HomeMy WebLinkAboutBLD2000-00700 Geotechnical Report 12;28/00 16:42 FAX 206 695 6777 SHANNON & W'ILSON IZ002
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SHANNON6WILSON INC.
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GECTE'ChNICAL AND SNVIRONVENTAL CONzLJLTANTS LCUA
30_1T4N
December 28, 2000 pECIEodE
JAN u 2001
Mr.Denny Fish
Route 1,Box 9 JEFFERSON COUNTY
Harrison, Idaho 83833 DEPT.OF COMMUNITY DEVELOPMENT
RE: GEOLOGIC SLOPE STABILITY EVALUATION OF
2314 EAST MARROWSTONE ROAD,NORDLAND,WA
This letter summarizes our observations,conclusions and recommendations regarding the
stability and development of the referenced property for a single-family residence,including
septic system, These conclusions are based on observations made during our visit to the site on
December 22,2000, an undated site plan by Casal Construction,available published area
geologic,topographic and soil maps, and areal photos provide by Mr.Fish.
SITE DESCRIPTION
The referenced property is located on Admiralty Inlet on the cast side of Marrowstonc Island,as
shown on Figure 1. The property is bisected by East Marrowstone Road,with most of the
planned development to be located on the portion of the property east of the road. As indicated
on Figure 2,this portion of the property extends from East Marrowstone Road on the west to
Admiralty Inlet on the east. This portion of the property is approximately 262 to 264 feet long
(east-west)by 109 to 117 feet wide(north south). Topography across this portion of the consists
of the following(from east to west);
■ A beach
b` A steep waterfront bank, (approximately 70 feet high)that slopes from the beach up to
the west with a near-vertical upper bluff.
A relatively flat upland at the top of the waterfront bank that slopes back down to the
west at about 1 to 2 degrees.
A generalized profile that shows the topography is shown on Figure 3.
400 \ICRTH 34TH STREET•SUJTE 7no
RD, EQX 300303 21-1-09273-001
SEATTLS, WASHINGTON 98103
205.632.8020 FAX 205.533.8777
TCO: 1,800.633•5388
12/28/0u 16:12 FAX 206 695 6777 SHANNON & WILSON 003
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Mr.Denny Fish SHANNON€sWILSON,INC.
December 28, 2000
Page 2
At the time of our site visit, the beach and lower reaches of the waterfront bank were inaccessible
from the upland portion of the site. We estimate that the height of the bank is approximately 70
feet, with about the upper 30 feet a near vertical bluff and about the lower 40 feet a steep slope
that descends down to the east at about 45 to 50 degrees. These estimates are reasonably
consistent with the site plan provided by Castle Construction which indicate the bank is 69 feet
high,with the upper 31 feet near vertical and the lower 38 feet sloping down to the east at 45
degrees. As observed from the top,the bluff portion of the bank (upper 30 feet)does not support
any vegetation. Vegetation on the sloping, lower reaches of the bank consists of grasses and
scotch broom with scattered small (less than 1/2 feet in diameter)madrona trees. Salt grass was
observed along the toe of bank along the beach behind scattered drift wood.
The upland portion of the property is largely cleared and vegetated mostly with grass with
scattered scotch broom and madrona and fir trees (up to about 1 foot in diameter). This
vegetation is indicative of relatively well-drained near surface soil conditions.
Conceptual locations of the proposed residence and septic/drainfield on the property are on the
upland portion of the site, the approximate locations of which are indicated 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. To the north and south of the site,these maps
indicate the till is 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 preexisting land surface,in front of the continental Vachon Stade ice sheet that advanced
from Canada across the Puget Sound region approximately I7,000 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 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 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
114-O 273-001-1�1rwpnMCI 21-1-09273-001
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Mr. Denny Fish SHANNON 5WILSQN.ING.
December 28, 2000
Page 3
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 duc 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, observed
soils exposed on the non-vegetated portions of the upper 30 feet of the bank confirm the presence
of the till. The till observed on the bank appeared to be a very dense, gray, slightly silty to silty,
gravelly sand. At the base of the bluff, scattered exposures of stratified sand were observed
which likely correspond to the advance outwash and which probably underlies the remainder of
the waterfront bank below the till bluff.
At the crest of the bank an the upland portion of the site,it appears that the upper 2-to 3-feet of
the till at the top of the bank has an orange mottled appearance and has weathered to a medium
dense to dense condition. The vegetated portion of the waterfront bank(the lower slope)
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,hydrophilic vegetation or other indication of near surface
water were observed on the waterfront bank or upland portion of the site or on the adjacent
properties during the site visit.
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps of the area c]aasify the steep waterfront slope immediately to the north
and south of the site as unstable. The lack of vegetation on the bluff and the sparse vegetation on
the lower slope is indicative of slope movements that occur at a sufficient frequency to preclude
growth of trees larger than the scattered small madrona on the lower slope.
Based on our observations of the site and our experience with similar sites in the Puget Sound
region, it appears that the bluff face may be susceptible to spilling, while the unstable soils on
the lower slope are primarily the topsoil and colluvium,. The risk of deep-seated slope movement
21.1.09273.001•L1IWP/Moi 21-1-09273-001
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Mr.Denny Fish SHANNON€iWLSON,INC,
December 28, 2000
Page 4
is relatively low, in our opinion. Signs of deep-seated slope movements(i.e., extensive failure
through the very dense glacial soils beneath the site)were not observed during our site visit.
