Earth Solutions Response dated 1.27.16.pdfJanuary 27, 2016
ES -3670.01
Echelbarger Investments, LLC
4001-198 1h Street Southwest, #2
Lynnwood, Washington 98036
Attention: Mr. Todd Echelbarger
Subject: Response to Comments
Proposed Casper Plat
307 Caspers Street
Edmonds, Washington
Reference: Insight Engineering Co.
Road, Drainage and Utility Plan
September 16, 2015
Earth Solutions NW, LLC
Geotechnical Evaluation
ES -3670
December 23, 2014
Dear Mr. Echelbarger:
Earth Solutions NW LLC
• Geotechnical Engineering
Construction Monitoring
• Environmental Sciences
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As requested, Earth Solutions NW, LLC (ESNW) has prepared this letter addressing the City of
Edmonds comments regarding the proposed rain gardens. ESNW prepared the referenced
geotechnical evaluation for the proposed project. The comments pertinent to the geotechnical
aspects of the project are provided below, followed by our responses.
City of Edmonds Comment — The geotechnical report does not contain all the site
characterization criteria found in Volume III, Section 3.3 of the SWMMWW. The design
infiltration rate used in the modeling is from two test pit sample. The location and depth of
these test pits as well as the grain size distribution are not included on the submittal. For
design purposes, soil samples must be collected at the location of the proposed
bioretention/rain gardens at or below the proposed depth of the native soil bioretention soil
interface.
1805 - 13601 Place N F., Suite 201 • Bellevue, WA 98005 • (425) 449-4704 • FAX (425) 449-4711
Echelbarger Investments, LLC ES -3670.01
January 27, 2015 Page 2
Also, Volume II, Section 7.7.2 of the 2005 Ecology Manual states:
A minimum of 3 feet of clearance is necessary between the lowest elevation of the
bioretention soil, or any underlying gravel layer, and the seasonal high groundwater
elevation or other impermeable layer if the area tributary to the rain garden meets or
exceeds any of the following limitations:
o 5,000 square feet of pollution -generating impervious surface; or
o 10,000 square feet of impervious area; or
o 3/ acres of lawn and landscape.
If the tributary area to an individual rain garden does not exceed the areal limitations
above, a minimum of 1 foot of clearance is adequate between the lowest elevation of the
bioretention soil (or any underlying gravel layer) and the seasonal high groundwater
elevation or other impermeable layer.
ESNW Response — ESNW recently conducted five additional test pits within the vicinity of each
proposed rain garden location. The test pits were excavated to depths of five to five and one-
half feet below existing grades. Soil conditions encountered at the test pit locations consisted
of fine sand to slightly gravelly sand (USDA Classification) and were relatively consistent
throughout the depths and locations of the test pit excavations. Based on the soil conditions
encountered at the proposed rain garden depths and locations, we recommend using a long
term infiltration rate of 2.0 in./hr. for design of the rain garden facilities. The referenced
geotechnical evaluation report provided the depth, location, and grain size distribution of the
test pits and soils samples as attachments to the document. The Test Pit Location Plan, Test
Pit Logs, and Grain Size Distribution for all eight test pits excavated on-site are attached to this
document.
Groundwater seepage representing seasonal high conditions was encountered at depths of
three to four feet below existing grades. We understand based on the limited tributary area for
the project, only a one foot separation between the seasonal high level and base of rain
garden will be necessary.
Earth Solutions NW, LLC
Echelbarger Investments, LLC
January 27, 2015
ES -3670.01
Page 3
We trust this response to the City of Edmonds review comments meets your current needs.
Should you have questions, or if additional information is required, please call.
Sincerely,
EARTH SOLUTIONS NW, LLC
Henn T. fright, E.I.T.
Project Engineer
Attachments: Vicinity Map — Plate 1
Test Pit Location Plan — Plate 2
Test Pit Logs
Grain Size Distributions
Raymond A. Coglas, P.E.
Principal
cc: Insight Engineering Co.
Attention: Mr. Santhosh Moolayil (Email only)
Earth Solutions NW, LLC