HomeMy WebLinkAbout964500401 Geotech AssessmentGeologic Slope Stability Evaluation
Broilers Property (Tax Parcels 964-500-401,
964-501-102, & 964-501-103)
Gardiner Road
Jefferson County, Washington
May 2001
GEDTECHNICAL AND ENVIPONMENTAL CONSULTANTS
Ai Shannon & Wilson, our mission is to be a progressive, well-
managed professional consulting firm in the fields of engineering
and applied earth sciences. Our goal is to perform our services
with the highest degree of professionalism tivith due consideration
to the best interests of the public, our clients, and our employees.
Submitted To:
Mr. Ray Broilers
5571 OId Gardiner Road
Port Townsend, Washington 98368P
By:
Shannon & Wilson, Inc.
400 N 34`h Street, Suite 100
Seattle, Washington 98103
21-1-09302-001
~% SH~-NNON WILSON INC SEATTLE
RICHLAND
FAIRBANKS
.
~ ANCHORAGE
"' GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS DENVER
SAINT LOUIS
BOSTON
°" May 11, 2001
Mr. Ray Broders
557101d Gardiner Road
Port Townsend, Washington 98368
Y
RE: GEOLOGIC SLOPE STABILITY EVALUATION, BRODERS PROPERTY
(TAX PARCELS 964-500-401, 964-501-102 & 964-501-103),
GARDINER ROAD, JEFFERSON COUNTY, WASHINGTON
Dear Mr. Broders:
This letter summarizes our observations, conclusions, and recommendations regarding the
stability and development of the referenced properties for single family residences. These
conclusions are based on observations made during our visit to the site on February 3, 2001,
available published geologic, topographic, and soil maps, and cross-sections and site plans by
JSF Engineering, dated January 30, 2001, and Apri112, 2001, respectively. Preliminary
observations and conclusions were provided to Mr. Broders orally upon completion of the site
visit.
SITE DESCRIPTION
The referenced site is located on the west side of Discovery Bay, as shown on Figure 1. As
~- indicated on Figure 2, the site will be developed into 4 contiguous waterfront properties and will
be the result of consolidation of a number of smaller lots. The 4 contiguous properties are
approximately 760 feet long (northeast-southwest, parallel to the shoreline) by 165 to 210 feet
wide (northwest-southeast, perpendicular to the shoreline). The topography across the site
includes the following.
- A beach
- A waterfront slope, (approximately 35 to 40 feet high) that slopes from the beach up to
the northwest between about 38 and 45 degrees.
- A flat bench approximately 40 to 55 feet wide.
- A cut bank (approximately 7 to 11 feet high) that slopes up to the northwest at about 38
to 42 degrees.
400 NORTH 34TH STREET • suiTE 100 21-1-09302-001
,.,, P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206.632.3020 FAX 206.695.6777
TDD: 1.800.833.6388
Mr. Ray Broilers
May 11, 2001
Page 2
SHANN®N ~~ILS®N, iN~C,
- A flatter slope (approximately 40 to 60 feet wide) that slopes up to Old Gardiner Road at
about 11 degrees.
w A generalized profile that shows the approximate ground surface topography is shown on
Figure 3.
The beach consists mostly of sand and gravel. At some locations at the base of the slope a very
narrow, vegetated (i.e., salt grass) backshore has developed. At most locations, however, no
backshore is present, and the foreshore extends up to the toe of the waterfront slope.
The waterfront slope is vegetated with madrona and fir trees up to about 21/z feet in diameter and
scattered alders. Undergrowth is sparse and includes salal, ivy, and grasses. This vegetation is
indicative of relatively well drained near surface soil conditions. Many of the tree trunks are
°° bowed down slope, which is indicative of soil creep. Soil creep is the slow, gradual down slope
movement of near surface soils under the effects of gravity and water and occurs on most slopes
to some degree. Recent, shallow slides have removed vegetation off of portions of the slope,
exposing very dense sand and gravel beneath the slope. The approximate locations of the recent
slide scarps are shown on Figure 2.
