20-105047-UP-SOILS REPORT-12.29.2020-V1GEOTECH
CONSULTANTS, INC.
Memo
3450 Southwest 320th Street
Federal Way, WA
Client Twin Lakes Plaza, LLC
13256 NE 20t" Street, Suite 16
Bellevue, WA 98005
(425) 747-5618 FAX 747-8561
J N: 16458
Engineer: Marc R. McGinnis, P.E.
Dater November 14, 2016
Email: cbpixtonCa ouaetsoundcommercial.com
Subject: Assessment of Subsurface Infiltration Potential
On November 11, 2016 the undersigned principal engineer visited the subject site to assess the
subsurface conditions for the potential use of on -site infiltration for disposal of storm water. The
vacant lot in the southwestern corner of the Twin Lake Plaza property is to be developed with a new
brew pub. The building would be located on the southern portion of the lot, with a terrace to the west
of the building, and paved parking covering most of the area north and northeast of the building. A
storm bioswale would be located in the southwestern corner of the lot, with a potential storm
infiltration system in the northwestern corner.
The ground on the site slopes gently from northeast to southwest. Along the lower, southern side of
the property is a sidewalk, and then Southwest 320th Street. To the west of the site is the
maintenance yard for the adjacent golf course and driving range.
During our visit, two test holes were excavated in the northwest portion of the lot. These were
conducted near the northeast and southwest corners of the potential storm infiltration area. Beneath
the grass, both test pits found fill soils consisting of medium -dense to dense, gravelly, silty sand.
This fill contained some chunks of asphalt. The fill was 2-1/2 feet thick in the southwestern test pit,
and extended to the 5-1/2 foot depth of the northeastern test pit. Underling the fill in the
southwestern test pit was native, very dense, gravelly, silty sand. This soil is referred to as glacial till.
No groundwater seepage was encountered. However, the test pits were conducted early in the fall,
and the near -surface soils already had an elevated moisture content from recent heavy rainfall. It is
relatively common for a perched groundwater condition to form on top of the essentially impervious
glacial till soils following extended wet weather.
Based on the observed soil conditions, the site subsurface and surface conditions make the use of
subsurface infiltration or dispersion infeasible. The near -surface glacial till soils will not allow
downward percolation of any shallow infiltrated water, which would simply move laterally toward the
sidewalk and street to the south and the maintenance yard to the west. This is the same reason why
dispersion should not be used. Attempting to infiltrate or disperse into the upper soils will adversely
impact the neighboring properties.
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