Report (19) .\ \c'k-(1)35o
FEB 7 2019
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CITY OF TIGARD CeoP
BUILDING DIVISION! Engineering,Inc.
Real-World Geotechnical Solutions
Investigation•Design•Construction Support
March 9, 2018
Project No. 17-4800
Nicholas Peets
Lennar Northwest, Inc. OFFICE COpy
11807 NE 99th Street, Suite 1170
Vancouver, Washington 98682
Via email: Nicholas.Peetsna.Lennar.com
SUBJECT: GEOTECHNICAL INVESTIGATION
TOUCHSTONE TOWNHOMES
SW OAK STREET —TIS R1W SECTION 35AA TAX LOT 3800
TIGARD,WASHINGTON COUNTY, OREGON
This report presents the results of a geotechnical engineering study conducted by GeoPacific
Engineering, Inc. (GeoPacific)for the above-referenced project. The purpose of our investigation
was to evaluate subsurface conditions at the site and to provide geotechnical recommendations for
site development. This geotechnical study was performed in accordance with GeoPacific Proposal
No. P-6318, dated December 7, 2017, and your subsequent authorization of our proposal and
General Conditions for Geotechnical Services.
SITE DESCRIPTION AND PROPOSED DEVELOPMENT
The subject site is located off the north side of SW Oak Street in Tigard, Washington County,
Oregon (Figure 1). The site is approximately 0.7 acres in size and topography is gently sloping to
the southeast. The property is currently occupied by SW Akilean Terrace and SW Elena Lane,
which were likely constructed during development of the property to the south. Vegetation consists
primarily of short grasses (Figure 2).
We understand that the proposed development will consist of 28 townhome units which will likely
be constructed as 3 to 4 structures and associated underground utilities. The structures will likely
be two to three stories in height.
REGIONAL AND LOCAL GEOLOGIC SETTING
Regionally, the subject site lies within the Willamette Valley/Puget Sound lowland, a broad
structural depression situated between the Coast Range on the west and the Cascade Range on
the east. A series of discontinuous faults subdivide the Willamette Valley into a mosaic of fault-
bounded, structural blocks (Yeats et al., 1996). Uplifted structural blocks form bedrock highlands,
while down-warped structural blocks form sedimentary basins.
14835 SW 72nd Avenue Tel (503) 598-8445
Portland,Oregon 97224 Fax(503)941-9281
Touchstone Townhomes
Project No. 17-4800
The site is underlain by the Quaternary age (last 1.6 million years) Willamette Formation, a
catastrophic flood deposit associated with repeated glacial outburst flooding of the Willamette
Valley(Yeats et al., 1996). The last of these outburst floods occurred about 10,000 years ago.
These deposits typically consist of horizontally layered, micaceous,silt to coarse sand forming
poorly-defined to distinct beds less than 3 feet thick. Regional studies indicate that the thickness of
the Willamette Formation in the vicinity of the subject site is less than 30 feet(Madin, 1990).
Regional geologic mapping indicates the Willamette Formation is underlain by the Columbia River
Basalt Formation (Gannett and Caldwell, 1998; Madin, 1990). The Miocene aged (about 14.5 to
16.5 million years ago) Columbia River Basalts are a thick sequence of lava flows which form the
crystalline basement of the Tualatin Valley. The basalts are composed of dense,finely crystalline
rock that is commonly fractured along blocky and columnar vertical joints. Individual basalt flow
units typically range from 25 to 125 feet thick and interflow zones are typically vesicular,
scoriaceous, brecciated, and sometimes include sedimentary rocks.
REGIONAL SEISMIC SETTING
At least three major fault zones capable of generating damaging earthquakes are thought to exist in
the vicinity of the subject site. These include the Portland Hills Fault Zone, the Gales Creek-
Newberg-Mt. Angel Structural Zone, and the Cascadia Subduction Zone.
