HomeMy WebLinkAboutBLD2006-00199 Geotechnical Report - cA°\
ALASKA
III SHANNON 6WILSON, INC. pRA O
FLORIDA
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GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS OREGON
WASHINGTON
PERMIT # BID
June 23,2006
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Mr. Jerry Coburn JEE1tf SSO►� LOi1iVT� �ICD
83 Timber Meadow Drive
Port Ludlow,WA 98368
RE: GEOLOGIC SLOPE STABILITY EVALUATION,385 OLD OAK BAY ROAD,
PORT HADLOCK,WASHINGTON
Dear Mr. Coburn:
•
This letter summarizes our field observations, conclusions, and recommendations regarding the
stability and development of the property referenced above for a single-family residence,
including septic system. These conclusions and recommendations are based on observations
made during our visit to the site on May 9,2006; site plans by Mitchell Design, dated May 16,
2006; our experience with nearby properties; and available published geologic, topographic and
soil maps of the area. Preliminary observations and conclusions were provided to you orally at
the completion of the site visit.
SITE DESCRIPTION
The referenced property is located approximately 2 miles southeast of Port Hadlock, on the west
side of Oak Bay, as shown in Figure 1. The property extends from Old Oak Bay Road on the
west to Oak Bay on the east. The property is approximately 714 feet long(east-west) and varies
in width(north-south) between approximately 25 and 190 feet. The proposed building site is
located at the east end of the property(see Figure 2), where the width (north-south) is relatively
uniform at about 116 feet. The topography across the east end of the property and in the vicinity
of the proposed building site is illustrated in Figure 3 and consists of the following(from east to
west):
► A beach
► A steep waterfront slope, (approximately 40 feet high)that slopes from the beach up to
the west at about 50 to 55 degrees with near-vertical section(about 20 feet high)at the
base.
► A gently sloping upland that slopes up to the west towards Old Oak Bay Road at about
5 to 10 degrees.
400 NORTH 34TH STREET•SUITE 100 21-1-20519-001
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
205-632.8020 FAX 206.695.6777
TOO; 1.800.833.5388
www.shannonwilson.com
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Mr. Jerry Coburn
SHANNON&W ,INC.
June 23,2006 !
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The lower,near-vertical section of the steep waterfront slope has little vegetation, consisting
typically of scattered grasses and small alder trees (up to about 4 inches in diameter). Vegetation
on the upper portion of the slope includes grasses,blackberries, alders,and cedar trees up to
about 1 foot in diameter. In addition, hydrophilic vegetation, indicative of near-surface water or
damp soil conditions,was observed in scattered locations on the lower reaches of the upper
slope.
The upland portion of the site slopes up toward the west to Old Oak Bay road at about 5 to
10 degrees. The proposed building site on the upland portion of the property is cleared and
vegetated mostly with grass with some cedar trees(up to about 21/2 feet in diameter). As shown
in Figure 2, an existing drainage easement crosses the proposed building site and extends down
along the north property line to the beach. We understand that an existing drainage system,
including perforated subsurface drains, tightlines, catch basin, and beach discharge point for the
residence on the property immediately to the south are located within the existing easement.
We understand that you propose to construct a garage and a two-to three-story,wood-framed
residence with an east-facing, day-lighting basement on the building site. The approximate
location and dimension of the garage and residence are shown in Figure 2. Prior to our site visit,
we understand that the proposed residence was located farther east and closer to the waterfront
slope. After completion of our site visit and discussions with you,the location of the proposed
residence has been relocated to the west, farther away from the waterfront slope, to the current
proposed location indicated in Figure 2. To accommodate the currently proposed location,we
understand that the existing drainage easement on the site will be vacated, and the new easement
and drainage system will be relocated to the east of the proposed building site. The existing and
new drainage easements are also shown in Figure 2. We understand that the proposed septic
drain field will be located about 200 to 300 feet west of the proposed building site, near Old Oak
Bay Road.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the site is underlain by Pleistocene-age(13,500
to 17,000 years old) glacial deposits,including Vashon Lodgment Till and Vashon Advance
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SHANNON f.MILSON,INC.
Mr. Jerry Coburn
June 23, 2006
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Outwash,underlain by Tertiary-age sandstone. The occurrence of these geologic units near the
waterfront slope is shown in the generalized profile in Figure 3.
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 and 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. As the
glaciers receded,meltwater deposited sand and gravel (recessional outwash) at some locations on
the newly exposed land.
Since the retreat of the glaciers,the upper few feet of glacial deposit exposed at the ground
surface has typically 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 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 slope on and near the site confirm the presence of
sandstone and siltstone and the likely presence of glacial till and advance outwash deposits.
Most of the subsurface material exposed on the slope at the site is sandstone and siltstone
exposed in the near-vertical lower portion of the slope. The sandstone/siltstone is very low
strength,gray to orange-brown,weakly cemented, slightly indurated,medium-bedded,medium-
jointed,moderately to highly weathered rock. The rock bedding observed in the slope has an
apparent dip of about 4 to 5 degrees to the north. On nearby properties, exposures of till and
advance outwash were observed on the upper portions of the slope.
