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• Hydro Geo&Environmental,Inc
8525 SW 67°i Ave.,
• Portland.OR 97223
Phone:503.892.2000
• c-mail: rvdrvgso, »,«...»
& Environmental, IncHydro Geo
July 13, 2017
Project# 2388.17
Attn: Diane
RE: SUMMARY OF SOIL INFILTRATION TESTING FOR SITE @ 15970 SW 76th
AVE.,TIGARD, OREGON
In accordance with your authorization Hydro Geo & Environmental, Inc., (HGE) has
completed soil infiltration testing at the subject property in the approximate locations
indicated on the attached site plan. The purpose of the infiltration testing was to provide
field infiltration rates for use in design of a storm water disposal system for the proposed
development. The scope of work for this project consisted of excavation of one shallow
test pit. and then angering of exploratory borehole from the bottom of the test pit,
evaluation of field infiltration rates using standardized methods and equipment, and
preparation of this letter report.
This letter report has been prepared for exclusive use of the owner and their agents, for
specific application to the referenced scope of services, in accordance with generally
accepted geotechnical engineering practices. No other warranty, expressed or implied, is
made. In the event that changes in the nature. usage, or layout of the proposed site
improvements are made the conclusion contained in this report shall not be considered
valid unless the changes are reviewed by HGE in writing.
The field work was conducted on July 11, 2017 consisting of excavating of one shallow
test pit and conducting the percolation tests at the depth of 4.5 feet. The hand-augered
exploratory borehole was advanced then from the bottom of test pit to a depth of 7 feet and
was terminated at that depth due to full auger refusal. An engineering associate from HGE
coordinated and observed the subsurface conditions and infiltration testing.
The site is located at 15970 SW 76th Ave., at the north corner of intersection with SW
Durham Rd., in Tigard, OR (Figurel). The site is fairly level at 147-150 above Mean Sea
Level (Beaverton Quad, USGS 7.5 Minute Map) and slopes slightly eastward. The existing
2372 s.f. single residential house was demolished prior of our site explorations. The site is
bounded on the west by paved SW 76th Ave., on the south, north and east by adjacent
residential houses (see Figure 1,Vicinity Map).
GEOLOGY & SUBSURFACE CONDITIONS
The near-surface geology of the project area consists.of Late Pleistocene age (recent to•
July 13, 2017
Page 2
approximately 1_5 un1111tm ycu�, u ,v) s.A;nic„iD «�l, ol, warp elopite1tPA lw eatactrnnhic
floods of the Columbia River. These glacial floods originated in the Missoula valley of
Montana, passed through eastern Washington, and followed the Columbia River
downstream. When these large floods entered the Portland Basin they flowed up the
Willamette River and its tributaries flooding most of the Willamette and Tualatin Valley
up to approximately elevation 350. These episodes of flooding deposited thick layers of
silt, sand and gravel that mantle the Willamette Valley and Portland Basin.
Based on a review of the Soil Survey of Washington County, the near-surface soils at the
site are mapped as Hillsboro Loam, 0 to 3 percent slopes. This well drained soil is on
broad valley terraces that formed in mixed silty and loamy, old alluvium on terraces.
At the time of our visit we hand dug one test pit to a depths 4.5 feet. Subsurface materials
represented in the test pit # 1 consisted of a 1.5- foot surface layer of dark-brown moist
organic silt fill with some grass and tree roots. Underlying the fill and organic topsoil, the
native soil consists of yellowish-brown, moist stiff, low plasticity sandy silt, underlain at
the depth of 4.5 feet by yellowish-brown damp to moist, dense fine-grained silty sand, in
turn, underlain at the depth of 7 feet by dark-grey basalt rock fragments. Our test boring
was terminated at the depth of 7 feet due to full auger refusal
No evidence of perched or static groundwater or seasonal perched subsurface water was
encountered in the test pit and exploratory boring. Based on Oregon Water Resources
Department data, groundwater in the vicinity was encountered in 196 feet deep water well at the
depth of 25 feet below the surface. (See Washington County Water Well Report # 011634
attached).
