HomeMy WebLinkAboutBLD2008-00515 Geotechnical Report _INC. a
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March 18, 2008
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Mr. Jerry Coburn
385 Old Oak Bay Road
Port Hadlock, WA 98339
RE: GEOLOGIC SLOPE STABILITY EVALUATION,385 OLD OAK BAY ROAD,
PORT HADLOCK,WASHINGTON
Dear Mr. Coburn:
At your request, we reviewed your plans for stairs to your beach at the referenced site to evaluate
the potential impact of the stairs on slope stability. This letter provides our opinion regarding the
potential impact on the stability of the steep slope above the beach. We understand that
Jefferson County permit review requires an evaluation of the proposed stairs on the stability of
the steep slope. We evaluated the stability of the slope in 2006 for residential development of
the site. We provide this evaluation for the proposed stairs as an additional service under our
original contract.
Our understanding of the proposed stairs is based on a set of hand-drawn plans and a letter you
sent to us dated January 25, 2008. The drawings and letter show that the stairs will be
approximately 55 feet long and 3 feet wide with a vertical rise of about 30 feet. The bottom half
of the stairs will be supported on 8-inch-square wood posts placed vertically in approximately
3Y2-foot-deep holes excavated on the beach and backfilled with concrete. The upper half of the
stairs will be supported on three pairs of 6-inch-square wood posts placed vertically in
approximately 4-foot-deep holes excavated on the slope and backfilled with concrete. The wood
timbers supporting the stair rungs will be supported on the vertical posts a few inches to a few
feet above the slope surface.
Our evaluation of the stability of this waterfront slope is provided in our letter report dated
June 23, 2006. This letter report describes some potential for both shallow and deep-seated slope
movement, with a higher potential for shallow slope movement and a lesser potential for
21-1-20519-003
400 NORNORTH 34TH STREET-SUITE 100
P.O. BOX 300303
SE:T TLE, WASHINGTON 98103
06.632.8020 FAX 206•695.6777
-DD: 1.800.833.6388
r. w.snannonwiison.corr
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Mr. Jerry Coburn
March 18, 2008
Page 2
deep-seated movement. Based on our previous slope stability evaluation and our understanding
of the proposed stairs, it is our opinion that the proposed stairs will not adversely impact the
stability the slope, provided that the applicable risk reduction measures discussed in our June 23,
2006, letter report are implemented. The applicable risk reduction measures include the
vegetation and erosion mitigation recommendations.
While it is our opinion that the proposed stair will not adversely impact the stability of the slope,
it is also our opinion that the stairs will not significantly improve the stability of the slope. The
landslide potential discussed in our June 23, 2006, letter report remains. Therefore, the owner
must be prepared to accept the risk of future instability(shallow or deep-seated) present on this
slope.
Recommendations for design of the stairs are not included in our scope of services and are not
included in this letter. We would be pleased to provide these additional services if requested.
We appreciate the opportunity to provide geologic engineering services to you, and are available
to answer questions regarding our conclusions in this letter.
Sincerely,
SHANNON & WILSON, INC.
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William Joseph Pc
William J. Perkins, L.E.G.
Associate
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