HomeMy WebLinkAbout821265007 Geotech Assessment~~b ~ ~~
STRATUM GROUP
1451 Grant Street, Bellingham, WA 98225
Phone (360) 714-9409
October 14, 2005
Rob Scott
3549 Sterne Street
San Diego, CA 92106
Re: Geology Hazard Evaluation, Shoreline Bluff
Tax Parce1821265007
Port Ludlow, WA
Dear Mr. Scott:
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Stratum Group is pleased to present the results of our geology hazard evaluation of the above
referenced property on a shoreline bluff. The purpose of this geology evaluation was to
qualitatively evaluate the risk of slope failures impacting the proposed residence on the subject
property and provide general site development and maintenance recommendations. This
evaluation included a visual inspection of the property and vicinity, a visual inspection of the
slopes on the property and vicinity, review of available geologic mapping in the area, and
inspection of the shoreline at the base of the bluff.
The property is located on an approximately 60-foot high bluff on the west side of Hood Canal.
The location of the property is indicated on Figure 1. The property is located in the northeast
quarter of Section 26, Township 28N, Range 1 east 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 the slope
conditions, shoreline conditions, and relevant conditions in the vicinity of the
property.
2) Observed surface soil conditions on the slope and on the uplands above the slope.
3) Prepared this report summarizing our findings, including a qualitative evaluation
of the slope stability in regards to the proposed residence on the property,
recommendations for site development, and recommendations for further
investigation, if necessary.
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October 14, 2005
Parcel #82165007, White Rock Lane, Jefferson County, WA
Geologic Evaluation of Shoreline Bluff
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 eroded and a relatively thick layer of glacial
related and interglacial fluvial sediments were deposited over the underlying bedrock in the
vicinity of the subject property.
The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle. Washinston
(Yount, Minard, and Dembroff, 1993) and the Geologic Map of the Uncas-Port Ludlow Area,
Jefferson County, Washington (Hanson, 1976) indicate the subject property is underlain by
Vashon glacial till and Vashon advance outwash deposits. The till consists of nonsorted,
nostratified clay, silt, sand and gravel up to boulder size. The till was deposited directly by glacial
ice. The advance outwash deposits consist of clay to gravel stratified deposits deposited by
meltwater from the advancing glaciers at the start of the ice age.
Observations on the subject property, slopes in the vicinity of the property and upland areas
above the subject property are consistent with the mapping described above. Till deposits are
present underlying the uppermost portion of the bluff and the level to gentle sloping northeast
portion of the upland area. The southwest portion of the upland area is underlain by sand and
gravel and is mapped as glacial advance outwash.
Sediments exposed below the glacial till on the steep shoreline bluff slope consist of very dense
sand and gravel which is in turn underlain by very compact layered silt and clay. The sediments
below the till are consistent with the bluff being underlain by glacial advance outwash, as the
sediment becomes coarser upward. Some of the layered silts and clays may be older preglacial
deposits.
The Slope Stability Map of the Uncas-Port Ludlow Area Jefferson County, Washington (Hanson,
1976) map indicates that the bluff slopes in the vicinity of the subject property are both former
landslide areas and known areas of recently active rapid downslope movement. The upland area
of the property as an area believed to be stable.
Observations confirm that there are potential unstable slopes and areas of recent slope failures on
the steep shoreline bluff on the subject property and the vicinity. The shoreline at the base of the
bluff is erosional and thus the slopes above the shore are potentially unstable and in some places
are unstable.
Stratum Group File: 12.30.04C
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October 14, 2005 `~ ~' ~ '
Parcel #82165007, White Rock Lane, Jefferson County, WA
Geologic Evaluation of Shoreline Bluff
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SPECIFIC SITE OBSERVATIONS g ~
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A sketch map and cross section of the property with pertinent features indicated are provided in
Figures 2 and 3. The property is crossed by two roads, White Rock Lane and Paradise Bay Road.
The building site on the northeast portion of the property is accessed via a gravel driveway off of
White Rock Lane. The building site area is located on a level to very gently sloping area adjacent
to and above the steep shoreline bluff slope on the northeast side of the property. The width of
the very gentle sloping to level area is approximately 188 feet. This area is mostly grass-covered
with a few scattered trees and areas of brush. The upland area of the property continues to gently
slope up above this area to the southwest. The level to gently sloping upland area is underlain by
glacial till.
