HomeMy WebLinkAboutAquifer Recharge 021332021
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May 8, 2006
Mr. Scott Cassill
290 Smith Road
Nordland, W A 98368
RE: GEOLOGIC SLOPE STABILITY EVALUATION, 2710 EAST MARROWSTONE
ROAD, MARROWSTONE ISLAND, JEFFERSON COUNTY, WASHINGTON
Dear Mr. Cassill:
This letter report summarizes our observations, conclusions, and recommendations regarding the
stability and development ofthe property referenced above for a single-family residence,
including septic system. These conclusions and recommendations are based on observations
made during our visit to the site on February 17,2006; a site plan by Laurie Stewart dated
AprilS, 2006; and available published geologic, topographic, and soil maps of the area.
Preliminary results of observations and conclusions were provided to you orally upon completion
of the site visit.
SITE DESCRIPTION
The referenced property is located on Admiralty Inlet on the east side of Marrowstone Island, as
shown in Figure 1. As indicated in Figure 2, the property extends from East Marrowstone Road
on the west to Admiralty Inlet on the east. The property is approximately 441 to 457 feet long
(east-west) by 110 feet wide (north-south). Topography across the site is illustrated in Figure 3
and consists of the following features (from east to west):
~ A beach.
~ A steep waterfront bluff (approximately 60 feet high) that slopes from the beach up to the
west at about 50 degrees with local near-vertical sections.
~ A relatively flat upland at the top of the bluff.
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May 8, 2006
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SHANNON &'\NILSON, INC.
The bluff consists of steeply sloping, sparsely vegetated sections with steeper, near-vertical faces
on which little, if any, vegetation exists. Where present, vegetation on the slope consists mostly
of grasses, with scattered, small (up to about 4 inches in diameter) fir and alder trees.
The upland portion of the site is relatively flat, generally sloping at about 4 to 5 degrees down to
the west. The eastern part of the upland portion of the property is cleared and vegetated mostly
with grass. An existing m-obile home/trailer is located on the eastern part of the upland within
approximately 18 feet of the edge of the bluff. The western part of the upland is vegetated,
which includes fir, madrona, and maple trees (up to about 2Y2 feet in diameter) with an
undergrowth of sword fern, grasses, and salal. The native madrona and salal vegetation is
indicative of relatively well-drained, near-surface soil conditions.
We understand that you propose to remove the existing mobile home/trailer from the site to
construct a new residence and septic system, The conceptual locations of the proposed residence
and septic system, and the location of the water well on the property are indicated in Figure 2.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the site is underlain by Pleistocene-age (13,500
to 17,000 years old) Vashon Lodgment Till underlain by Vashon Advance Outwash. Vashon
Advance Outwash typically consists of sand with lesser amounts of silt and gravel. The advance
outwash was deposited on the pre-existing land surface, in front of the continental Vashon Stade
ice sheet that advanced from Canada across the Puget Sound region approximately 17,000 years
ago. Lodgment Till is typically an unsorted mixture of clay, silt, sand, and gravel with
occasional cobbles and boulders that was deposited directly beneath the ice sheet as the glacier
advanced over the area. The Vashon Lodgment Till was deposited directly beneath the Vashon
Stade ice sheet that covered this l:lfea approximately 13,500 to 17,000 years before present. The
ice sheet that overrode the till and t~underlying soils (including the advance outwash) is
estimated to have been up to 4,000 feet thick in this area. Consequently, the till and the
underlying advance outwash have been compacted to a very dense or hard state. As the glaciers
receded, meltwater deposited sand and gravel (recessional outwash) at some locations on the
newly exposed land. Since the retreat of the glaciers, the upper few feet of glacial deposit
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. Mr. Scott Cassill
May 8, 2006
Page 3
exposed at the ground surface has typically loosened and weathered, and topsoil and/or
colluvium has developed at the ground surface, as illustrated in Figure 3, 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.
Subsurface explorations were not performed at this site for this evaluation; however, soils
exposed on the non-vegetated portions of the bluff confirm the presence of the till and advance
outwash. The till was observed in approximately the upper quarter (about 15 feet) ofthe bluff
and appeared to be a very dense, gray, silty, gravelly SAND. Advance outwash was observed in
approximately the lower three-quarters (about 45 feet) ofthe bluff and appeared to be dense to
very dense, cross-bedded, fine to medium SAND with a trace of fine gravel.
