HomeMy WebLinkAbout502313004 Geotech Assessment
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STRATUM GROUP
1451 Grant Street, Bellingham, W A 98225
Phone (360) 714-9409
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February 27,2006
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John Ewald
P.O. Box 207
Lilliwaup, W A 98555
Re:
Geotechnical Report (Vt\~ O~-13 i', (~"DO(Q- 00 1'34\
Tax Parcel 502313004 I ) )
Forest Drive
Brinnon, Washington
Dear Mr. Ewald:
Stratum Group is pleased to present the results of our geology evaluation of Tax Parcel
502313004 on Forest Drive south of Brinnon, Washington. The purpose of this geology
evaluation was to qualitatively evaluate the risk of slope failures impacting proposed
development on the subject 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.
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. The property is located in the southwest
quarter of Section 31, Township 25N, Range 2 West W,M.
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 ~fthe property.
2) Observed surface soil conditions on the slopes of the subject property and vicinity.
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.
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
February 27,2006
Tax Parcel 502313004, Forest Drive, BriOOD, W A
Geology Hazard Assessment
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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' ~uadrangle. Washington
(Yount, Menard and Dembroff, 1993) indicates that the subject property and vicinity is underlain
by Vashon recessional outwash deposits. The recessional deposits were deposited by melt water
streams from the retreating glacial ice near the end of the last glacial period approximately
13,000 years ago. Observations on the subject property are generally consistent with this
mapping. However, the deposits on the property are more consistent with the site being underlain
by ice-wasting deposits. As the glacier melted it left behind sand and gravel and boulders in a
poorly sorted manner.
SPECIFIC SITE OBSERVATIONS
The subject property is located on a slope above Hood Canal and u.S. Highway 101 on the west
side of Hood Canal. The location of the subject property is indicated on Figure 1, Site Vicinity
Map, A site plan sketch showing pertinent features is provided on Figure 2. The property is
accessed via Forest Drive. Forest Drive bounds both the upper and lower parts of the lot.
However, the proposed building area is on the upper, western side of the lot and is accessed via a
driveway from the upper side of the Forest Drive.
A level building pad area has been graded on the southwest portion of the property. Otherwise
the property slopes down to the east at an angle of 26 degrees or less. Vegetation on the lot
consists of a mix of brush and cut back blackberry vines. The lower east side of the lot is covered
by thick growth of blackberries with a few stands of alders.
At the time of my site visit in late January 2006 there were a few area of minor water seepage
approximately half way down the slope, but otherwise the site was free of standing water and test
pits for septic drain fields were free of water. It should be noted that my site visit coincided with
a period of extremely wet weather.
No areas of past slope movement or landsliding are present on the subject property or on
properties in the immediate vicinity of the subject property.
CONCLUSIONS AND RECOMMENDATIONS
It is my opinion that the subject property is stable and that there are no geologically hazardous
slopes on the site. Furthermore, development of the property will not increase the risk of
landslides or slope movement on the property or elsewhere.
Stratum Group
File: 1.16.058
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It is my understanding that the proposed home may be constructed with a daylight basement. A . " I
daylight basement will be feasible on this site as long as adequate drainage is provided ar~~d--'" - ",j
the daylight basement walls and floor. At the time of our site visit the building area appeared to
be well drained as was no standing water in the proposed building area despite very high rain fall
the morning of my site visit and over the previous month prior to my site visit in late January
2006. I recommend that all roof drains and paved surfaces be directed into a level dispersion
trench located on the lower slope below the home site as indicated on the attached site plan
sketch map. Foundation drainage and access road drainage should be constructed to prevent soil
saturation beneath the foundation. Where possible, the land adjacent to buildings should be
sloped away from them at a minimum 2 percent slope.
February 27, 2006
Tax Parcel 502313004, Forest Drive, Brinon, W A
Geology Hazard Assessment
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Drain systems are recommended around the perimeter of the proposed house, which should
consist of 4-inch perforated pipe surrounded by washed (coarse sand). Structural fill should
consist of clean sandy gravel with less than 5% passing the #200 sieve based on a wet sieve
analysis of that portion passing the Y..-inch sieve (GW by USCS Classification).
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
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Licensed Engineering Geologist
Stratum Group
File: 1.16.05B
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Figure 2
Site Sketch Plan