Slight to moderate seepage was observed on the lower reaches of the upper portion of the steep
waterfront slope and in the rock located in the lower near-vertical face. At the time of our site
00411 * RIP
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Mr. Jerry Coburn
June 23,2006
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visit,the seepage extended from the south property line, north approximately 36 feet. Similar
seepage zones on the waterfront slope were observed on properties to the north and south of the
site. We also observed moderate flow(estimated less than about 1/2 gallon per minute) into the
existing catch basin along the north property line(see Figure 2) from drains that are reportedly
subsurface trench drains buried in the existing drainage easement south of the catch basin.
CONCLUSIONS AND RECOMMENDATIONS
Slope Stability
Geologic hazard maps of the area identify areas of recent slope movements on the waterfront
slope in the vicinity of the site. Based on our experience and observations on other projects in
the immediate vicinity,these slope movements consist of both relatively shallow and deep-seated
slides.
Typically, shallow slides occur in the in the colluvium, weathered glacial deposits, and/or
weathered rock. The weathered materials are not as strong as the un-weathered glacial deposits
or rock and are susceptible to movement on steep slopes where the underlying unweathered
materials are relatively stable. With enough time, movement of colluvium and slide debris
toward the base of the slope and continued weathering and erosion of the glacially overridden
soil and rock upslope would result in a flatter,more stable slope. However, wave erosion at the
toe of the slope does not allow the colluvium and slide debris to accumulate at the toe of the
slope and maintains the slope in an over-steepened condition. Consequently, continued
movement of colluvium and weathered glacial deposits and weathered rock on the steep
waterfront slope should be expected in the future.
Deep-seated slides in the vicinity of the site that we have observed appear to be related to
relatively high hydraulic gradients or groundwater in the rock and soil, and the relatively low
strength of the sandstone/siltstone exacerbated by the presence of very low-strength lenses or
beds of highly weathered claystone or shale. Deep-seated slides may extend a few tens to a few
hundreds of feet west beyond the crest of the waterfront slope. The slight to moderate seepage
observed on the slope indicates that groundwater is present within the rock and overlying soils
and is an indicator of the slope's susceptibility to deep-seated slope movements. However, the
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Mr. Jerry Coburn SHANNON iWILSON,INC.
June 23, 2006
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very low-strength lenses or beds of highly weathered claystone or shale were not observed in the
rock exposed in the slope at the site.
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 the risk
reduction measures discussed in this letter are implemented,it is our opinion that the proposed
development will not adversely impact the stability of the adjacent properties.
Measures to Reduce the Risk Posed by Slope Movement
In general, the risk of soil movement on a slope can be reduced by not over-steepening the slope
(e.g., do not excavate the toe of the slope),not increasing the weight on the slope(e.g., do not
place yard debris or fill at the crest of the slope), maintaining the slope as dry as possible(e.g.,
locate septic drain fields away from the bank,route roof downspouts, yard drains, trench
subdrains, and footing 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 maintaining a
vegetative cover on the slope.
Septic Drain field 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 drain field and building setback. An appropriate setback is a function
of the rate of slope regression(i.e.,how fast the slope moves landward by erosion and/or
landsliding), the design life of the structure,the amount of water the drain field 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 a few feet per year.
The presence of effluent in the soils near the waterfront slope could contribute to higher
or increased groundwater levels and an increased potential for slope movements. In our opinion,
a minimum septic drain field setback of 100 feet from the top of the steep waterfront slope
RiD
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Mr. Jerry Coburn SHANNON ZIWILSON,INC.
June 23,2006
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should be used at this site. In our opinion, building setback of at least 50 feet from the top of the
slope should be used at 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 50 feet to the top of the steep waterfront slope,
provided the owner is willing to accept a greater risk of slope movement affecting these
components. We recommend that a minimum setback of 40 feet for these components be used.
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
shallow or deep-seated 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. Therefore, we recommend that a trench subdrain(curtain
drain)be constructed in the new drainage easement(see Figure 2), footing drains be constructed
around the residence, and that surface drains 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. Given the known slope instabilities in the vicinity of the site,it is our
opinion that all water collected in trench, footing, and surface drains should be routed via
tightline to the base of the steep waterfront slope. In our opinion, the existing tightline would be
adequate for discharge from these drains.
Based on our understanding of the limited, single-residence development of this property,
it is our opinion that the anticipated discharge from the drains described in this letter and 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. •
The following provides additional recommendations for the trench and footing drains.
Trench Subdrain. We recommend that a trench subdrain(curtain drain)be constructed
in the north-south oriented portion of the new drainage easement. The purpose of the subdrain is
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Mr. Jerry Coburn SHANNON FiWILSON,INC.