At the crest of the bluff, it appears that the upper 1 to 2 feet of the till has weathered to a medium
dense to dense condition, The till located farther to the west on the upland portion of the site
appears to be overlain by a thin mantle of recessional outwash. Specifically, in test pits
excavated previously at the site in the vicinity ofthe proposed septic system, the till appeared to
by overlain by about 4Y2 to 5~ feet of recessional outwash consisting of medium dense, clean to
slightly silty SAND with a trace of fine gravel.
The vegetated and lower portion of the bluff appeared to be covered with a relatively thin mantle
(i.e., no more than a few feet thick) of colluvium.
No signs of springs, seeps, damp soils, or other indications of near-surface water were observed
on the bluff, on the site, or on the property to the south. Approximately 200 to 300 feet north of
the property, slight groundwater seepage was observed at a silt lens near the top of the advance
outwash exposed in the bluff.
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps of the area classify the bluff as unstable, with recent slope movement. We
understand that about 10 years ago, spalling at the cr~s~ of!h~_~lQP.ecaus@d about a 20-foot-wide
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May 8, 2006
Page 4
SHANNON {':I V\liLSON,INC.
(north-south) portion of the bluff in front of the mobile home/trailer to recede toward the west
about 5 to 8 feet. The lack of vegetation on the bluff indicates that spalling and other slope
movements occur frequently enough to preclude the growth of most vegetation other than
grasses on the bluff, If slope movements were relatively infrequent, we would anticipate more
vegetation, including larger trees, to be present on the slope.
Based on our observations of the site, it appears that the unstable soils are primarily the topsoil
and colluvium on the slope and that the risk of deep-seated slope movement (i.e" failure through
the very dense glacial soils beneath the site) is relatively low, in our opinion. Signs of deep-
seated slope movements (e.g., topographic bowls, step-downs, or a serrated bluff crest) were not
observed during our site visit. The very dense, glacially overridden soils that underlie the bluff
may be stable beneath relatively steep slopes (e.g., 50 degrees or more). However, the relatively
loose topsoil and colluvium that weather from these soils are not as strong and are susceptible to
slope movements on slopes on which the underlying glacially overridden soils are relatively
stable.
With enough time, movement of colluvium and topsoil toward the base of the bluff and
continued weathering and erosion of the glacially overridden soil up slope would result in a
flatter, more stable slope. However, wave erosion at the toe of the bluff does not allow the
colluvium and topsoil to accumulate at the toe of the slope and maintains the slope in an
oversteepened condition. Consequently, continued movement of topsoil and colluvium on the
bluff 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, which may cause sliding. The following paragraphs provide
further discussion of risk reduction measures that may be effective at this site. Provided 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 the adjacent properties.
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Mr. Scott Cassill
May 8, 2006
Page 5
Measures to Reduce the Risk Posed by Slope Movement
In general, the risk of soil movement on a slope can be reduced by not oversteepening 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 at the crest of the slope), maintaining the slope as dry as possible (e,g.,
locate septic drainfields away from the bank, route roof downspouts and yard drains to the base
of the slope or storm drain system, and minimize the amount of surface water that could flow
down the face of the slope), and maintaining a vegetative cover on the slope. In addition,
measures that can be taken to reduce or minimize rate of wave erosion at the toe of the slope
(e.g., construction of a seawall, not removing large wood debris or driftwood near the top of the
beach) will decrease the rate at which the slope erodes,
Septic Drainfield and 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 and impact of slope movement is to
provide an adequate septic drainfield and building setback. An appropriate setback is a function
of the rate of slope regressesion, the design life of the structure, the amount of water the
drainfield may discharge into the soils, and the risk the owner of the structure is willing to
assume. The regression rate for this specific slope is unknown; however, based on regression
rates measured elsewhere in the Puget Sound area, the regression rate could be on the order of a
few inches to 1 foot per year, The presence of effluent in the soils near the edge of the bluff may
increase the regression rate. In our opinion, a minimum septic drainfield and building setback
equal to the height ofthe slope (i.e" 60 feet) would be adequate for this site. Greater risk
reduction can be achieved with larger setbacks. Components of the septic system that do not
discharge water into the soils at the site (e.g., sand filters and septic tanks) could be located
closer than 60 feet to the crest of the bank, provided the owner is willing to accept a greater risk
of slope movement affecting these components. We recommend that a minimum setback of 30
feet for these components be used.