Portland Hills Fault Zone
The Portland Hills Fault Zone is a series of NW-trending faults that include the central Portland Hills
Fault, the western Oatfield Fault, and the eastern East Bank Fault. These faults occur in a
northwest-trending zone that varies in width between 3.5 and 5.0 miles. The combined three faults
vertically displace the Columbia River Basalt by 1,130 feet and appear to control thickness changes
in late Pleistocene (approx. 780,000 years)sediment(Madin, 1990). The Portland Hills Fault occurs
along the Willamette River at the base of the Portland Hills, and is about 6.1 miles northeast of the
site. The Oatfield Fault occurs along the western side of the Portland Hills, and is about 4 miles
northeast of the site. The Oatfield Fault is considered to be potentially seismogenic(Wong, et al.,
2000). Madin and Mabey(1996) indicate the Portland Hills Fault Zone has experienced Late
Quaternary(last 780,000 years)fault movement; however, movement has not been detected in the
last 20,000 years. The accuracy of the fault mapping is stated to be within 500 meters(Wong, et al.,
2000). No historical seismicity is correlated with the mapped portion of the Portland Hills Fault Zone,
but in 1991 a M3.5 earthquake occurred on a NW-trending shear plane located 1.3 miles east of the
fault(Yelin, 1992). Although there is no definitive evidence of recent activity, the Portland Hills Fault
Zone is assumed to be potentially active (Geomatrix Consultants, 1995).
Gales Creek-Newberg-Mt. Angel Structural Zone
The Gales Creek-Newberg-Mt. Angel Structural Zone is a 50-mile-long zone of discontinuous, NW-
trending faults that lies about 14.2 miles southwest of the subject site. These faults are recognized
in the subsurface by vertical separation of the Columbia River Basalt and offset seismic reflectors in
the overlying basin sediment (Yeats et al., 1996; Werner et al., 1992). A geologic reconnaissance
and photogeologic analysis study conducted for the Scoggins Dam site in the Tualatin Basin
revealed no evidence of deformed geomorphic surfaces along the structural zone (Unruh et al.,
1994). No seismicity has been recorded on the Gales Creek Fault or Newberg Fault(the fault
closest to the subject site); however, these faults are considered to be potentially active because
they may connect with the seismically active Mount Angel Fault and the rupture plane of the 1993
M5.6 Scotts Mills earthquake (Werner et al. 1992; Geomatrix Consultants, 1995).
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Cascadia Subduction Zone
The Cascadia Subduction Zone is a 680-mile-long zone of active tectonic convergence where
oceanic crust of the Juan de Fuca Plate is subducting beneath the North American continent at a
rate of 4 cm per year(Goldfinger et al., 1996). A growing body of geologic evidence suggests that
prehistoric subduction zone earthquakes have occurred (Atwater, 1992; Carver, 1992; Peterson et
al., 1993; Geomatrix Consultants, 1995). This evidence includes: (1)buried tidal marshes recording
episodic, sudden subsidence along the coast of northern California, Oregon, and Washington, (2)
burial of subsided tidal marshes by tsunami wave deposits, (3) paleoliquefaction features, and (4)
geodetic uplift patterns on the Oregon coast. Radiocarbon dates on buried tidal marshes indicate a
recurrence interval for major subduction zone earthquakes of 250 to 650 years with the last event
occurring 300 years ago(Atwater, 1992; Carver, 1992; Peterson et al., 1993; Geomatrix
Consultants, 1995). The inferred seismogenic portion of the plate interface lies roughly along the
Oregon coast at depths of between 20 and 40 miles.
FIELD EXPLORATION
Our site-specific exploration for this report was conducted on December 28, 2017. A total of 3
exploratory test pits were excavated with a small sized trackhoe to depths of 8.5 to 10 feet. The
approximate locations of the explorations are shown on Figure 2. It should be noted that
exploration locations were located 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.
