HomeMy WebLinkAboutBLD2008-00040 Geotechnical Report 0.�
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STRATUM GROUP
1451 Grant Street,Bellingham, WA 98225 E ES 2 0 2008
Phone(360)714-9409
_January 8, 2008
Dan Carlson
2019 NE 179th Street
Apartment Z113
Ridgefield, WA 98642
Re: Geology Hazard Assessment
120 Rocky Point Road
Tax Parcel 502294010
Brinnon, Washington
Dear Mr. Carlson:
Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel
502294010 at 120 Rocky Pont Road south of Brinnon, Washington. The purpose of this geology
evaluation was to evaluate the risk of slope failures impacting proposed development on the
property and to provide general site maintenance recommendations for the property. This
evaluation included a visual inspection of the property and vicinity and review of available
geologic mapping in the area. A home is already located on the property. A garage is proposed to
be constructed on the property upslope from the home on an existing level area.
The property is located on the east side of the Olympic Peninsula on a slope above Hood Canal.
The location of the property is indicated on Figure 1.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including slope
conditions and relevant conditions in the vicinity of the property.
2) Observed surface soil conditions on the slopes and on the uplands above the
slopes.
3) Prepared this report summarizing our findings, including a qualitative evaluation
of the stability in regards to the proposed garage and residence on the property,
recommendations for site development, and recommendations for further
investigation, if necessary.
•
t 2 0 2008
January 8,2008
Tax Parcel 502294010,Brinon,WA
Geology Assessment .r
GENERAL GEOLOGY
Northwestern Washington has been occupied by continental glaciers at least four times during
the Pleistocene Epoch(1.6 million to 10,000 years ago). During these glacial and accompanying
interglacial periods,the underlying bedrock was deeply eroded. Glacial related deposits were
deposited over the bedrock in various places on and in the vicinity of the subject property.
The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington
(Yount, Menard and Dembroff, 1993) indicates that the subject property and vicinity is underlain
by bedrock. Observations on the subject property are consistent with this mapping. The Bedrock
Geologic Map of the Seattle 30' by 60' Quadrangle, Washington(Yount and Gower, 1991)
indicates that mapped bedrock in the vicinity is basalt of the lower Eocene Crescent Formation.
The property is underlain by massive reddish brown to black basalt that has been very slightly
metamorphosed. Pillow structures are apparent within the basalt exposed on the property.
SPECIFIC SITE OBSERVATIONS
The subject property is located on a slope above Hood Canal between U.S. Highway 101 and the
water on the west side of Hood Canal. The property is access via a gravel road that is part of the
access to several other properties west of the subject property.
A home is already present on the property. A garage had been formerly located on the level area
at the end of the road. The proposed garage foot print has been shifted from the original garage
site. The level area for the proposed garage is bounded on the south by a reinforced concrete
retaining wall downslope of the proposed garage location to support the level area and maintain
the septic tank location below. A cut slope is located on the northwest side of the level building
area.
The slope above the level garage area slopes up towards Highway 101 at an angle of 30 degrees
and is approximately 20 feet high. The top of the slope is Highway 101 itself. The slope is tree-
covered with straight Douglas fir trees, a few madrone trees and a thick understory of salal and
evergreen huckleberry. The lower part of the slope has been cut with the cut slope approximately
5 to 8 feet high. The cut slope is cut into bedrock with the exception of the uppermost topsoil
layer that is approximately 2 feet thick. The bedrock is hard and competent with the exception of
minor fracturing on the upper part of the exposed bedrock.
Stratum Group File: 12.20.07
2
• •
January 8,2008
Tax Parcel 502294010,Brinon,WA
Geology Assessment
CONCLUSIONS AND RECOMMENDATIONS
It is my opinion that proposed garage building site is not located within a landslide hazard area.
The slope above the level building area is stable and the cut into bedrock at the base of the slope
will not cause any slope stability problems. Beyond minor raveling of occasional loose material
on the cut slope, the slope and the cut slope are stable.
Based on my assessment of the geology conditions on the slope,the proposed garage site and the
slope above the garage site are not located in a geologic hazardous area. No set back is required
from the base of the slope. However, since the base of the slope is a bedrock cut slope and some
fractures are present in the rock, the garage should be located 8 feet from the base of the cut slope
unless a retaining structure is built on the slope or the wallof the garage is designed to support
the weight of up to 4 feet of soil. i ~
Stratum Group appreciates 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 .0 _ ._.„hlry~
47
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Dan McShane, L.E.G., M.Sc. (.1, ;t:
Licensed Engineering Geologist `, do \ ,-:, i'.,,
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TDanl 'McShane
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FEE 2 0 2008
Stratum Group Pile: 1210.07
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