Report (3) OFFICE COPY
xx 7 HARDMAN
GEOTECHNICAL
r SERVICES INC.
2we1 cal.Cost-Effective Geotechnical Solutions
July 14, 2020
HGSI Project No. 20-2607 ---�
OCIDO
U ,VA1RECEIVEV
Daniel Silvey / HUYY1bSD
DBS Group LLC / DEC 3 0 1011
2115 SE Tenino Street
Portland,Oregon 97202 CITY OF TIGARD
danielsilvey(a)kniperealty.com BUILDING DIVISION
Copy: Annemarie Skinner, annemarieemeriodesign.com
7
Via email with hard copies mailed on request
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Subject: GEOTECHNICAL ENGINEERING REPORT AND INFILTRATION TESTING
7460 SW HERMOSO WAY
TIGARD,OREGON
This report presents the results of a geotechnical engineering study conducted by Hardman Geotechnical
Services Inc. (HGSI)for the proposed development at 7460 SW Hermoso Way in Tigard, Oregon. The
purpose of this study was to evaluate subsurface conditions at the site and to provide geotechnical
recommendations for site development. This geotechnical study was performed in accordance with HGSI
Proposal No. 20-554a, dated June 24, 2020, and your subsequent authorization of our proposal and General
Conditions for Geotechnical Services.
SITE DESCRIPTION AND PROPOSED DEVELOPMENT
The site area is comprised of a roughly 15,000 square feet lot that is rectangular in shape. The property
currently supports a residential home of unknown construction date. The site has a small concrete driveway
from SW Hermoso Way to the built-in garage. Immediately surrounding the home is landscaped garden
areas and a small inner-fenced-in-yard in the back. Landscaping appears to have been done to the back yard
up to the south property line where there is a remnant depression indicative of a former water/pond feature
surrounded by small basalt boulders and rounded river rock while the remainder of the property is vegetated
with grasses and some trees.
Slopes on site are gentle, descending to the west and north. A slight grade break on the west portion of the
site is clearly visible by the line of small boulders that create the short step. The steepest slopes on site are at
a garden bed just south of the home, at about 5H:1 V(Horizontal:Vertical) approximated from pacing on site.
There are no steep slope hazards mapped on the site. Based on information provided on the Oregon
Department of Geology and Mineral Industries (DOGAMI)online utility, SLIDO,there are no mapped
landslides or history of sliding indicated for the property and vicinity.
We understand the planned development consists of a maximum five-story multi-use building with
driveways, parking, underground utilities and stormwater facilities also to be included as part of site
development.
10110 SW Nimbus Avenue,Suite B-5 Tel(503)530-8076
Portland,Oregon 97223 www.hgsirocks.com
July 14, 2020
HGSI Project No. 20-2599
REGIONAL GEOLOGY AND SEISMIC SETTING
The subject site lies within the Portland Basin, a broad structural depression situated between the Coast Range
on the west and the Cascade Range on the east. The Portland Basin is a northwest-southwest trending
structural basin produced by broad regional downwarping of the area. The Portland Basin is approximately 20
miles wide and 45 miles long and is filled with consolidated and unconsolidated sedimentary rocks of late
Miocene,Pliocene and Pleistocene age.
Geologic maps indicate the subject site is underlain by Quaternary age (last 1.6 million years)Willamette Silt,
a windblown silt deposit that mantles older deposits, basalt bedrock, and elevated areas in the Portland region
(Madin, 1990; Schlicker and Deacon, 1967). The loess generally consists of massive silt deposits following
repeated catastrophic flooding events in the Willamette Valley,the last of which occurred about 10,000 years
ago. In localized areas,the loess includes buried paleosols that developed between depositional events. In the
site area the Willamette Silt formation included some to numerous cobbles. Regionally,the total thickness of
loess ranges from 5 feet to greater than 100 feet.
Underlying the Willamette Silt, regional geologic mapping indicates the subject site is underlain by the Boring
Lava lithologic unit which consists of basaltic and basaltic andesite lava flows erupted from a series of local
volcanic vents during Plio-Pleistocene time (about 600,000 thousand to 2.6 million years ago)(Trimble, 1963;
Madin, 1990). The total thickness of the Boring Lava unit ranges from greater than 600 feet near vents to less
than 50 feet on the outer margins.
At least three major seismic source zones capable of generating damaging earthquakes are known to exist in
the region. These include the Portland Hills Fault Zone, Gales Creek-Newberg-Mt.Angel Structural Zone, and
g g
the Cascadia Subduction Zone. These potential earthquake source zones are included in the determination of
seismic design values for structures, as presented in the Seismic Design section.
FIELD EXPLORATION
Exploratory Borings
The site-specific exploration for this study was conducted on July 8, 2020 and consisted of three boreholes
(designated B-1 through B-3), excavated to a maximum depths of 6.4 to 8 feet below ground surface(bgs)at
the approximate locations shown on the attached Site Plan, Figure 2. It should be noted that exploration
locations were determined in the field by pacing or taping distances from apparent property corners and other
site features shown on the plans provided. As such,the locations of the explorations should be considered
approximate.
