HomeMy WebLinkAbout501031005 Geotech Assessment Geologic Slope Stability Evaluati
1364 Hazel Point Re
Jefferson County, Washing1
February 2(
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Submitted
Ms. Francine R~
121 Wolf R~
Quilcene, Washington 98:
Shannon & Wilson, I
400 N 34th Street, Suite '
Seattle, Washington 98'
21-1-09559-(
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GEOTECHNICAL AND ENVIRONMENTAL CONS DENVER
SAINT LOUIS
BOSTON
February 20, 2002
Ms. Francine Rose
121 Wolf Road
Quilcene, WA 98376
RE: GEOLOGIC SLOPE STABILITY EVALUATION, 1364 HAZEL POINT ROAD,
JEFFERSON COUNTY, WASHINGTON
Dear Ms. Rose:
This letter summarizes our observations, conclusions, and recommendations regarding slope
stability and development of the property referenced above for a single-family residence. We
understand that Ms. Rose would like to locate the proposed residence as close to the crest of the
slope on the site as reasonable. Jefferson County Geologic Hazard Area Maps indicate that the
landslide hazard rating of slopes on the site range from low to high. As the proposed building
location may be within a landslide hazard area buffer, we have prepared this report in accordance
with the Unified Development Code for Jefferson County to evaluate the potential for slope
movement and provide recommendations for development of the proposed building site and near
the crest of the slope on the property in general with respect to slope stability. These conclusions
and recommendations are based on observations made during our visits to the site on
September 25 and November 23, 2001; available published geologic, topographic, and soil maps;
and a site plan by Tillman Engineering dated December 21,2001. Preliminary observations and
conclusions were provided to Ms. Rose orally upon completion of the site visits.
SITE DESCRIPTION
The site is located on Hood Canal at the southeast tip of the Toandos Peninsula, as shown on
Figure 1. The irregularly shaped 26-acre parcel is about 1,900 to 2,200 feet long (north-south)
by about 450 feet wide (east-west), except in the middle third of the property where it widens to
about 725 to 825 feet. The approximate location of the proposed building site is shown on
Figure 1; a detailed plan of the general building site is shown on Figure 2.
400 NORTH 34TH STREET' SUITE 100
P.O. BOX 300303 21-1-09559-001
SEATTLE, WASHINGTON 98103
206.632-8020 FAX 206.695.6777
TDD: 1.800.833-6388
Ms. Francine Rose SHANNON ~WlLSON. INC~
February 20, 2002
Page 2
The topography across the site rises from sea level at Hood Canal, to about 380 feet to the north
and includes the following:
· A beach.
· A steep, near vertical waterfront bluff, (approximately 10 to 20 feet high).
· A lower slope, approximately 170 to 190 feet high, with ravines and an overall slope up
to the north-northwest of about 15 to 20 degrees.
· An upper slope, approximately 170 to 190 feet high, that slopes up to the north-northwest
at about 27 to 29 degrees.
· A relatively flat upland that slopes down to the north-northwest at 6 degrees or less.
A generalized subsurface profile drawn perpendicular to the upper and lower slope with these
topographic features is shown on Figure 3.
The marine bluff is sparsely vegetated with alder, maple, and scattered fir. Where the vegetation
is absent, the exposed soils consist of laminated to bedded, hard clay and silt and very dense
sandy gravel. Horsetails, which are indicative of wet soil conditions, are present at the top of the
bluff. A nearly continuous zone of slight to moderate seepage was observed near the top of the
bluff.
Vegetation on the lower slope includes maple, fir, cedar and madrona trees up to about 2 to 3 feet
in diameter, with scattered alder trees and an undergrowth of sword ferns. Some of the tree
trunks on the lower slope are bowed down slope, which is indicative of soil creep. Soil creep is
the slow, gradual down slope movement of near surface soils under the effects of gravity and
water and occurs on most slopes to some degree. A fine to medium sand is exposed in old road
cuts near the top of the lower slope. The sand in the face of the cuts appears medium dense and
becomes very dense within about a foot of the face of the cut.
Most of the upper slope and upland portion of the site were reportedly logged about 16 years
ago. vegetation on these portions of the site includes fir and madrona trees (up to about 1 foot in
diameter) with an undergrowth of salal and evergreen huckleberry. This vegetation is indicative
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Ms. Francine Rose SHANNON &WILSON, lNG.
February 20, 2002
Page 3
of well-drained soil conditions near the ground surface. On the upper slope, very dense sandy
gravel with cobbles was observed in near vertical road cuts.
