HomeMy WebLinkAbout601061014 Geotech Assessment~ I
i T
STRATUM GROUP
PO Box 2546, Bellingham, WA 98227
Phone (360) 714-9409
September 15, 2007
Robert Bindschadler
12212 Apache Tears Circle
Laurel, MD 20708
Re: Geology Evaluation, Shoreline Bluff
Gustavson Road Lot
Parce1601061014
Quilcene, Washington
Dear Mr. Bindschadler:
We are pleased to present the results of our geology evaluation of the above referenced property
on a shoreline bluff. The purpose of this geology evaluation was to 1) determine the suitability of
the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3)
provide general site development and maintenance recommendations for development of the
property adjacent to a potentially unstable bluff. This evaluation was limited to a visual
inspection of the property and vicinity, a visual inspection of-the bluff face, review of available
geologic mapping in the area, and inspection. of the shoreline at the base of the bluff..
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including the bluff
face slope conditions, shoreline conditions, and relevant conditions in the vicinity of the
property.
2) Observed surface soil conditions on the bluff face and on the uplands above the
bluff by excavating shallow hand dug test pits.
3) Prepared this report summarizing our findings, including an evaluation of the
feasibility of building a residence on the subject property, a qualitative evaluation of the
shoreline bluff stability, 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
September 15, 2007
Parcel 6011061014, Gustavsen Road, Jefferson County, WA
Geology Evaluation, Shoreline Bluff
interglacial periods, the underlying bedrock was eroded and glacial related and interglacial fluvial
sediments were deposited over the underlying bedrock in the vicinity of the subject property.
The Geolo ig c Ma_p of Surficial Deposits in the Seattle 30' b~60' Quadrangle, Washington
(Yount, Minard, and Dembroff 1993) indicate the upland portion of the property and vicinity is
underlain by glacial till and that the steep shoreline bluff exposed on the subject property is
underlain undifferentiated Pleistocene deposits. The Geolo ig c Map of East-Central Jefferson
County (Birdseye, 1976) indicates that the steep bluff is underlain by Double Bluff Till at the
base of the bluff, Pre-Vashon Stratified sediments on most of the bluff, and that the top of the
bluff is underlain by Vashon Till.
Observations on the bluff face on the subject property and on the bluff face in the vicinity of the
property are consistent with the mapping described above. The steep bluff face is underlain by
very compact glacial till and very compact sand, silt and gravel layers.
SPECIFIC SITE OBSERVATIONS
A site location map is provided on Figure 1 and a general site plan sketch is provided on Figure
2. The property consists of a very gently sloping upland area that is bounded on the south by a
steep south facing shoreline bluff. The upland portion slopes gently towards the south and is
covered primarily with second growth Douglas fir with an understory of salal and bracken fern as
well as other brush. The property is accessed from Gustavson Road which bounds the north side
of the property.
The overall slope of the steep shoreline bluff averages approximately SO degrees; however,
portions of the slope are steeper. The total height of thebluff is approximately 200 feet. The bluff
slope is covered with a mix of mature Douglas fir, relatively young trees, brush covered areas
and a few areas with very little vegetation. Note that there are several thick patches of poison oak
on the bluff face and at the top edge of the bluff.
Essentially, the entire bluff face is underlain by very compact silty, sandy gravel and gravelly
sand. Very compact glacial till is present at the base of the bluff.
The bluff is eroded primarily by wave action that undermines the base of the bluff. The shoreline
at the site is exposed to a fairly long fetch of open water to the south such that south winds .will
drive waves against the base of the bluff. Hence south winds coinciding with high tides will
generate large waves that erode the base of the bluff. Because the bluff is underlain by very
compacted soils the apparent erosion rate is low. Evidence of past landslides is readily apparent
throughout the bluff slope. Based on the age of trees and vegetation, it appears that slides have
taken place on portions of the bluff within the last few years. However, older slide scarps are
Stratum Uroup File: 7.1.07
September l5, 2007
Parcel 6011061014, Gustavsen Road, Jefferson County, WA
Geology Evaluation, Shoreline Bluff
apparent as well. All slope failures observed appeared to be shallow slab-type failures or very
small circular failures none of which exceeded 10 feet of depth. The cause of the slides is
oversteepening of the slope by the undermining of the base of the slope by wave action. Once the
base of the slope fails, the upper, slopes loose support and the slides progresses through a series
of shallow soil failures to the top. of the bluff.