Standard Penetration Test(SPT) sampling was performed in general accordance with ASTM D1586 using a
2-inch outside diameter split-spoon sampler and a 140-pound hammer equipped with a rope and cathead
mechanism. During the test, a sample is obtained by driving the sampler 18 inches into the soil with the
hammer free-falling 30 inches. The number of blows for each 6 inches of penetration is recorded. This
resistance, or N-value, provides a measure of the relative density of granular soils and the relative
consistency of cohesive soils. At the completion of the borings,the holes were backfilled with bentonite
"chips", and capped with cold-mix asphalt compound.
Explorations were conducted under the full-time observation of HGSI personnel. Soil samples obtained from
the borings were classified in the field and representative portions were placed in relatively air-tight plastic
bags. These soil samples were then returned to the laboratory for further examination. Pertinent information
including soil sample depths, stratigraphy, soil engineering characteristics, and groundwater occurrence was
recorded. Soils were classified in general accordance with the Unified Soil Classification System.
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HGSI Project No. 20-2599
Summary exploration logs are attached to this report. The stratigraphic contacts shown on the individual
yl
borehole logs represent the approximate boundaries between soil types. The actual transitions may be more
gradual. The soil and groundwater conditions depicted are only for the specific dates and locations reported,
ii and therefore, are not necessarily representative of other locations and times.
Infiltration Testing
i
On July 8, 2020, HGSI performed falling head infiltration tests using the open hole method in borings B-1
t and B-3. Infiltration tests were performed by measuring the water level at one minute intervals using
THOBOTM data loggers,which measures water pressure corrected for temperature and barometric pressure.
I See the attached HOBOTM water level data logger plots. The infiltration rates were determined based on the
slope of the water depth line near the end of each test. Table 1 presents the results of the falling head
infiltration tests.
Table 1. Summary of Infiltration Test Results
Depth Infiltration Hydraulic Head
Boring (feet) Soil Type Rate(in/hr) Range during
Testing(inches)
B-1 8 Clayey Silt(ML) 1.6 69.6—66.2
B-3 7.5 Clayey Silt(ML) 1.8 43.3—39.7
SUBSURFACE CONDITIONS
The following discussion is a summary of subsurface conditions encountered in our explorations. For more
detailed information regarding subsurface conditions at specific exploration locations, refer to the attached
exploration logs. Also, please note that subsurface conditions can vary between exploration locations, as
discussed in the Uncertainty and Limitations section below.
Soil
On-site soils are anticipated to consist of undocumented fill, topsoil, and native soil as described below.
Topsoil—Topsoil was encountered in two of the three boreholes, B-1 and B-2, and extended to
depths of about 6 to 8 inches. These soils generally consisted of dark brown, organic silt(OL), with
trace gravel, and the up
per approximately
pp matelinches y 2 of topsoil being highly organic.
Native Silt Soil—Silt soils were encountered in all of the boreholes at depth beyond the topsoil down
to about 6 to 7 feet bgs. The soil consisted of stiff to very-stiff, moist, orange and dark-brown silt
with trace organics, and is slightly micaceous.
Weathered Boring Lava—Underlying the Willamette Silt, we encountered very-stiff to hard, dry,
dark gray weathered rock extending to the maximum depth of exploration at 6.4 to 8 feet bgs.
Orange mottles present through the rock indicate that weathering has occurred to varying degrees.
Groundwater
During the field exploration, no groundwater seepage was encountered in the borings. While no groundwater
was visible, records suggest that the water table at this site is approximately 20 feet bgs(Snyder, 2009).
Perched groundwater conditions often occur over fine-grained native deposits such as those beneath the site,
particularly during the wet season. It is anticipated that groundwater conditions will vary depending on the
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season, local subsurface conditions, changes in site utilization, and other factors. The groundwater
conditions reported above are for the specific date and locations indicated, and therefore may not necessarily
be indicative of other times and/or locations.
ii
CONCLUSIONS AND RECOMMENDATIONS
Results of this study indicate that the proposed development is geotechnically feasible,provided that the
recommendations of this report are incorporated into the design and construction phases of the project. No
slope stability hazard areas were identified on site, and none are indicated by available geologic and hazard
mapping at the time of this report. Recommendations are presented below regarding site preparation,
engineered fill,wet weather earthwork, spread footing foundations, below grade structural retaining walls,
perimeter footing drains, seismic design, stormwater infiltration systems, excavating conditions and utility
trench backfill,typical pavement sections, and erosion control considerations.
i
Site Preparation
The areas of the site to be graded should first be cleared of vegetation and any loose debris; and debris from
clearing should be removed from the site. Organic-rich topsoil should then be removed to competent native
` soils. We anticipate that the average depth of topsoil stripping will be 6 to 8 inches over most of the site.
The final depth of stripping removal may vary depending on local subsurface conditions and the contractor's
methods, and should be determined on the basis of site observations after the initial stripping has been
performed. Stripped organic soil should be stockpiled only in designated areas or removed from the site and
, stripping operations should be observed and documented by HGSI. Existing subsurface structures (tile
drains, old utility lines, septic drain fields, etc.)beneath areas of proposed structures and pavement should be
removed and the excavations backfilled with engineered fill.
While undocumented fill was not encountered in our explorations,there is potential for old fills to be present
on site in areas beyond our exploration locations. Where encountered beneath proposed structures,
pavements, or other settlement-sensitive improvements, undocumented fill should be removed down to firm
inorganic native soils and the removal area backfilled with engineered fill (see below). HGSI should observe
removal excavations(if any)prior to fill placement to verify that overexcavations are adequate and an
appropriate bearing stratum is exposed.
