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Myers
Biodynamics inc.
__ geo~echnical engineering · coastal processes * biological sciences
February 12, 2003
Peggy and Wayne Siscoe
1403 North 37t~ Place
Renton, Washington 98056
Re:
Results of Geotechnical Slope Evaluation
709 Lindsay Hill Road
Quilcene, Washington
Dear Mr. and Mrs. Siscoe:
APR 2 9 2003
JEFFERSON COUNTY
DEPT. OF COMMUNITY DEVELOPMENT~
This letter report presents the results of a slope evaluation conducted for your
property located at 709 Lindsay Hill Road in Quilcene, Washington. We understand
that you are planning to construct a single family residence on the property and seek
information on the general stability of the site's steep slope in addition to POtential
slope stability risks. The purpose of our work is to provide you with our opinion on
the Stability of the slope and general recommendations for site development to help
reduce slope stability risks. Our work included a review of site and local reference
mapping, discussions with you, a site visit, reconnaissance of the site slope, and
preparation of this letter rePOrt. Our work was conducted in general accordance with
our letter of agreement dated January 6, 2003.
Site .and Project Description
The subject property is approximately 5 acres and located at 709 Lindsay Hill Road in
Quilcene, Washington. The site is rectangularly shaped with the principle axis oriented
east to west. The property is bordered on the east by a gravel easement drive. The east
portion of the site is relatively level. The central and west side of the property slopes
steeply down to the west.
Currently,- a driveway and garage/shop are constructed on the southeast side of the
properly, 30 feet or more east of the steep slope crest. Project plans include construction
of a single family residence on the property north of the garage. Current plans also call
for the residence to be located 30 feet or more away from the steep slope crest.
Reference Mapping
As a part of our work, we reviewed available reference mapping of the area which
included coastal zone mapping (State of Washington Department of Ecology Coastal
Zone Arias, Volume 11, Jefferson County, July, 1978) and soil mapping (U.S.
Department of Agriculture Soil Conservation Service, Soil Survey of Jefferson
Siscoe Slope Evalm~tion 031043-5 · Febntat? 12, 2003
County Area, Washington, 1975). Coastal zone mapping indicates the east portion of
the subject property is cmnprised of Vashon till; a very dense mixture of gravel, sand,
silt, and clay. The steep slope is underlain by stmtitied sediments composed of
gravel, sand, silt, and clay massively to thinly bedded. The sediments are in a very
dense or hard condition due to the weight of glacial ice from past glaciation of the
area. Soil mapping indicates the site and local area are mantled with a gravelly sandy
loam soil derived from the underlying sediments. Coastal zone mapping categorizes:
the steep slope as "unstable" with regard to slope stability. The relat}~ve_N~t
portion of the site is mapped as "stable".
Site Observations
Site conditions were evaluated by conducting a reco~maissance of t e propera~~
coastal slope on January 21, 2003. Observations of the prope~~~'~0p~4[~-r
steep
time of our site visit including topography, soil and groundwater, vegetation,
drainage, and evidence of slope instability are discussed in the sections to follow.
Topography
The subject property is situated in an area that descends west to the Quilcene Bay
shoreline. The subject property is rectmigularly shaped, with the principle axis
oriented east to west. From the east property line, topography is characterized by
relatively level to gentle grades. Approximately 350 feet from the east property line,
the site slopes steeply down to the west. The steep slope crest alignment is arcuate in
shape on thc north and south sides of the site. Im~nediately west of the proposed
residence, the slope crest has been artificially modified by fill placement which is
supported by a. low log retaining structure. The logs are currently deteriorated and
likely have limited future performance.
The steep slope descends approximately 400 feet from the slope crest down past the
west property line and west of the subject property to Quilcene Bay. Steep slope
angles range from approximately 40 to 45 degrees near the slope crest to average
grades along the slope face of 25 to 35 degrees. Topographic draws in the slope face
are located below the arcuate shaped slope crest areas. Locally, several overgrown
logging roads cross the slope face. On the west side of the property, a local
residential access road intersects the southwest comer of the property.
Soil and Groundwater
Due to the well vegetated nature of the slope face, soil exposures observed at the time
of our site visit were limited. At and near the steep slope crest, near-surface soils
were composed of dense, silty sandy gravel to silty gravelly sand. Soils observed on
the middle portion of the site slope face were similar in composition. Near the base
of the site slope, hard silt soils were observed exposed along a road cut.
