HomeMy WebLinkAboutRS_Geotechnical_Infiltration_Evaluation_170727_v1July 13, 2017
ES -5048.02
Harbour Homes, LLC
400 North 34th Street, Suite 300
Seattle, Washington 98103
Attention: Mr. Chris Burrus
Subject: Infiltration Evaluation
Cedars at the Highlands
14120 - 160th Avenue Southeast
Renton, Washington
Reference: Earth Solutions NW, LLC
Geotechnical Engineering Study
Cedars at the Highlands
ES -5048, Dated February 23, 2017
Earth Solutions NW, LLC
Infiltration Consultation
Cedars at the Highlands
ES -5048.01, Dated May 16, 2017
Dear Mr. Burrus:
Earth
I Solutions
NWLIC
Earth Solutions NW LLC
• Geotechnical Engineering
• Construction Monitoring
• Environmental Sciences
In accordance with your request, Earth Solutions NW, LLC (ESNW) has prepared this letter
providing a summary of our infiltration evaluation for the proposed residential development at
the above-mentioned address in Renton, Washington.
Project Description
The project area consists of a property located on the east side of 160th Avenue Southeast, and
north of the intersection with Southeast 8th Street in Renton, Washington. The site is
rectangular in shape; and is currently developed with two barns, and associated improvements.
Site plans include the construction of a residential development, and associated improvements,
The feasibility of utilizing infiltration of stormwater runoff from impervious surfaces is being
investigated as a part of the re -development plans for the subject site, as was required by the
City of Renton reviewer.
1805 - 136th Place N.E., Suite 201 0 Bellevue, WA 98005 0 (425) 449-4704 • FAX (425) 449-4711
Harbour Homes, LLC ES -5048.02
July 13, 2017 Page 2
A series of test pits was excavated across the subject site (July 11, 2017) for the purposes of
determining the subsurface conditions on the site, and performing pilot infiltration testing (PIT)
at depths between four to eight feet in accordance with the City of Renton requirements. The
four test pits were advanced to a minimum of ten feet.
Suhsurfacp
ESNW representatives performed in-situ PIT testing, observed, logged and sampled at four test
pit locations across the site.
Topsoil
Topsoil was encountered at the test pit locations, and was observed in thickness of eight
inches. If topsoil is encountered during site grading activities, it is not suitable for use as
structural fill nor should it be mixed with material to be used as structural fill. Topsoil or
otherwise unsuitable material can be used in landscaping areas if desired.
Fill
Fill was encountered at all four of the test pit locations. The fill was observed ranging in
thickness from one to three feet, and consisted of silty sand with gravel (Unified Soil
Classification, SM) in a loose to medium dense condition.
Given fill was encountered in all four test pits, fill is likely present in most areas of the site.
Native Soil
Underlying the topsoil and fill, native soils consisting predominately medium dense to very
dense silty sand with gravel (SM), dense gravel with silt and sand (GM), and dense poorly
graded sand with silt (SP -SM) were encountered extending to the maximum exploration depth
of 11 feet below existing grades.
We observed lenses of silt (ML) in the southeastern portion of the site at test pit location TP -
104. The silt lenses were observed below the gravel with sand and silt soils, and in our opinion
represent a confining layer in regards to infiltration.
Assessment of Infiltration Characteristics
We performed in-situ PIT testing in accordance with the City of Renton requirements at four
locations across the subject site. Three of the test pits (TP -101, TP -102, and TP -103) were
located in areas underlain by glacial till, where no infiltration was observed during our testing
protocol.
Earth Solutions Nw. LLC
Harbour Homes, LLC ES -5048.02
July 13, 2017 Page 3
One of the test pits (TP -104, located in the southeast portion of the site) was observed to
expose gravel with sand and silt (GM) soil below fill soil and native soils consisting of silty sand
(SM). The gravel with silt and sand was encountered at a depth of six feet, and extended to
nine and one-half feet. We preformed infiltration testing at this location and observed limited
amounts of infiltration at a depth of eight feet. However, in advancing the test pit deeper than
the infiltration test depth, we observed a confining layer of silt (ML) in a six inch thickness at a
depth of nine and one-half feet. The confining layer limits the capacity of the underlying
subgrade to infiltrate. Furthermore, stormwater will travel laterally on a confining layer such as
the one observed at test pit TP -104. This lateral migration of water may create flooding related
issues on the neighboring property to the south of the subject site since it is sited lower in
elevation than the subject site.