A GeoPacific geologist continuously monitored the field exploration program and logged the
explorations. Soils observed in the explorations were classified in general accordance with the
Unified Soil Classification System (USCS). During exploration, our geologist also noted
geotechnical conditions such as soil consistency, moisture and groundwater conditions. Logs of
the test pits are attached to this report. The following report sections are based on the exploration
program and summarize subsurface conditions encountered at the site.
SUBSURFACE CONDITIONS
Results of the field exploration program indicate the site is underlain by undocumented fill and soils
belonging to the Willamette Formation. The observed soil and groundwater conditions are
summarized below.
Undocumented Fill—Undocumented fill was encountered in test pits TP-1 through TP-3. In test
pits, the fill generally consisted of medium stiff to stiff, clayey silt(ML)with trace gravel. The fill
extended to a depth of approximately 1.5 to 2.5 feet below the existing ground surface, as
indicated on Figure 2. A thin, topsoil horizon with a low organic had developed at the ground
surface. It is possible that other areas and thicker areas of fill may exist outside of our explorations
- especially in the vicinity of the existing roadways.
Willamette Formation—The fill in test pits TP-1 through TP-3 was directly underlain by soils
belonging to the Willamette Formation. These soils typically consisted of stiff to very stiff clayey silt
(ML) that exhibited subtle to strong orange and gray mottling. In test pits, soils belonging to the
Willamette Formation extended beyond the maximum depth of exploration (10 feet).
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Groundwater
On December 28, 2017, soils encountered in test pits were moist. Neither static groundwater nor
groundwater seepage was encountered in explorations to a maximum depth of 10 feet. Regional
geologic mapping indicates that static groundwater is present at a depth of less than 20 feet below
the existing ground surface (Snyder, 2008). Experience has shown that temporary perched storm-
related groundwater conditions often occur within the surface soils 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 season, local subsurface conditions, changes in
site utilization, and other factors.
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. Our explorations indicate the soils on site are suitable for development utilizing
conventional spread footing foundations. The primary geotechnical constraint to project
development is the presence of undocumented fill. Fill was encountered in test pits TP-1 through
TP-3 to depths of 1.5 to 2.5 feet below the ground surface.
Site.Preparation
Areas of proposed buildings, streets, and areas to receive fill should be cleared of vegetation and
any organic and inorganic debris. Existing fill should be completely removed. Fill was
encountered in all of our explorations conducted for this study and ranged in thickness from 1.5 to
2.5 feet. Other areas and thicker areas of fill may be encountered on site, especially in the vicinity
of the existing roadways. Existing buried structures such as septic tanks, should be demolished
and any cavities structurally backfilled. Inorganic debris should be removed from the site.
Organic-rich topsoil should then be stripped from native soil areas of the site. The estimated depth
range necessary for removal of topsoil in cut and fill areas is approximately 6 to 9 inches,
respectively. The final depth of soil removal will be determined on the basis of a site inspection
after the stripping/excavation has been performed. Stripped topsoil should preferably be removed
from the site due to the high density of the proposed development. Any remaining topsoil should
be stockpiled only in designated areas and stripping operations should be observed and
documented by the geotechnical engineer or his representative.
Once topsoil stripping and removal of organic and inorganic debris is approved in a particular area,
the area must be ripped or tilled to a depth of 12 inches, moisture conditioned, root-picked, and
compacted in-place prior to the placement of engineered fill or crushed aggregate base for
pavement. Exposed subgrade soils should be evaluated by the geotechnical engineer. 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, over-excavated and replaced
with engineered fill (as described below), or stabilized with rock prior to placement of engineered
fill. The depth of overexcavation, if required, should be evaluated by the geotechnical engineer at
the time of construction.
