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REPORT OF GEOTECHNICAL ENGINEERING SERVICES
11300 SW Bull Mountain Road - Site Improvements
Tigard, Oregon
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May 6, 2021
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May 6,2021 bama-21-I-gi
BAMA; auryn@bamadesign.com
REPORT OF GEOTECHNICAL ENGINEERING SERVICES
Infiltration and Pavement Improvements
11300 SW Bull Mountain Road, Tigard, Oregon
As authorized, we appreciate the opportunity to present this report of geotechnical engineering services
for the proposed grade and pavement improvements for the commercial facility at 1 1300 SW Bull
Mountain Road, Tigard, Oregon. Improvements are to pavements and infiltration. The purpose of our
work was to provide geotechnical recommendations for design. Our specific scope of work included
the following:
. Provide principal level project management including management of field and subcontracted
services, report writing, analyses, and invoicing.
. Review previous reports, geologic maps, and vicinity geotechnical information in our files as
indicators of subsurface conditions.
. Complete a site reconnaissance to observe surface features relevant to geotechnical issues, such as
topography, vegetation, presence and condition of springs, exposed soils and rock, and evidence of
previous grading.
Explore subsurface conditions by drilling in 2 locations, one to a depth of up to 20 feet and one to a
'1 depth of up to 5 feet or refusal with a solid stem auger. Remove cuttings and patch the holes.
. Classify and sample materials encountered and maintain a detailed log of the explorations.
1 . Complete infiltration testing in the deeper hole using open hole falling head methods.
.? . Provide recommendations for pavement earthwork including site stripping and preparation, seasonal
material usage, use of granular working pads, fill preparation and compaction, and trench backfill
preparation and compaction.
. Provide recommendations for pavement subgrade preparation and asphalt concrete, portland
cement concrete, and base rock thickness and materials.
. Provide an infiltration rate to the civil engineer, along with backfill material recommendations and
infiltrate strata and depth.
Provide a written letter report summarizing the results of our geotechnical evaluation.
SITE OBSERVATIONS AND CONDITIONS
Surface Conditions
The site at 1 1300 SW Bull Mountain Road in Tigard, Oregon as shown on the attached Site Plan. The
site is bordered by light commercial properties. The site is on the eastern lower flank of Bull Mountain
on a gently sloping site that was likely cut flatter for development. The southern portion of the site is
paved with asphalt concrete, and the northern includes the one-story building and abutting concrete
parking and landscaping. Aerial photos show the site as undeveloped in 1999, and with a structure after
about 2001, with no significant changes since.
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Subsurface Conditions
General—The site was explored on May 5, 2021 by completing two borings to refusal in hard basalt at
depths of 5 to 8 feet at the approximate exploration locations shown on the attached Site Plan. In
general, the site consisted of Rooty silt to a depth of about 6 inches in landscape/grass areas, and several
feet of medium stiff to stiff silt, quickly transitioning to very stiff silt that represents residual soils of
severely weathered basalt. The silt became increasingly stiff with higher gravel and intact rock content
with depth, until refusal was met at depths of 8 and 5.5 feet in hard basalt. Moisture contents ranged
from 15%to 31%, and terminal blow counts (No) were over 50.
Detailed descriptions of the subsurface conditions encountered in our explorations are provided in the
attached Boring Logs, and results of moisture content testing are also attached.
Groundwater— Ground water seepage was not observed. It is possible that perched ground water is
present at shallow depths above the rock in the wet season. Seasonal high ground water is mapped at
depths of 120-140 feet(USGS 2007).
Infiltration Testing—We completed open hole infiltration testing in B-2 at a depth of 5.5 feet. Testing
was conducted after an initial wetting period and measurements were taken over time. Geotechnical
recommendations for infiltration rate are provided in the Infiltration section of this report.
Exploration Soil Tested Unfactored rate (in3/hr/in2) *
B-2@ 5.5 ft weathered basalt and silt <0.01*
*this value is unfactored and not to be used for design.
CONCLUSIONS AND RECOMMENDATIONS
General
Based on the results of our explorations and analyses, the proposed improvements are feasible following
the recommendations herein. The near surface soils at the site consist of silt which is moisture sensitive
and require protection in the wet season. Infiltration of storm water is not feasible due to very low
rates and potential for seasonal shallow perched ground water.