Myers Biot(vmtmics. Inc. page 2 of 7
Sixcoe Slope Evahtation 031043-5 · Februao, 12, 2003
No evidence of surface water flow or daylighting groundwater seepage was observed
on the property at the time of our site visit. Within the topographic-draws on the
slope face and on adjacent property immediately west of the subject property,
vegetation suggests seasonally very moist to wet subsurface soil conditio~~
associated with precipitation and stormwater runoff toward the topograph
base ofthe slope.
VegetationILl {J~ AP~ ~ AP~
The proposed residence area has been cleared of vegetation except lbr grlS-s_eS.~
, [Pr Or CO,~M~J~
the west side of the property, site vegetation near and below the slope
dominated by a moderate to dense canopy of mature conifer and madrone trees with
an understory of~voody shrubs and herbaceous groundcover typically indicative of
granular, well-drained soil conditions, hnmediately west of the proposed residence
location, the upper portion of the steep slope has been cleared of mature trees. We
understand the tree removal occurred approximately 10 years ago. Lower on the
slope face, vegetation is well established with second or third growth forest.
LOCalized topographic draws w4thin the slope face contain red alder, big leaf maple,
and Western red cedar trees.
Drainage
No evidence of daylighting groundwater seepage or significant surface water runoff
or surface water-related soil erosion was observed on the property at the time of'our
site visiL Due to a minor topographic rise near the slope crest, the majority of the
surface water flow in the proposed residence area is directed east, away from the
steep slope crest.
Evidence of Slope Instability
No evidence of significant recent landslide activity was noted on the steep slope at the
time of our site visit. However, the topographic draws and steep, arcuate shape of the
slope crest suggest historic landslide activity has occurred on the steep slope face
west of the proposed residence. Elsewhere in the local area, landslide activity has
occurred along road cuts and fills associated with Lindsay Hill Road and other local
roadways. This evidence confirms reference ~napping interpretations ~vhich
designate the steep slope as "unstable",
Slope Evaluation
Based on our site observations, the steep coastal slope on the subject property shows
no indication of significant, recent landslide activity. However, evidence on the
property and published reference mapping indicate that the slope has been subject to
past landsliding and slope crest retreat. In our opinion, there is a risk of future
landsliding on the property's steep slope that would result in a progressive retreat of
._----___a
COUNTY
FY DEVELOPMENT
Myers Biodynamics, h~c. page 3 of 7
Siscoe Slope Evahmtion 031043-5 . Februa~7 12, 2003
the slope crest to the east, toward the proposed residence area. Landsliding and slope
crest retreat often occurs episodically after many years of inactivity and usually in
conjunction with major storm events.
The risk of large-scale, sudden slope crest retreat that would suddenly undermine the
proposed residence location, in our opinion, is low due to the dense to hard soils
comprising the slope and absence of observed daylighting grotmdwater seepage on
the subject property. This opinion assumes static (non-earthquake) conditions. It
should be recognized, however, that large-scale, deep-seated landsliding
encompassing multiple properties has occurred at other locations in Jefferson County.
Seismic activity and/or other on-site and off-site factors such as increased surface
water runoff, development, mass vegetation clearing/harvesting, heavy precipitation,
and other factors can increase the risk of both localized, shallow-seated and large
scale, deep-seated landsliding.
It is our opitfion that construction of a single family residence on the property can
occur without adverse impacts to the existing condition of the site slope. This
opinion assumes static (non-earthquake) conditions and that the reconunendations
presented below are incorporated into site development plans, Our opinion is also
based on the lack of evidence of recent landslide activity on the property, no observed
daylighting groundwater seepage, and the apparent dense/hard condition of the
underlying soils comprising the slope. As a minimum, we recommend the residence
be located in accordance with Jefferson County regulations, no closer than 30 feet
from the steep slope crest.
As previously mentioned, changes to local land use including vegetative clearing,
increases in stormwater runoff, and other thctors could impact local and regional
slope stability. Long-term maintenance and management of the property should focus
on mitigating the risks associated with local site slope stability and landslide risk.
General site recommendations to help reduce potential impacts to the local site slope
are presented in the General Site Recommendations section to follow.
General Site Recommendations
General factors which adversely affect slope stability include, but are not limited to,
rainfall events, groundwater, water infiltrating from septic systems, stonnwater
runoff, clearing or disturbance of vegetation, excavation or fill placement, loading the
top of a slope, and cutting at the toe of the slope. Although it is not possible to
control all of these factors, measures can be taken to help reduce some of their
impacts to slope stability. The tbllowing general recommendations for site
development, vegetati.on management, drainage, and other considerations should be
incorporated into development, maintenance, and long-tem~ management of the
MversBiodynamic~',lnc. L---~ tage4of7
- jEFFERSO..N~C.. ...... ,'-,pCEN'T '~ .
