HomeMy WebLinkAboutBLD1988-00359 Bluff/Geo Evaluation . - NORTHWE•RN TERRITORIES, INC. 1111 RECEIVED
Engineers ■ Land Surveyors ■ Planners OCT 5 1988
-'�... Construction Coordination ■ Materials Testing
NP1,,I JEFFERSON COUNTY
PL.t.T.a!NG &BIDG OEPT
Clan�socutrs
BLUFF SETBACK EVALUATION REPORT
Sections 19 & 30 , Township 30 North , Range 1 West , W . M .
Jefferson County , Washington
Prepared For
MR. DICK WORTLEY , OWNER
Prepared By
CLARK ASSOCIATES , A Division of
NORTHWESTERN TERRITORIES , INC .
August , 1988
717 SOUTH PEABODY, PORT ANGELES, WA 98362 (206) 452-8491 1-800-654-5545
d
411 II!
BLUFF SETBACK EVALUATION
FOR
MR. DICK WORTLEY
BACKGROUND
On July 26 , 1988 , the undersigned , accompanied by Mr . Wortley
and his son , made a field inspection of the subject 5 acre
waterfront property . The lot is described as Parcel 41 of the
survey recorded in Volume 8 of Surveys , Page 126 , records of
Jefferson County , Washington , and is located in Sections 19 and
30 , Township 30 North, Range 1 West , W. M . , Jefferson County ,
Washington .
A profile of the topograpic map through the proposed building
site 100 feet northwesterly of the southeasterly property line
(Figure 1 ) indicates a very steep bluff ( 65° average ) rising
from the shore of Discovery Bay for approximately 90 feet . The
bluff flattens into a "bench area" ( 35° average ) over a
distance of 90 feet , and then rises steeply again ( 50° average )
for another 70 feet for a total vertical rise of 160 feet over
a horizontal distance of about 250 feet . The property then
slopes back over 200 feet at an average slope of 10° forming a
relatively level building site .
Within this profile , near vertical faces were observed along
the top bluff and at the base of the lower bluff . Whereas the
exposed face at the upper bluff is cemented silty sand , loose
stratified sand is more prominent along the lower bluff face at
the beach .
The intermediate "bench area" is generally unaccessible and ,
therefore, only observed from a distance .
STABILITY OF THE BLUFF
For purposes of evaluating the bluff stability for construction
of a single family residence , a somewhat conservative pragmatic
approach is presented based on assumed homogenous granular soil
conditions .
It would appear that the three ( 3 ) zones of geotechnical
interest are : 1 ) the lower bluff along the beach , 2 ) the
intermediate "bench area" and , 3 ) the upper bluff. Reference
to setback distances for conventional footings is from the top
a1
edge of the upper bluff which is 20 to 30 feet west of the
property survey markers in place along the east-west property
lines .
The lower bluff is very unstable and subject to active wave and
tidal action . As such , periodic slumping of large chunks of
loose silty sand can be expected along the shoreline for many
years into the future . If this were to continue at a long term
average rate of one ( 1 ) foot per year , it would take 40 to 50
years for the lower bluff to retreat to a point that the upper
bluff would become subject to the wave and tidal action .
The intermediate "bench area" may be the remains of a larger
land mass that dropped over 80 feet vertically as a result of
major undercutting of its original base over many decades . The
current profile suggests that such an event could occur again
within the next 100 years resulting in the loss of
approximately 50 feet (horizontal ) of the upper bluff area .
Based on this scenario, a 50 foot setback from the upper bluff
edge would provide a stable foundation for the next 100 years ,
with a less than desireable distance from the home to the edge
of the bluff at that point in time .
The upper bluff face appears to be the exposed failure plane
resulting from the dropping of the larger land mass discussed
above . As such , it appears very stable in its present
condition with the "bench area" acting as a "buttress " and
protecting its base from wave and tidal erosion.
From the foregoing , it could be assumed that the "bench area"
will remain in place for the next 40 to 50 years and a setback
distance could be determined on the basis of the stability of
the upper bluff . Assuming the base of the upper bluff at the
point shown on the profile (Figure 1 ) , the "long term stable
angle of repose" ( 30° to 35° ) intercepts the proposed building
site between 60 and 90 feet or an average of 75 feet east of
the bluff edge .
CONCLUSIONS AND RECOMMENDATIONS
1 . Based on the foregoing analysis , the minimum setback for
the proposed home with conventional spread footings, is 75
feet , however , 90 feet or more is highly recommended .
2 . All surface and subsurface drainage should be controlled to
prevent erosion of the bluff face .
` • •
3 . Vegetation should be maintained wherever possible along the
upper bluff area to further enhance the stability of the
soil .
STANDARD DISCLOSURE STATEMENT
This report is based on a limited budget investigation . Only
visual inspection of the existing site conditions and/or
facility was conducted . No scientific measurements , tests or
calculations were performed . The findings and report are
limited to the normal standard of care of the industry for a
limited budget investigation without benefit of scientific data
and calculations . Unless foundation plans are reviewed and the
construction work is inspected by the Engineer , no warranty is
made by the Engineer concerning conformity of the construction
to the conclusions and recommendations of this report .
ERRYNE * °�
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a J . R . Jerry Newlin , PE
soF•.. .? j4 , .•,.j,•', Principal Engineer
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