HomeMy WebLinkAbout021212006 Geotech Assessment Geologic Slope Stability Evaluat
Bahlburg and Hutchinson ProF~
Schwartz Road, Marrowstone:
Jefferson County, Washi~
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
Geologic Slope Stability Evalual
Bahlburg and Hutchinson Proi:
Schwartz Road, Marrowstone
Jefferson County, Washi
Jul~
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
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Tillman Engir~
141 Oak Ba,
P.O. Bo.'..
Port Hadlock, Washington
Shannon & Wilsc-
400 N 34TM Street, Sun
Seattle, Washington
21-1-091
it SHANNON &WILS, ON. !NC.
~ $~OT-,.,.!~"~t"~I,~AL AND~ ENVIRONMENTAL CONS LTANTS
July 28, 2000
Mr. Ryan Tillman
Tillman Engineering
141 Oak B ay Rd.
P.O. Box 1375
Po~ Hadlock, WA 98339
GEOLOGIC SLOPE STABILITY EVALUATION OF BAHLBURG AND
HUTCHINSON PROPERTIES, SCHWARTZ ROAD,
~RROWSTONE ISLAND, JEFFERSON CO~TY, WASHINGTON
Dear Mr. Tillman:
This letter summarizes our observations, conclusions and recommendations regarding the
stability and development of the Bahlburg (parcel No. 012-212-006) and Hutchinson properties.
We understand that a single-family residence will be constructed on each of the properties.
These conclusions are based on observations made during our visit to the site on July 27, 2000,
and published area geologic, topographic and soil maps. Preliminary observations and
conclusions were provided to Mr. Bahlburg orally upon completion of the site visit.
SITE DESCRIPTION
The referenced properties are located on Admiralty Inlet on the northeast section of Marrowstone
Island, as shown on Figure 1. As indicated on Figure 2, the properties are adjacent and extend
from Schwartz Road on the west to Admiralty Inlet on the east. 'The Bahlburg property is
approximately 480 to 500 feet long (east-west) by 107 feet wide north-south). We understand
that the Hutchinson property is also about 500 feet long by about 100 feet wide. The topographic
features across both sites are similar and consist of the following (from east to west);
A beach
· A steep waterfront bank, (approximately 70 feet high) that slopes from the beach up to
the west at about 56 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 15 degrees and less.
Tillman Engineering SHANNON ~WILSON, INC.
Mr. Ryan Tillman
July 28, 2000
Page 2
A generalized profile of the above described topographic conditions are sketched on Figure 3.
The beach consists mostly of sand and gravel. At some locations at the base of the steep bank, a
very narrow, vegetated (i.e., salt grass) backshore has developed behind scattered driftwood. At
most locations, however, no backshore is present, and the foreshore extends up to the toe of the
bank.
The bank consists of steeply sloping vegetated sections with steeper, near-vertical sections on
which little if any vegetation exists. Vegetation on the slope includes alder trees (up to about 8
inches in diameter), grasses, and black berries.
The upland portion of the properties are vegetated with native fir, cedar and madrona trees up to
about 2 feet in diameter, with an undergrowth of salal. This native vegetation is indicative of
relatively well-drained near surface soil conditions. On the Hutchinson property, a landscaped
lawn area is located at the top of the bank and extends to the west to a driveway, west of which
most of the site includes the native vegetation previously described. Structures on, or near the
landscaped portion of the Hutchinson property include a covered concrete fire pit and storage
sheds, a water well, and various campers, trailers, and cargo container boxes.
The proposed residence, septic/drainfield, and proposed water well on the Bahlburg property are
located on the upland portion of the site, the approximate locations of which are indicated on
Figure 2. The existing well on the Hutchinson property and the property north of the Bahlburg's
are also shown on Figure 2. We understand that the location for the proposed residence and
septic system for the Hutchinson property have not yet been determined.
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 and older 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
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Tillman Engineenng SHANNON &WILSON. INC.
Mr. Ryan Tillman
July 28, 2000
Page 3
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 soils have been compacted to a very dense or hard state. Since the retreat of the
l 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 bank confirm the presence of the till and
underlying advance outwash beneath the site. Specifically, till, consisting of very dense, non-
sorted, gravelly silty sand with scattered cobbles, was observed in the upper 25 feet of the bank.