In construction areas, once stripping has been verified,the area should be ripped or tilled to a depth of 12
inches, moisture conditioned, and compacted in-place prior to the placement of engineered fill. Exposed
subgrade soils should be evaluated by HGSI. For large areas,this evaluation is normally performed by
proof-rolling the exposed subgrade with a fully loaded scraper or dump truck. For smaller areas where
access is restricted,the subgrade should be evaluated by probing the soil with a steel probe. Soft/loose soils
identified during subgrade preparation should be compacted to a firm and unyielding condition or over-
excavated and replaced with engineered fill,as described below. The depth of overexcavation, if required,
should be evaluated by HGSI at the time of construction.
Engineered Fill
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In general, we anticipate that on-site soils will be suitable for use as engineered fill in dry weather conditions,
provided they are relatively free of organics and are properly moisture conditioned for compaction. Imported
fill material must be approved by the geotechnical engineer prior to being imported to the site. Oversize
material greater than 6 inches in size should not be used within 3 feet of foundation footings, and material
greater than 12 inches in diameter should not be used in engineered fill.
Engineered fill should be compacted in horizontal lifts not exceeding 8 inches using standard compaction
equipment. We recommend that engineered fill be compacted to at least 90 percent of the maximum dry
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density determined by ASTM D1557(Modified Proctor)or equivalent. On-site soils may be wet or dry of
optimum; therefore, we anticipate that moisture conditioning of native soil will be necessary for compaction
operations.
Proper test frequency and earthwork documentation usually requires daily observation and testing during
stripping, rough grading, and placement of engineered fill. Field density testing should conform to ASTM
D2922 and D3017, or D1556. Engineered fill should be periodically observed and tested by HGSI.
Typically, one density test is performed for at least every 2 vertical feet of fill placed or every 500 yd3,
whichever requires more testing.
Wet Weather Earthwork
The on-site soils are moisture sensitive and may be difficult to handle or traverse with construction
equipment during periods of wet weather. Earthwork is typically most economical when performed under
dry weather conditions. Earthwork performed during the wet-weather season will probably require
expensive measures such as cement treatment or imported granular material to compact fill to the
recommended engineering specifications. If earthwork is to be performed or fill is to be placed in wet
weather or under wet conditions when soil moisture content is difficult to control,the following
recommendations should be incorporated into the contract specifications.
• Earthwork should be performed in small areas to minimize exposure to wet weather. Excavation or
the removal of unsuitable soils should be followed promptly by the placement and compaction of
clean engineered fill. The size and type of construction equipment used may have to be limited to
prevent soil disturbance. Under some circumstances, it may be necessary to excavate soils with a
backhoe to minimize subgrade disturbance caused by equipment traffic;
• The ground surface within the construction area should be graded to promote run-off of surface
water and to prevent the ponding of water;
• Material used as engineered fill should consist of clean, granular soil containing less than about 7
percent fines. The fines should be non-plastic. Alternatively, cement treatment of on-site soils may
be performed to facilitate wet weather placement;
• The ground surface within the construction area should be sealed by a smooth drum vibratory roller,
or equivalent, and under no circumstances should be left uncompacted and exposed to moisture.
Soils which become too wet for compaction should be removed and replaced with clean granular
materials;
• Excavation and placement of fill should be observed by the geotechnical engineer to verify that all
unsuitable materials are removed and suitable compaction and site drainage is achieved; and
• Bales of straw and/or geotextile silt fences should be strategically located to control erosion.
If cement or lime treatment is used to facilitate wet weather construction, HGSI should be contacted to
provide additional recommendations and field monitoring.
Spread Footini Foundations
Shallow, conventional isolated or continuous spread footings may be used to support the proposed structures,
provided they are founded on competent native soils, or compacted engineered fill placed directly upon the
competent native soils. We recommend a maximum allowable bearing pressure of 3,500 pounds per square
foot(psf)for designing spread footings bearing on undisturbed native soils or engineered fill. The
recommended maximum allowable bearing pressure may be increased by a factor of 1.33 for short term
transient conditions such as wind and seismic loading. All footings should be founded at least 18 inches
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below the lowest adjacent finished grade. Minimum footing widths should be determined by the project
engineer/architect in accordance with applicable design codes.
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Assuming construction is accomplished as recommended herein, and for the foundation loads anticipated, we
estimate total settlement of spread foundations of less than about 1 inch and differential settlement between
two adjacent load-bearing components supported on competent soil of less than about '/2 inch. We anticipate
that the majority of the estimated settlement will occur during construction, as loads are applied.
Wind, earthquakes, and unbalanced earth loads will subject the proposed structures to lateral forces. Lateral
forces on a structure will be resisted by a combination of sliding resistance of its base or footing on the
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underlying soil and passive earth pressure against the buried portions of the structure. For use in design, a
coefficient of friction of 0.45 may be assumed along the interface between the base of the footing and
subgrade soils. Passive earth pressure for buried portions of structures may be calculated using an equivalent
fluid weight of 390 pounds per cubic foot(pcf), assuming footings are cast against dense, natural soils or
engineered fill. The recommended coefficient of friction and passive earth pressure values do not include a
safety factor. The upper 12 inches of soil should be neglected in passive pressure computations unless it is
protected by pavement or slabs on grade.