The planned residence will be located on the upland portion of the site near the crest of the upper
slope as shown on Figure 2. The size and location of the proposed residences indicated on
Figure 2 are conceptual and may change. ·We also understand that the existing septic system are
located north of the proposed residence on the upland portion of the site.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the site is underlain by undifferentiated
Pleistocene deposits and capped by younger, Pleistocene-age Vashon Till. Subsurface
explorations were not performed at this site for this evaluation. However, the soils observed in
the old road cuts down the slope and in the marine bluff are consistent with undifferentiated
'Pleistocene deposits indicated on the map. These soils include:
Laminated to bedded, hard clay and silt and very dense sandy gravel beneath the marine
bluff.
· Very dense, fine to medium sand that appears to underlie much of the lower slope.
· Very dense sandy gravel with cobbles that appears to underlie the upper slope and
upland.
A generalized profile showing these geologic units is shown on Figure 3.
While indicated on the published geologic maps, till was not observed in the road cut at the crest
of the slope. Till may be locally thin (i.e., only a few feet thick) or absent along the crest of the
slope. The Vashon till, where present, and the underlying undifferentiated Pleistocene soils were
overridden by the Vashon Stade ice sheet that covered this area approximately 13,500 to
17,000 years before present. This ice sheet is estimated to have been on the order of 3,000 to
4,000 feet thick in this area. Consequently, the till and the underlying undifferentiated
Pleistocene soils have been compacted to a very dense or hard state.
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Ms. Francine Rose SH/~NON ~WILSON, lNG.
February 20, 2002
Page 4
Since the retreat of the glacier, the upper few feet of the very dense/hard soil has loosened and
weathered, and topsoil, colluvium and/or slide deposits have developed at the ground surface.
Colluvium is weathered material that has reached its present location due to the forces of water
and gravity and is typically found on, and at the base of steep slopes. Slide deposits also
appeared to be present at various locations at the base of the marine bluff.
As previously indicated, slight to moderate groundwater seepage was observed along a nearly
continuous zone near the top of the marine bluff at the base of the lower slope. No signs of
springs, seeps, damp soils, or other indication of near surface water were observed farther up on
the lower slope or on the upper slope.
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Evidence of recent slope movement along the marine bluff was observed during our site visit.
However, evidence of slope movement decreases higher up the slope. This suggests that while
the slopes in the vicinity of the marine bluff may be highly susceptible to slope movements, the
risk of slope movement decreases away and above the marine bluff, resulting in a relatively low
risk to the proposed development at the top of the slope, in our opinion. While there is some risk
that a large, deep seated slide could impact a structure at the top of the slope, based on our site
observations, the slope topography and geology, and our experience with similar sites in the
region, it is our opinion that the risk posed by slope movements to a building site at the crest of
the slope is relatively low. Consequently, building set-backs from the crest of the slope are
based on likely rates of erosion on the upper slope and not on potential deep-seated slope
movements that, in our opinion, have a very low probability of occurrence.
We note that geologic hazard maps classify the upper and lower slope as unstable with recent
slides along the marine bluff. Evidence of recent slides along the marine bluff was observed
during our site visits. Based on our observations of the site, it is our opinion that the slope
movements on and near the marine bluff are the result of two conditions.
21-1-09559-001-L1/wp/lkd 21-1-09559-001
Ms. Francine Rose SHANNON &WILSON, lNG.
February 20, 2002
Page 5
The first condition affecting the stability of the marine bluff is wave erosion at the base of the
bluff. The dense to very dense glacial soils that presumably underlie the bluff may be stable at
relatively steep slopes (e.g., 40 degrees or more). However, the relatively loose topsoil and
colluvium that weather from these soils are not as competent and are susceptible to movement on
slopes on which the underlying glacial soils may be relatively stable. With enough time,
movement of colluvium, topsoil and/or slide debris toward the base of the slope would result in a
flatter, more stable slope. However, wave erosion at the toe of the bank does not allow the slide
deposits, colluvium, or topsoil to accumulate at the toe of the slope and maintains the slope in an
over-steepened condition. Consequently, slope movement on the bluff should be expected in the
future. In our opinion, slope movement on the marine bluff would likely be local in nature and
consist of movement of shallow topsoil/colluvium or spalling of the very dense/hard soils.
The second condition affecting the stability of the slope is the perched groundwater at the contact
between the clay and silt of the marine bluff and the sand in the lower slope. Because the sand in
the lower slope is relatively pervious, while the clay and silt soils in the marine bluff are not,
perched groundwater develops within the lower portion of the lower-slope sand at the contact
with the underlying bluff clay and silt. Where this contact daylights near the top of the bluff,
springs and seeps develop from the perched groundwater. Pressure gradients or build-up of
hydrostatic pressures associated with the perched water often result in deep-seated slope
instability. These contacts of sand over silt or clay with perched groundwater have historically
been the locations of numerous slides on slopes in the region. While we did not observe
evidence or recent deep-seated movements near the base of the lower slope, the ravines along the
lower portion of the lower slope may be remnants of head scarps that have been subsequently
eroded into ravines.
