HomeMy WebLinkAboutBLD2005-00320 Geotechnical Report '- �5.:3yci
ALASKA
SHANNON 6WILSON, INC. CMISSOURI
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FLORIDA
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS OREGONWASHINGTON
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Mr. Michael Hoskins OkJefferson County ?Department of Community Development
Building Division
621 Sheridan Street
Port Townsend, WA 98368
RE: GEOTECHNICAL
AT 592 BROAD SPIT CONSTRUCTION
RESIDENCEROAD, QUILCENE WASHINGTON
MLA NO.: MLA05-00305, CASE NO.: BLD05-00320
Dear Mr. Hoskins:
In response to the letter from the Jefferson County Department of Community Development
Building Division to Mr. Alberti, dated August 17, 2006, and a subsequent phone conversation
with you and Ms. Farfan of your office on August 31, 2006, I visited the site on September 8,
2006, to observe and review specific geotechnical construction issues of concern to the County.
These construction issues are:
► The stability of a log retaining wall.!_`
► The location and stability of the exterior building footing at the northwest corner of the`j
residence.
► The stability of a deck footing along the south side of the residence. t
While at the site, we met with you and Mr. Alberti and provided our observations and
conclusions regarding these issues. The following provides a brief description of our
observations and conclusions.
STABILITY OF LOG RETAINING WALL
A log about 20 to 30 feet long and about 2 to 3 feet in diameter was placed on the north side of
the driveway on the north side of the residence near the crest of the slope. The log retains about
3 to 4 vertical feet of fill. The west end of the log has been wedged against a tree to provide
400 NORTH 34TH STREET• SUITE 100 21-1-09885-002
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206.632.8020 FAX 206.695.6777
TDD: 1.800.833.6388
www.shannonwilson.com
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Mr. Michael Hoskins SHANNON FiWILSQ
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Jefferson County Ville VA � 1`�'
Department of Community Development
October 10, 2006 , `, ,u
Page 2 Vi
lateral support. Mr. Alberti indicated that this log had been placed to widen the driveway
temporarily for construction access and would be removed.
Based on our observations, we recommend that the temporary wall not be left in place and that
the slope should be regraded to its original geometry. Best Management Practices (BMP) should
be used to reduce erosion of the regraded slope during the upcoming wet weather season. If a
permanent wall will replace the log, the permanent wall should be engineered.
RESIDENCE NORTHWEST EXTERIOR FOOTING LOCATION AND STABILITY
At the northwest corner of the residence, the bottom of the exterior wall footing is about 4 feet
above the existing driveway elevation, and the driveway is about 4 feet horizontally from the
wall footing. Because of the elevation difference and proximity of the footing and driveway, you
expressed concern for the stability of the footing. We reviewed with you the recommendations
in our geotechnical report to place the building foundations at an elevation of 114 feet or lower.
We understand that this was accomplished by overexcavating the footing foundations to an
elevation of 114 feet or lower, backfilling the excavation with lean-mix concrete, and
constructing the footings directly on the lean-mix concrete trench backfill. At the northwest
corner of the residence, Mr. Alberti indicated that the lean-mix backfilled trench extends about
6 feet below the footing, which is about 2 feet below the elevation of the adjacent driveway.
Therefore,based on the reported elevation of the base of lean-mix concrete below the footing at
this corner of the residence, it is our opinion that the footing is stable.
DECK FOOTING STABILITY ON THE SOUTH SIDE OF THE RESIDENCE
You expressed concerned as to whether or not the footing at the southeast corner of the deck off
the south side of the residence was in native soil. The footing is near the crest of about a 3-foot-
high slope and along the edge of the excavation for building basement. I observed dense, fine
sand with faint laminations at the same elevation of the bottom of the footing about 1 foot to the
east of the footing. I probed the subgrade adjacent to the footing with a 'A-inch-diameter,
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Mr. Michael Hoskins SHANNON 8WILSON,INC.
Jefferson County
Department of Community Development
October 10, 2006
Page 3
bull-nosed, steel T-probe and found it to be dense. Based on these observations, it is our opinion
that the deck footing is on dense native subgrade suitable for bearing. I noted that the slope
below the footing is at about 13/ Horizontal to 1 Vertical (13/H:1V) and is adjacent to a concrete
block wall. We recommend that the base of the concrete block wall be extended below the
footing and the slope below the footing regraded so that there is a minimum of 18 inches of soil
above the bottom of the footing and the slope is no steeper than 2H:1V.