Stabilization and Soft Areas-After stripping, we must be contacted to evaluate the exposed
subgrade. This evaluation can be done by proof rolling in dry conditions or probing during wet
conditions. Soft areas will require over-excavation and backfilling with well graded, angular crushed
rock compacted as structural fill, overlying a separation geosynthetic such as a Propex Geotex 801 or
equivalent. If particularly soft areas are observed a geogrid may also be required, such as a Tensar
BXSQ2020 or equivalent punched and drawn biaxial or multiaxial geogrid.
Working Blankets-Construction equipment must not operate directly on the subgrade, as it is
susceptible to disturbance and softening. Rock working blankets placed over a geosynthetic in a
thickened advancing pad can be used to protect subgrades in wet conditions. We recommend that
sound, angular, pit run or crushed basalt with no more than 6 percent passing a#200 sieve be used to
construct haul roads and working blankets, overlying the preceding separation geosynthetic. Working
blankets must be at least 6 inches thick.
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20978 S Springwater Road, Estacada, OR 97023 503.869.8679;don@geotechsolutionsinc.com
May 6, 202 I bama-2 I-I-gi
The preceding rock and amendment thicknesses are the minimum recommended. Subgrade protection
is the responsibility of the contractor and thicker sections may be required based on subgrade
conditions during construction and type and frequency of construction equipment.
Earthwork
Preparation- Prior to earthwork construction, the site should be prepared by removing existing
structures,foundation elements, utilities, topsoil, and any encountered undocumented fill. Any
excavation resulting from the aforementioned preparation should be brought back to grade with
structural fill.
Slopes- Permanent slopes should be inclined no steeper than 2H:I V for slopes up to 8 feet high. The
face of fill slopes should be cut back into compacted materials with a smooth bucket excavator. Erosion
control is critical to maintaining fill slopes and should be as described for cut slopes. Drainage must be
routed away from slope faces.
Fill-The on-site fine-grained soils, or site gravel fill or crushed pavement (less than 2 inches in size) can
be used for structural fill if properly moisture conditioned. This will not be feasible during wet
conditions for the silt. In dry summer conditions the soils will require drying by scarification and
frequent mixing in thin lifts. Once moisture contents are within 3 percent of optimum, the material
should be compacted to at least 92 percent relative to ASTM D-I 557 (modified proctor) using a
tamping foot or sheeps-foot type compactor. Fill should be placed in lifts no greater than I 0 inches in
loose thickness. In addition to meeting density specifications, fill will also need to pass a proof roll using
a loaded dump truck, water truck, or similar size equipment.
In wet conditions, fill should be imported granular soil with less than 6 percent fines, such as clean
crushed or pit run rock. This material should also be compacted to 95 percent relative to ASTM D-
1557. Demolition materials such as concrete and masonry building rubble, demolished pavements, and
excavated base rock that are free of organic and other deleterious materials and crushed to no greater
than 2 inches in any dimension and are well graded may be suitable for fill. Such material must be
compacted in a manner to prevent voids and provide a dense, incompressible material. Recycled fill
materials should be placed in lifts no greater than 12 inches in loose thickness. In addition to meeting
density specifications, fill will also need to pass a wheel roll using a loaded dump truck.
Trenches - Utility trenches may encounter groundwater seepage and caving should be expected where
seepage is present. Shoring of utility trenches will be required for depths greater than 4 feet and where
groundwater seepage is present. We recommend that the type and design of the shoring system be the
responsibility of the contractor,who is in the best position to choose a system that fits the overall plan of
operation.
Depending on the excavation depth and amount of groundwater seepage, dewatering may be necessary
for construction of underground utilities. Flow rates for dewatering are likely to vary depending on
location, soil type,and the season during which the excavation occurs. The dewatering systems, if necessary,
should be capable of adapting to variable flows. Flowing conditions in soils with appreciable sand content may
occur and dewatering and shoring in these soils, if encountered, is critical, particularly to protect adjacent
infrastructure.
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Pipe bedding should be installed in accordance with the pipe manufacturers' recommendations. If
groundwater is present in the base of the utility trench excavation,we recommend over-excavating the trench
by 12 inches and placing trench stabilization material in the base. Trench stabilization material should consist
of well-graded,crushed rock or crushed gravel with a maximum particle size of 4 inches and free of
deleterious materials. The percent passing the U.S. Standard No.200 Sieve shall be less than 5 percent by
weight when tested in accordance with ASTM C 117.
Trench backfill above the pipe zone should consist of well graded, angular crushed rock or sand fill with
no more than 7 percent passing a#200 sieve. Trench backfill should be compacted to 92 percent
relative to ASTM D-1557, and construction of hard surfaces, such as sidewalks or pavement, should not
occur within one week of backfilling.
