HomeMy WebLinkAboutGEO Slope Assessment - GEO General - 6/1/1993 r
Bradley-Noble Geotechnical Company RECEIVED
Soils Engineers and Geologists FEB 2 2 2016
PL Testing and Inspecting Services
615W. Alder Street
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1 June 1993
Mariette Homes
8411 Granite Drive N.W.
Gig Harbor, Washington 98329
Subject: Evaluation of slope stability and
recommended setback for the proposed
residence, Lot 7, Tax Parcel Number
12108-75-90032, Island Crest Road,
Stretch Island, Mason County,
Washington.
Gentleman:
As requested, we have conducted a reconnaissance
survey of the marine bluff and upland area at the
above site. This letter will present our opinions as
to the stability of the present slope, the erosional
agents actively working on the slope, and a minimum
setback from the bluff for the proposed construction.
The marine bluff at this site has estimated vertical
relief of about 50 feet. The bluff is steep, with
estimated slope angles in the 60 degree range.
Portions of the bluff face have nearly vertical
relief. Cropping out in the bluff face are soils that
we interpret as being Vashon advance outwash
deposits. These soils are primarily sands generally
referred to as either Clovis or Esperance sands.
Estimated maximum thickness of the advance outwash
unit is about 200 feet. Area geology is discussed in
the Geology of Southeastern Mason County, Washington
by John B. Noble and Dee Molenarr, Water-Supply
Bulletin 29, Plate 1, and the Coastal Zone Atlas Mason
County Washington, Map MA 13. In the Coastal Zone
Atlas, the slopes and areas immediately adjacent to
the slope are considered to be unstable, while the
upland areas behind the slope are considered stable.
Our impression of the site concurs with these
findings.
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2401 Bristol Court S.W. • P.O. Box 10267 Olympia, WA 88502 • Phone(206)357-7883
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The most active erosional agents working on the bluff
are freeze-thaw cycles resulting in ice plucking, ice
wedging during periods of prolonged subfreezing
weather, and sheet washing of loosened soils by rain.
Recent debris falls have carried vegetation from the
bluff and the edge of the upland area to the beach.
we found trees and vegetation with parts of their root
balls unsupported and overhanging at the top of the
bluff because of loss of soil support by erosion. We
expect that erosion of this bluff is predominantly by
small debris falls and sloughing of loosened soils
from the bluff face. We did not observe any
indication that major slumping, sliding, or mass
wasting events occur in this area. Generally, we
consider the erosion of the marine bluff at this site
to be slow.
Placement of the new residence no closer than 40 feet
from the present top of the bluff should result in a
useful life-span for the structure of at least 40
years. This estimate is based on the rate of erosion
observed not accelerating over the life of the
structure. Placement of low value structures such as
decks is permissible between the residence and the top
of the bluff.
Control of storm water from impervious surfaces is
required by the owner. Disposal of storm water by use
of dry wells between the residence and the bluff
should not be permitted. Dry wells result in point
discharge of water, creating area of localized
saturation with the risk of inducing slope
instability. We recommend that all storm water from
impervious surface be collected and disposed of in the
natural drainage areas on the south and east sides of
the property. Preventing water from being directed
towards the bluff will improve slope stability and
lessen erosion caused by concentrated water flows.
You can expect that continued small debris falls and
sloughing of soils from the bluff face will continue
on a random basis. These debris falls will typically
be of small volume, generally a few 10's of cubic
meters or less. Between events, continued slow
erosion rates and apparent slope stability will
develop.
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At the recommended setback from the present top of the
bluff, we expect that the structure will be safe from
soil movements affecting foundation support for at
least 40 years. Prediction of erosion rates is not a
quantifiable science; rates are estimated based on
observed rates in similar soils and exposures.
Construction adjacent to marine bluffs always involves
some risk of loss by mass wasting. If the general
recommendations of this report are included in the
design, we expect the risk from construction activity
to be slight. As long as the owner is aware that
risks do exist for construction adjacent to the
bluffs, we have no objection to the proposed
construction at the recommended minimum setback.
If you have any questions, or if we may be of
additional service to you, please contact us at our
Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL COMPANY
David C. Strong
Engineering Geologist