Footing excavations should be trimmed neat and the bottom of the excavation should be carefully prepared.
r Loose,wet or otherwise softened soil should be removed from the footing excavation prior to placing
reinforcing steel bars. HGSI should observe foundation excavations to verify that adequate bearing soils
have been reached. Due to the high moisture sensitivity of on-site soils, construction during wet weather
may require overexcavation of footings and backfill with compacted, crushed aggregate.
Below-Grade Structural Retaining Walls
Lateral earth pressures against below-grade retaining walls will depend upon the inclination of any adjacent
slopes,type of backfill, degree of wall restraint, method of backfill placement, degree of backfill compaction,
drainage provisions, and magnitude and location of any adjacent surcharge loads. At-rest soil pressure is
exerted on a retaining wall when it is restrained against rotation. In contrast, active soil pressure will be
exerted on a wall if its top is allowed to rotate or yield a distance of roughly 0.001 times its height or greater.
If the subject retaining walls will be free to rotate at the top,they should be designed for an active earth
pressure equivalent to that generated by a fluid weighing 35 pcf for level backfill against the wall. For
restrained walls, an at-reset equivalent fluid pressure of 54 pcf should be used in design, again assuming
level backfill against the wall. These values assume that the recommended drainage provisions are
incorporated, and hydrostatic pressures are not allowed to develop against the wall.
During a seismic event, lateral earth pressures acting on below-grade structural walls will increase by an
incremental amount that corresponds to the earthquake loading. Based on the Mononobe-Okabe equation
and peak horizontal accelerations appropriate for the site location, seismic loading should be modeled using
the active or at-rest earth pressures recommended above, plus an incremental rectangular-shaped seismic
load of magnitude 5H, where H is the total height of the wall.
We assume relatively level ground surface below the base of the walls. As such,we recommend passive
earth pressure of 390 pcf for use in design, assuming wall footings are cast against competent native soils or
engineered fill. If the ground surface slopes down and away from the base of any of the walls, a lower
passive earth pressure should be used and HGSI should be contacted for additional recommendations.
A coefficient of friction of 0.5 may be assumed along the interface between the base of the wall footing and
subgrade soils. The recommended coefficient of friction and passive earth pressure values do not include a
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safety factor, and an appropriate safety factor should be included in design. The upper 12 inches of soil
should be neglected in passive pressure computations unless it is protected by pavement or slabs on grade.
The above recommendations for lateral earth pressures assume that the backfill behind the subsurface walls
will consist of properly compacted structural fill, and no adjacent surcharge loading. If the walls will be
subjected to the influence of surcharge loading within a horizontal distance equal to or less than the height of
the wall,the walls should be designed for the additional horizontal pressure. For uniform surcharge
pressures, a uniformly distributed lateral pressure of 0.3 times the surcharge pressure should be added.
The recommended equivalent fluid densities assume a free-draining condition behind the walls so that
hydrostatic pressures do not build up. This can be accomplished by placing a 12-to 18-inch wide zone of
crushed drain rock containing less than 5 percent fines against the walls. A 3-inch minimum diameter
perforated,plastic drain pipe should be installed at the base of the walls and connected to a sump to remove
water from the crushed drain rock zone. The drain pipe should be wrapped in filter fabric(Mirafi 140N or
other as approved by the geotechnical engineer)to minimize clogging. The above drainage measures are
intended to remove water from behind the wall to prevent hydrostatic pressures from building up. Additional
drainage measures may be specified by the project architect or structural engineer, for damp-proofing or
other reasons.
HGSI should be contacted during construction to verify subgrade strength in wall keyway excavations,to
verify that backslope soils are in accordance with our assumptions, and to take density tests on the wall
backfill materials.
Concrete Slabs-on-Grade
Preparation of areas beneath concrete slab-on-grade floors should be performed as recommended in the Site
Preparation section. Care should be taken during excavation for foundations and floor slabs,to avoid
disturbing subgrade soils. If subgrade soils have been adversely impacted by wet weather or otherwise
disturbed,the surficial soils should be scarified to a minimum depth of 8 inches,moisture conditioned to
within about 3 percent of optimum moisture content, and compacted to engineered fill specifications.
Alternatively, disturbed soils may be removed and the removal zone backfilled with additional crushed rock.
For evaluation of the concrete slab-on-grade floors using the beam on elastic foundation method, a modulus
of subgrade reaction of 200 kcf(115 pci) should be assumed for the soils anticipated at subgrade depth. This
value assumes the concrete slab system is designed and constructed as recommended herein,with a
minimum thickness of crushed rock of 8 inches beneath the slab.
Interior slab-on-grade floors should be provided with an adequate moisture break. The capillary break
material should consist of ODOT open graded aggregate per ODOT Standard Specifications 02630-2. The
minimum recommended thickness of capillary break materials on re-compacted soil subgrade is 8 inches.
The total thickness of crushed aggregate will be dependent on the subgrade conditions at the time of
construction, and should be verified visually by proof-rolling. Under-slab aggregate should be compacted to
at least 90%of its maximum dry density as determined by ASTM D1557 or equivalent.