INFILTRATION TESTS- The test was conducted by driving a six-inch diameter
infiltrometer stand pipe into the soil at the above pointed interval. The infiltration test was
conducted as Encased Falling Head tests based on methodology of Portland Stormwater
Management Manual—January 2014.
We have embedded a solid 6-inch diameter casing into the native medium-grained sand soil
at the elevation of 4.5 feet below the surface. That embedment has provided a good seal
around the pipe casing so that percolation was limited to the 6-inch plug of the material
within the casing. The pipe with clean water approximately of 12 inches above the soil was
maintained at this depth for at least 4 hours to presoak the native fine-grained sand soil.
Total of three trials of infiltration test were conducted during our study. After each trial,the
water level was readjusted to the 12 inch level.
The water level was measured to the nearest 0.01 foot ('/x inch) at 10-minute intervals for a
total period of 2 hours. Successive trials were run until the percent change in measured
infiltration rate between two successive trials is minimal. All test results are summarized in
the data table. The infiltration rate noted below is a last of three observations of actual
infiltration rate measured in the field in undisturbed medium-grained sand soil and do not
include a factor of safety.
Location Soil Test depth Field infiltration rate
TP-1 sand 4.5 feet 16 inches/hour
July 13, 2017
Page 3
After infiltration testing, test pit was further augured to verify soil conditions beneath the
test location. The soils observed below the test depth appeared consistent with sandy soil
above and across the site.
In accordance with AASHTO classification, tested soils refer to A-2 and A-3 groups of this
classification. In accordance with Unified Soil Classification System, tested soil refer to
SM class group symbol (well-graded fine-grained sand).
CONCLUSIONS- Based oil the results of the infiltration test, observation of subsurface
conditions, and our office review, the native site soils appear to have moderate permeability at the
depth of sandy soils encountered and are suitable for subsurface discharge of storm water. We
believe that the onsite infiltration capability has not been compromised by the past construction use
on the site and is suitable for subgrade of pervious pavement.
Field infiltration rates recorded during this study generally correspond to the range of permeability
values reported in the Soil Survey of Washington County, Oregon.
Differences in infiltration test results noted above may be due to slight areal and depth variations in
soil gradation, density,and in-situ moisture content. In addition, it has been observed that the
permeability of undisturbed native soils such as those found on this site can be substantially
different than soils that have been disturbed by construction activities.
It is recommended that HGE be contacted to observe subsurface conditions at the time of
construction to correlate actual soil conditions with those observed during this study. It is also
advisable to test the infiltration system to confirm adequate capacity.
We appreciate the opportunity to assist you on this project. If you have any questions or would like
additional information please feel free to contact the undersigned at(503) 892-2000.
Truly yours,
Hydro Geo& Environmental,Inc.
i O
BERG
@E174 4 NI ,f
Mike Go berg, t E.G
Principal Engineer g Geologist lrr`-e- /P/307
July 13, 2017
Page 4
TROT PIT 1 lr.
Hydro Geo& Environmental, Inc Project# 2388.17
8525 SW 67th Ave.,
Portland, OR 97223
Site Address: 15970 SW 76th Ave.,Tigard,OR
Test Pit/borehole# I Depth- 7 feet. 6/08/17
Depth(ft) Soil Description:
0.0-1.5 Top soil-organic silt fill with tree roots and wood debris
1.5-4.5' Native yellowish-brown,damp to moist low plasticity stiff sandy silt
4.5-7.0' Yellowish-brown damp to moist,dense,well-graded, fine-grained silty sand
7.0- Dark-grey weathered basalt rock fragments
No seepage
Test pit was terminated at the depth of 4.5 feet and then hand-augered test boring was
advanced to a depth of 7 feet.Test boring was terminated at the depth of 7 feet due to full
auger refusal
Test pit backfilled upon infiltration test completion.
M. Golberg, Engineering Geologist
Completed by(name and title) �, g g g
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- trials infiltration test were conducted dein. : our study. Ali
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level was my A : I to the nearest 0.01 foot inch) at 10-minu
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