June 23, 2006
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to intercept near-surface groundwater in order to reduce seepage and groundwater levels in the
steep waterfront slope. A typical trench subdrain is shown in Figure 4. As indicated,the pipe's
circumference should be surrounded by washed pea gravel. The remainder of the trench should
be backfilled with compacted drainage sand and gravel meeting specifications outlined in the
figure. The drain pipe should be approximately 8 to 12 feet below the adjacent ground surface or
as low as practical to still provide gravity flow into the existing tightline drain system. Some of
the existing tightline system may need to be modified (e.g., lowered)to provide the gravity flow.
At the connection between the tightline and the perforated subdrain pipe, an impervious ditch
dam should be placed around the pipe to force water in the trench backfill into the tightline. A
ditch dam may be constructed by placing sacks of concrete in the bottom of the trench, around
the pipe to a height of approximately 18 inches above the top of the pipe. A clean-out at the
ground surface at the upstream end (south end) of the drain should be provided to facilitate
periodic cleaning or other maintenance.
Footing Drains. We recommend that footing drains be installed around the perimeter
and on the upslope side of interior footings to improve soil drainage in the immediate vicinity of
the structure. Footing subdrains should consist of slotted,4-inch-diameter minimum,plastic pipe
bedded in washed, 3/8-inch pea gravel. Typical installation details for these drains are shown in
Figure 5. Figure 5 also includes subdrainage and foundation wall backfill recommendations.
Note that the perimeter subdrain invert should be located at least 18 inches below the lowest
adjacent grade or floor slab. Roof or other drains should not be connected to the footing
subdrains. All outside drains should slope away from the residence.
Vegetation
Maintaining a healthy vegetative cover above and on the slope can reduce erosion and the
rate of slope regression. In general, native vegetation should be used on and near the 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.
Erosion Hazard
According to published USDA soil maps, surficial soils on the upland portion of the site
are classified as Cassolary sandy loam and Alderwood gravelly loam on 15 to 30 percent slopes.
�ERI�I1 # BID
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Mr. Jerry Coburn SHANNON& WILSON.INC.
June 23,2006
Page 8
The USDA maps indicate that these soils have moderate to severe erosion hazard. To reduce the
potential for soil erosion and associated hazards, the following wet weather earthwork
recommendations are presented. Provided these wet weather earthwork recommendations and
prudent construction practices are used, it is anticipated that the proposed development would
not significantly affect soil erosion and associated hazards on the site.
Wet Weather Earthwork
In western Washington, wet weather generally begins about mid-October and continues
through about May, although rainy periods may occur at any time of the year. Therefore,it
would be advantageous to schedule earthwork during the normally dry weather months of June
through mid-October. Earthwork performed during the wet winter months will generally prove
more costly.
The on-site soils may be susceptible to changes in moisture content, and could become
muddy and unsuitable if wet and/or subjected to construction traffic. The following
recommendations are applicable if earthwork is to be accomplished in wet weather or in wet
conditions:
► Fill material should consist of clean, granular soil, of which not more than 5 percent by
dry weight passes the No. 200 mesh sieve,based on wet-sieving the minus 3/-inch
fraction. Any fines should be non-plastic.
► The ground surface in and surrounding the construction area should be sloped and sealed
with a smooth-drum roller to promote runoff of precipitation away from work areas and
to prevent ponding of water.
► Earthwork should be accomplished in small sections to reduce exposure to wet
conditions. If vehicular traffic is expected to be over the exposed subgrade during
construction, the subgrade should be protected with a compacted layer(generally
8 inches or more) of clean crushed rock. The size or type of equipment may have to be
limited to prevent soil disturbance.
► No soil should be left exposed to moisture or uncompacted. A smooth drum vibratory
roller,or equivalent, should be used to seal the surface. Soils that become too wet for
compaction should be removed and replaced with clean crushed rock.
P. Excavation and placement of structural fill during wet weather should be observed on a
full-time basis by a geotechnical engineer/engineering geologist(or representative)
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Mr. Jerry Coburn SHANNON&VVILSON.INC.
June 23,2006
Page9
experienced in wet weather earthwork, to determine that all unsuitable materials are
removed and suitable compaction is achieved.
Covering work areas, soil stockpiles, or slopes with plastic, sloping, ditching,installing
sumps, dewatering, and other measures should be employed, as necessary,to permit proper
completion of the work. Straw bales and/or geotextile silt fences should be aptly located to
control soil movement and erosion.
LIMITATIONS
The conclusions and recommendations presented 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. During
subsequent site activities (e.g., construction), if 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 pleased to discuss these with you if the need arises.
PERMIT # RED
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SHANNON iWILSON,INC.
Mr. Jerry Coburn
June 23,2006
Page 10
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,
SHANNON &WILSON, INC.
e of Washit)
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William Jose•h Perkins
William J. Perkins, L.E.G.
Associate
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Enclosures: Figure 1 —Vicinity Map
Figure 2—Site Plan
Figure 3—Generalized Profile A—A'
Figure 4—Typical Trench Subdrain
Figure 5—Subdrainage and Backfilling
Important Information About Your Geotechnical Report
PERMIT # Hip
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