Infiltration
Based on the results of our testing and analyses, infiltration rates in the silt unit are very low, and with
potential for perched shallow seasonal groundwater, infiltration is not feasible.
Concrete Walkways, Hardscaping, Trash Enclosure
Concrete walkways and hardscaping and the trash enclosure slab and walls can be supported on 4
inches of compacted crushed rock over undisturbed medium stiff or better native soils.
Retaining Walls
General-The following recommendations are based on the assumptions that (I) Walls are
conventional concrete cantilever walls (2) Wall backfill consists of level, well-drained, angular, granular
material, (3) Walls are less than 6 feet in height, and (4) No surcharges such as stockpiled soil or
equipment are placed within 6 feet of the wall.
Walls restrained against rotation (braced prior to backfilling) should be designed using an equivalent fluid
pressure of 48 pcf. Walls not restrained against rotation should be designed using an equivalent fluid
pressure of 29 pcf. These forces can be resisted by passive pressure at the toe of the wall using an
equivalent fluid pressure of 450 pcf(this should exclude the top 12 inches of embedment) and friction
along the base using a friction coefficient of 0.45. Wall foundations can use a bearing pressure of 3,000
psf on the native medium stiff or better silt.
Backfill- Retaining walls should be backfilled with clean, imported,granular soil with less than 6 percent
fines, such as clean sand or rock. This material should also be compacted to a minimum of 92 percent
relative to ASTM D-1557 (modified proctor). Within 3 feet of the wall, backfill should be compacted to
not more than 90 percent relative to ASTM D-1557 using hand-operated equipment.
Retaining structures typically rotate and displace up to I percent of the wall height during development
of active pressures behind the wall. We therefore recommend that construction of improvements
adjacent to the top of walls be delayed until approximately two weeks after wall construction and
backfill.
Wall Drainage
All retaining walls must include a drain constructed as described in the following section. Foundation
and retaining wall drains should consist of a composite drain board such as an Amerdrain 500/520 or
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equivalent that extends full height to within one foot of the ground surface. The drain board can either
contact weep holes constructed through the lower external face of the wall at 8-foot centers, or be
integrated to a drain pipe routed to suitable down gradient discharge as determined by the civil
engineer. In either case one foot of low permeability soil (such as the on-site silt) should be placed over
the fabric protected top of the drain board to isolate the drain from surface runoff.
Pavement
Asphalt Concrete—At the time of this report we did not have specific information regarding the type
and frequency of expected traffic. We therefore developed new asphalt concrete pavement thicknesses
for areas exposed to passenger vehicles only and areas exposed to up to 3 trucks per day, such as a 3-
axle garbage or delivery truck, based on a 20-year design life. The sections herein can also support a
75,000 GVW fire truck. Traffic volumes can be revised if specific data is available.
Our pavement analyses are based on AASHTO methods and subgrade of structural fill or undisturbed
medium stiff or better native silt having a resilient modulus of 6,000 psi and prepared as recommended
herein. We have also assumed that roadway construction will be completed during an extended period
of dry weather. The results of our analyses based on these parameters are provided in the table below.
Each of these sections can support a 75,000 GVW fire truck.
Traffic 18k ESAL's AC (inches) CR (inches)
Passenger Vehicle Only - 2.5 6
Up to 3 Trucks Per Day 17,000 3 8
The thicknesses listed in the above table are the minimum acceptable for construction during an
extended period of dry weather where the roadway is not used as a haul road or working blanket.
Increased rock thicknesses and stabilization will be required for such uses and for construction during
wet conditions per the Stabilization section in this report. Crushed rock must conform to ODOT
base rock standards and have less than 6 percent passing the#200 sieve. Asphalt concrete must be
compacted to a minimum of 91 percent of a Rice Density.
Portland Cement Concrete -We developed PCC pavement thicknesses at the site for the assumed
one-way traffic levels as shown in the table below. Each of these sections is based on AASHTO
methods with no reduction for wander and a composite modulus of subgrade reaction of 350 pci
(AASHTO Figure 3.3 with Mr = 6,000 psi and 6 inches crushed rock base). Other parameters include
4,000 psi compressive strength portland cement concrete (PCC), and plain jointed concrete without
load transfer devices or tied concrete shoulders. PCC pavements over trench backfill should not be
placed within one week of fill installation unless survey data indicates that settlement of the backfill is
complete.
Traffic I8k ESALS PCC (inches) CRB (inches)
Up to 3 Trucks Per Day 17,000 5 6
Subgrade Preparation -The pavement subgrade must be prepared in accordance with the Earthwork
and Site Preparation recommendations presented in this report. All pavement subgrades must pass a
proof roll prior or probing by the geotechnical engineer prior to paving. Soft areas must be repaired
per the preceding Stabilization section.
