HomeMy WebLinkAbout701344049 Geotech Assessment (2006)September 20, 2006
Paul Coover
2545 NE Kevos Pond Drive
Poulsbo, WA 98370
STRATUM GROUP
1451 Grant Street, Bellingham, WA 98225
Phone (360) 714-9409
Re: Geotechnical Report, Addendum
Kennard Lot 2, Tax Parce1701334049
Township 27N, Range 1 W, Section 34
Jefferson County, Washington
Dear Mr. Coover:
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This addendum to my geology hazard assessment of Tax Parcel 70133049 addresses the potential
impacts of the septic drain field on the property to slope stability within a ravine located
northeast. of the subject property. As noted by Jefferson County, the ravine area is an area of
unstable slopes and slope failures took place on Beach Drive along the upper slope of the ravine
in the past few years. Based. on my observations of the ravine and the underlying geology, it is
my opinion that the additional water from the drain field poses no risk to slope stability at the
ravine.
I visited two properties on the lower portions of Beach Drive several years ago. At that time I
inspected the previously failed portion of Beach Drive and did an assessment of the road failure. I
concluded that the failure was the result of poor road drainage, fill soils being placed on a very
steep already potentially unstable slope, soils underlying the road fill area being subject to failure
when saturated, and a major storm event.
Due topography of the area, the location of the septic drainfield on the east side of the subject
property will not cause an increase of water at the area where Beach Drive failed.
The ravine northeast of the subject property is the result of periodic surface water runoff from the
glacial till upland area flowing down into the ravine. The till soils on the upland area are very
compact and resistant to erosion. However, the sandy soils immediately underlying the till are
readily erodable. Once surface water flow drops off the till surface it rapidly incises downward
through the sandy soils. The headwall of the ravine is located east of the subject property across
Beach Road.
Clay soils are present on the lower slopes and the bottom of the ravine that slow the rate of
erosion down such that the downward cutting at the bottom of the ravine is now minimal.
Perched water is present on the silty/clay soil in the bottom of the ravine such that some. water
flow is present for most if not all year.
September 20, 2006
Kennard Lot 2, Dabob Cove, Tax Parcel # 701334049, Jefferson County, WA
Geotechnical Report, Addendum
Surface water flow from the till upland area into the ravine takes place only during conditions
where surface soils become saturated. Typically these events are during rain on snow events or
during very intense rainfall events preceded by extended very wet weather. The contribution of
water from the drainfield to the total amount of water that could potentially impact the catchment
area of the stream above the ravine is less than 1 % of the total water that should be expected to
reach the ground surface for the entire year. In addition, unsaturated flow rates from the
drainfield to the ground water are typically orders of magnitude slower than saturated flow rates.
Hence the contribution of the drainfield during an erosion-causing event is insignificant.
Clearing of trees on the eastern half of the property should be limited to that necessary for
construction of the drain filed and maintenance of the access road to the home site. Native
vegetation under the trees in this area should be left in place. The ability of trees, particularly
evergreens, and native undisturbed soil to intercept and store rainfall greatly reduces the
frequency and peak rates of storm water runoff.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions please contact our office at (360) 714-9409.
Sincerely yours,
Str um Group
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
Stratum Group
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File: 8.S.OIBII
2
reen Mountain
eoServices, LLC
~=P.O.-Box 823
~Seabeck, WA 98380
~; ;~Phone:360.830.5612
Job number: 05-002
Date: February q11, 2005
Client: Paul & Jan Coover
Parcel Id Number(s): 701344049
Acreage: 5 acres total
Data source: PSLC LIDAR
LIDAR Aquisition Date: 2002
Vertical resolution: 1ft.
Horizontal resolution: 12 ft.
Rotation angle: 90
0 25 50 100
Elevation Contours
Contour interval: 2 feet
-- Intermediate contour
Index contour
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