On the upland portion of the site, the flat bench above the waterfront slope is an old railroad
grade that was reportedly constructed around 1912. The cut bank on the northwest side of the
bench was presumably excavated when the railroad grade was constructed. Gravelly, sandy soils
with cobbles were observed on the surface of the cut bank. Above the cut bank, the ground
-, slopes gently up to Old Gardiner Road. Except along the crest of the waterfront slope, most of
the upland portion of the site has been cleared over time, and existing vegetation consists mostly
of grasses, and salal, and other shrubs and small trees.
The proposed residences, septic drainfields and reserve drainfields are located above the cut bank
as shown on Figure 2. We understand that the sizes and locations of the proposed residences
indicated on Figure 2 are conceptual. We also understand that portions of the cut bank south of
the proposed septic drainfields will be regraded to 2 horizontal to 1 vertical.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the site isunderlain byPleistocene-age Vashon
Recessional Outwash. While subsurface explorations were not performed at this site for this
21-1-09302-001-L1/WPlMGI 21-1-09302-001
Mr. Ray Broilers
May 11, 2001
Page 3
SHANNt~N ~U~IfLSON, 9NC.
evaluation, recessional outwash and older, Vashon Drift and Vashon Lodgement Till are exposed
in portions of the waterfront slope. A generalized profile of the site stratigraphy is shown in
Figure 3. The following provides a brief description of the observed stratigraphy.
Lodgement till is typically an unsorted mixture of clay, silt, sand and gravel with occasional
.cobbles and boulders that is deposited directly beneath glacial ice. 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 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. The till at the site
~.. was observed in the lower 10 to 15 feet of the slope and appeared to be a very dense, slightly
silty to silty, sandy gravel and gravelly sand.
`-
Inscattered locations, the till appears to be overlain by up to 5 feet of a drift consisting of very
dense, fine sand to fine sandy silt with faint bedding. The drift may be a very localized ice
~' contact deposit. Otherwise, above the till, the remainder of the slope appeared to be underlain by
very dense recessional outwash, consisting of slightly silty, sandy gravel with scattered cobbles
and boulders. As the Vashon ice sheet that covered much of northwestern Washington receded
north towards the Straight of Juan de Fuca, melt water emanating from the ice sheets carried
large amounts of sand and gravel that was deposited as.recessional outwash across the pre-
' existing land surface. The recessional outwash deposit that underlies the west side of Discovery
Bay where the site is located is indicated as being deposited in a delta or alluvial fan.
Since the retreat of the glaciers, the upper few feet of the very dense/hard soil has loosened and
weathered, and topsoil, colluvium and/or slide deposits 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. Recent landslide deposits
~, were observed at the base of the slide scarps along the toe of the waterfront slope.
No signs of springs, seeps, damp soils, hydrophilic vegetation, or other indication of near surface
water were observed on either of the waterfront slope or bench on, or adjacent to the site.
21-1-09302-001-L1/WP/MGI 21-1-09302-001
Mr. Ray Broilers ~HANN®N ~~iVILSON, lNC.
May 11, 2001
Page 4
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps classify the area as unstable. Based on our observations of the site, the
.~ unstable soils are primarily the topsoil and colluvium on and near the crest of the waterfront
slope that have weathered from the underlying recessional outwash, drift and till, and existing
landslide deposits located along the toe of the slope. Based on our observations, it is our opinion
that the risk of deep-seated slope movement is relatively low. The dense to very dense glacial
soils that presumably underlie the waterfront slope maybe stable beneath relatively steep slopes
~, (e.g., 40 degrees or more). However, the relatively loose topsoil and colluvium that weather
from these soils are not as competent and are susceptible to slope movements on slopes on which
the underlying glacial soils may be relatively stable. The slide scarps observed on the slope
appear to be consistent with relatively shallow movements of topsoil and colluvium. These slope
movements reportedly occurred during the severe snowmelt and rain storm of December 31,
-- 1996/ January 1,1997. Signs of deep-seated slope movements (i.e., failure through the dense to
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 waterfront slope
and continued weathering and erosion of the glacial soils up slope would result in a flatter, more
~. stable slope. However, wave erosion at the toe of the bank does not allow the slide deposits,
colluvium, and topsoil to accumulate at the toe of the slope and maintains the slope in an over-
steepenedcondition. Consequently, continued movement of topsoil, colluvium and existing slide
deposits 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
slope movements).