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Engineered Fill
All grading for the proposed construction should be performed as engineered grading in
accordance with the applicable building code at time of construction with the exceptions and
additions noted herein. Proper test frequency and earthwork documentation usually requires daily
observation and testing during stripping, rough grading, and placement of engineered fill. 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% of the
maximum dry density determined by ASTM D1557(Modified Proctor)or equivalent. Field density
testing should conform to ASTM D2922 and D3017, or D1556. All engineered fill should be
observed and tested by the project geotechnical engineer or his representative. Typically, one
density test is performed for at least every 2 vertical feet of fill placed or every 500 yd3, whichever
requires more testing. Because testing is performed on an on-call basis, we recommend that the
earthwork contractor be held contractually responsible for test scheduling and frequency.
Site earthwork will be impacted by soil moisture and shallow groundwater conditions. Earthwork in
wet weather would likely require extensive use of cement or lime treatment, or other special
measures, at considerable additional cost compared to earthwork performed under dry-weather
conditions.
Excavating Conditions and Utility Trenches
We anticipate that on-site soils can be excavated using conventional heavy equipment such as
trackhoes to a depth of at least 10 feet. 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 soil is classified as Type B Soil and
temporary excavation side slope inclinations as steep as 1 H:1 V may be assumed for planning
purposes. This cut slope inclination is applicable to excavations above the water table only.
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.
Saturated soils and groundwater may be encountered in utility trenches, particularly during the wet
season. We anticipate that dewatering systems consisting of ditches, sumps and pumps would be
adequate for control of perched groundwater. 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 prevent loss of ground support and possible distress to existing or previously
constructed structural improvements.
PVC pipe should be installed in accordance with the procedures specified in ASTM D2321. We
recommend that trench backfill be compacted to at least 95% of the maximum dry density obtained
by Standard Proctor ASTM D698 or equivalent. Initial backfill lift thickness for a 3/"-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,
then the lifts for large vibrating plate-compaction equipment(e.g. hoe compactor attachments) may
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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.
Erosion Control Considerations
During our field exploration program, we did not observe soil types that would be considered highly
susceptible to erosion. In our opinion, the primary concern regarding erosion potential will occur
during construction, in areas that have been stripped of vegetation. Erosion at the site during
construction can be minimized by implementing the project erosion control plan, which should
include judicious use of straw wattles and silt fences. If used, these 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.
Wet Weather Earthwork
Soils underlying the site are likely to be 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 5
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;
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➢ 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
➢ Geotextile silt fences, straw wattles, and fiber rolls should be strategically located to control
erosion.
If cement or lime treatment is used to facilitate wet weather construction, GeoPacific should be
contacted to provide additional recommendations and field monitoring.
Spread Foundations
Based on our understanding of the proposed project and the results of our exploration program,
and assuming our recommendations for site preparation are followed, native deposits and/or
engineered fill soils should be encountered at or near the foundation level of the proposed
structures. These soils are generally stiff to very stiff, and should provide adequate support of the
structural loads.
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. For footing subgrade soils at depths of 4 feet or
less, we recommend maximum allowable bearing pressures of 1,500 pounds per square foot (psf).
The recommended maximum allowable bearing pressures may be increased by 1/3 for short term
transient conditions such as wind and seismic loading. All footings should be founded at least 18
inches below the lowest adjacent finished grade. Minimum footing widths should be determined by
the project engineer/architect in accordance with applicable design codes.
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 1/2 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 structure to lateral
forces. Lateral forces on a structure will be resisted by a combination of sliding resistance of its
base or footing on the underlying soil and passive earth pressure against the buried portions of the
structure. For use in design, a coefficient of friction of 0.42 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 320 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. Loose, wet or otherwise softened soil should be removed from the footing excavation
prior to placing reinforcing steel bars.
The above foundation recommendations are for dry weather conditions. 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. GeoPacific should observe foundation
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excavations prior to placing formwork and reinforcing steel, to verify that adequate bearing soils
have been reached.
Concrete Slabs-on-Grade
Preparation of areas beneath concrete slab-on-grade floors should be performed as recommended
in the Site Preparation and Undocumented Fill Removal 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 150 kcf(87 pci)should be assumed for the medium stiff native silt
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 10-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 and mold prevention issues, which are outside
GeoPacific's area of expertise.