Please call if you have any questions regarding our observations or conclusions regarding these
or other geotechnical aspects of the site.
Sincerely,
��SHANNON & WILSON, INC. ��� r .Gam' •
William J. Pe ns, L.E.G.
Associate
WJP:CAR/wjp
c: John Alberti
21-1-09885-002-L1/wp/LKD 21-1-098 85-002
• Geologic Slope Stability Evaluation
IFIberti Property Off Broad Spit Road
on Dabob Bay
Quilcene, Washington
June 2003
SHANNON 6WILSON,INC. s
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At Shannon & Wilson, our mission is to be a progressive, well-
managed professional consulting firm in the.frelds of engineering
and applied earth sciences. Our goal is to perform our services
with the highest degree of professionalism with due consideration
to the best interests of the public,our clients,and our employees.
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Submitted To:
Mr.John Alberti
1100 University Street, Apt. 14-D
Seattle,Washington 98101
1Thr N i( c By:
Shannon &Wilson, Inc.
J t^iu L6E-R I !
400 N 34th Street, Suite 100
ij ' wily D25 i l( a 1`�,'`p Seattle,Washington 98103
S L `iTLr •�' 21-1-09885-001
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be,SHANNON 6WILSON, INC. `YE^
I GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS 4,e.,:oRa E
DEN,.ER
SANN1 LO'JIS
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June 4, 2003
I
I Mr. John Alberti
1100 University Street, Apt. 14-D
Seattle, WA 98101
I RE: GEOLOGIC SLOPE STABILITY EVALUATION, ALBERTI PROPERTY
OFF BROAD SPIT ROAD ON DABOB BAY, QUILCENE, WASHINGTON
IDear Mr. Alberti:
IThis letter summarizes our observations, conclusions, and recommendations regarding slope
stability and development of your property (parcel No. 701204002 and 701204010)referenced
le above for a single-family residence. Jefferson County Geologic Hazard Area Maps indicate that
the landslide hazard rating of slopes on the site range from moderate to high. As the proposed
building location may be within a landslide hazard area buffer, we have prepared this report in
accordance with the Unified Development Code for Jefferson County to evaluate the potential
for slope movement and provide recommendations for development of the site with respect to
aslope stability. These conclusions and recommendations are based on observations made during
our visit to the site on March 24, 2003; available published geologic, topographic, and soil maps;
$ and a site plan by Tillman Engineering, dated April 22, 2003. Preliminary observations and
conclusions were provided to Mr. Alberti and Tillman Engineering orally upon completion of the
site visits.
SITE DESCRIPTION
III
The site is located on Dabob Bay on the east side of the Bolton Peninsula, as shown on Figure 1.
The property is over 780 feet long (east-west) by approximately 300 feet wide (north-south).
The approximate location of the proposed building and septic system drain field is shown on
Figure 2.
The topographyacross the site rises from sea level at Dabob, to about 260 feet to the west and
III
includes the following.
400 NORTH 34TH STREET•SUITE 100
P.O. BOX 300303
SEATTLE, WASHINGTON 98103
206.632.8020 FAX 206.695.6777 21 1 09885 001
TDD: 1.800.833.6388
IN i •
Mr. John Alberti S ;'- NO Ni �'�i' Ss :..I;w C.
June 4, 2003
Page 2
IN
► A beach.
NAn approximately 36 feet long (east-west) by 40 feet wide (north-south) backshore area.
► A steep, waterfront bank (approximately 25 to 40 feet high) that slopes up to the
111 southwest at about 45 to 60 degrees.
► A ravine that is incised into the waterfront bank at about 38 degrees (overall height of
I about 80 feet).
rn ► An approximately 160- to 240-foot-high upper slope above the ravine and waterfront
bank that is inclined up to the west and northwest at about 29 to 33 degrees (local
inclinations may be as low as about 11 degrees).
if ► A northeast-southwest trending flattened ridge (up to about 260 feet above sea level).
Slopes north and northwest of the ridge top slope down to the north-northwest at about 30
INto 35 degrees (locally may be as steep as 38 degrees).
The approximate locations of these features are shown on Figure 2.
4 The small backshore area is located at the toe of the ravine above the ordinary high water line.
Wood debris and the location of this backshore area suggest that backshore soils may consist of
itslide debris deposited at the mouth of the ravine. The backshore area appears to be partially
protected from wave action and erosion by driftwood and an abandoned, beached floating dock.