While there is a potential for deep-seated instability on the lower slope, the lack of clear
evidence of deep-seated movement, coupled with the size and inferred ages of the trees, suggests
that, the rate at which deep-seated movements may occur could be relatively low (e.g., on the
order of hundreds of years). The fact that the possible indicators of old slope movement on the
lower slope generally do not extend onto the upper slope, coupled with the slope topography,
suggest that deep seated slope movements in the lower slope do necessarily result in movement
in the upper slope. That is, deep-seated slope movements in the lower slope do not necessarily
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Ms. Francine Rose SHANNON ~WlLSON, lNG.
February 20, 2002
Page 6
extend into the upper slope. Multiple slope movement in the lower slope may be required to
result in movement in the upper slope and would therefore suggest an even lower rate or risk of
slope movement in the upper slope than in the lower slope. The relatively low rate or risk of
deep seated slope movements within the upper slope is further supported by fact that the
inclination of the upper slope (about 27 to 29 degrees) is approximately the angle of repose for
the relatively loose topsoil/colluvium. This suggests that the rate at which a slope is over-
steepened due to deep-seated slope movements is very low in order for the slope to reach the
angle of repose of the loose, weathered soils that form on the surface of the slope.
Please note that there is some risk of future instability (shallow or deep-seated) present on all
hillsides, which the owner must be prepared to accept. Such instability could occur because of
future water line breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or
other actions or events on a slope that may cause sliding. The following provides further
discussion of risk reduction measures that may be effective at this site. Provided that the risk
reduction measures discussed in this letter are implemented, it is our opinion that the proposed
development will not adversely impact the stability of adjacent properties.
Measures to Reduce the Risk Posed by Slope Movement
In general, the risk of soil movement on a slope can be reduced by not over-steepening the slope
(e.g., do not excavate the toe of the slope), not increasing the weight on the slope (e.g., do not
place yard debris or fill on or at the crest of the slope), maintaining the slope as dry as possible
(e.g., locate septic drain fields away from the slope, route roof downspouts and yard drains away
from the slope, and minimize the amount of surface water that could flow down the face of the
slope), and maintain a vegetative cover on the slope.
Building Setback
The measures discussed above may reduce the risk of soil movement on a slope. One of
the most cost-effective measures to reduce the potential impact of slope movement is to provide
an adequate building setback. An appropriate setback is a function of the rate or risk of slope
movement (regression rate), the design life of the structure, and the risk the owner of the
structure is willing to assume (the greater the setback, the lower the risk). The regression rate for
this specific slope is unknown. However, based on the existing angle of the upper slope, the
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Ms. Francine Rose SHANNON &WILSON, lNG.
February 20, 2002
Page 7
absence of observed indications of past slope instability on the upper slope, and the growth
position of the trees on the slope, it is our opinion that the regression rate on the upper slope is
relatively low (e.g., less than a few inches per year) and appears to be controlled largely by
surficial erosion. In our opinion, a minimum building setback of 20 feet from the crest of the
slope on the property would be adequate. While at the site, we flagged the crest of the slope
from which the 20-foot setback should be measured. Tillman Engineering subsequently
surveyed the crest of slope. The surveyed location of the slope crest and 20-foot building
setback are shown on Figure 2.
Drainage
In general, reducing the amount of water entering and discharging onto the slope can
reduce the risk of slope movement. Drains should be constructed and maintained to collect water
from impermeable surfaces on the property (e.g., roof, decks, patios, and driveways) and directed
in a tightline to a suitable discharge point away from the crest of the slope. To provide adequate
drainage away from the residence without significantly impacting the stability of the slope or
increase the surface water discharge or sedimentation to adjacent properties beyond pre-
development conditions, we recommend that drains, such as roof, footing and storm water
drains, discharge on the upland portion of the site at least 100 feet from the crest of the slope and
the residence and no closer than 30 feet of adjacent properties. The discharge point should be
constructed to allow dispersion of the water and dissipation of energy to reduce the potential for
erosion. Discharge into a perforated pipe buried in a shallow, gravel-filled trench would be one
method to provide water dispersion and energy dissipation. Based on our understanding of the
limited, single-residence development of this property and the relatively well-drained nature of
the soils that underlie the upland portion of the site, it is our opinion that the anticipated
discharge of roof and footing drains as outlined above will not significantly affect the
pre-development drainage conditions on the adjacent properties.
Impermeable surface around the residence (e.g., paved drives) should be minimized to
reduce potential changes in the existing site drainage characteristics and impacts on adjacent
sites.
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Ms. Francine Rose SHANNON ~WILSON, lNG.