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 breakslleaks, 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.
-- Z1-1-09302-001-LI/WP/MGI 21-1-09302-001
Mr. Ray Broilers SHA~N~~ ~lLSC~, !l~.
May 11, 2001
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 on or at the crest of the waterfront slope), maintaining the slope as dry as
~' possible (e.g., locate septic drainfields away from the waterfront slope, 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 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, in
our opinion.
Septic 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 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 waterfront slope is unknown; regression rates 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 waterfront slope 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 35 feet on the south to
about 40 feet on the north) would be adequate for this site. Greater risk reduction can be
achieved with larger setbacks.
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
in a tightline to a suitable discharge point (e.g., bottom of the waterfront slope above the ordinary
high water mark in one of the backshore areas). If water is discharged to the toe of the slope, 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 slope. The stakes should be placed at the top and middle of the bank with a maximum 20
"" 2I-t-09302-001-L1/WPlMGI 21-1-09302-001
Mr. Ray Broilers S1-1ANN®N ~~UIL.~t~N,1NC.
May 11, 2001
_ Page 6
~. foot spacing. Stainless or galvanized steel cable and clamps can be used to attach the tightline to
stakes. Splices in the tightline on the slope should be avoided. If tightline splices occur on the
~- slope, 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 slope immediately
above. 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".
Based on our understanding of the limited, single-residence development of these
_ properties 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
and the rate of slope regression. In general, native vegetation should be used on and near the
waterfront slope 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 at the crest or on the slope 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 slope 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 the trees and vegetation on the slope and vegetation that may be planted.
..,, 21-1-09302-001-LI/WPIMGI 21-1-09302-001
Mr. Ray Broilers
May 11, 2001
Page 7
Erosion Hazard
SHANNON ~W~LSON, lNC.
We note that the according to published USDA soil maps, surficial soils on the upland portion of
the site are classified as Hoypus gravelly loamy sand C on 0 to 15 percent slopes. The USDA
maps indicate that these soils have a slight to moderate .erosion hazard. It is anticipated that the
~' proposed development on the site will not significantly affect the erosion and associated hazard
of the soils on the site provided 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 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 assessments 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.
21-1-09302-001-LI/WP/MGI 21-1-09302-001
Mr. Ray Broders SHANN®N ~~V~L~(~N, lNC.
May 11, 2001
Page 8
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.
!/ pR`n\
William J. Perkins, R.P.G.
Principal Engineering Geologist
WJP:GJB:wjp
Enclosures: Figure 1-Vicinity Map
Figure 2 -Site Plan
Figure 3 -Generalized Subsurface Profile A-A'
Important Information About Your Geotechnical Report
c: Mr. Jerry Smith, Department of Community Development,
Jefferson County, Washington
21-1-09302-001-L1/WP/MGI 21' 1-09302-001
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NOTE
Map adapted from 1:24,000 USGS topographic
map of Port Townsend, South, Washington,
dated 1953, photo revised 1981.
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... SHANNON & WILSON, INC. Attachment to and part of Report 21-1-09302-001
_ Geotechnical and Environmental Consultants
Date: May 11, 2001
_ To: Mr. Ray Broilers
-... Port Townsend, Washington
Important Information About Your GeotechnicaUEnvironmental 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 geotechnicallenvironmental 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 byscope-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 adj scent 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 geotechnicallenvironmental 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 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 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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