The recurrence interval or length of time between slab failures at any given location on the bluff
face appears to be on the -order of 50 to 100 years or more as a number of mature trees are present
on the bluff slope at various locations..
No beach armoring is present in the vicinity of the property and the beach appears to be in good
condition. That is, the level of the beach appears to be such that downward erosion is minimized
by the erosion of the entire bluff along this section of shoreline.
We did not observe any evidence (tension cracks or trees rotated inward away from the top of the
bluff or indications of past uplift of the beach area) indicating an incipient large-scale global-type
or deep-seated failure on the subject property. Slope failures appear to be limited to fairly
shallow failures of upper soil as the base of the bluff is eroded by wave action.
CONCLUSIONS AND RECOMMENDATIONS
Based on my inspection of the subject property and vicinity, I conclude that a residence can be
located on the upland portion of the property such that bluff erosion or landsliding would not
threaten the structure within the expected life of the structure (100 years) as long as my
recommendations are followed.
The residence should be located at .least 50 feet from the top of the shoreline bluff as indicated in
the Site Sketch Map (Figure 2). Native vegetation should be maintained within the setback
buffer.
I do not anticipate that the development of the subject property will cause any negative impacts
on the stability of the slopes or the shoreline bluff as long as my recommendations are followed,
Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic
basis. The only way to prevent continued erosion of the base of the bluff is to construct hard
armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder
bluff (erosion of the bluff provides sediment) for the beaches east and west of the subject
property, and any shoreline armoring will have a negative impact on properties down drift from
the bluff. The construction of shoreline armoring on other properties may cause an increase of
erosion of the subject property. The owner of the subject property should contact Jefferson
Stratum Group File: 7.1.07
September 15, 2007
Parcel 6011061014, Gustavsen Road, Jefferson County, WA
Geology Evaluation, Shoreline Bluff
County officials regarding any proposed shoreline protection projects along the bluff or any
shoreline protection. construction. There are currently no shoreline-armored areas along this
stretch of coastline.
Soils on the upland portion of the property appear to be relatively well drained, and therefore we
do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm
water catch basins must not be introduced into the perimeter footing drain. I recommend that roof
drains and any other drainage be discharged to a water infiltration system that will- distribute
water into the ground at the site. All infiltration systems should be set back at least 50 feet from
the top of the bluff slope. The access road to the building site should be graded such that storm
water run off is directed to the forest floor instead of all being channeled to the same discharge
point.
Site grading soils or debris, landscape debris, or any other material should not be disposed of
over the bluff face or placed at the top of steep bluff.
Trees on the bluff face and within the setback buffer may be limbed or trimmed for view
purposes as long as limbs and debris generated is removed from the bluff face and the top of the
bluff. No trees greater than 10-inches in diameter at chest height should be removed from the
native vegetation buffer.
The septic drain field should not cause problems for slope stability as long as it is setback at least
50 .feet from the top of the steep bluff slope.
It is possible to construct a trail down the bluff slope to the beach. In order to construct a route to
the shoreline that is more user friendly. and in less need of routine maintenance, a series of wood
ladder type stairs can be used. This approach will minimize the need to cut into the slope and will
have essentially no impact on the stability of the slope. If shallow soil failures do impact the
slope and the ladder stairs, the stairs can be readily readjusted. Each end of the ladder stairs can
be supported either by a wood landing supported on wood piles set in the slope or the ends of the
ladder can be supported directly by wood piles or pins embedded in the slope. The deeper the
pilings the less likely. shallow surface failures will impact the piles. and stairs.
Based on slope aspects, the best route down the slope would be located on the west side of the
property. The slope is slightly less steep in this area and no shallow failures are present on this
portion. of the slope.
The landing at the base of the bluff can be constructed on a deck like platform on woodpiles at the
bottom of the slope immediately above the ordinary high-water mark. Wood stair steps to the beach
from the platform will minimize erosion from foot traffic.
Stratum Group File: 7.1.07
September 15, 2007
Parcel 6011061014, Gustavsen Road, Jefferson County, WA
Geology Evaluation, Shoreline Bluff
Please note that there are inherent risks associated with building on lots near or adjacent to steep
slopes. These are risks that the building owner should recognize and be willing to accept. If
conditions appear different than those described in this report, or other concerns arise, we request
that we be notified so we can review those areas and modify our recommendations as required.
We appreciate the opportunity to be of service to you. Should you have any questions regarding
our reconnaissance please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
i
Dan McShane, M.S., L.E.G.
Licensed Engineering Geologist
Stratum Group File: 7.1.07
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