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20978 S Springwater Road, Estacada, OR 97023 503.869.8679;don@geotechsolutionsinc.com
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LIMITATIONS AND OBSERVATION DURING CONSTRUCTION
We have prepared this report for use by BAMA and the design and construction teams for this project
only. The information herein could be used for bidding or estimating purposes but must not be
construed as a warranty of subsurface conditions. We have made observations only at the
aforementioned locations and only to the stated depths. These observations do not reflect soil types,
strata thicknesses, water levels or seepage that may exist between observations. We must be consulted
to observe all foundation bearing surfaces, subgrade stabilization, proof rolling of slab and pavement
subgrades, installation of structural fill, subsurface drainage, and cut and fill slopes. We must be
consulted to review final design and specifications in order to see that our recommendations are
suitably followed. If any changes are made to the anticipated locations, loads, configurations, or
construction timing, our recommendations may not be applicable, and we must be consulted. The
preceding recommendations must be considered preliminary, as actual soil conditions may vary. In
order for our recommendations to be final, we must be retained to observe actual subsurface
conditions encountered. Our observations will allow us to interpret actual conditions and adapt our
recommendations if needed.
Within the limitations of scope, schedule and budget, our services have been executed in accordance
with the generally accepted practices in this area at the time this report was prepared. No warranty,
expressed or implied, is given.
We appreciate the opportunity to work with you on this project and look forward to our continued
involvement. Please call if you have any questions.
Sincerely,
t'J
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Don Rondema, MS, PE, GE
Principal annalt
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Attachments— Site Plan, Guidelines for Classification of Soil, Boring Logs, Moisture Contents.
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20978 S Springwater Road,Estacada, OR 97023 503.869.8679;don@geotechsolutionsinc.com
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GUIDELINES FOR CLASSIFICATION OF SOIL
Description of Relative Density for Granular Soil
Standard Penetration Resistance
Relative Density
(N-values) blows per foot
very loose 0-4
loose 4- 10
medium dense 10- 30
dense 30-50
very dense over 50
Description of Consistency for Fine-Grained (Cohesive)Soils
Standard Penetration Torvane
Consistency Resistance(N-values) Undrained Shear
blows per foot Strength,tsf
very soft 0-2 less than 0.125
soft 2-4 0.125 -0.25
medium stiff 4-8 0.25-0.50
stiff 8- 15 0.50- 1.0
very stiff 15 - 30 1.0-2.0
hard over 30 over 2.0
Grain-Size Classification
Description Size
Boulders 12- 36 in.
Cobbles 3- 12 in.
Gravel '/a-3/4 in. (fine)
3/4-3 in. (coarse)
Sand No. 200- No.40 Sieve (fine)
No.40- No. 10 sieve(medium)
No. 10- No.4 sieve(coarse)
Silt/Clay Pass No.200 sieve
Modifier for Subclassification
Percentage of Other
Adjective
Material In Total Sample
Clean/Occasional 0-2
Trace 2- 10
Some 10- 30
Sandy,Silty,Clayey, etc. 30- 50
Soil and Rock Description Samples and Data
Oft—
Medium stiff to stiff, brown SILT; moist. (root depth 6"). w=20%
Stiff,sandy SILT,with some weathered basaltic gravel; moist. 21 w= 15%
Stiff to very stiff, brown and black severely WEATHERED BASALT; moist. 26 w=31%
Becomes hard to very hard BASALT- refusal at 8'.
10—
Boring completed on 5/5/21 and backfilled with bentonite.
20—
30
Nbo=SPT blowcount
w= moisture content
40— f= percent fines
yd=dry unit weight
tfflch BORING B-I
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Soil and Rock Description Samples and Data
Oft—
Medium stiff to stiff, brown SILT; moist. (root depth 6").
`^'=24%
Stiff, sandy SILT,with some weathered basaltic gravel; moist.
Very hard BASALT or boulder-refusal at 5.5' Soi w=22%
10—
Boring completed on 5/5/21 and backfilled with bentonite.
20—
30—
Nbo=SPT blowcount
w= moisture content
40— f= percent fines
yd=dry unit weight
to ch BORING B-2
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Exploration Depth, ft Moisture Content
B-1 1.0 20%
B-1 2.5 15%
B-1 5.0 31%
B-2 2.5 24%
B-2 5.0 22%
G P�Qtech MOISTURE CONTENTS
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