HomeMy WebLinkAbout021332005 Geotech Assessmentn-
- ALASKA
FLORIDA
~' ~ ~ SHAIV NON ~ WI LSON, i NC. M3SOUR~
- GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS OREGON
WASHINGTON
February 1, 2006
Mr. Kevin Miller
Windermere Port Ludlow
9526 Oak Bay Road
Port Ludlow, WA 98365
RE: GEOLOGIC SLOPE STA$ILITY EVALUATION TAX PARCEL
N0.021332005, EAST MRROWSTONE ROAD, MARROW5TONE ISLAND,
JEFFERSON COUNTY, V~JAST3INGTON
Dear Mr. Miller:
This letter summarizes our observations, conclusions, and recommendations regarding the
stability and development of the property referenced above for asingle-family residence,
including septic system. These conclusions and recommendations are based on observations
made during our visit to the site on December 13, 20,05; a site plan by Tillman Engineering
dated November 22, 2004; and available published geologic, topographic, and soil maps of the
area. Preliminary observations and Conclusions were provided to Mr. Howland :orally upon
completion of the site visit.
SITE DESCRIPTION
The referenced property is located on Admiralty Inlet on the eastside of Marrowstone Island,
as shown in Figure 1. As indicated Figure 2, the property extends from East Marrowstone
Road on the west to Admiralty Inlet on the east. The property is approximately 409 `to 416 feet
long (east-west) by 94 feet wide nor(h-sosath, Topography across the site is illustrated in
Figure 3 and consists of the followir~!g (from east to west):
~~
- A beach.
- A steep waterfront bluff (approximately 55 to 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.
400 NORTH 34TH STREET • SUITE 100
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206.632.8020 FAX 206.695.6777
TDD: 1.800.833.6388
www. shannonwilson.com
21-1-20439-001
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 2
SHANNON FaWILSON, INC.
The bluff consists of steeply sloping, sparsely vegetated sections with steeper, near-vertical
sections on which little, if any, vegetation exists. Where present, vegetation on the slope
consists mostly of grasses, with scattered madrona and deciduous trees (up to about 18 inches
in diameter) along the crest.
The eastern half of the upland portion of the property is cleared and vegetated mostly with
grass. Vegetation on the western half of the upland portion of the site includes fir, cedar, alder,
and maple trees (up to about 2%2 feet in diameter) with an undergrowth of sword fern, salmon
berry, grasses, and salal. The native madrona and salal vegetation is indicative of relatively
well-drained, near-surface soil conditions.
Conceptual locations of the proposed residence, septic/drainfield, and proposed water well on
the property are on the upland portion of the site, the approximate locations of which are
indicated in Figure 2.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the site is underlain byPleistocene-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 area approximately 13,500 to
17,000 years before present. The ice sheet that overrode the till and the 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. Since the retreat of the glaciers, the upper few feet of the very denselhard
soil has 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
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F. . ,
21-1-20439-001
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 3
SHANNON F~WILSON. INC.
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 third (about 20 feet) of the bluff
and appeared to be a very dense, gray, silty, gravelly SAND. Advance outwash was observed
in approximately the lower two-thirds (about 40 feet) of the 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 and in the septic test pits on the upland portion of the site, it appears
that the upper 1 to 2 feet of the till hes weathered to a medium dense to dense condition. In
addition, 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 indication ofnear-surface water was observed
on the bluff on the site or on the property to the south. Approximately 300 to 400 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.
The lack of vegetation on the bluff indicates that movement occurs frequently enough to
preclude the growth of mast vegetation other than grasses. If slope movements were relatively
infrequent, we would anticipate more vegetation, including trees, to be present on the slope and
not only at the crest.
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 ofdeep-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., topogr~~ibowls, step-downs, or a serrated bluff crest)
zi-i-ZOas9-ooi-LiiwpiLxn ~ ~ 21-1-20439-001
~~ ? ~
~,
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 4
SHANNON F~WILSON, INC.
were not observed during our site visit. The very dense, glacially overridden soils that underlie
the bluff have a relatively high strength and would 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 upslope 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 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 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 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 beyond
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 rate of wave erosion at the toe of
the slope (e.g., construction of a seawall and not removing large wood debris or driftwood near
the top of the beach) will decrease the rate at which the slope erodes.
21-1-20439-001-LI/wp/LKD 21-1-20439-001
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 5
Septic Drainfield and Building Setback
SHANNON ~WIL.SON, INC.
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 of the slope (i.e., 55 feet) would be adequate for this site. Greater
risk reduction can be achieved with longer 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 55 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 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).
In our opinion, the water collected in the drainage systems could be discharged to the
road ditch. It may be possible to use the swale along the north property line (see Figure 2) to
convey the water to the road ditch. Because'af the relatively flat topography of the upland
zi-i-zoa39-ooi-r.v~iucn a 21-1-20439-001
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 6
SHANNON WILSON, INC.
portion of the site, we could not readily discern the direction that water would likely flow in the
Swale. Discharging water to the road ditch via the Swale or some other means could require
collecting the water in a catch basin or holding tank and pumping it to the road ditch. A survey
of the elevation of the road ditch and the planned elevation of the residence and drains will be
required to determine if a holding tank and pump are required.
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 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. Unhealthy trees, snags, or other trees with a
weak root system on the crest of the bluff could be removed or limbed to reduce the risk of
potential erosion and slope movement cause by potential uprooting during heavy winds. If
trees are cut from the crest of the slope, the stumps should not be removed and the area around
the stump should be re-vegetated. A professional landscaper, landscape architect, arborist, or
other qualified professional should be consulted in assessing the health of the trees and
vegetation on the slope and vegetation that may be planted.
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. 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 th a"~~ pi~t~l ied on the maps. However, based on the apparent
zi-i-zoa39-ooi-LV~~cn ~ ~~ ~ . ~ ~~ ~~ ~ ~ ~~ 21-1-20439-001
Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 7
SHANNON F~WILSON. INC.
active nature of the slope movement on the bluff, it is our opinion that the erosion potential of
the soils maybe 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 site 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 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 was prepared. We make
no other warranty, either express 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 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 pleasedlto discuss these with you if the need arises.
zi-i-zoa39-oo~-ui~R,[cn 21-1-20439-001
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Mr. Kevin Miller
Windermere Port Ludlow
February 1, 2006
Page 8
SHANNON F~WILSON, INC.
Shannon & Wilson has prepared the enclosed, "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,
SFIANNON & WILSON, INC.
/v (Q
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
c: Mr. Larry Howland
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SHANNON & WILSON, INC.
Geotechnical and Environmental Consultants
Attachment to and part of Report 21-1-20439-001
Date: February 1, 2006
To: Mr. Kevin Miller
Windermere Port Ludlow
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVIRONMENTAL
REPORT
CONSULTING SERVICES ARE PERFORMED FOR SPECIFlC 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 otte 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 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 warehouse will be built instead of an
unrefi-igerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or co~guration 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 acent site. Consultants cannot aecvpt 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 geotechxucal/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 adj acent 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 maybe 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.
~~~:
~~ e
Page 1 of 2 1/2006
~ SHANNON 8~ WILSON, INC. Attachment to and part of Report 21-1-20439-001
- Geotechnical and Environmental Consultants
Date: February 1, 2006
To: Mr. Kevin Miller
Windermere Port Ludlow
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 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.
AgeotechnicaUenvironmental 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 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 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 acent 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 maybe 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.
t + ~ , ~,-.
,~ Page 1 oft 1/2006