HomeMy WebLinkAbout921042033 Geotech Assessment L~t~UIUylL; .31UlJe ~.~LitUIIII, y r-VillUill, lUil UI I,.UV~ rroper[y
East Marrowstone Road, Marrowstone Island
,Jefferson County, Washington
December 2000
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
Ar Shannon & Wilson. our mission is to be cz progressive, well-
mai?aged p/'qf~s.s'io,al co~s~dting.firm i~ ~he fie[ds q/'e~zgineering
~md applied earth sciences. Our goal is t(; per~brm our se~ices
wt'm r/~e highe.~'r degree rg'pm&ssionalism ;v/th due consideration
ro d~e best interests (}f the p~b/ic, our c[ieJ,rs, rind our employees.
Submitted To:
Event Management Corporation
93 Kercheval Avenue, Suite 6
Grosse Pointe Farms, Michigan 48236-3618
By:
Shannon & Wilson, Inc.
400 N 34m Street, Suite 100
Seattle, Washington 98103
21-1-09217-001
SHANNON&WILSON INC
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
December I, 2000
Mr. John Love
Event Management Corporation
93 Kercheval Ave, Suite 6
Grosse Pointe Farms, MI 48236-3618
RE: GEOLOGIC SLOPE STABILITY EVALUATION OF LOVE PROPERTY,
EAST MARROWSTONE ROAD, MARROWSTONE ISLAND,
JEFFERSON COUNTY, WASHINGTON
This letter summarizes our observations, conclusions and recommendations regarding the
stability and development of the Love property (parcel No. 921-042-0~) for a single-family
residence, including septic system. These conclusions are based on observations made during
our visit to the site on November 11, 2000, a site plan by Tillman Engineering dated February
22, 2000, and published area geologic, topographic and soil maps.
SITE DESCRIPTION
The referenced property is located on Admiralty Inlet on the east side of Marrowstone Island, as
shown on Figure 1. As indicated on Figure 2, the property extends from East Marrowstone Road
on the west to Admiralty Inlet on the east. The property is approximately 415 to 430 feet long
(east-west) by 105 feet wide north-south. Topography across the site consists of the following
(from east to west);
· A beach.
· A steep waterfront bank, (approximately 90 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 waterfront slope that continues to slope up to the
west at about 2 to 3 degrees.
A generalized profile of the site (A-A') is sketched on Figure 3.
At the time of our site visit, the beach and lower reaches of the slope were inaccessible from the
upland portion of the site. As observed from the top, the bank consists of steeply sloping
vegetated sections with steeper, near-vertical sections on which little if any vegetation exists.
.tO0 NORTH 34TH STREET' SUITE 100
RO. BOX 300303 21-1-09217-001
SEATTLE. WASHINGTON 98103
206.632.8020 FAX 206.633-6777
TDD: 1-800.833-6388
Mr. John Love SHANNON ~WILSON. INC.
Event Management Corporation
December 1, 2000
Page 2
Vegetation on the slope includes alder trees (up to about 8 inches in diameter), grasses, and
blackberries. Many of the alder trees at the base of the slope are in ".jack straw" positions, that
is, lean into and out of the slope, which is indicative that the ground beneath these trees has most
likely experienced previous slide movements.
The upland portion of the property is cleared and vegetated mostly with grass. Native vegetation
on the lots to the north and south include fir, cedar, and madrona trees with an undergrowth of
sword fern and salal. Based on our experience, this native vegetation is indicative of relatively
well-drained near surface soil conditions.
Conceptual locations of the proposed residence, septic/drainfield, and proposed water well along
the upland portion of the site are shown on 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 Lodgement 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. Lodgement 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 Lodgement Till was depbsited directly beneath the Vashon
Stade ice sheet that covered this area approximately 13,500 to 1%000 years before present. The
ice sheet that overrode the till and the underlying soils (including the advance outwash) is
estimated to be on the order of 3,000 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. Since the
retreat of the glaciers, the upper few feet of the very dense/hard soil has loosened and weathered,
and topsoil and/or colluvium 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.
