HomeMy WebLinkAboutRS_GeoTech_Infiltration_Assessment_180301_v1OEOTEC'H
CONSULTANTS, INC.
Sazei Design Group, LLC
6608 —110th Avenue Northeast
Kirkland, Washington 98033
Attention: Hamid Korasani
via email. hamidkorasani@yahoo.com
Subject: Infiltration Assessment Report (Updated)
Proposed Short Plat
1909 Northeast 14th Street
Renton, Washington
Dear Mr. Korasani:
24011 Oth Ave E
Seattle, Washington 98102
(425) 747-5618
JN 17532
January 2, 2018
This report presents our geotechnical conclusions related to the potential for utilizing infiltration to
dispose of storm water from the proposed new short plat to be constructed on the subject lot. The
site is located on the southern side of Northeast 14th Street. The lot currently contains a small,
single story residence, which will be removed. No detailed plans were available, but from our
discussions with you, we are aware that the property will be subdivided for new single-family
homes.
The subject site generally slopes gently downward from east to west. There are short retaining
walls along the west and south property lines, with the neighboring lots being lower than the site.
These neighboring lots are developed with homes.
On October 3, 2017, an engineer from our firm visited the site to assess the subsurface conditions
exposed in test holes excavated in a potential infiltration area located in the front and back yard of
the existing lot. The approximate locations of these two test holes are indicated on the attached
copy of the site plan. Test hole logs are also attached. The northern test hole (Test Hole 1) was
performed in the front yard of the lot and encountered 2.5 feet of loose fill beneath a thin layer of
topsoil. The fill was dry and loose, indicating that it was poorly compacted during the original
grading of the lot. The fill was underlain by native silty sand that was initially loose, but became
dense and intact at a depth of 3 feet. The dense silty sand was fine-grained, and was encountered
to the maximum explored depth of 3.5 feet. The south test hole (Test Hole 2) was excavated in the
backyard of the lot. Test Hole 2 encountered approximately 1.5 feet of loose fill was encountered
beneath a thin layer of topsoil. Loose, native, silty sand with gravel was encountered beneath the
fill and was observed to become dense and intact at a depth of 2.5 feet. The dense silty sand was
encountered to the base of the test hole at 3 feet.
The dense soil encountered in our test holes has been glacially compressed. This soil is mapped
on the Washington Geologic Information Portal (Department of Natural Resources) as Glacial Drift.
Sazei Design Group JN 17532
January 2, 2018 Page 2
The test holes were excavated during the dry season and no evidence of groundwater seepage
was observed. Following extended wet weather, it is not uncommon to encounter subsurface water
perched on top of, and within, the silty glacially -compressed soil.
Conclusions
Based on the results of the test holes and our previous experience in the surrounding area, the site
is underlain by fill soils, beneath which is dense, fine-grained silty sand that is mapped as Glacial
Drift. A seasonal water table is likely present in at least localized areas perched above the dense
silty sand following extended wet weather.
The dense silty sand encountered at the subject site is cemented and is essentially impermeable,
preventing downward percolation of water, which often causes a perched water table to form
following extended heavy rainfall. The dense silty sand encountered in our test holes is very
compact and is comprised of fine-grained sand, with a high silt (fines) content. As a result, there
are no large or continuous pore spaces in the soil that can transmit water. Often, the impermeable
nature of the soil causes a shallow seasonal perched water table to form where the ground surface
is not covered by an impervious layer. This is a common problem in the wet season throughout the
Pacific Northwest.
Infiltration tests conducted in dense, silty, fine-grained soils such as this should yield test rates less
than 0.1 inches per hour.
Considering the observed soil and wet season groundwater conditions, it is our professional opinion
that infiltration of concentrated storm water is not feasible for this site. The silty, fine-grained soils
would rapidly clog, even in a well-maintained infiltration system. Large volumes of water infiltrated
or dispersed into the upper few feet of soil would migrate lateral toward the west and south
retaining walls and the adjacent properties. This would likely result in an increased risk of surface
flooding in the downslope lots surrounding the site, and potentially causing increased subsurface
water in below -grade spaces beneath those houses.
It is our understanding that the City of Renton has expressed a desire for test holes to be at least 4
feet in depth. Our test holes reached refusal in the glacially -compressed soil using hand
implements at depths of 3 to 3.5 feet. While the 4 -foot minimum depth is a good general guideline
to strive for, it is not always necessary, depending largely on the soil and groundwater conditions
that are encountered. As a result, the minimum test hole depth should to be left up to the
geotechnical professional's judgement. In the case of this site, the soils encountered in the test
holes were fine-grained and glacially compressed at depths of 2.5 to 3 feet. The soil beneath this
depth will also have been compressed under the weight of glacial ice, and so it will not become less
dense or more permeable with increasing depth. The test holes were extended as deep as was
practical using hand implements. Excavating deeper test holes would not reveal more permeable
soils, and it would require the use of a backhoe, which would involve extensive site disturbance.
Considering the above discussion, it is our professional opinion that the test holes were conducted
for this study were sufficiently deep to adequately assess the soil and groundwater conditions.
GEOTECH CONSULTANTS, INC.
Sazei Design Group
January 2, 2018
A 17532
Page 3
Please contact us if you have any questions regarding this letter, or if we can be of further service.
Attachments:
• Site Plan
• Test Hole Logs
MKM/MRM: mw
Respectfully submitted,
GEOTECH CONSULTANTS, INC.
- /�
Marc R. McGinnis, P. .
Principal
GEOTECH CONSULTANTS, INC.
Legend:
Test Hole Location
GEOTECH
CONSULTANTS, INC.
SITE EXPLORATION PLAN
1909 Northeast 14th Street
Renton, Washington
Job No:
Date:
Plate:
1
17532
Dec. 2017
No Scale
1
TEST HOLE 1
d:
Test Hole was term inate . I at a depth of 3.5 -feet on October 3, 201 7.,
No groundwater seepage was observed in the. test hole.
TEST HOLE -2
Test. Hole was -terminated at a depth of 3 feet on October 3, 2017.
No groundwater seepage was observed in the test hole.
GEOTECH
CONSULTANTS, INC.
TEST HOLE LOGS
1909 Northeast 14th Street
Renton, Washington
Job No:
Date:
Plate:
17532
1 Dec. 2017
1
1