Where the bank is near vertical, portions of the glacially overridden soil that form the bluff may
spell off from time to time. Based on our experience, occasional spalling of the bluff may cause
portions of the crest of the bank to recede 2 to 10 feet during a single event. However,the time
between spelling events may be several years during which no noticeable regression of the crest
of the bank occurs.
The very dense glacially overridden soils that presumably underlie the lower, sloping portion of
the bank may be stable beneath relatively steep slopes (e.g., 50 degrees or more), However,the
relatively loose topsoil and colluvium that weather from the glacially overridden soils are not as
strong and are susceptible to slope movements on slopes on which the underlying glacially
overridden soils are relatively stable.
tx1 t1 a„tingh time,movement of oolluxaum and topsoil toward the baoa of the lsnitlt Arid
continued weathering and erosion and spelling 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.
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
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slope movement,the rate of slope regression may also be reduced.
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,
21-1-)9173-001•L1/RP/MG! 21-1-09273-001
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Mr.Denny Fish SHANNON 8VVELSON,INC.
December 28, 2000
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 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 bank or on 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 in tightlines, and minimize the amount of
surface water that could flow down the face of the bank), 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,not removing large wood debris or driftwood
near the top of the beach)will decrease the rate at which the slope erodes,in our opinion.
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 bluff may increase
the regression rate. In our opinion, a minimum septic drainfield and building setback equal to
the height of the bank(i.e., about 70 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,septic tanks)could be located closer than 70 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
maintained for these components even if the owner is willing to accept a great risk of slope
movement.
214-09273-001-L1/WP340 1 21-1-09273-001
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Mr. Denny Fish SE-'�kNNOE�i WEl.SDN.INC
December 28, 2000
Page 6
Drainage
In gcnersl,reducing the amount of water entering and discharging onto the slope can
reduce the risk of slope movement. Drains should be constructed and maintained to collect water
from impermeable surfaces on the property(e.g., roof,decks,patios, and driveways) and directed
in a tightline to a suitable discharge point(e.g.,bottom of the bank or road ditch). Because the
upland portion of the site slopes down to the west towards the road, we recommend that water be
discharged to the road ditch. While it is possible to discharge the water at the toe of the
waterfront bank, it is our opinion that construction costs and maintenance would favor
discharging water to the road ditch. Recommendations to convey water over the face of the
waterfront bank and discharge at the toe of the slope can be provided if 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 on and at the crest of the bank can reduce erosion
und tho sato of elepa oseraanis,,. Tn 6anaral,viativa vab.,tatis..s1,s�,111,, uoi.1 u a uuL1 uk..eu t11c:
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 at the crest or on the bank 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 off the crest of the bank or on the lower slope,the stumps should not be removed,
and the area surrounding the stump should be vegetated. A professional landscaper, landscape
architect, arborist or other qualified professional should be consulted in assessing the health of
wale uwo u11u vi.g;LGliull ulL uiC.574.41.JG 411U vc6ci.L1 1U1) li146 clay un i.IliiJu.eU.
1-1-04273-001•1.1/WP/M61 21-1-09273-00 1
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Mr. Denny Fish SHANNON&WILSON,INC.
December 28, 2000
Page 7
Erosion Hazard
We note that according to published US Department of Agriculture(USDA) soil maps, surficial
soils on the upland portion of the site are classified as Whidbey gravelly sandy loam Con 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 bank. Based on the apparent
active nature of the slope movement, 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 bank
provided the recommendations in this letter are followed and prudent construction practices with
respect to erosion are used.
LINIITATIONS
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
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 Mr. Fish 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.
21-1-09273-00 .LInvriatct 21-1-09273-001
12/26/G0 16:47 FAX 206 695 6777 SHANNON & WILSON 421009
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Mr.Denny Fish SHANNON L WILSDN.INC.
December 28, 2000
Page 8
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.
William J. Perki .P.G.
Principal Engine ng Geologist
WJP:H :wjp
Enclosures: Figure 1 —Vicinity Map
Figure 2—Site Plan
Figure 3—Generalized Profile A—A'
Important Information About Your Geotechnical Report
c: Mr.Cary Casal,Casal Construction
21 a-09273.ao1•L1 PNWMGt 21-1-09273-001
12/28/00 1648 FAX 206 695 6777 SHANNON & WILSON zoio
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i
SHANNON &WILSON, INC. Attachment to and part of Report 21-1-0g273-001 ,
Geotechnlcal and Environmental Consultants
Date; December 2B,2000 _
To: Mr.Denny Fish
Harrison,Idaho
Important Information About Your GeotechnicalfEnvironmental 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. Na 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 gcotcchnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors.
Depcndieg 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
unrcfrigerated 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/envirotmrental 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 obeervc 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. Cnly the
consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's
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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`cn misinterpretation of a geotechnicallenvironmental
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 afield 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
geotechnicalienvironmertal 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 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 limitattortc,assuming that a contractor was not one of the specific persons for whom the
purposesforwhich it wasle
report was prepared,and that developing construction cost�tnnates was not one of the specific bleb 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 geotechnicallenvirorunental 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`,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 yuur 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
ASFEi'Association of Engineering Firms Practicing in the Geosciences,Silver Spring,Maryland
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