Correspondence •
1 ;PI' REDMOND & ASSOCIATES
Uv Project No. 174.009.G
Page No. 1
August 26, 2002
Mr. Ronald James.Ped
Ronald James Ped Architect
1 4742 Liberty Road South, # 194
Salem, Oregon 97302
1 Dear Mr. Ped:
Re: Geotechnical Investigation, Proposed Bank of Portland. Site, Tax Lot No. 6505,
1 . SW 69th Avenue, Tigard (Washington County), Oregon
INTRODUCTION
In accordance with the request and authorization by Mr. Ronald James Ped of Ronald James
1 Ped Architect, we have completed our Geotechnical Investigation at the above subject proposed
bank site. The site a rectangular shaped property, encompasses approximately 0.5 acres and is
located just east of SW 69th Avenue and to the south of Pacific Highway (99W) in Tigard
1 (Washington County), Oregon.
We understand that present plans are to develop the site by constructing a new bank building.
Specifically, we understand that the bank building project will consist of a one -story structure
1 which will be constructed with wood framing. The new bank structure is anticipated to be
supported primarily by conventional continuous and /or individual spread footing systems and will
include a concrete slab -on -grade floor. Additionally, due to the existing sloping site grades, we
• anticipate that the north side of the proposed bank building will be constructed with a below
I grade retaining wall. Other associated site improvements for the project are anticipated to
include new asphalt pavements for automobile drives and parking, concrete curbs and sidewalks,
and utility services.
1 SITE DESCRIPTION
The proposed bank building site, located within Township 1 South, Range 1 West, and Section
36 of the Willamette Meridian, is presently improved and contains an existing paved surface.
The site is bounded to the west by SW 69th Avenue, to the north by an existing commercial
property, to the south by an existing residential property, and to the east by undeveloped land.
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1 P.O. Box 301545 • Portland, OR 97294 • Phone: (503) 760 -1060 • Fax: (503) 760 -1372
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Topographically, the northerly portion of the site is characterized as gently to moderately sloping
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terrain (0 to.20 percent) descending down to the south with overall topographic estimated at
I about 5 to 6 feet and is estimated at about Elevation 330 feet.
Vegetation across the site consists primarily of a Tight to moderate growth of grass, weeds and
1 brush as well as occasional small trees.
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SCOPE OF WORK
1 The purpose of our Geotechnical studies is to evaluate the overall site subsurface soil and
ground water characteristics as well as any associated impacts or concems with regard to the
planned new construction and development of the site. Specifically, our Geotechnical
1 investigation included the following scope of work items:
1. Site exploration by means of two (2) exploratory drilled test boring excavations. The
exploratory test borings were drilled near to the ends of the proposed new bank building
I location as shown on the Site Exploration Map, Figure No. 2 to a depth of about 14.0 feet
beneath existing site grades. Detailed logs of the exploratory test boring excavations,
presenting conditions encountered at each location explored, are presented on the Boring
I Log, Figure No's. 5 and 6. Additionally, representative samples of the subsurface soils
were collected and retumed to our laboratory for further examination and testing.
2. A laboratory testing program to assess the pertinent physical and engineering
I characteristics of the subsurface soils. The laboratory program consisted of tests to
. evaluate the natural (field) moisture content, Atterberg Limits, and gradational properties.
Results of the moisture content tests are shown on their respective test boring logs, Figure
I No's: 5 and 6. Results of the Atterberg Limits and gradation tests are shown graphically
on Figure No's. 7 and 8, respectively.
3. Recommendations and our final written report presenting the . results of our investigation.
I Our report includes and overall assessment as to natural site and soil hazards as well as
recommendations for site preparation and grading including any overexcavation of
unsuitable materials revealed by the explorations, placement and compaction of any
I required structural fill(s), suitability of the on -site soils for use as structural fill as well as
criteria for import fill materials, and preparation of pavement and foundation areas.
4. Recommendations for foundation support and design including allowable contact bearing
I pressures. for proportioning footings, minimum width and embedment depths, estimates of
foundation settlement, and lateral earth pressures for below grade retaining walls.