In areas where moisture will be detrimental to floor coverings or equipment inside the proposed structure,
appropriate vapor barrier and damp-proofing measures should be implemented. A commonly applied vapor
barrier system consists of a l0-mil polyethylene vapor barrier placed directly over the capillary break
material. Other damp/vapor barrier systems may also be feasible. Appropriate design professionals should
be consulted regarding vapor barrier and damp proofing systems, ventilation, building material selection,
radon and mold prevention issues, which are outside HGSI's area of expertise.
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Perimeter Footing Drains
We recommend the outside edge of perimeter footings be provided with a drainage system consisting of
3-inch minimum diameter perforated pipe embedded in a minimum of 1 ft3 per lineal foot of clean, crushed
drain rock. The drain pipe and surrounding drain rock should be wrapped in non-woven geotextile(Mirafi
140N, or approved equivalent)to minimize the potential for clogging and/or ground loss due to piping.
Water collected from the footing drains should be directed into the local storm drain system or other suitable
outlet. A minimum 0.5 percent fall should be maintained throughout the drain and non-perforated pipe
outlet. The footing drains should include clean-outs to allow periodic maintenance and inspection.
Down spouts and roof drains should collect roof water in a system separate from the footing drains in order
to reduce the potential for clogging. Roof drain water should be directed to an appropriate discharge point
well away from structural foundations. Grades should be sloped downward and away from buildings to
reduce the potential for ponded water near structures.
Seismic Design
Structures should be designed to resist earthquake loading in accordance with the methodology described in
the current Oregon Structural Specialty Code(OSSC). We recommend Site Class C (Very Dense Soil and
Soft Rock)be used for design per the OSSC, which references ASCE 7. Design values determined for the
site using the ASCE 7-16 Hazard Tool are summarized on Table 3, for Risk Category III.
Table 3. Recommended Earthquake Ground Motion Parameters (ASCE 7-16)
Parameter Value
Location (Lat, Long), degrees 45.4306, -122.7536
Mapped Spectral Acceleration Values
(MCE, Site Class B):
Short Period, S, 0.865 g
1.0 Sec Period, Si 0.394 g
Design Values for Site Class C (Very Dense Soil and Soft Rock):
Peak Ground Acceleration PGAM 0.471
Fa 1.2
F„ 1.5
SD,=2/3 x Fa x Ss 0.692 g
SDI =2/3 x F, x SI 0.394 g
Soil liquefaction is a phenomenon wherein saturated soil deposits temporarily lose strength and behave as a
liquid in response to earthquake shaking. Soil liquefaction is generally limited to loose, granular soils
located below the water table. Following development, on-site soils will consist predominantly of
engineered fill and stiff to very stiff silt and clay, which are not considered susceptible to liquefaction.
Therefore, it is our opinion that special design or construction measures are not required to mitigate the
effects of liquefaction.
Stormwater Infiltration Systems
nfiltration test locations were limited by the presence of existing improvements and trees. Infiltration rates of
1.6 and 1.8 inches per hour were measured at depths of 7.5 to 8 feet in B-1 and B-3.
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Based on results of the infiltration testing,the site is not well suited for use of deep infiltration systems such
as dry wells due to fine-grained soils. Some opportunity for infiltration may be realized in the upper part of
the soil profile, where an ultimate infiltration rate of 1.6 inches/hour is recommended for design of
stormwater planters, swales, pervious pavements, or other shallow facilities. We recommend the system
designer select an appropriate infiltration rate based on the depth and location of proposed facilities. The
jI infiltration rates do not incorporate a factor of safety; many local agencies require a minimum factor of safety
of 2.0 be applied to the measured(ultimate)infiltration rate.
il Infiltration test methods and procedures attempt to simulate the as-built conditions of the planned disposal
system. However, due to natural variations in soil properties, actual infiltration rates may vary from the
measured and/or recommended design rates. All systems should be constructed such that potential overflow
is discharged in a controlled manner away from structures, and all systems should include an adequate factor
of safety. Infiltration rates presented in this report should not be applied to inappropriate or complex
hydrological models such as a closed basin without extensive further studies.
Excavating Conditions and Utility Trench Backfill
We anticipate that on-site soils can be excavated to depths of at least 8 feet using conventional heavy
equipment such as trackhoes. The small drill rig used for this exploration encountered refusal at depths of
6.4 to 8 feet bgs. It is likely that a large excavator would be capable of digging deeper into the subsurface
materials on site without excessive effort. Should the combined depth of any planned cuts and utilities
exceed about 10 feet,we recommend additional subsurface explorations to evaluate rippability at the deep
cut location(s).
Maintenance of safe working conditions, including temporary excavation stability, is the responsibility of the
contractor. Actual slope inclinations at the time of construction should be determined based on safety
requirements and actual soil and groundwater conditions. All temporary cuts in excess of 4 feet in height
should be sloped in accordance with U.S. Occupational Safety and Health Administration (OSHA)
regulations (29 CFR Part 1926), or be shored. The existing native soils classify as Type C Soil and
temporary excavation side slope inclinations as steep as 1H:IV may be assumed for planning purposes. This
cut slope inclination is applicable to excavations above the water table only. Flatter temporary excavation
slopes will be needed if groundwater is present, or if significant thicknesses of sandy soils are present in
r excavation sidewalls.