Drains
The outside edge of perimeter walls should be provided with a drainage system consisting of
3-inch diameter, slotted, flexible plastic pipe embedded in a minimum of 1 ft3 per lineal foot of
clean, free-draining gravel or 1 1/2"- 3/4" 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. Down
spouts and roof drains should not be connected to the foundation drains in order to reduce the
potential for clogging. The footing drains should include clean-outs to allow periodic maintenance
and inspection. Grades around the proposed structure should be sloped such that surface water
drains away from the building.
Footing drains are recommended to prevent detrimental effects of surface water runoff on
foundations—not to dewater groundwater. Footing drains should not be expected to eliminate all
potential sources of water entering a basement or beneath a slab-on-grade. An adequate grade to
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a low point outlet drain in the crawlspace is required by code. Underslab drains are sometimes
added beneath the slab when placed over soils of low permeability and shallow, perched
groundwater.
Seismic Design
The Oregon Department of Geology and Mineral Industries(Dogami), Oregon HazVu: 2017
Statewide GeoHazards Viewer indicates that the site is in an area where severe ground shaking is
anticipated during an earthquake (Dogami HazVu, 2017). Structures should be designed to resist
earthquake loading in accordance with the methodology described in the 2015 International
Building Code(IBC)with applicable Oregon Structural Specialty Code (OSSC) revisions (current
2014). We recommend Site Class D be used for design per the OSSC, Table 1613.5.2 and as
defined in ASCE 7, Chapter 20, Table 20.3-1. Design values determined for the site using the
USGS (United States Geological Survey)2017 Seismic Design Maps Summary Report are
summarized in Table 1, and are based upon existing soil conditions.
Table 1. Recommended Earthquake Ground Motion Parameters (2010 ASCE-7)
Parameter Value
Location (Lat, Long), degrees 45.445, -122.768
Mapped Spectral Acceleration Values(MCE):
Peak Ground Acceleration PGAM 0.459 g
Short Period, SS 0.984 g
1.0 Sec Period, S, 0.425 g
Soil Factors for Site Class D:
Fa 1.107
F„ 1.575
SD, = 2/3xFaxS$ 0.7269
SD, = 2/3 x F„x Si 0.446 g
Seismic Design Category D
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. According to the Oregon HazVu: Statewide
Geohazards Viewer, the majority of the subject site is regionally characterized as having a high risk
of soil liquefaction (DOGAMI:HazVu, 2018).
Our explorations indicate liquefaction hazard in the upper 10 feet of soils is low; however,
statewide hazard mapping indicates a high liquefaction hazard for the site. GeoPacific can further
evaluate the effects of seismic induced liquefaction hazards such as vertical settlement and lateral
deformation, if desired. We anticipate that our additional explorations on the site for the purpose of
evaluating seismic hazards would include at least one cone penetrometer test and analysis.
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UNCERTAINTIES AND LIMITATIONS
We have prepared this report for the owner and their 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, GeoPacific 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. The
checklist attached to this report outlines recommended geotechnical observations and testing for
the project. 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, GeoPacific attempted to execute these
services in accordance with generally accepted professional principles and practices in the fields of
geotechnical engineering and engineering geology 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.
We appreciate this opportunity to be of service.
Sincerely,
GEOPACIFIC ENGINEERING, INC. PROre
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EXPIRES:06/30/20,1i
Beth K. Rapp, C.E.G. James D. Imbrie, G.E., C.E.G.
Senior Engineering Geologist Principal Geotechnical Engineer
Attachments: References
Figure 1 —Vicinity Map
Figure 2—Site and Exploration Plan
Test Pit Logs (TP-1 through TP-3)
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REFERENCES
Atwater, B.F., 1992, Geologic evidence for earthquakes during the past 2,000 years along the Copalis River,
southern coastal Washington:Journal of Geophysical Research,v.97, p. 1901-1919.