' SUBSURFACE CONDITIONS
Our understanding of the subsurface conditions which underlie the site was developed by means
of two . (2) exploratory test borings drilled on August 12, 2002 with truck - mounted, solid -stem,
I hollow- flight, auger drilling equipment at the approximate locations shown on Figure No. 2. The
test borings revealed. that the site is underlain by native soils composed of Catastrophic Flood
deposits of Pleistocene age. Specifically, the native soil materials were comprised of an upper
I unit of medium stiff, sandy, clayey silt to a depth of about 4 to 5 feet beneath existing site
grades. These sandy, clayey silt soils possess low to moderate plasticity and are best
characterized by relatively low to moderate strength and moderate compressibility. These upper
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clayey silt soils were intum underlain by loose to medium stiff, clayey, silty sand to sandy silt to
I the maximum depth explored of about 14.0 feet beneath existing site grades.
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' Project No. 174.009.G
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These clayey, silty sand to sandy silt subgrade soils are best characterized by relatively low to
moderate strength and moderate compressibility.
' Ground water was encountered at the site during our exploration work at a depth of about 10 feet
beneath existing site grades. However, seasonal ground water elevations can be expected to
' raise in the wetter winter and spring months and may approach to near ground surface elevation.
All soils encountered at the site were classified in accordance with the Unified Soil Classification
System (USCS) which is outlined on Figure No. 4.
1 CONCLUSIONS AND RECOMMENDATIONS
From a Geotechnical engineering and constructability standpoint, we are of the opinion that the
site is presently stable and void of any significant soil related hazards other than potential soil
erosion in open cuts. As such, it is our opinion that the site is suitable for the planned new bank
development provided that the recommendations contained within this report are properly
incorporated into the design and construction of the project.
' The primary features of concem at the site are 1) the low to moderately plastic characteristics of
the upper near surface clayey silt soils across the site and 2) the relatively shallow depth in which
1 ground water seepage could be encountered.
With regard to the low to moderately plastic clayey silt subgrade soils at the site, we are of the
opinion that the exposed subgrade soils within the floor slab areas should be pre -wetted prior to
' the placement of the structural concrete. Additionally, due to the anticipated construction of a
retaining wall along the northerly .side of the proposed new bank building, we are of the opinion
that the bank structure will require the use of a perimeter foundation and /or retaining wall drain
1 system.
The following sections of this report present specific recommendations for site preparation and
grading as well as foundation design and construction for the bank building development project.
' SITE PREPARATION
' In general, we recommend that all planned structural improvement areas for the bank building
and pavements be stripped of all existing vegetation, topsoil materials, and any deleterious
materials as well as cleared of any existing structures and/or associated site improvements
present at the time of construction. In general, we envision that about 6 to 12 inches of topsoil
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striping may be required to remove existing topsoil materials. Holes resulting from the removal
of any buried obstructions, such as old foundation remnants and/or tree root areas, should be
backfilled and compacted with structural fill materials. Areas resulting in deeper stripping and
removals should be evaluated at the time of construction by the.Geotechnical Engineer. The
' stripped and cleared materials should be properly disposed of as they are generally not
considered suitable for use/reuse as structural fill.
1 Following the stripping and clearing operations, and prior to the placement of any required
structural fills and/or structural improvements, the exposed subgrade soils within the planned
building and pavement areas should be inspected by the Geotechnical Engineer and possibly
proof - rolled with a fully-loaded dump truck. Areas found to be soft or otherwise unsuitable for
' support of structural loads or improvements should be scarified and recompacted or
overexcavated and replaced with structural fill. During wet or inclement weather conditions,
proof - rolling as recommended above may not be appropriate.
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The on -site native clayey silt and/or silty sand to sandy silt subgrade soil materials are
considered suitable for use/reuse as structural fill provided that they are free of organic
I materials, debris, and rock fragments in excess of 6 inches in dimension. If grading is conducted
during wet weather, the use of the on -site clayey silt soils will be difficult at best, due to the
amount of fine grained soil particles, and the use of an import granular fill material may be
I required. In general, we recommend that a free - draining (clean) granular fill (sand & gravel)
containing no more than about 5 percent fines be used during wet weather grading.
Representative samples of the material(s) to be used as structural fill should be submitted to our
laboratory for approval and to determine the maximum dry density and optimum moisture
1 content for compaction.
In general, we do not recommend that site grading and earthwork construction be performed
I during wet or inclement weather conditions due to the extreme moisture sensitivity of the near
surface fine - grained clayey silt soils at the site. However, should wet weather grading and
construction be planned or required, the use of a granular working surface of at least 12 inches
as well as possibly a geotextile fabric such as Mirafi 600nx may be needed to protect the
1 sensitive silty subgrade soils from disturbance due to repetitive wheel loading.
