The Site Coefficient should be assumed to be S3.
The Uniform Building Code/City of Tigard requirements may be used to determine the minimum
footing widths and depths of embedment. Continuous wall footings are typically on the order of
12 to 18 inches for office building projects, and column footings typically have a minimum width
of 18 to 24 inches. All perimeter footings should be founded at least 18 inches below the lowest
adjacent grade which should be taken as the finished floor elevation or exterior grade, whichever
is lower. Interior footings may be founded at least 12 inches below finished floor grade.
Each footing excavation should be evaluated by a qualified Geotechnical Engineer to confirm
suitable bearing conditions and to determine that all topsoil, loose materials, organics and
unsuitable soil or non - engineered fill have been removed. If such unsuitable materials are
encountered at footing locations, we recommend that the unsuitable material be removed. If you
desire to raise the footing grade after excavation, the engineered fill should be placed according to
our recommendations shown by Figure 4.
Footing Settlement. Based on our knowledge of the project and our settlement analysis, total
footing settlement is estimated to be, approximately, I inch, but probably less. Our settlement
estimate assumes that no disturbance to the foundation soils will be permitted during excavation
and construction and that the thick topsoil layer and the underlying soft, saturated clay layer have
Bennett Porter Office Building
April 19, 1999
Page 5
been completely removed from footing locations. Therefore, we recommend that a smooth - bucket
trackhoe be used to excavate footings. Or, the final few inches should be removed with a hand
shovel.
Our settlement estimate also assumes that minimal engineered fill will be required to raise the site
grade. Some fill to raise the site grade may be advantageous to reduce the risk of ponded surface
water /shallow groundwater seepage. If the change in elevation is more than 2 to 3 feet, then we
recommend that we be allowed to evaluate settlements caused by large, areal fill loadings.
To evaluate differential settlements accurately, additional information is required such as footing
location, footing size and a more precise breakdown of loading. However, differential settlements
for similar projects are typically on the order of 1/2 to 1 -inch as long as all the adjacent, nearby
footings bear on the same soil stratum near the same elevation. If the structure is considered to
be relatively intolerable to total or differential settlements, then we recommend that we be allowed
to review our settlement estimates when more precise information is available.
Floor Slabs. All non - basement floor slabs -on- grade, if any are planned for this project, should be
founded on a minimum 6 -inch layer of free - draining, well - graded sand and gravel or crushed rock
with a maximum particle size of 1 -1/2 inches and containing not more than 5 percent passing the
No. 200 sieve (based on a wet sieve analysis). We also recommend that a moisture vapor barrier
be installed beneath the slab in heated areas, if any, as additional protection as well.
All underslab granular materials should be compacted to a dry density of at least 98 percent of the
standard Proctor maximum dry density (ASTM D698) or as approved by the Geotechnical
Engineer.
A concrete slab should also be provided for trash container areas and aprons, if any. All concrete
slabs and aprons should be designed assuming an effective modulus of subgrade reaction, k, of
125 pounds per square inch per inch for the silt subgrade soils typical to the site. This
recommendations also assumes that a 6 -inch layer of compacted aggregate base is placed beneath
the concrete slab.
Retaining/Basement Walls. At the present time, small cantilever retaining walls and/or below -
grade basement walls do not appear to be present in this project. If, later in the design, such
structures are to be included into the project, we recommend that we be allowed to review the
situation and provide you with geotechnical design recommendations at that time in the design.
Bennett Porter Office Building
April 19, 1999
Page 6
Drainage. Groundwater is not anticipated to be a significant Tong -term design consideration for
this project unless the elevation of the slab area, if any below -grade or at -grade slabs are planned
for this project, results in a significant cut excavation. For such a situation, we recommend that
we be allowed to evaluate the need for slab underdrains. Of course, if the site is raised a few feet
than drainage becomes a less critical issue. The Landscaper may also desire to evaluate drainage
considerations outside of the building that may cause a continually wet condition.
