HomeMy WebLinkAbout821351015 Geotech AssessmentJames B. Scott, P.E. ~ ~ ~ ~~ ~ ~ ~
GEOTECHNICAL CONSULTANT
360-293-6044
FAX 360-293-6044
Email geoscott26~n,comcast.net '~~~~ ~ fwd l~
J. B. SCOTT & ASSOCIATES
An Engineering Corporation
3601 West 5~ Street
Anacortes, WA 98221
October 10, 2005 Project File 05-1249
Mr. Andrew Lesko
c% Waves Waterfront Properties, Inc.
P. O. Box 1668
Port Townsend, WA 98368
RE: Geotechnical evaluation of Parcel 821351015, Tax 12 which is located in the
NE'/. of Sec. 35, T. 28 N., R. 1 E.W.M., Jefferson County, WA
Dear Mr. Lesko:
At the request of Ms. Barbara Blowers, Waves Waterfront Properties, Inc., and
with your authorization, a geotechnical evaluation was made of the subject lot as
shown in Photo 1.
The reason for this evaluation is that conditions exist that is classed as critical, as
defined in the Jefferson County Critical Areas Ordinance. Therefore, a
Geological Hazardous Area Site Assessment will be required by the county
before permits can be issued for development purposes. This report should
satisfy that requirement. The field reconnaissance phase of the evaluation,
accompanied by Ms. Blowers, was made on October 5, 2005. Photographs were
taken to document field conditions.
The subject evaluation consisted of the review of (1) Coastal Zone Atlas of
Washington, volume 11, Jefferson County, Washington Department of Ecology,
1978; (2) Soil Survey of Jefferson County, Washington, USDA Soil Conservation
Service, 1975; (3) Coastal Zone Aerial Photograph JEF-0314, 7/11/94,
PHOTO 1-AERIAL PHOTO OF PROJECT AREA
Department of Ecology, 1994; and (4) the field reconnaissance which consisted
of walking over the near level portion of the parcel and also looking down the
bluff slope toward the shoreline to inspect surtace conditions, slope stability,
attitude of conifer trees, soil and formation exposures, and surface drainage.
SUMMARY AND RECOMMENDTIONS -Based on the Coastal Zone Atlas, as
shown by the blue arrow in Photo 2 the entire general area in the bluff slope
portion of the parcel is classed as ether being Unstable, Old Landslide or is a
Recent Landslide. That means that any setback of any proposed structure from
the bluff must allow for loss of bluff slope material. Yet that classification is not
confirmed by the attitude of existing conifer trees both adjacent to the bluff slope
and down the upper portion of bluff slope as shown Photos 3 & 4.
An average bluff slope angle of 35° was measured, looking down the bluff. In
addition, based on the Coastal Zone Map showing the topography of the bluff, a
slope angle of less than 30° is indicated. This is well below the angle of repose
for the existing formations, both in the upper and lower portions of the bluff slope.
Another factor which will have an effect on slope stability is surface water
drainage. As was noted during the field inspection, all surface water drainage
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PHOTO 2 -STABILITY MAP OF PROJECT AREA
PHOTO 3 -SOUTH ALONG TOP OF BLUFF PHOTO 4 -EAST TOWARD TOP OF BLUFF
will flow from west to east toward the bluff. Also, because of recent upslope
grading, any runoff during a major storm, could result in concentrated surface
flows. This could result in excessive erosion both in the near level area and most
certainly down the bluff slope.
There#ore, to reduce or lessen the effect of erosion, placement of a curtain drain
and/or surface drains is strongly recommended. A standard design for such a
water collection system is attached as Attachment A. Placement of such a
system will increase the existing factor of safety in regard to both slope stability
and erosion.
Soil conditions, based on both exposures and the Soil Survey Map; indicates the
presence of sandy loam (SM). This soil zone will also have a very high
permeability rate to a depth of about 2.0 feet where the soil then grades into (CL)
silt loam and silty clay loam. Temporary perched ground water conditions might
overlay the silt loam and silty clay loam during the winter months. As already
stated, the near surface soil zone because of its sandy nature is quite prone to
erosion, especially on steep slopes.
_ ( ~
PROPOSED STRUCTURE \ \
Scale: 1"~~ 50°-
VIEW LOOKING NORTH
High Tide Level
FIGURE 1 - PROFlLE OF BLUFF SLOPE AND PROPOSED BUILDING SITE
As shown in Figure 1, with the placement of such a system and allowing for a
bluff slope height of 100 feet and the existing slope angle, having a setback of 60
feet from top of the bluff would indicate a slope angle of 28° to a point 10 feet
back from the proposed setback location of any proposed structure. This is 7°
less than the existing slope.
Therefore, the projected slope angle is well below the existing bluff slope, what
would be considered as having a factor of safety of 1.0. Therefore, a setback of
60 feet or more is considered as very reasonable and would allow for a project
life of 100 years or more.
Note: All distances are approximate
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The acxess road down slope to the proposed building site from the County Road
will require some grading and placement of a base course of crushed rock or
gravel.
We appreciate this opportunity to be of service to you. If you have any questions
regarding this report, please contact the undersigned.
Sincerely,
James B. Scott, P.E.
Attachment
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