All required structural fill materials placed within the building and pavement (structural) areas
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should be moistened or dried as necessary to near (within 3 percent) optimum moisture
conditions and compacted by mechanical means to a minimum of 92 percent of the maximum
dry density as determined by the ASTM D -1557 (AASHTO T -180) test procedures. Fill materials
should be placed in lifts (layers) such that when compacted do not exceed about 8 inches.
I FOUNDATION SUPPORT
I Based on the results of our investigation, it is our opinion that the proposed new bank building
structure may be supported directly on the native medium stiff, sandy, clayey silt subgrade soils
and/or properly compacted structural fill materials with conventional continuous and individual
I spread footings. As such, were foundations are constructed on approved medium stiff, native,
sandy, clayey silt subgrade soils or on properly compacted structural fill materials, an allowable
contact bearing pressure of about 2,000 pounds per square foot (psf) is recommended for
design. This allowable contact bearing pressure is intended for dead Toads and sustained live
I Ioads and maybe increased by one -third for the total of all loads including short-term wind or
seismic loads.
In general, continuous strip footings should have a minimum width of at least 16 inches and be
I embedded at least 18 inches below the lowest adjacent finish grade (includes frost protection).
• Individual column footings (if required) should be embedded at least 16 inches below grade and
have a minimum width of about 24 inches.
1 Total and differential settlements of foundations constructed as recommended above and
supported directly by approved native subgrade soils or on properly placed and compacted
structural fill materials are expected to be well within tolerable limits for this type of structure and
1 generally Tess than 1 -inch and 1/2 -inch, respectively.
Allowable lateral frictional resistance between the base of the footings and the clayey silt or a
I granular subgrade soil can be expressed as the applied vertical load multiplied by a coefficient of
friction of 0.30 and 0.45, respectively. In addition, lateral loads may be resisted by passive
pressures on footings poured "neat' against in -situ native soils or properly compacted structural
fill materials. For passive earth pressure resistance we recommend that an equivalent fluid
1 density of 300 pounds per cubic foot (pcf) be used for design.
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' W GRADE G RADE WALLS
I Below grade walls should be designed to resist lateral earth pressures imposed by native soils or
granular backfill materials as well as any adjacent surcharge Toads. For walls which are
restrained from rotation at the top and supporting level backfill, we recommend that at -rest earth
I pressures be computed on the basis of an equivalent fluid density of 55 pcf and 60 pcf for silty
soils and granular backfill, respectively. For walls which are free to rotate at the top and retaining
level backfill, we recommend that active earth pressures be computed on the basis of an
equivalent fluid density of 35 pcf and 30 pcf for silty soils and granular backfill materials,
U respectively. The above recommended values assume that the walls will be adequately drained
to prevent the buildup of hydrostatic pressures. Where wall drainage will not be present and/or if
adjacent surcharge loading is present, the above recommended values will be significantly
, I higher.
Backfill materials . behind walls should be compacted to 90 percent of the maximum dry density
determined by the ASTM D -1557 (AASHTO T -180) test procedure. Special care should be taken
I to avoid overcompaction near the wall(s) which could result in higher lateral earth pressures than
those indicated herein. In an area within 3 to 5 feet behind walls, we recommend the use of hand
operated compaction equipment.
1 EXCAVATIONS/SLOPES
Temporary excavations within native silty soils of up to four (4) feet in depth are expected to
1 remain fairly stable at near vertical inclinations. Excavations to depths of between four (4) feet to
ten (10) feet should be properly braced and shored or backcut to inclinations of about 1 to 1
(Horizontal to Vertical). Where excavations are planned to exceed ten (10) feet, this office
1 should be consulted.
All shoring and/or bracing for the project should be the responsibility of the excavation
subcontractor. Additionally, depending on the time of year in which the site grading and
I excavation work is performed, excavations below a depth of about three (3) feet may encounter
ground water. Further, due to the potential for some soil erosion of open excavations and slopes,
we recommend the use of straw bales and/or silt fences as well as revegetation immediately
111 following site grading and construction.
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Permanent slopes at the site should be constructed no steeper than about 2 to 1 (Horizontal to
Vertical).