In our opinion, based on the soil conditions observed at the test sites, and past experience on
similar sites, the subject property is not a good candidate for infiltration of stormwater runoff.
This conclusion is based on the highly cemented nature of the native soils observed within the
potential infiltration locations across the majority of the subject site, the stratified nature of soil
adjacent to the property line, proximity of neighboring structures, and presence of near -surface
groundwater seepage in areas underlain by glacial till soil.
Glacial till material, and the silt lenses observed in the southern portion of the site, represent a
confining layer of soil, which is a limiting factor when investigating stormwater infiltration. In our
opinion, due to the presence of a confining layer and groundwater seepage, alternative
measures should be considered for stormwater management on the subject site.
I imitatinnc
The recommendations and conclusions provided in this geotechnical report are professional
opinions consistent with the level of care and skill that is typical of other members in the
profession currently practicing under similar conditions in this area. A warranty is not
expressed or implied. Variations in the soil and groundwater conditions observed at the test
locations may exist, and may not become evident until construction. ESNW should reevaluate
the conclusions in this geotechnical engineering study if variations are encountered.
Earth Solutions NW, LLC
Harbour Homes, LLC
July 13, 2017
ES -5048.02
Page 4
We trust this information meets you needs. If you have any questions or require additional
information, please call.
Sincerely,
EARTH SOLUTIONS NW, LLC
Stephen H. Avril
Project Geologist
Attachments: Test Pit Logs
Test Pit Location Plan
WAs�
Kyle R. Campbell, P.E.
Principal
Earth Solutions NW, LLC
Test Pit Logs
ES -5048.02
TP -101
0'-1' SM Brown Silty Sand with Gravel, Loose, Moist (Fill)
1'-2.5' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till)
2.5'-11' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till)
TP -102
0'-3' SM Brown Silty Sand with Gravel, Loose, Moist (Fill)
3'-4' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till)
4'-11' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till)
TP -103
0'-8"
TPSL
Topsoil, Loose, Moist
8"-2'
SM
Brown Silty Sand with Gravel, Medium Dense, Moist (Fill)
2'-4'
SM
Brown Silty Sand with Gravel, Dense, Moist (Weathered Till)
4'-11'
SM
Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till)
TP -104
0'-2'
SM
Brown Silty Sand with Gravel, Loose, Moist (Fill)
2'-4'
SM
Brown Silty Sand with Gravel, Dense, Moist (Weathered Till)
4'-6'
SM
Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till)
6'-9.5'
GM
Brown Gravel with Silt and Sand, Dense, Moist
9.5'-10'
ML
Brown Silt, Stiff, Moist
10'-11'
SP -SM
Brown Poorly Graded Sand with Silt and Gravel, Dense, Moist
Earth Solutions NW, LLC
I
�'tPlde Tract B
Workshop Serf .dive Are,
Shed �
I TP -5 TP -2
- t
l4
TP -4
— TP -3 TractA TP -11 — I
— ■ — Storm Drama( )o j
LEGEND
TP -1 Approximate Location of
— — ESNW Test Pit, Proj. No.
ES -5048, Jan. 2017
Subject Site
Existing Building
NOTE: The graphics shown on this plate are not intended for design
purposes or precise scale measurements, but only to illustrate the
approximate test locations relative to the approximate locations of
1 existing and / or proposed site features. The information illustrated
I is largely based on data provided by the client at the time of our
study. ESNW cannot be responsible for subsequent design changes
or interpretation of the data by others.
NOTE: This plate may contain areas of color. ESNW cannot be
responsible for any subsequent misinterpretation of the information
resulting from black & white reproductions of this plate.
0 50 100 200
1"=100' Scale in Feet
Earth
Solutions
NW irc
Test Pit Location Plan
Cedar of the Highlands
Renton, Washington
Drwn. MRS
I Date 02/10/2017
1 Proj. No.
5048
Checked SHA
Date Feb. 2017
Plate
2