The steep shoreline bluff is .approximately 60 feet high. The average slope of the bluff on the
subject property is approximately 65 degrees. However, portions of the bluff are very steep and
in some areas are vertical. The uppermost portion of the bluff is underlain by glacial till. The
approximate middle third of the bluff is underlain by very dense sand and gravel and the lower
portion of the bluff is underlain by very compact silt and clay. The slope is covered with a mix of
trees and brush with alders being the predominant species. A few areas of bare soil are present on
the close to vertical sections of the slope.
Past shallow soil failures are evident on the steep shoreline slope of the subject property and on
the shoreline bluff in the vicinity. No deep-seated large rotational slides are evident on the
property or on any other properties in the vicinity.
The beach at the base of the bluff consists of a wave cut platform beach on the underlying very
hard clay covered with a mix of cobbles and boulders eroded from the past failures of till soils
from the top of the bluff. White Rock, is a large glacial erratic boulder just off the shoreline from
the subject property. The beach conditions along the section of shoreline are in a fairly natural
condition with only a few properties having shoreline armoring.
Several relatively recent landslides are apparent along the shoreline bluff north of the property.
All the slides appear to be primarily the result of erosion at the base of the bluff from wave
action. As the bluff becomes oversteepened the lower slopes fail first and overtime the slope
failure progresses up the slope to the top of the bluff. All the landslides observed appear to be
shallow slab type failures involving only a few feet thick sections of soil.
The toe of the bluff is underlain by very compact silt and clay that is fairly resistant to erosion.
Wave erosion at the toe of the slope is limited to periods of very high tide combined with storm
events. Wave size is fairly limited on this section of coast as the site is located in a fairly well
protected area of the northern part of Hood Canal. Hence the shore in not open to any long fetch
Stratum Group File: 12.30.04C
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October 14, 2005
Parcel #82165007, White Rock Lane, Jefferson County, WA
Geologic Evaluation of Shoreline Bluff
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that would generate large waves. The only appreciable fetch for wave generation is from the
north.
CONCLUSIONS AND RECOMMENDATIONS
Based on my visual inspection of the subject property and vicinity, I conclude that a residence
can be located on the gentle sloping upland area as indicated in the site plan sketch. A residence
located at this site should not be at risk from bluff erosion or landsliding that would threaten the
structure within the expected life of the structure (75 years) as long as our recommendations are
followed.
The residence should be located at least 40 feet from the top of the shoreline bluff as indicated in
the Site Sketch Map (Figure 2). This distance will be adequate to provide protection for a
residence for at least 75 years. Please note that this setback distance may be precluded by
shoreline setback regulations, which may require a greater setback. Development of the subject
property will not cause any negative impacts on the stability of the slopes or the shoreline bluff as
long as our recommendations are followed.
A deck may be constructed within the setback from the top of the steep slope as long as it is not
an integral part of the structure of the residence. That is if a slope failure occurs, the deck will
detach from the house.
No debris should be placed on the top of the bluff. Thinning and trimming of trees on the bluff
slope and within the setback area should be limited to topping of alders and maples and limbing
for view corridors of Douglas fir and cedar. Any debris from thinning on the bluff slope itself
should be removed from the slope.
Storm water run off from the roof gutters should not be routed towards the bluff and should be
infiltrated into the ground in a dispersed manner either north, west or south of the building. This
site will qualify as full dispersion site as long as at least 65% of the tree canopy on the upland
portions of the property is left in place.
The septic drain field should not be located within the setback area. The addition of water from
the drainfield outside the setback will have minimal impact on the hydrology of the site and will
not pose a risk of causing slope stability problems.
CLOSURE
Please note that there are inherent risks associated with lots near or adjacent to steep slopes.
These are risks that the building owner should recognize and be willing to accept. If conditions
Stratum Group
File: 12.30.04C
4
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October 14, 2005
Parcel #82165007, White Rock Lane, Jefferson County, WA
Geologic Evaluation of Shoreline Bluff
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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
please contact our office at {360) 714-9409.
Sincerely yours,
Stratum Group
E~ ~;;~
Dan McShane, M.S., L.E.G.
Licensed Engineering Geologist
Stratum Group
File: 12.30.04C
5
Figure 1, Site Vicinity Map
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