. SURFACE DRAINAGE/GROUND WATER
I We recommend that positive measures be taken to properly finish grade the site so that drainage
waters from the apartment buildings and landscaping areas as well as adjacent properties or
pavement areas are directed away from the new apartment buildings. All roof drainage should be
directed into conduits that carry runoff water away from the apartment buildings to a suitable
I outfall. Roof downspouts should not be connected to foundation drains. A minimum ground slope
of about 2 percent is generally recommended in unpaved areas around the buildings.
I In this regard, based on our current understanding that some of the proposed new apartment
buildings may contain a partial and/or below grade floor level, we are of the opinion that a
perimeter footing/foundation drainage system should be constructed around the exterior
perimeter of the apartment building(s). A recommended perimeter footing/foundation drainage
' detail is presented on Figure No. 3.
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PAVEMENTS
Pavement design for the project was determined on the basis of projected traffic volume and
loading conditions relative to assumed subgrade soil strength characteristics. Based on an
assumed subgrade "R" -value of 25 (CBR = 2.5) and utilizing the Oregon State Highway Flexible
Pavement Design Procedures, we recommend that the asphaltic concrete pavement section for
' automobile parking and drive areas consist of the following:
Asphaltic Concrete Crushed base Rock
' Thickness (inches) Thickness (inches)
Automobile Parking 3.0 6.0
and Drive Areas
Note: Where heavy vehicle traffic is anticipated, we recommend that the pavement
1 section be increased by adding 0.5 inches of asphalt and 4.0 inches of aggregate base
rock.
The above recommended pavement section(s) assume that the subgrade will be prepared as
recommended herein, that the exposed subgrade soils will be properly protected from rain and
' construction traffic, and that the subgrade is firm and unyielding at the time of paving.
Additionally, it assumes that the subgrade is graded to prevent any ponding of water which may
tend to accumulate in the base course. Further, the above recommended flexible pavement
1 section(s) assumes a design life of about 20 years.
Pavement base course materials should consist of well- graded 1112-inch and/or 3/4 -inch minus
' crushed base rock having less than 5 percent fine materials passing the No. 200 sieve. The base
course and asphaltic concrete materials should conform to the requirements set forth in the latest
edition of the Oregon Department of Transportation, Standard Specifications for Highway
Construction. The base course materials should be compacted to at least 95 percent of the
' maximum dry density as determined by the ASTM D -1557 (AASHTO T -180) test procedures.
The asphaltic concrete materials should be compacted to at least 90 percent of the theoretical
maximum density as determined by the ASTM D -2041 (Rice Gravity) test method.
1 SEISMIC DESIGN CONSIDERATIONS
Subgrade acceleration coefficients for the project were obtained from the seismic hazard/design
I mapping project performed by Geomatrix Consultants. Geomatrix mapping indicates a peak
ground acceleration on bedrock soils at the site of approximately 0.19g with a 500 -year return
period. The UBC seismic zone factor (Z) for the subject site is 0.30. Additionally, the soil profile
type for the site based on conventional shallow foundation support on either a well compacted
structural fill material or the native, medium stiff, sandy clayey silt subgrade soils is So.
Additionally, based on the results of our field exploration work at the site as well as our past
experience in the area of the subject site, we are of the opinion that the potential for liquefaction
' at the site within the underlying loose to medium stiff, clayey, silty sand to sandy silt subgrade
soils located below a depth of about 10 feet is considered to be low for the earthquake events
typically associated with the Tigard area.
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1 REDMOND & ASSOCIATES
111 Project No. 174.009.G
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USE OF REPORT
This report is intended for the exclusive use of the addressee and their representatives to use to
design the proposed bank building structure and associated improvements described herein and
to prepare any construction documents. The data, analyses, and recommendations may not be
appropriate for other structures or purposes. We recommend that parties contemplating other
1 structures or purposes contact our office. In the absence of our written approval, we make no
representation and assume no responsibility to other parties regarding this report.
• LEVEL OF CARE
Services performed by the Geotechnical Engineer for this project have been conducted with that
level of care and skill ordinarily by members of the profession currently practicing in
' the area under similar budget and time restraints. No warranty, either expressed or implied, is
made.
We will be pleased to provide such additional assistance or information as you may require in
1 the balance of the design phase of this project and to aid in construction control or solution of
unforeseen conditions which may arise during the construction period.
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Sincerely, 0 PRO/
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N E F
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Daniel M. Redmond, P.E.
1 President/Principal Geotechnical Engineer J • r
OREGON
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