Perched groundwater conditions often occur over fine-grained native deposits such as those beneath the site,
particularly during the wet season. If encountered,the contractor should be prepared to implement an
appropriate dewatering system for installation of the utilities. At this time, we anticipate that dewatering
• systems consisting of ditches, sumps and pumps would be adequate for control of groundwater where
encountered during construction conducted during the dry season. Regardless of the dewatering system
used, it should be installed and operated such that in-place soils are prevented from being removed along
with the groundwater.
Vibrations created by traffic and construction equipment may cause some caving and raveling of excavation
walls. In such an event, lateral support for the excavation walls should be provided by the contractor to
e
prevent loss of ground support and possible distress to existing or previously constructed structural
1 improvements.
Utility trench backfill should consist of 3/4"-0 crushed rock, compacted to at least 90% of the maximum dry
l density obtained by Modified Proctor(ASTM D1557)or equivalent. Initial backfill lift thick nesses for a
3/4"-0 crushed aggregate base may need to be as great as 4 feet to reduce the risk of flattening underlying
flexible pipe. Subsequent lift thickness should not exceed 1 foot. If imported granular fill material is used,
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then the lifts for large vibrating plate-compaction equipment(e.g. hoe compactor attachments)may be up to
2 feet, provided that proper compaction is being achieved and each lift is tested. Use of large vibrating
compaction equipment should be carefully monitored near existing structures and improvements due to the
potential for vibration-induced damage.
Adequate density testing should be performed during construction to verify that the recommended relative
compaction is achieved. Typically, one density test is taken for every 4 vertical feet of backfill on each 200-
lineal-foot section of trench.
Pavement Sections
We recommended the following minimum pavement section for dry weather construction conditions.
Table 4. Recommended Minimum Dry Weather Pavement Sections
Material Layer Minimum Thickness (inches)
y Streets Compaction Standard
92%of Rice Density(top lift)
Asphaltic Concrete(AC) 3 91% of Rice Density(lower lifts)
AASHTO T-209
Crushed Aggregate Base 2 95% of Modified Proctor
3/4"-0 (leveling course) ASTM D1557
Crushed Aggregate Base 95% of Modified Proctor
1'/z"-0 8 ASTM D1557
Recommended Subgrade 12 95% of Standard Proctor
or approved native
In new pavement areas,the native soil subgrade should be ripped or tilled to a minimum depth of 12 inches,
moisture conditioned, and recompacted in-place to at least 95 percent of ASTM D698 (Standard Proctor)or
equivalent. In order to verify subgrade strength, we recommend proof-rolling directly on subgrade with a
loaded dump truck during dry weather and on top of base course in wet weather. Soft areas that pump, rut,
or weave should be stabilized prior to paving.
If pavement areas are to be constructed during wet weather, HGSI should review subgrade at the time of
construction so that condition specific recommendations can be provided. Wet weather pavement
construction is likely to require soil amendment or geotextile fabric and an increase in base course thickness.
For planning purposes an overexcavation and increased base rock thickness of 6 inches,with geotextile
fabric(Mirafi 500x or better), may be assumed. This overexcavation thickness is subject to field verification
during construction based on actual soil moisture and weather conditions at the time. Where the
overexcavation and increased base rock thickness method of providing a wet weather pavement section is
used,the requirement for proof-rolling the subgrade soils should be waived.
During placement of pavement section materials, density testing should be performed to verify compliance
with project specifications. Generally, one subgrade, one base course, and one AC density test is performed
for every 100 to 200 linear feet of paving.
Erosion Control Considerations
Based on our subsurface exploration we did not encounter soils considered particularly susceptible to
erosion. Erosion during construction can be minimized by implementing the project erosion control plan,
20-2607-7460 SW Hermoso Way_GR_Inf 10 HARDMAN GEOTECHNICAL SERVICES INC.
July 14, 2020
HGSI Project No. 20-2599
which should include judicious use of bio-bags, silt fences, or other appropriate technology. Where used,
erosion control devices should be in place and remain in place throughout site preparation and construction.
Erosion and sedimentation of exposed soils can also be minimized by quickly re-vegetating exposed areas of
soil, and by staging construction such that large areas of the project site are not denuded and exposed at the
same time. Areas of exposed soil requiring immediate and/or temporary protection against exposure should
be covered with either mulch or erosion control netting/blankets. Areas of exposed soil requiring permanent
stabilization should be seeded with an approved grass seed mixture, or hydroseeded with an approved seed-
mulch-fertilizer mixture.
UNCERTAINTIES AND LIMITATIONS
We have prepared this report for the owner and his/her consultants for use in design of this project only.
This report should be provided in its entirety to prospective contractors for bidding and estimating purposes;
however,the conclusions and interpretations presented in this report should not be construed as a warranty of
the subsurface conditions. Experience has shown that soil and groundwater conditions can vary significantly
over small distances. Inconsistent conditions can occur between explorations that may not be detected by a
geotechnical study. If, during future site operations, subsurface conditions are encountered which vary
appreciably from those described herein, HGSI should be notified for review of the recommendations of this
report, and revision of such if necessary.
Sufficient geotechnical monitoring, testing and consultation should be provided during construction to
confirm that the conditions encountered are consistent with those indicated by explorations.
Recommendations for design changes will be provided should conditions revealed during construction differ
from those anticipated, and to verify that the geotechnical aspects of construction comply with the contract
plans and specifications.