February 20, 2002
Page 8
Erosion Hazard
We note that the according to published U.S. Department of Agriculture (USDA) soil maps,
surficial soils on the upland portion of the site are classified as Sinclair gravelly sand loam C and
Dabob very gravelly sandy loam C on 0 to 15 percent slopes. The Dabob series likely extends
down the upper slope and would be more consistent with D (Dabob soils on 15 to 30 percent
slopes). The USDA maps indicate that the upland Sinclair and Dabob C soils have a slight to
moderate erosion hazard. The lower slope is mapped as Kitsap silt loam E on 30 to 50 percent
slopes. The Kitsap E soils are indicated to have a severe erosion hazard. It is anticipated that the
proposed development as located on the site will not significantly affect soil erosion and
associated hazards on the site, provided prudent construction practices with respect to erosion are
used.
LIMITATIONS
The conclusions in this letter are based on site conditions visually observed during our site
reconnaissances and inferred from published geologic, soils, topographic, and hazard maps and
assume that observed conditions are representative of the subsurface conditions throughout the
site; i.e., the subsurface conditions are not significantly different from those inferred from the
site reconnaissance or indicated on geologic maps. If, during subsequent site activities (e.g.,
construction), subsurface conditions different from those inferred in this letter are observed or
appear to be present, we should be advised at once so that we can review those conditions and
reconsider our conclusions where necessary.
Within the limitations of scope, schedule, and budget, the conclusions presented in this letter
were prepared in accordance with generally accePted geologic engineering principles and
practices in this area at the time this letter was prepared. We make no other warranty, either
expressed or implied.
This letter was prepared for the use of the Owner in the evaluation of the stability of this site.
With respect to possible future construction, it should be made available for information on
factual data only and not as a warranty of subsurface conditions, such as those interpreted from
the site visits and discussion of geologic conditions included in this letter.
21-1-09559-001-L1/wp/lkd 21-1-09559-001
Ms. Francine Rose SH/~NON &WILSON, iNC.
February 20, 2002
Page 9
Please note that the scope of our services did not include any environmental assessments or
evaluation regarding the presence or absence of wetlands or hazardous or toxic material in the
soil, surface water, groundwater, or air on, or below, or around this site. We are able to provide
these services and would be pleased to discuss these with you if the need arises.
Shannon & Wilson has prepared the attached, "Important Information About Your Geotechnical
Report," to assist you in understanding the use and limitations of our report.
We appreciate the opportunity to provide geologic services to you, and are available to answer
any questions regarding our observations and conclusions contained in this letter.
Sincerely,
SHANNON & WILSON, INC.
William J. P/e?kinst
Senior Prinbfpal Engineering Geologist
WJP:TMG/wjp
Enclosures: Figure 1 - Vicinity Map
Figure 2 - Site Plan
Figure 3 - Generalized Subsurface Profile
Important Information About Your Geotechnical Report
21-1-09559-001-L1/wp/lkd 21-1-09559-001
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~ I ~ ~ ~ I I 1364 Hazel Point Road
~ Scal~ in Milos Oofforson Goun~, Washington
~~ Map adapted from ~ :24,000 HSGS topographic
} map of Soabeck, photorovised ~ ~73.
~ Februa~ 2002 2
~t~hniml and finvimnm~n~l Con~ul~n~
SITEPLAN
2002
....
~~ROPOSED~ RESIDENCE ~P.)
/
/
,
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0 30 60
~ Scale in Feet
~' NOTES
~ 1. Site plan based on topographic site plan by Tillman
v Engineering, dated 12-21-01. 1364 Hazel Point Road
~ Jefferson County, Washington
'- 2. The proposed residence is shown for illustrative
purposes only. The actual dimensions and location
9 may vary, except for the recommended minimum
~- 20-foot set back from the top of the slope. SITE PLAN
~ 3. Topographic contour interval is one foot. Datum is
~ February 2002 21-1-09559-001
9 arbitrary.
~ SHANNON & WILSON, INC. FIG. 2
~. Geotechnlcal and Environmental Consultants
Geotechnical and Environmental Consultants
Date: February 20, 2002
To: Ms. Francine Rose
Quilcene, Washington
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVlRONMENTAL
REPORT
CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS.
_, Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate
for a construction contractor or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly
for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first
conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first
conferring with the consultant.
THE CONSULTANT'S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS.
A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors.
Depending on the project, these may include: the general nature of the structure and property involved; its size and configuration; its
,., historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking
lots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly
problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations.
Unless your consultant indicates otherwise, your report should not be used: (1) when the nature of the proposed project is changed (for
example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an
unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project
is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for
application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors
which were considered in the development of the report have changed.
SUBSURFACE CONDITIONS CAN CHANGE.
Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is
based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose
adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for
example, groundwater conditions commonly vary seasonally.
Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect
subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant should be kept apprised
of any such events, and should be consulted to determine if additional tests are necessary.
MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS.
Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were
extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface
between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from
those predicted in your report. While nothing can be done to prevent such situations, you and your consultant can work together to help
reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect.
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