DNS BLD2021-0812+Geotechnical_Report+7.23.2021_8.59.55_AM+2317876LL:n
COBALT
GEOSCIENCES
June 25, 2019
Mr. Luke Shamp
Luke (cry stoneandwindllc.com
RE: Plan Review Letter with Comment Responses
Proposed Residential Development
79xx — 240th Street SW
Edmonds, Washington
Dear Mr. Shamp,
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
present responses to City of Edmonds comments, a discussion of plan review, and additional
geotechnical recommendations.
In preparation of this letter, we have reviewed the following documents:
• Civil Plans prepared by ICEO dated September 6, 2oi8
• City comments letter dated May 1, 2019
Plan Review & Comments
We have reviewed the civil plans showing the location and anticipated depth of an infiltration
gallery. The plans indicate a system that will be 22 feet by 35 feet in area with a depth of 15 feet
below grade (320 feet base elevation). The plans show the location of the system to be between
Lot's 2 and 3 and in the eastern portion of the property.
The planned depth of 320 feet below grade is anticipated to be at or near the contact between
glacial till and advance outwash; however, since this contact is not always horizontal, it may be
necessary to locally overexcavate or remove and replace any till -like soils at the base of the
excavation. These materials may be replaced with washed rock or structural fill with less than 5
percent fines by weight and a maximum grain size of 3 inches.
The geotechnical engineer must verify soil conditions at the base of the infiltration system. This
should include at least one large-scale Pilot Infiltration Test (PIT) to confirm the anticipated
infiltration rate.
Comment Letter Responses
The following items are paraphrased geotechnical-related comments from the City letter followed
by our discussions and recommendations.
Comment discussing infiltration rate determination for wells and/or flat surfaces (infiltration
basin).
www.cobaltgeo.com (2o6) 331-1097
June 25, 2019
Page 2 of 3
Plan Review and Comment Responses
The infiltration rate of 20 gallons per minute (gpm) for injection wells was determined through
field testing. The procedure included inflow of clean water into the 2-inch diameter well with
maintenance of the water level halfway up the screened zone of the well (approximate elevation
295.5 feet) for 1.5 hours; maintenance of the water level at the top of the screen (approximate
elevation 305.5 feet) for about 1.5 hours; and, increasing the rate to maintain a groundwater level
about 5 feet below grade for an additional I to 1.5 hours.
The 20 gpm rate for an anticipated 6 inch diameter well was calculated based on the volume
increase from the test well diameter (2 inches) to the 6 inch diameter production well from the
steady-state inflow rate determined through the testing. It is our understanding that injection
wells will not be used at this site.
The proposed system is a more typical shallow infiltration gallery, although the depth is greater
than a typical 2 to 6 feet. For the proposed gallery system, we recommended a preliminary
infiltration rate of 2 inches per hour in our report.
This rate was based on a combination of the falling head portion of the injection analysis (over the
24 hour period after testing) in conjunction with grain size distributions (sieve analyses) and the
logarithmic equation in the Department of Ecology manual. This equation is not recommended
for glacially consolidated soils; therefore, it was only a partial factor in the infiltration rate. The
results of the sieve analysis correlations are attached with this letter and yielded long term rates of
10.4 to 15.6 inches per hour with applied. correction factors for site variability (0.33), testing
(0.4), and influent control (o.9) .
Again, these rates were a minor factor in the preliminary rate determination. Typically,
infiltration rates in advance outwash vary from 1 to 4 inches per hour in most areas, which is
much lower than the spreadsheet values. This is primarily due to the typically high density of the
outwash.
While we anticipate that the infiltration rate in advance outwash will be at or above 2 inches per
hour, we do recommend in situ testing utilizing a full scale Pilot Infiltration Test (PIT) prior to
completion of the infiltration system. In other words, a PIT should be completed on the
excavated subgrade soils prior to system construction. If the PIT yields a rate lower than 2 inches
per hour, the size of the system may need to be increased.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site.
www.cobaltgeo.com (2o6) 331-1097
June 25, 2019
Page 3 of 3
Plan Review and Comment Responses
Sincerely,
Cobalt Geosciences, LLC
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Exp 6-26-2020
Phil Haberman, PE, LG, LEG
Principal
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www.cobaltgeo.com (2o6) 331-1097
Grain Size Analysis Infiltration Estimation
Soil Grain Size Analysis Method (Stormwater Management Manual for Western Washington Vol III, pgs
3-79 and 3-82, Aug 2012):
Table 331
loglo(K.tdr)=-1.57+1.90DIo +0.015D60-0.013D90-2.08ff�
Where, Dio, D60 and Dyo are the grain sizes in mm for which 10 percent,
60 percent and 90 percent of the sample is more fine and ffi„., is the
fraction of the soil (by weight) that passes the niunber-200 sieve ((K..is in
cm/s).
Total Correction Factor, CFT = CFr x CF, x CF.
CFT is used in step 5 of the Design of Infiltration Facilities (Section 33.4)
to adjust the measured (initial) saturated hydraulic conductivity.
K:aa::;Lnd>=K..atX CFT
Long Term
Log10 Correction Infiltration
D10 D60 D90 Fines (Ksat) K(sat) K(sat) CF(v) CF(t) CF(m) Factor (total) Rate
(cm/s) (in/h) (in/h)
B-2 30 0.224 0.4 0.7 3.0%-1.2093 0.062 87.53 0.33 0.40 0.90 0.1188 10.4
B-2 35 0.3 0.72 5.0 1.0% -1.075 0.084 119.25 0.33 0.40 0.90 0.1188 14.2
B-2 40 0.28 0.718 1.8 3.0%-1.11238 0.077 109.42 0.33 0.40 0.90 0.12 13.0
B-2 50 0.3 0.565 0.9 1.5%-1.034425 0.092 130.93 0.33 0.40 0.90 0.1188 15.6
..
Correction Factors to be Used With In -Situ Saturated Hydraulic Conductivity
Measurements to Estimate Design Rates.
Partial Correction Factor
Issue
Sire a�atiabilin� and aovtber of loca[wnz tested CF = 0.33 to 1.0
Test Method
PIT CF, = 0-75
Small-scale Pir = 0.50
Orber small-scale (e I!. Double rmg.=alltng head) = 0 40
Gram Size Method = 0-i0
Degree of itdloem control to pretest silEanon and bio- CFn = 0.9
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