Within the limitations of scope, schedule and budget,HGSI executed these services in accordance with
generally accepted professional principles and practices in the field of geotechnical engineering at the time
the report was prepared. No warranty, expressed or implied, is made. The scope of our work did not include
environmental assessments or evaluations regarding the presence or absence of wetlands or hazardous or
toxic substances in the soil, surface water, or groundwater at this site.
20-2607-7460 SW Hennoso Way_GR_Inf 1 1 HARDMAN GEOTECHNICAL SERVICES INC.
All
July 14, 2020
HGSI Project No. 20-2599
O.O
We appreciate this opportunity to be of service.
Sincerely,
HARDMAN GEOTECHNICAL SERVICES INC.
.1 0,101\ 7
Scott L. Hardman, P.E., G.E. •
Geotechnical Engineer
EXPIRES: 06-30 2014.
Attachments: Reference
Figure 1 —Vicinity Map
Figure 2—Site Plan
Logs of Borings B-1 through B-3
Infiltration Test Results, B-1 and B-3
ASCE 7-16 Seismic Hazard Tool Output(3 pages)
®�®�®�OO �OO �OO�OO �®�®�®�OO �OO �OOt�0O �00 �00 �®�®�OO �OO �OO �OO �OO
REFERENCES
Madin, I.P., 1990, Earthquake hazard geology maps of the Portland metropolitan area, Oregon: Oregon
Department of Geology and Mineral Industries Open-File Report 0-90-2, scale 1:24,000, 22 p.
Schlicker, H.G., and Deacon, R.J., 1967, Engineering geology of the Tualatin Valley region: Oregon
Department of Geology and Mineral Industries, Bulletin 60,
Trimble, D.E., 1963, Geology of Portland, Oregon and adjacent areas; US Geological Survey Bulletin B-
1119, Scale 1:62,500.
20-2607-7460 SW Hermoso Way_GR_Inf 12 HARDMAN GEOTECHNICAL SERVICES INC.
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B-X Site Boundary
. Boring Location
Project: 7460 SW Hermoso Way
Tigard, Oregon Project No. 20-2607 FIGURE 2
BOREHOLE LOG
Project: 7460 SW Hermoso Way
Tigard, Oregon Project No. 20-2607 Boring No. B - 1
Do w o
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o ? o c o . o o Material Description
jm o 0 0
TOPSOIL: Stiff, Dry, Brown SILT, with organics & rock fragments
1 Stiff to Very-Stiff, Moist, Orange-Dark Brown SILT (ML) with Orange Mottles and
trace organics, Non-Plastic, Noncohesive, Slightly Micaceous (Willamette
Formation)
2
447
3
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Very-Stiff, Moist to Dry, Dark Brown SILT (ML)with Orange Mottles and increas-
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over Boring Lava Formation)
1
S-2
7
19.50
X 50+ Hard, Dry, Dark Gray, Weathered Rock, Non-Plastic, Noncohesive (Boring Lava
8 Formation)
S-3 Drilling refusal at 8 feet
No apparent seepage or water encountered
�,, 1 HARDMAN LEGEND X
GEOTECHNICAL
SERVICES INC. Date Drilled: 7 / 8 /2020
.e.11,-.31 4,mA hltei u.e Geob rt x iv S:ifatr>x%
10110 SW Nimbus Avenue,Suite B-5 S-1 Water Level at Logged By: JAC
4 Portland,Oregon 97223 Soil Sample Depth Time of Drilling
(503)530-8076 Interval and Designation
BOREHOLE LOG
Project: 7460 SW Hermoso Way Project No. 20-2607 Boring No. B - 2
Tigard, Oregon
So �
tU o .� N
c z to �'o
a) Ti52
r6 Material Description
0 cn—5 o o
F- — ��-' � g 2
0- o U 0
TOPSOIL: Stiff, Dry, Brown SILT, with organics & rock fragments
Very-Stiff, Moist to Dry, Dark Brown SILT (ML)with Orange Mottles and trace to
1 some Weathered Rock, Non-Plastic, Noncohesive, Slightly Micaceous
(Willamette Formation over Boring Lava Formation)
2
6.13.14
3
27
4
S-1
5
24.21.31
Hard, Dry, Dark Gray, Weathered Rock, Non-Plastic, Noncohesive (Boring Lava
Formation)
6 50+
S-2 Drilling refusal at 6.4 feet
No apparent seepage or water encountered
7
8 ----
GEO GEOTECTEC N LEGEND
.:1 HNiC11L
I SERVICES INC. Date Drilled: 7 / 8 /2020
loinSW Nimbus Avenue Suite B-5 S 1 Water Level at Logged By: JAC
Portland,Oregon 97223 Soil Sample Depth Time of Drilling
(503)530-8076 Interval and Designation
BOREHOLE LOG
Project: 7460 SW Hermoso Way
Tigard, Oregon Project No. 20-2607 Boring No. B - 3
T o
N �q O N ca
E , � c -. 3
EL co Z o o o o Material Description
m 0 U (7
Very-Stiff, Dry, Dark Brown SILT (ML) with gravel and trace organics
1 Stiff, Moist, Orange-Dark Brown SILT (ML) with Orange Mottles and trace gravel,
Non-Plastic, Noncohesive, Slightly Micaceous (Willamette Formation)
2
547
3
11
4 S-1
Very-Stiff, Moist, Orange-Dark Brown SILT (ML) with Orange Mottles and
increasing Weathered Rock, Non-Plastic, Noncohesive, Slightly Micaceous
5 — (Willamette over Boring Lava Formation)
7.9.14
6 23
S-2 Hard, Dry, Dark Gray, Weathered Rock, Non-Plastic, Noncohesive (Boring Lava
7 Formation)
.__.41.75.100
50+ *Switched split-spoon sampler for 3-inch diameter"California" Sampler*
8
S-3 Drilling refusal at 8 feet
No apparent seepage or water encountered
x rlHARDMAN LEGEND X
I GEOTECHNiCAL
I SERVICES INC. Date Drilled: 7 / 8 / 2020
0 ;k •.'<';exteekvs
10110 SW Nimbus Avenue,Suite B-5 S-1 Water Level at Logged By: JAC
Portland,Oregon 97223 Soil Sample Depth Time of Drilling
(503)530-8076 Interval and Designation
I
,-, .._., I HARD _
.F INFILTRATION TEST DATA
L SERVICES INC.