Below the upper 25 feet, soils exposed on the non-vegetated portions of the bank included very
dense, interbedded sand, silty fine sand, fine sandy silt, and silt.
l In addition to the very dense till and advance outwash, it appeared that the upper 1 1/2- to 2-feet
of the till at the top of the bank had 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 about 2 feet thick) of colluvium.
No signs of springs, seeps, damp soils, or other indication of near surface water were observed
on the steep bank.
~ CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps of the area classify the steep bank as unstable. 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. The very
dense glacially overridden soils that presumably underlie the slope may be stable beneath
relatively steep slopes (e.g., 56 degrees or more). However, the relatively loose topsoil and
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Tillman Engineering SHANNON &WILSON. INC.
Mr. Ryan Tillman
July 28, 2000
Page 4
colluvium that weather from these soils are nOt as strong and are susceptible to slope movements
on slopes where the underlying glacially overridden soils are relatively stable. Recent
movements of topsoil and colluvium on the slope are indicated by the areas on the slope where
the vegetation is very sparse or absent. Signs of deep-seated slope movements (i.e., failure
through the very dense glacial soils beneath the site) were not observed during our site visit.
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 would 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.,
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.
21-1-09117-0Ol -L1. DOC/WP/SJM 21 - 1-091 17-00 i
Tillman Engineering SHANNON 8WILSON, INC.
Mr. Ryan Tillman
July' 28, 2000
Page 5
Building Set Back
The measures discussed above may reduce the risk of soil movement on a slope. One of
the most cost-effective measures to reduce the impact of slope movement is to provide an
adequate building set-back. An appropriate building set-back is a function of the rate of slope
regression, the design life of the structure, 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. In our opinion, a minimum building set-back equal to the height
of the slope (i.e., 70 feet) should be considered for this site. Greater risk reduction can be
achieved with larger building set-backs.
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). 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 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, and could be in
one of the small backshore areas behind some of the large driftwood on the bank immediately
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Tillman Engineering SHANNON ~WILSON, INC.
Mr. Ryan Tillman
July 28, 2000
Page 6
above. An energy dissipator should be located at the discharge point. The energy dissipator 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 dissipator from time to time due to high waves and
movement of sediment that could remove rock from around the "T".
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 the slope 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 could be
removed or limbed to reduce the risk of potential erosion and slope movement caused 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, or qualified professional be consulted in assessing the
arborist
other
should
health of the trees and vegetation on the slope and vegetation that may be planted.
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
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Tillman Engineering SHANNON ~WILSON, INC.
Mr. Ryan Tillman
Jul5, 28, 2000
Page 7
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
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 Tillman 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 pieased to discuss these with you if the need arises.
21-1-09117-001-L1.DOC/WP/SJM 21-1-091 17-001
Tillman Engineering SHANNON &WILSON, INC.
Mr. Ryan Tillman
July 28, 2000
Page 8
Shannon & Wilson has prepared the atta6hed, "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. Perkins, R.P.G.
Principal Engineering Geologist
WJP:GJB/wjp
Enclosures' Figure 1 - Vicinity Map
Figure 2 - Site Plan
Figure 3 - Generalized Profile
Important Information About Your Geotechnical Report
21-1-09117-00 l-Ll .DOC/WP/SJM 21-1-09 1 17-00 1
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- I i t i I I Bahlburg/Hutchinson Prope~ies
~ S~le in Miles Ma~owstone Island, Washington
~ NOTE
- VICINI~ MAP
Map adapted from 1:24,000 USGS topographic
map of Nordland, WA quadrangle, dated 1953,
photomvised 1973. July 2000 21-1-09117-001
~ _ SHANNON & WILSON, INC. J FIG 1
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II 1 SHANNON & WILSON, INC. Attachment tO and part of Report 21-1-09117-001
Geotechnical and Environmental Consultants
Date: July 28, 2000
To: Mr. Ryan Tillman
Tillman Engineering
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
ll for and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first
you
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/environmental report. The consultant should be kept apprised
l 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
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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 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 P/-acticing in the Geosciences, Silver Spring, Maryland
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