Siscoe SIo?e Evoluation 031043-5 · February 12, 2(X~3
Vegetation Management
'Maintain as much mature, native vegetation on the property as possible.
This reduces adverse impacts from stormwater runoff on the site and slope
associated with clearing oflbrested areas.
Aggressively plant and augment existing site vegetation to maintain a
dense, continuous strip (15 to 20 feet in width) of deep-rooted vegetation
including small trees, woody shrubs, and/or herbaceous groundcover along
the site slope crest. Live vegetative root systems help provide a stabilizing
effect by increasing the apparent shear strength of shallow slope crest
soils.
· Maintain and encourage existing vegetation on the site slope face,
particularly where mature trees were cleared for views.
Do not dispose of yard debris, fill, or stock pile materials, equipment, or
other heavy items on or near the slope crest or on the slope face. These
materials can accumulate over time and create an unstable mass on the
slope which is subject to landsliding.
Drainage
Site development plans should include a comprehensive drainage system
that tightlines and routes all site drainage (roof downspouts, foundation
drains, driveway drainage, etc.), to a suitable discharge location away
from the crest of the steep slope.
Regularly monitor and clean-out drainage pipes and catchbasins to
maintain unobstructed flow and provide rapid identification and repair of
breaks and/or leaks in the drainage systmn. As a gninimum, clean-out and
check the performance of site drainage systems at least once annually,
prior to the wet weather season. This can be simply accomplished by
utilizing a garden hose (during dry weather) to introduce water into each
of the site catch basins, downspouts, etc. and checking discharge pipes for
similar flow volumes.
Myers Biodynamics, Ira:.
Incorporate Water saving measures (Iow flush toilets, water restrictors,
etc,) in the residence design to reduce water usage and subsequent flow of
wastewater into the site wastewater treatment system. This will reduce the
amount of water introduced into the system and surrounding soils which
could adversely impact site slope stability during wet weather periods.
JEFFERSON COUNTY ~
..... rmu~k~UNITY DEVELOPME N_~
Siscoe Slope Evaluation 031043-5 · February 12, 2003
Other Considerations
Remove the existing fill and decomposing logs used as a retaining system
near the slope crest. Revegetate the area as soon as possible after fill
removal. Do not place new fill on or near the slope crest.
The following homeowner manuals can provide you with some additional
general information and guidance for vegetation management, slope
revegetation, and drainage control. The manuals are: Slope Stabilization
and Erosion Control Using Vegetation; Vegetation Management: A
Guide for Puget Sound Bluff Property Owners; and Surface Water and
Groundwater on Coastal Bh~fs: A Guide fi>r Puget ,gound Property
Owners published by the Washington State Department of Ecology,
Shorelands and Coastal Management Program in Olympia, Wastfington.
For manual copies, contact the Shorelands and Coastal Management
Program directly at (360) 407-7472. Alternatively, copies can be obtained
on the internet at the Department of Ecology's website,
http://www.ecy.wa.gov/programs/se~shorlan.htrnl, under the heading
Property Owner Guides.
Myers Biodynamicx. Inc. page 6 of 7
$ixcoe Slope Evaluation 031043-5 · February 12, 2003
Closure
This letter report was prepared for the exclusive use of Peggy and Wayne Siscoe for
specific application to the property located at 709 Lindsay Hill Road in Quilcene,
Washington as described herein. The conclusions and interpretations within this
letter report should not be construed as a warranty of subsurface conditions.
Within the limits of scope, schedule, and budget, the evaluation and reconunendations
presented in this letter report were prepared in general accordance with accepted
professional geotechnicai engineering principles and practices in the area at the time
this letter report was prepared. No other warrant),, whether expressed or implied, is
made. The evaluation presented herein was based on our observations of the subject
property at the time of our site visit.
If there is a substantial lapse of time, conditions have changed at the site, or if
conditions appear different from those described in this letter report, we should be
contacted m~d retained to review the changed conditions. The purpose of the review
is to determine the applicability of the considerations and recmnmendations presented
in this letter report considering the time lapse and/or changed conditions.
We appreciate the opportunity to provide you with geotechnical engineering services.
If you have any questions regarding the evaluation and recommendations presented
herein or we may be of further assistance, please contact our office at your
convenience.
Sincerely Yours,
MYERS BIODYNAMICS, INC.
J~an; N. Myers, P.E.,~.
Principal Geotechnical Engineer
JNM:saf
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