RESUB1 BLD2022-0561+Geotechnical_Memo+10.19.2022_8.30.22_AM+3172817RECEIVED BLD2022-0561
Oct 19 2022
CITY OF EDMONDS 17311-135th Ave. N.E. Suite A-500
DEVELOPMENT SERVICES
DEPARTMENT Woodinville, WA 98072
NELSON GEOTECHNICAL (425) 486-1669
ASSOCIATES. INC. www.nelsongeotech.com
MEMORANDUM
DATE: August 26, 2022
TO: Shekhar Sharma — seattlens@gmail.com
CC: Site Development Services — Dave Dougherty — davesds49@gmail.com
FROM: Khaled M. Shawish, PE
Thor Christensen, PE
RE: Geotechnical Infiltration Plan Review Memorandum
Sharma Residence
8422 — 238`h Street SW
Edmonds, Washington
NGA File No. 1269821
INTRODUCTION
This memo presents our geotechnical engineering review of the proposed infiltration system for the
proposed residence located at 8422 — 238`h Street SW in Edmonds, Washington. We previously prepared
a geotechnical engineering evaluation for the project dated August 27, 2021.
We have been provided with a grading and drainage plan prepared by Site Development Services dated
)22
April 28, 2022. We have been informed that City of Edmonds requires that a geotechnical engineer review
and approve the use of the proposed stormwater infiltration trenches and overflow.
The provided plan shows that roof runoff from the proposed residence will be directed to a catch basin a
short distance south of the residence. From the catch basin, stormwater will be directed to a series of
three infiltration trenches. The trenches are to have a width of 4 feet and a total length of 121 feet. The
infiltration trenches will be excavated about 2 feet below existing grade, and washed rock will be placed
over the bottom of the trenches. We understand that the 484 square feet of the infiltration trenches was
based on the 2014 DOE Stormwater Management Manual which lists 75 linear feet of 2-foot-wide trench
per 1,000 square feet of roof area for loamy sand. We understand that infiltration testing is not required
provided the infiltration system includes an overflow.
Geotechnical Infiltration System Plan Review Memorandum NGA File No. 1269821
8422 — 2381" Street SW August 26, 2022
Edmonds, Washington Page 2
During heavy rain events, if water is directed to the infiltration system at a rate that exceeds the rate of
infiltration, the water will back up and could eventually flow out of the catch basin and onto the ground
surface. Because the terrain south of the residence declines gently toward the southwest, any water that
emerges from the catch basin would flow in that direction, away from the proposed residence.
Our previous test pit explorations encountered silty sand that was loose to medium dense to depths of 3
to 3.2 feet, and then became medium dense to dense. The silty sand can reasonably be considered loamy
sand. In our previous study we completed infiltration testing at a depth of 4.5 feet. That testing found
that the capacity of the site soil at that depth was very low. However, that finding was for soil at a depth
of 4.5 feet and those soils are much denser and thus have much lower permeability than the loose to
medium dense at the proposed infiltration depth of 2 feet.
In our opinion, the proposed infiltration system, including the catch basin overflow, is suitable for the
proposed development.
We trust this memorandum should satisfy your needs at this time. Please contact us if you have any
questions or require additional services.
NELSON GEOTECHNICAL ASSOCIATES, INC.