HomeMy WebLinkAboutBLD2013-00158 Geotechnical Report rY1 t '- lU - i l
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STRATUM GROUP
P.O.Box 2546,Bellingham,WA 98227
Phone(360)714-9409
January 13, 2014
Dennis Shields
Kelley Shields Inc.
260 Kala Point Drive
Port Townsend, WA 98368
Re: Drywell Locations and Geology Hazard
Stormwater
Tax Parcel 976200102
2522 E Marrowstone Road
Nordland, WA
Dear Mr. Shields:
During construction of the new garage on the site the stormwater drainage for the house was
redirected into new drywells installed to the south and north of the home as indicated on the site
plan, and a new dry well was installed to the west of the existing home to infiltrate water from
the garage. The dry wells are located more than 70 feet back from the top edge of the steep
shoreline bluff.The dry wells will not cause any slope stability problems at the bluff face.
The Geologic Map of Northeastern Jefferson County, Washington(Gayer, 1976)and the
Surficial Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region,
Washington(Pessl, Dethier, Booth and Minard, 1989) indicate the upland area of the property is
underlain by glacial till and the steep shoreline bluff exposed on the subject property is underlain
glacial till and advance glacial deposits. Observations on the site and on the bluff face in the
vicinity of the property are consistent with the mapping described above.The upper 10 to 15 feet
of the bluff face and the entire upland area on the property are underlain by very compact glacial
till.The till at this location consists of very poorly sorted sediments ranging from clay to
boulders deposited directly by glacial ice approximately 17,000 years ago when glacial ice
covered the Puget lowlands. Till at this location is somewhat sandy due to the underlying units
being predominantly sand.The lower 50 feet of the bluff consists of compact advance glacial
outwash sand with some very minor silt. There are no perching layers along this reach of the
shoreline bluff. These units were deposited by melt water streams associated with the advancing
glacial ice approximately 18,000 years ago.
The primary slope stability issue at the site is periodic erosion at the toe of the slope from waves
associated with storm surge and high tides.The toe erosion has caused portions of the slope to
become over steep and hence the outer few feet of the bluff will be subject to slope failures as
erosion continues.A very conservatively high erosion rate of 4 inches per year would yield a
setback of 33 feet for a 100 year period. The top of the bluff will not recede at a rate of 4 inches
per year,but will instead stay static for many years before the outer few feet of the bluff will
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January 13,2014
Tax Parcel 976200102,2522 E Marrowstone Road
Nordland,WA
Drywells and Geology Hazard Assessment
collapse onto the slopes below and the erosion and over steepening would then repeat itself.
Based on bluff observations at the site and vicinity over the past 20 years as well as LiDAR
imagery of the area, when top of bluff failures do take place in this geologic setting, up to 5 feet
of the outer edge of the bluff may peal away.
The dry wells are located adequately far back from the bluff slope such that they will not be at
risk from landslides and the infiltration of stormwater at these locations will not increase the risk
of landslides on or off of the subject property.
We appreciate the opportunity to be of service to you. Should you have any questions regarding
please contact our office at(360) 714-9409.
Sincerely yours,
Stratum Group
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
Stratum Group File:1.10.14
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