IIVegetation on the waterfront bank includes small-diameter(typically 1 foot or smaller in
diameter) fir and cedar trees with lesser numbers of alder, maple, and madrona. Undergrowth
Iincludes sword fern, salal, and grasses. The tree trunks are typically bowed downhill, indicative
of downhill movement of the soil in which they are located. In the immediate vicinity of the
IIravine and along various locations near the beach, some of the trees are toppled, apparently
caused by recent slope instabilities and movement. The presence of madrona and salal are
I indicative of well-drained surficial soils beneath the bank. Where the vegetation is absent on the
bank and within the ravine, the exposed soils consist of cross-bedded, very dense, trace to silty,
fine to medium sand with lenses of very dense sandy gravel and fine sandy silt. Scattered
Igroundwater seeps were observed in some of the gravel lenses.
I Vegetation on the east-southeast facing upper slope between the top of the waterfront bank and
the ridge top includes maple and alder trees up to about 2 feet in diameter with an undergrowth
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Mr. John Alberti `'4`L` 3 f' �' ° "a
LsoN,I N C.June 4, 2003
Page 3
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of sword ferns and scattered salal. Up to 3-foot-diameter trunks of fir trees were also observed
11 on this slope. As shown on Figure 2, evidence of a recent slide scarp was observed towards the
top of this slope. The scarp appeared to be about 40 feet wide and extend down slope about
1111 65 feet. At the time of our site visit, the scarp was only vegetated with small alder(only a few
feet high) and grasses. It appears that the slide was confined to the upper 5 or 6 feet of soil near
the surface of the slope and possibly included some side-cast fill from an abandoned skidder trail
1111 that appeared to cross the slope in the vicinity of the scarp.
11 The ridge at the top of the upper slope appears to have been recently leveled with no vegetation
at the time of our site visit. Native soils exposed on the leveled ridge top are typically dense to
very dense slightly silty to silty, fine to medium sand. It appears that in flattening the ridge top,
IIIthe excavated soil was cast over the sides of the ridge. The approximate location of the cut and
resulting side-cast fill are shown on Figure 2.
111
The slopes north and northwest of the ridge top are vegetated with large (up to 3 feet in diameter)
trees, which include fir, maple, and cedar and scattered madrona and alder. Undergrowth
IIIIincludes sword fern,Oregon grape, and salal. The presence of madrona, Oregon grape, and salal
are indicative of relatively well-drained surficial soils. As shown on Figure 2, a dirt road extends
from the gravel road on the west side of the site to the proposed building at the ridge top by
traversing the side of the northwest-facing slope. Also shown on Figure 2 is the approximate
I location of recent slide scarp observed on the cut portion of the slope above the dirt road. The
scarp is about 25 feet wide and about 30 feet high. The slide that caused the scarp appeared to be
recent as no vegetation was present in the scarp at the time of our site visit. It appears that the
1111 slide was confined to the upper 5 or 6 feet of soil near the crest of the slope and included the
recent side-cast fill. Native soils exposed in the scarp included very dense, silty fine sand to fine
IIIgravelly sand. No sighs of springs or seeps were observed.
It is proposed to locate the planned residence on the ridge top at the approximate location shown
lifi on Figure 2. The size and location of the proposed residences indicated on Figure 2 are
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conceptual and may change. It is proposed to locate the planned septic system on a relatively flat
portion of the upper slope southeast of the residence as indicated on Figure 2.
S
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Mr. John Alberti
S-ANNON W!L SON.INC.
June 4, 2003
Page 4
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GEOLOGIC CONDITIONS
SPublished geologic maps of the area indicate that pre-Vashon-Till Pleistocene deposits of
undetermined origin underlie the site. Subsurface explorations were not performed at this site
MIfor this evaluation. However,the soils observed in road cuts and in the waterfront bank at the
site are consistent with undifferentiated Pleistocene deposits indicated on the map. Based on the
observed soil exposures, these soils are generally very dense, trace to silty, fine to medium sand
with scattered lenses or layers of fine sandy silt and sandy gravel.
01 The Vashon Stade ice sheet that covered this area approximately 13,500 to 17,000 years before
present overrode the pre-Vashon-Till Pleistocene soils. The ice sheet is estimated to have been
on the order of 3,000 to 4,000 feet thick in this area. Consequently, the till and the underlying
undifferentiated Pleistocene soils have been compacted to a very dense or hard state.
111 Since the retreat of the glacier, the upper few feet of the very dense/hard soil has weathered and
loosened, 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. Slide deposits also appear
to be present at various locations at the base of the waterfront bank.