Subsurface explorations were not performed at this site for this evaluation; however, soils
exposed on the non-vegetated portions of the upper 15 feet of the bank confirm the presence of
21-1-09217-001-L I.doc/WP/SJM 21-1-09217-001
Mr. John Love SHANNON ~WILSON, INC.
Event Management Corporation
December 1, 2000
Page 3
the till. The till observed on the bank appeared to be a very dense, gray, gravelly silty sand to
gravelly sandy silt. The soils located farther downslope on the bank were not adequately
exposed for visual classification from the top of the slope. However other properties on the east
side of the island in the vicinity of the site have about a 20- to 30-foot thick till cap underlain by
advance outwash consisting of very dense, interbedded sand and silt.
At the crest of the bank on the upland portion of the site, it appears that the upper 1 V2- to 2-feet
of the till at the top of the bank has weathered to a medium dense to dense condition. In
addition, the vegetated portion of the slope appeared to be covered with a relatively thin mantle
(i.e., no more than a few feet thick) of colluvium. The jack straw trees at the bottom of the slope
indicate that some slide debris may be present at the base of the hillside.
No signs of springs, seeps, damp soils, or other indication of near surface water were observed
along the portion of the steep bank and upland portion of the site that were visible from the top
of the bank.
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps of the area classify the steep bank as unstable. Evidence of recent slope
movements was observed during our site visit. Specifically, topography and absence of
vegetation delineate two recent slides. One scarp is about 35 feet wide at the top with the south
edge of the scarp about 18 feet north of the south property line. The south edge of the second
scarp is approximately 15 feet south of the north property line and extends approximately 25 feet
north of this property line. The presence of mostly small alder trees (8-inches or smaller in
diameter) across the remainder of the slope indicate that the areas beyond the recent slide scarps
have most likely undergone ground movement in the recent past. If slope movements were
relatively infrequent, we would anticipate larger diameter and other species of trees to be present
on the bank.
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 is relatively low, in
our opinion. Signs of deep-seated slope movements (i.e., failure through the very dense glacial
21-1-09217-001-LI.doc/WP/SJM 21-1-09217-001
Mr. John Love SHANNON &WILSON, INC.
Event Management Corporation
December 1, 2000
Page 4
soils beneath the site) were not observed during our site visit. The very dense glacially
overridden soils that presumably underlie the slope may be stable along 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 therefore susceptible to slope movements.
With enough time, movement of colluvium and topsoil toward the base of the bank 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 bank does not allow the
colluvium and topsoil to accumulate at the toe of the slope and maintains the slope in an over-
steepened condition. Consequently, continued movement of topsoil and colluvium on the slope
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 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 the 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 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 maintain a vegetative cover on the slope. In addition, measures
that can be taken to reduce or minimize the 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.
21-1-09217-001-Ll.doc/WP/S.IM 21-1-09217-001
Mr. John Love SHANNON ~WILSON, INC.
Event Management Corporation
December l, 2000
Page 5
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 regression, 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
one foot per year. The presence of effluent in the soils near the edge of the bank would likely
increase the regression rate. In our opinion, a minimum septic drainfield and building setback
equal to the height of the slope (i.e., 90 feet) would be appropriate 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, septic tanks) could be located closer
than 90 feet to the crest of the bank, provided that the owner is willing to accept a greater risk of
slope movement affecting these components. We recommend that a minimum setback of 50 feet
be used for these components.
The actual rate of slope regression will likely 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). By implementing the measures 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).
If water is discharged to the toe of the bank, we recommend that it be conveyed in a
flexible tightline on the surface of the bank and periodically inspected and maintained. The
tightline can be supported by steel stakes driven into the soils on the bank. The stakes should be
placed at the top and middle of the bank with a maximum 20 foot spacing. Stainless or
galvanized steel cable and clamps can be used to attach the tightline to stakes. Splices in the
21-1-09217-001 -LI .doc/WP/SJM 2 1 - 1-09 2 17-00 1
Mr. John Love SHANNON &WILSON, INC.