The actual rate of slope regression likely will vary from year to year (e.g., some years, no
noticeable regression may occur while in other years the slope may regress by several feet due to
slope movements as reportedly occurred about 10 years ago). By irifplementing the measures
Mr. Scott Cassill
May 8, 2006
Page 6
SHANNON &VVILSON.INC.
outlined in this letter for reducing the risk of slope movement, the rate of slope regression may
also be reduced.
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
to a suitable discharge point (e.g., bottom of the bank or road ditch).
In our opinion, the water collected in the drainage systems could be discharged to the
road ditch or to an infiltration area on the western part of the site, west of the proposed residence
(i.e., more than 60 feet from the crest ofthe bluff). Discharging water to the road ditch could
require collecting the water in a catch basin or holding tank and pumping it to the road ditch. If
an infiltration area is used, the location of the area on the western portion of the site relative to
the septic drainfield should be determined by the septic system designer.
Based on our understanding of the limited, single-residence development of this property
and the relatively well-drained nature ofthe soils that underlie the upland portion ofthe site, it is
our opinion that the anticipated discharge of roof and footing drains as recommended above will
not significantly affect the drainage conditions on the adjacent properties from pre-development
conditions. Impermeable surfaces surrounding the residence (e.g., paved drives) should be
minimized to reduce potential changes in the existing site drainage characteristics and impacts on
adjacent properties.
Vegetation
Maintaining a healthy vegetative cover at the crest of the bluff can reduce erosion and the
rate of slope regression. In general, native vegetation should be used on and near the bluff to
eliminate the need for irrigation and wetting the soils on or near the slope. A healthy vegetative
cover may include large, healthy trees.
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Mr. Scott Cassill
May 8, 2006
Page 7
SHANNON &WILSON, INC.
Erosion Hazard
We note that according to published U.S. Department of Agriculture (USDA) soil maps, surficial
soils on the upland portion of the site are classified as Whidbey gravelly sandy loam C on 0 to 15
percent slopes and Dick sandy loam C on 0 to 15 percent slopes. The USDA maps indicate that
these soils have only a slight to moderate erosion hazard. The soil maps do not classify the soils
on the waterfront slope, and no indication of the erosion potential is provided on the maps.
However, based on the apparent active nature of the slope movement on the bluff, it is our
opinion that the erosion potential of the soils may be relatively high. However, it is anticipated
that the development on the upland portion of the site will not significantly affect the erosion and
associated hazard of the soils on the slope provided the recommendations in this letter are
followed and prudent construction practices are used with respect to erosion.
LIMITATIONS
The conclusions and recommendations presented in this letter report are based on site conditions
visually observed during our site reconnaissance and inferred from published geologic,
topographic, and haza/rd 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 and recommendations where
necessary.
Within the limitations of scope, schedule, and budget, the recommendations and 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 report was prepared. We make no other
warranty, either express or implied.
This letter report 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
'1 ~,
Mr. Scott Cassill
May 8, 2006
Page 8
SHANNON &VVILSON, INC.
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.
Shannon & Wilson has prepared the enclosed, "Important Information About Your Geotechnical
Report," to assist you in understanding the use and limitations of our letter 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 report.
Sincerely,
William J. Perkins, L.E.G.
Associate
;.
WJP:JW/wjp
Enclosures: Figure 1 - Vicinity Map
Figure 2 - Site Plan
Figure 3 - Generalized Profile A-A'
Important Information About Your Geotechnical Report
21-1-20468-001-L1lwp/LKD
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--'" ~arrowstone Island, Washington
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SHANNON & WILSON, INC.
Geotechnical and Environmental Consultants
Attachment to and part of Report 21-1-20468-00 I
Date: May 8, 2006
To: Mr. Scott Cassil\
Nordland, Washington
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVIRONMENTAL
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 CONSULTANrS 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: (I) 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
unrefiigerated 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, ~1IDb~.particularly beneficial in this respect.
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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 if another
party is retained to observe construction.
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THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION.
Costly problems can occur when other design professionals develop their plans based on misinterpretation 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.
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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 fmal boring logs and data are customarily included in
geotechnical/environmental reports. 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.
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To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete
geotechnical engineering/environmental report 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.
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READ RESPONSIBILITY CLAUSES CLOSELY.
Because geotechnical/environmental 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.
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The preceding paragraphs are based on information provided by the
ASFE/Association of Engineering Finns Practicing in the Geosciences, Silver Spring, Maryland
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