Grading Operations
Subgrade Preparation. The subgrade preparation should include the stripping and removal of
all surficial organic and/or unsuitable soft soil (sod and topsoil and probably the soft, wet clayey
silt layer just beneath the topsoil layer) and unsuitable non - engineered fill, if any is found, as well
as existing debris from the demolition of existing buildings/pavements from all new building and
pavement areas as determined by a qualified representative of the Owner (preferably, the
Geotechnical Engineer). The removal of trees/stumps will likely cause soft holes in the subgrade.
Any soft or disturbed areas that are observed to be present, preferably by the Geotechnical
Engineer, should be removed and backfilled with engineered fill. The actual amount of material to
be excavated may need to be determined in the field, and we recommend that the specifications
include a unit cost bid item for any over excavation normally required beyond that excavation
required to be conducted by the Contractor in fulfillment of the Contract.
Construction operations may need to be modified to minimize site disturbance especially during
wet weather conditions when soil moistures are above optimum moisture content or in deeper
excavations where saturated soils are encountered such that pumping or rutting of the subgrade is
observed by the Owner's representative. Any disturbed soil shall either be compacted to
acceptable standards or removed and replaced with engineered fill. Due to the nature of the
underlying surficial clayey silt soils, we recommend that the site work be conducted during the
normal summer /early fall construction season when extended periods of dry, warm weather is
normally common. Site work during the winter or early spring season or during extended periods
of wet weather will likely require a workpad to protect the clayey silt subsurface soils from
becoming soft and disturbed by pumping.
Because of the presence of ponded water, the clayey silt subgrade will probably need to be
protected throughout the year. We recommend that all construction traffic travel on a workpad
Bennett Porter Office Building
April 19, 1999
Page 7
and that the Excavator choose to excavate with a smooth - bucket trackhoe instead of
dozers/scrappers.
Engineered Fill. We recommend that a clean (not more than 5 percent passing the No. 200
sieve based on a wet sieve analysis) reasonably well - graded granular material such as a sand and
gravel or crushed rock be used for engineered fill for the following situations:
o beneath all footings,
o during the wet periods when there is insufficient time or dry, hot weather to dry
the soil moisture to optimum moisture content,
o when excess moisture that is present in the subgrade is observed to be migrating to
the fill layer during compaction such that pumping is observed by the Owner's
representative or specified compaction levels cannot be achieved using on -site
soils.
The gradation of the any granular import material selected by the Contractor should be checked to
determine its compatibility for use adjacent to on -site soils. The maximum particle size of the
granular engineered fill should not exceed 1-1/2 inches for testing purposes. We also recommend
that the Contractor submit, for approval, samples of fill material intended for use as engineered fill
prior to earthwork construction.
Engineered fills should be placed in about 9 to 12 -inch loose lifts for areas that are compacted
with large self - propelled rollers, and should be compacted to a dry density of at least 98 percent
of the standard Proctor maximum dry density (ASTM D698) within the proposed building and
pavement areas, if any, or as approved by the Geotechnical Engineer. Lift sizes for small vibrating
plates typically used in trenches vary from 4 to 6 inches. The size of the lifts and the number of
passes of the compactor may need to be modified by the Contractor to achieve the desired results
using the equipment selected. The engineered fill should be placed in horizontal lifts commencing
on a relatively level subgrade surface.
Dewatering. The Contractor should control any water, if any, in a manner that will not affect
excavation or fill construction. The surficial clayey silt may slough into any trench excavation
especially when wet. The Contractor should excavate in such a manner that nearby footings,
slabs, utilities and existing pavements designated to remain are not undermined by potential
sloughing. Water should not be allowed to pond in the bottoms of the footings. Exposed subgrade
or fill softened by ponded water should be removed and replaced with engineered fill.