Event Management Corporation
December l., 2000
Page 6
tightline on the bank should be avoided. If tightline splices occur on the bank, the splice should
be supported and reinforced using stainless or galvanized steel cable attached to the tightline
both above and below the splice. The water collected in the tightline should be discharged as
close to the beach elevation as allowable. This is typically at or immediately above the high
water elevation, (e.g., at the toe of the bank where vegetation is present immediately above large
driftwood on the beach that could provide some wave erosion protection). An energy dissipater
should be located at the discharge point. The energy dissipater may be constructed by
connecting 3 feet (total width) of perforated drain pipe to form a "T" at the end of the tight line.
The "T" should be located on a minimum 12-inch-thick layer of 6-inch or larger diameter
cobbles that extend a minimum of 3 feet beyond the ends of the perforated pipe that forms the
"T". The "T" may be covered with cobbles to help secure and hide the "T." It may be necessary
to add rock to the energy dissipater from time to time due to high waves and movement of
sediment that could remove rock from around the "T."
Alternatively, the water collected in the drainage systems could be discharged to the road
ditch. This would likely require collecting the water in a catch basin or holding tank and
pumping it up to the road. Considering the construction costs and the anticipated maintenance of
the tightline and energy dissipater for discharge to the beach, it may be more cost effective to
discharge water into the road ditch.
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 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 sites.
Vegetation
Maintaining a healthy vegetative cover on the bank can reduce erosion and the rate of
slope regression. In general, native vegetation should be used on and near the bank to eliminate
the need for irrigation and wetting the soils on or near the bank. A healthy vegetative cover may
include large, healthy trees. Unhealthy trees, snags or other trees with a weak root system on the
21-1-09217-001- L 1. doc/WP/S JM ' 21 - 1-09217 -001
Mr. John Love sHANNoN ~WILSON. INC.
Event Management Corporation
December 1, 2000
Page 7
bank could be removed or timbed to reduce the risk of potential erosion and slope movement
cause by potential uprooting during heavy winds. If trees are cut from the slope, the stumps
should not be removed, and the area around the stump should be vegetated. A professional
landscaper, landscape architect, arborist or other qualified professional should be consulted in
assessing the health of the trees, vegetation on the slope, and vegetation that should be planted in
deforested areas.
Erosion Hazard
We note that the according to published 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. 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 other than soil on rough or broken land. No
indication of the erosion potential is provided on the maps. However, based on the apparent
active nature of the slope movement and the observed debris flows on the bank immediately
above the beach, 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 with respect to
erosion are used.
LIMITATIONS ~
The conclusions 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. 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
21-1-09217-001-LI.doc/WP/SJM 21-1-09217-001
Mr. John Love SHANNON &WILSON. INC.
Event Management Corporation
December-1, 2000
Page 8
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 and Engineering 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 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 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.
Pripcipal Engif~ering Geologist
WJP:GJB/wjp
Enclosures: Figure 1 - Vicinity Map
Figure 2 - Site Plan
Figure 3 - Generalized Profile A-A'
Important Information About Your Geotechnical Report
c: Eric Page, Tillman Engineering
Kevin Miller, Windermere Real Estate (via fax)
21-1-09217-001 -L1 .doc/WP/SJM 21 - 1-09217 -001
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photorevised 1973.
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SHANNON &WILSON, INC. Attachment to andpart of Report 21-1-09217-001
Geotechnical and Environmental Consultants
Date: December I, 2000
To: Mr. John Love
Event Management Corporation
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 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/environmentat 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.
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
Page I of 2 112000.
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.
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.
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 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.
To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete
geotechnical engineerin~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.
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.
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 1/2000