HomeMy WebLinkAbout998200356 Geotech Assessment® SEATTUE
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SHANNOtV ~WiLSON FAIRBANKS
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, ANCHORAGE
OTECHNICAL AND ENVIRONMENTAL CONSULTANTS DENVER
GE SAINT LOUIS
BOSTON
Apri13, 2003
Mr. Edward Foster
c/o Ms. Catherine Hendy
John L. Scott Realty
40 Teal Lake Road
Port Ludlow, WA 98365
R~: DRAFT GEOLOGIC SLOPE STABILITY EVALUATION FOR
RESIDENTIAL DEVELOPMENT OF 650 TALA SHORES ROAD,
PORT LUDLOW, WASHINGTON
Dear Mr. Foster:
This letter report summarizes our observations, conclusions, and recommendations regarding
slope stability and development of the property referenced above for asingle-family residence.
Our review of the Jefferson County Geologic Hazard Area Maps indicates that the slopes on
the site may be susceptible to movement. Therefore, 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 site with respect to
slope stability. These conclusions and recommendations are based on observations made
during our reconnaissance of the site on February 27, 2003, available published geologic,
topographic, and soil maps, and an undated, hand drawn site plan prepared for the SPAAD
application.
SITE DESCRIPTION
The referenced property is located on the west side of Hood Canal near Tala Point, as shown on
Figure 1. Figure 2 indicates that the property is bounded by Tala Shores Road on the west and
Hood Canal on the east. From Tala Shares Road on the west, the property extends to the east
approximately 385 feet to Hood Canal; in the north-south direction, the property is
approximately 80 feet wide. Topography across the site consists of the following (from east to
west):
400 NORTH 34TH STREET • SUITE 100 21-1-09874-001
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206.632.8020 FAX 206.695.6777
TDD: 1.800.833.6388
Mr. Edward Foster
c/o Ms. Catherine Hendy
Apri13, 2003
Page 2
- Beach.
SHANNON F~WILSON, INC.
- Steep waterfront slope, (approximately 45 feet high) that extends from the beach on the
east to the upland portion of the site on the west.
- Relatively gently-sloping upland that slopes up to the west at about b to 8 degrees.
A generalized geologic profile that shows the approximate topography is sketched on Figure 3.
Th~beach consists mostly of sand and gravel, with cobbles and boulders and scattered
driftwood along the toe of the slope. Also lying along the toe of the slope are scattered, toppled
trees and root balls, which have apparently moved down the slope onto the beach. No
backshore is present, and the foreshore extends up to the toe of the slope.
The waterfront slope extends from the beach up to the west at about 45 to 50 degrees with near
vertical sections, particularly along the toe. Vegetation on the slope includes horse tails and
other hydrophilic species, salmon berry and small alder trees (up to about 1 foot in diameter}.
At the crest of the slope, an approximately 20-foot long (east-west) by 20 foot wide (north-
south} by about 15 to 20-foot deep scallop or bowl is present near the north property line, that
extends west, into the upland portion of the site (see Figure 2). The shape and location of this
feature at the crest of the slope suggest that it is a relatively recent slide scarp.
The eastern two-thirds of the upland portion of the site has been cleared of trees some time in
the past and is mostly vegetated with salmon berries and grass. The western part of the upland
portion of the site includes cedar and maple trees, up to about 2 feet in diameter.
We understand that the proposed development of the site includes construction of a single
.. family residence and on-site sewage disposal. The approximate location of the existing wells
on and adjacent to the site and the location and dimensions of the proposed residence and septic
drain fields for the sewage disposal system are shown on Figure 2.
21.1.09874-001-LRl.doc/wp/eet 21-1-09874-001
Mr. Edward Foster
c/o Ms. Catherine Hendy
April 3, 2003
Page 3
GEOLOGIC CONDITIONS
SHANNON WILSON, iNC.
Published geologic maps of the area indicate that the site is underlain by Pleistocene-age
Vashon Lodgment Till. Lodgment till is typically an unsorted mixture of clay, silt, sand, and
gravel with occasional cobbles and boulders, which is deposited directly beneath a glacier. The
Vashon Lodgment Till was deposited directly beneath Vashon Stade ice sheet that covered this
area approximately 13,500 to 17,000 years before present. The ice sheet that overrode the till
and ~he underlying soils 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 soils have been compacted to a very dense
or hard state.