Bennett Porter Office Building
April 19, 1999
Page 8
Cut and Fill Slopes. All permanent cut and fill slopes, if any, should be groomed to slopes no
steeper than 2 Horizontal (H): 1 Vertical (V) for stability purposes. Flatter slopes may be
necessary for ground cover and maintenance operations.
Because of safety considerations and the nature of temporary excavations, the Contractor should
be responsible for maintaining safe cut excavations and supports. We recommend that the
Contractor incorporate all pertinent safety codes during construction including the latest edition
of the OR -OSHA Standards for Construction Industry (Type C Soil). This classification should
be verified during excavation by a "competent person" as defined by OR -OSHA.
Underground Fuel Tanks. Underground fuel tanks, if any are known to be present or are found
during excavation, should be removed in accordance with Oregon Department of Environmental
Quality requirements and backfilled with engineered fill.
LIMITED LIQUEFACTION STUDY
We understand that the City of Tigard does require an opinion concerning liquefaction for this
project. Groundwater was observed at an approximate depth of 4 feet below current ground
surface in wet, winter conditions. The predominate soils at the groundwater table (or below)
generally consist of stiff clayey silts (with SPT values ranging from 11 to 13 blows per foot below
a depth of 4 feet) and weathered basalt which are not determined to be highly susceptible to
liquefaction.
The clayey silts in this area, also generally have more than 15% clay fraction which is generally
regarded as the level in which liquefaction is usually precluded by the finer fraction present in the
soil.
A more in -depth liquefaction study (using SHAKE91 Computer Program to identify the
ground /subsurface motions for three typical area earthquakes and Seed's Simple Analysis) is not
needed, in our opinion.
We assume that this information is sufficient to meet the City's requirement. If a more in -depth
analysis is still required or if the City requests a more detailed Seismic Hazard Study other than an
Engineer's opinion rendered herein about the potential for liquefaction, we recommend that we be
allowed to respond accordingly.
Bennett Porter Office Building
April 19, 1999
Page 9
We have reviewed available geologic information to determine the presence of active faults, if
any. Based on our review of Earthquake - Hazard Geology Maps of the Portland Metropolitan
Area (Report 0- 90 -2), no active faults appear present on the property. A SE/NW trending
"inferred fault" does appear to have been mapped by geologists about 1/2 mile north of the
project site. Please bear in mind, that inferred faults do not necessarily mean that the faults are
active nor does it necessarily indicate that the faults are inactive. Finding faults and determining
their activity is a very complex and expensive exercise that is not funded by private sector clients.
Normally, finding evidence of such faults in the Tualatin Valley (that has been filled in over time
with Undifferentiated Valley Alluvium) can only be conducted on a regional/state/federal funded
basis or, possibly, as a Geologic Study conducted by an Oil Company. Hence, as more in -depth
geologic studies are conducted, evidence of more faults (inferred or active) may be discovered.
LIMITATIONS
In reviewing this report, be advised that the Local Governing Agency may require additional
geotechnical or other studies in order to approve the development as part of the planning approval
process. Our Geotechnical Report(s) does not guarantee that the development will be approved
by the Local Governing Agency without additional studies being performed. Expenses incurred in
reliance upon our Report(s) prior to final approval of the Local Governing Agency are the
exclusive responsibility of the Developer. In no event shall West Coast Geotech, Inc., be
responsible for any delays in approval which are not exclusively caused by West Coast Geotech,
Inc..
It is recommended that close quality control be exercised during the preparation and construction
of building foundations and pavement sections. Fills and new asphalt pavement and base sections
should be monitored and tested by a qualified representative of the Owner. In addition, we also
advise that the subgrade preparation and the footing excavations be inspected by the Geotechnical
Engineer.
If there is a substantial lapse of time between the submission of this report and the start of work at
the site, if conditions have changed due to natural causes of construction operations at or adjacent
to the site, or if the basic project scheme is significantly modified from that assumed, it is
recommended that this report be reviewed to determine the applicability of the conclusions and
recommendations considering the changed conditions and time lapse.