Carver, G.A., 1992, Late Cenozoic tectonics of coastal northern California: American Association of
Petroleum Geologists-SEPM Field Trip Guidebook, May, 1992.
Gannett, M.W. and Caldwell, R.R., 1998, Geologic framework of the Willamette Lowland aquifer system,
Oregon and Washington: U.S. Geological Survey Professional Paper 1424-A, 32 pages text, 8
plates.
Geomatrix Consultants, 1995, Seismic Design Mapping, State of Oregon: unpublished report prepared for
Oregon Department of Transportation, Personal Services Contract 11688.
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(HazVu): https://gis.dogami.oregon.qov/hazvu/
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U.S. Geological Survey Professional Paper 1560, P. 183-222,5 plates, scale 1:100,000.
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24, no. 5, p.92.
4800-Touchstone Townhomes GR 11 GEOPACIFIC ENGINEERING,INC.
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Project:Touchstone Townhomes
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G oeoe Pc Portland,Oregon 97224 TEST PIT LOG
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Project: Touchstone Townhomes Project No. 17-4800 Test Pit No. TP-1
Tigard, Oregon
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Stiff, clayey SILT(ML), trace gravel, light brown, subtle orange and gray
mottling, trace roots, trace inorganic debris, 3 inch thick topsoil developed at
1 — 1.5 surface, moist(Undocumented Fill)
2— 2.5
3— 2.5 Stiff to very stiff, clayey SILT(ML), light brown to gray, micaceous, subtle to
strong orange and gray mottling,trace fine roots throughout upper 3.5 feet,trace
black staining, moist(Willamette Formation)
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LEGEND
Date Excavated: 12/28/2017
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..eiNgrai. 14835 SW 72nd Avenue
6 o nig Portland,Oregon 97224 TEST
Enalncenna.lnc. Tel: (503)598-8445 Fax:(503)941-9281 PIT LOG
Project: Touchstone Townhomes Project No. 17-4800 Test Pit No. TP-2
Tigard, Oregon
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light brown, subtle orange and gray mottling,trace roots, 5 inch thick topsoil
developed at surface, moist(Undocumented Fill)
2 0.5
3 2.0
Stiff to very stiff, clayey SILT(ML), light brown to gray, micaceous, strong orange
4 2.5 and gray mottling, trace fine roots throughout upper 3 feet,trace black staining,
moist(Willamette Formation)
5
6
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10—I Note: No seepage or groundwater encountered.
11-
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LEGEND
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,oDate Excavated: 12/28/2017
PA1 suet Logged By: B. Rapp
t,000 t • J
Bag Sample Bucket Sample Shelby Tube Sample Seepage Water Bearing Zone Water Level at Abandonment Surface Elevation:
14835 SW 72nd Avenue
GeoP Ifle Portland,Oregon 97224 TEST PIT LOG
ma nearing hit. Tel:(503)598-8445 Fax:(503)941-9281
Project: Touchstone Townhomes Project No. 17-4800 Test Pit No. TP-3
Tigard, Oregon
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Medium stiff to stiff, clayey SILT(ML),trace gravel up to 4 inches in diameter,
light brown,subtle orange and gray mottling, trace roots, 3 inch thick topsoil
1 , 1.0 developed at surface, moist(Undocumented Fill)
2.5
2 1.5
3 2.5
Stiff to very stiff, clayey SILT(ML), light brown to gray, micaceous, subtle orange
4—i 2.5 and gray mottling, trace fine roots throughout upper 3 feet, trace black staining,
moist(Willamette Formation)
5—
6
7
8
9 Test Pit Terminated at 8.5 Feet.
10 Note: No seepage or groundwater encountered.
11--$
12--5
LEGEND
0
Date Excavated: 12/28/2017
1000"" s Gai PA f/ Logged By: B. Rapp
Bucket LL
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Bag Sample Bucket Sample Shelby Tube Sample Seepage Water Bearing Zone Water Level at Abandonment Surface Elevation:
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