Subsurface explorations were not performed at this site for this evaluation; however, soils
exposed on the steep slope on and adjacent to the property confirm the presence of till beneath
the site. The till observed at the site consists of very dense, non-sorted, gravelly silty sand with
scattered cobbles and boulders that extends from the beach level at the toe of the slope up to
within about 15 to 20 feet of the top of the slope. While not indicated on geologic maps, the
upper 15 to 25 feet of the site appears to be mantled with Vashon recessional outwash.
Recessional outwash is typically a sand and gravel deposit with lesser amounts of silt that was
deposited from meltwater emanating from a receding glacier. The recessional outwash
observed on scattered exposures on the slope and in the septic drain field test pits appeared to
be a dense to very dense, slightly gravelly to gravelly, slightly silty to silty sand.
Since the retreat of the glaciers, the upper few feet of the dense soils have loosened and
weathered, and topsoil, colluvium, andlor 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. The topsoil and colluvium
that develop on the steep slopes typically have a lower shear strength (i.e., are not as strong) as
the underlying, dense to very dense glacial sediment. Consequently, the topsoil and/or
colluvium may move, resulting in slide deposits on and near the base of the steep slopes.
zi-~-ovs~a-oo~-ixi.a~W~i~c 21-1-09874-001
Mr. Edward Foster
c/a Ms. Catherine Hendy
Apri13, 2003
Page 5
SHANNON WILSON, iNC.
region. At this site, it appears that groundwater perches within the recessional outwash, on top
of the relatively impervious till. Where the recessional outwash/till contact and groundwater
daylights on the face of a slope, springs and seeps develop. The springs and seeps saturate the
soils on the slope and thereby decrease the shear strength of the soils and increase their
susceptibility to movement, particularly of the shallow topsoil and colluvium on the slope. In
addition, pressure gradients or build-up of hydrostatic pressures associated with this perched
water may result in deep-seated slope instability. At this site, it appears that the deep seated
ins~bility occurs within the recessional outwash above the till.
u
With enough time, movement of colluvium, slide debris and topsoil toward the base of the
slope and continued weathering and erosion of the glacial soils up-slope would result in a
flatter, more stable slope. However, wave erosion at the toe of the slope does not allow the
colluvium, topsoil, and slide debris to accumulate at the toe of the slope and maintains the slope
in an over-steepened condition. Consequently, continued slope movement should be expected
in the future.
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 maybe 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 a sufficient distance 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.
21-1-09874-0o]-l.R l.doc/wp/eet 21-1-09874-001
Mr. Edward Foster
c/o Ms. Catherine Hendy
Apri13, 2003
Page 7
SHANNON WILSON, INC.
impacting the stability of the slope or increase the surface water discharge or sedimentation to
adjacent properties beyond pre-development conditions. These discharge points include the
existing road ditch on the west side of Tala Shore Drive, an on-site drain field, or a cistern
system that collects and stores the water for other uses at the site (e.g., landscape irrigation).
If an on-site drain field is used for the discharge of water collected in the drains on the
property, we recommend that it be located as far as practical from the crest of the slope and
have a minimum setback of at least 50 feet. An on-site drain field should be designed to allow
r
dispersion of water and dissipation of energy to reduce the potential for erosion. Conceptually,
a perforated pipe buried in a shallow, gravel-filled trench could be one method to provide water
dispersion and energy dissipation. If an on-site drain field is used at this site, it should be
designed by a civil engineer of other qualified design professional. Besides the recommended
setback from the crest of the slope, the location of an on-site drain field should consider the
location of the septic drain field and reserve area and should be reviewed by the septic system
designer.
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.
Based on our understanding of the limited, single-residence development of this
property, it is our opinion that the anticipated discharge of roof, footing and other drains in
accordance with the recommendations outlined above will not significantly affect the pre-
development drainage conditions on the adjacent properties.