3
,1t tical.Cost-Effective Geotecnnical Solutions
t7= 20-2607 HOBO 1
- —Albs Pres,psi
�7 —Tome, 'F 78
_—_�
.— --t—�_ A.villrler Qatarhed
17_ __ s" --- ---- T Coupler:tached
0.;topped 76
End Of File
1 i 74
_ 1 fi
-1 Infiltration Rate Determined ff
105- 1 I 72
1 Using Slope of Line at Interval
�. Indicated = 1.6 in/hr
i �,
te-. r� rsd -4
i5.5-
1 / 51
{ K
62
H
;0
61
J
A L.1.,...%)
688
-I
1
I
14:5- t-R 50
88:00'00:04 10:00:00 PM 12.00:00 PM 02.00:00 PM 04:00:00 PM 0600.00 PM
0ii0%0 08:00.06 PM GMT-07:00 67518520 00:00:00 PM GMT-1
Project: 7460 SW Hermoso Wa Date Tested: 7/8/20
y Tested By: JAC / SLH Boring: B-1
Tigard, Oregon Depth: 7.5 Feet
Project No: 20-2607
HARDMIE
GEOTECH Ec, INFILTRATION TEST DATA
#x s s SERVICES INC,.
Tactical.Cost-Effective Geotecnnical Solutions
2D-2607 HOBO
—Abs Pres,psi
-
- —Temp, 'F
ii
' — # ls' f j A Coupler Detached
76
`L....., --' A COLIpler Attached
/ C StoppedInfiltration Rate Determined 1 :x End Of File
Using Slope of Line at Interval
^ Indicated = 1.8 in/hr T°
/
1/
16- f/ 0
I
I AP
!' 56
1
11
_
I I , i ilg
t i 00'00 NA 10:00.00 AM 12:00:00 PM 02:00:00 PM 04:00:00 PM v100:130 PM
07i08,-0 0800:00 Ari alf-07:00 tittaV'0 ua W.00 PM WI!
Project: 7460 SW Hermoso Way Date Tested: 7/8/20 Boring: B-3
Tested By: JAC / SLH
Tigard, Oregon Project No: 20-2607
Depth: 8 Feet
�II ASCE 7 Hazards Report
:4,1F_RC'.0 OTCFG,.kF'.. NEFq
Address: Standard: ASCE/SEI 7 16 Elevation: 218.41 ft (NAND 88)
No Address at This Risk Category: Ill Latitude: 45.430597
Location Soil Class: C - Very Dense Longitude: -122.753578
Soil and Soft Rock
! " :4 " ..' a' ig S's* .r � • . > r °
( "Ex f x ',
!i t i - yrr It,{
1
,
httrs://asce7hazardtool.online/ Page 1 of 3 Thu Jul 09 2020
Seismic
Site Soil Class: C - Very Dense Soil and Soft Rock
Results:
Ss : 0.865 So, : 0.394
Si : 0.394 TL : 16
Fa : 1.2 PGA : 0.393
F, : 1.5 PGA M : 0.471
S Ms 1.038 F PGA 1.2
SM, : 0.592 le : 1.25
i
1 Sos : 0.692 C„ : 1.066
Seismic Design Category D
MCER Response Spectrum Design Response Spectrum
06
0.
0 05 .
0�
i. 04
>6 Via,.
0,3 be
02t<
Sa(g)vs T(s) Sa(g)vs T(s)
MCER Vertical Response Spectrum Design Vertical Response Spectrum
1
'4 e 4N,
5 9.4€ 5 20 if^ 1.6 2_0
Sa(g) vs T(s) Sa(g)vs T(s)
Data Accessed: Thu Jul 09 2020
Date Source: USGS Seismic Design Maps based on ASCE/SEI 7-16 and ASCE/SEI 7-16
Table 1.5-2. Additional data for site-specific ground motion procedures in
accordance with ASCE/SEI 7-16 Ch. 21 are available from USGS.
https://asce7hazardtool.online/ Page 2 of 3 Thu Jul 09 2020
ASCE
AMER CA%SCCI-:T`Gf cult E`:G ft_EFS:
The ASCE 7 Hazard Tool is provided for your convenience,for informational purposes only,and is provided"as is"and without warranties of
any kind.The location data included herein has been obtained from information developed,produced,and maintained by third party providers;
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