Bennett Porter Office Building
April 19, 1999
Page 10
Oregon seismicity and the standards used to evaluate ground response is receiving closer attention
in recent years, and, our understanding of these events as a profession is increasing as regional
studies increase. This report is prepared for your use and is based upon the information/methods
that is currently available. As our understanding of these events increases, our profession will
further refine our procedures/methods.
Unanticipated soil conditions are commonly encountered and cannot be fully determined by
merely taking soil samples or drilling test borings. Such unexpected conditions frequently require
that additional expenditures be made to attain a properly constructed project. Therefore, a
contingency fund is recommended to accommodate such potential extra cost.
Very truly yours,
WEST COAST GEOTECH, INC. y �1 44'
,l 14278 4 Ns.,
By ti 9cat
Michael F. Schrieber, P.E.
President
Geotechnical Engineer et F.
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West Linn, Oregon
SOIL DESCRIPTION — ° _
STANDARD PENETRATION RESISTANCE
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WEATHERED BASALT (POSSIBLE BORING to + : ____;<ia_.uc_:3L__1 /_2;in.____
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BOTTOM OF BORING (COMPLETED 3129/99) --- - -- ; - - -- - - - -F -- - - -- r--- -:
NOTES: 0 25 5.
1. SOIL DESCRIPTIONS AND INTERFACES ARE INTERPRETIVE AND 0 WATER CONTENT IN PERCENT
ACTUAL CHANGES MAY BE GRADUAL.
2. WATER LEVEL IS FOR DATE SHOWN AND MAY VARY WITH TIME OF BENNETT PORTER OFFICE
YEAR.
LEGEND
Tigard, Oregon
2.0" O.D. SPLIT SPOON SAMPLE IMPERVIOUS SEAL
a 3.0" O.D. THIN -WALL SAMPLE WATER LEVEL LOG OF BORING B -1
• SAMPLE NOT RECOVERED SLOTTED TIP
P SAMPLE PUSHED ATTERBERG LIMITS Apr., 1999 W -1431
LIQUID LIMIT
USC UNIFIED SOIL CLASSIFICATION WEST COAST GEOTECH, INC.
1 -1: —{ NATURAL WATER Geotechnical Consultants FIG. 2
PLASTIC LIMIT West Linn, Oregon
SOIL DESCRIPTION
- STANDARD PENETRATION RESISTANCE
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----------------------------
BOTTOM OF BORING (COMPLETED 3/29/99)
0 25 50
NOTES:
1. SOIL DESCRIPTIONS AND INTERFACES ARE INTERPRETIVE AND 0 WATER CONTENT IN PERCENT
ACTUAL CHANGES MAY BE GRADUAL.
2. WATER
YEAR. LEVEL IS FOR DATE SHOWN AND MAY VARY WITH TIME OF
BENNETT PORTER OFFICE
Tigard, Oregon
LEGEND
2.0" O.D. SPLIT SPOON SAMPLE IMPERVIOUS SEAL
v LOG OF BORING B-2
t 3.0" O.D. THIN-WALL SAMPLE WATER LEVEL
SAMPLE NOT RECOVERED SLOTTED TIP
ATTERBERG LIMITS
P SAMPLE PUSHED Apr., 1999 W-1431
LIQUID LIMIT
USC UNIFIED SOIL CLASSIFICATION WEST COAST GEOTECH, INC.
--7-■:'---1 NATURAL WATER Geotechnical Consultants FIG. 3
PLASTIC LIMIT West Linn. Oregon
COMPACTED COMPACTED
FILL FILL
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2 COMPACTED FILL
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NATIVE SOIL
BENNETT PORTER BUILDING
Tigard, Oregon
LIMITS OF COMPACTED
FILL UNDER FOOTINGS
Apr., 1999 W-1431
WEST COAST GEOTECH, INC.
Geotechnical Consultants FIG. 4
West Linn, Oregon