Erosion Hazard
We note that according to published USDA soil maps, surficial soils on the upland portion of
the site are classified as Cassalory-Kitsap series on 0 to 30 percent slopes, while the soils at the
toe of the slope are indicated to be Coastal Beach. The soil maps do not classify the soils on
the slope, but based on the observed naturally occurring erosion on this steep slope, it is
expected that the erosion hazard associated with the topsoil, colluvium, and slide deposits
would be relatively high. The erosion hazard associated with the underlying till is expected to
2]-1-09874-001-uzl.doGwp/eet 21-1-09874-001
Mr. Edward Foster
c/o Ms. Catherine Hendy
April 3, 2003
Page 8
SF1Af'VNON WILSON, INC.
be relatively low. It is anticipated that the proposed residential development will not
significantly affect soil erasion and associated hazard on the site provided that prudent
construction practices with respect to erosion are implemented.
LIMITATIONS
The conclusions and recommendations presented in this letter are based on site conditions
visually observed during our site reconnaissance and inferred from published geologic,
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.
During subsequent site activities (e.g., construction}, if 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 report was prepared for the use of Mr. Foster in the evaluation of the stability of this
slope. 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 visit and discussion of geologic conditions included in this letter.
Please note that the scope of our services did not include any environmental assessment 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.
21-1-09874-001-LR1.doc/wpleer 21-1-09874-001
Mr. Edward Foster
c/o Ms. Catherine Hendy
Apri13, 2003
Page 9
SHANNON f,WILSOI~, II~IC.
Shannon & Wilson has prepared the attached, "Important Information About Your
Geotechnical Report," to assist you and others in understanding the use and limitations of our
report.
We appreciate the opportunity to provide geologic services to you, and aze available to answer
any questions regarding our observations and conclusions contained in this letter report.
Sincerely,
SHANNON & WILSON, INC.
~/~/03
William Joseph Perkins
William J. Perkins, L.E.G.
Senior Principal Engineering Geologist
WJP:JW:wjp
Enclosures: Figure 1-Vicinity Map
Figure 2 -Site Plan
Figure 3 -Generalized Subsurface Profile
Important Information About Your Geotechnical Report
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Scale in Feet
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NOTE. -
1. This topographic profile and geological conditions are 650 Tala Shores Road
based on field observations. Variations between the Port Ludlow, Washington
profile and actual topography and geological
conditions are likely. GENERALIZED
2. vertical Datum is arbitrary. SUBSURFACE PROFILE
March 2003 21-1-09874-001
SHANNON ~ WILSON, INC. FIG
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Geotechnlcat and Environmental Consultants
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SHANNON & W ILSON, INC. Attachment to and part of Report 21-1-09874-001
Geotechnical and Environmental Consultants
Date: Apri13, 2003
To: Mr. F.dwazd Foster
do Ms. Catherine Hendy, John L. Scott Realty
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAVENVIRONMENTAL
REPORT
CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS.
Consultants prepare reports to meet the specific needs of specific individuals. Areport 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 fast
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 geotechnicaUenvironmental report is based on a subsurface exploration plan designed to consider a unique set ofproject-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 wazehouse 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 adj scent 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 maybe affected as a result of natural processes or human activity. Because a geotechnicaUenvironmental 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 adj scent 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 geotechnicaUenvironmental 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.
Page 1 of 2 1/2003
A REPORT'S CONCLUSIONS ARE PRELIMINARY.
The conclusions contained in your consultant's report are preliminary because they must be based on the assumption that conditions revealed
through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned
only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the
consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's
recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The
consultant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations ifanother
party is retained to observe construction.
THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION.
Costly problems can occur when other design professionals develop their plans based onmisinterpretation of a geotechnical/environmental
report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant
geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative
to these issues.
BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE REPORT.
Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and
laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customaazily included in
geotechnical/environmentalreports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other
design drawings, because drafters may commit errors or omissions in the transfer process.
To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete
geotechpical engineering/environmentalrcport prepared or authorized for their use. If access is provided only to the report prepared for
you, you should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whom the
report was prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While
a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your
consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost
estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface
information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly
construction problems and the adversarial attitudes that aggravate them to a disproportionate scale.
READ RESPONSIBILITY CLAUSES CLOSELY.
Because geotechnicaUenvironmental engineering is based extensively on judgment and opinion, it is far less exact than other design
disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem,
consultants have developed a number of clauses for use in their contracts, reports and other documents. These responsibility clauses are not
exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identify where the
consultant's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take
appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your
consultant will be pleased to give full and frank answers to your questions.
The preceding paragraphs are based on information provided by the
ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland
Page 2 of 2 1/2003