As previously indicated, slight groundwater seepage was observed in some of the gravel lenses
exposed in the waterfront bank and ravine. No signs of springs, seeps, damp soils,or other
indication of near surface water were observed farther up on the upper slope or ridge top.
CONCLUSIONS AND REC
OMMENDATIONS
S N CO MMENDATI ONS
Slope Stability
Geologic hazard maps indicate that recent slope movements have occurred along the waterfront
111 bank and portions of the upper slope on the northeast corner of the property. Ample evidence of
recent slope movement along the waterfront bank and ravine are consistent with hazard maps and
indicate that the bank and ravine are highly susceptible to slope movements. Based on our
observations of the site, it is our opinion that the slope movements on and near the waterfront
I bank are the result of ongoing wave erosion and oversteepening at the toe of the bank. The
dense to very dense glacially overridden soils that presumably underlie the bank may be stable at
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Mr. John Alberti t rr 1"k' si(y£_SON.INC.
June 4, 2003
Page 5
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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 movement on
s
slopes on which the underlying glacial soils may be relatively stable. With enough time,
1111, movement of colluvium, topsoil, and/or slide debris toward the base of the slope would result in
a flatter, more stable slope. However, with the exception of the backshore area at the mouth of
the ravine, wave erosion at the toe of the bank does not allow the slide deposits, colluvium, or
NI
It topsoil to accumulate at the toe of the slope and maintains the slope in an oversteepened
condition. Consequently, slope movement on the bank should be expected in the future. In our
111 opinion, slope movement on the waterfront would likely consist of movement of shallow
topsoil/colluvium.
111 Evidence of slope instabilities on the upper slope appears to be relatively shallow, induced
instabilities associated with recent cuts and side-cast fills. Similar future slope instabilities
Itshould be anticipated where side-cast fills have been placed on the slope. Otherwise,the natural
slopes on the upper slope appear to be less susceptible to movement than the waterfront bank or
it
ravine. The relatively more stable condition of the upper slope as compared to the waterfront
bank is evidenced by the relatively large trees (up to 3 feet in diameter) on the upper slope.
IWhile the areal extent of the observed slope instabilities may be relatively large (e.g., the recent
slide scarp on the southeast side of the ridge on the upper slope), the slope instabilities observed
ill, across the site appear to be relatively shallow in nature, involving primarily the topsoil,
colluvium, and/or side-cast fill on the slopes. Evidences of recent or historic deep-seated slope
I, instabilities within the underlying Pleistocene soils were not observed during our site visit.
Please note that there is some risk of future instability (shallow or deep-seated) present on all
111 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
1111 development will not adversely impact the stability of adjacent properties.
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Mr. John Alberti k t°' 9'jN,irs.
111, June 4, 2003
Page 6
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Measures to Reduce the Risk Posed by Slope Movement
111 In general, the risk of soil movement on a slope can be reduced by not oversteepening a slope
(e.g., do not excavate the toe of a slope or place side-cast fill at the top) and not increasing the
weight on a slope (e.g., do not place yard debris or fill on or at the crest of a slope). In this
regard, removing the side-cast fills on the upper slope could reduce the potential for future slope
movement on this slope. The risk of soil movement on a slope can also be reduced by
maintaining a slope as dry as possible (e.g., locate septic drain fields away from slopes, route
roof downspouts and yard drains away from slopes, and minimize the amount of surface water
that could flow down slope faces), and maintaining a vegetative cover on slopes. The following
provides additional recommendations to reduce the risk of soil movement.
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 building setback so that if soil movement on the slope does occur, the hazard to the
structure is minimal. An appropriate setback is a function of the rate or risk of slope movement
(regression rate), the design life of the structure, and the risk the owner of the structure is willing
to assume. The regression rate for the upper slope is unknown. However, based on the existing
angle of the upper slope, the absence of observed indications of past slope instability on the
upper slope (other than those that appear to be human-induced), and the growth position and size
of the trees and stumps on the slope, it is our opinion that the regression rate on the upper slope
is relatively low (e.g., less than a few inches per year) and appears to be controlled largely by
surficial erosion.
In our opinion, a minimum base of building foundation setback of 20 feet from the edge
of the north-northwest facing slope would be adequate for this site. For the east and southeast
facing upper slopes, we recommend a minimum base of building foundation setback of 50 feet
from the natural face of the slope (the presence of side-cast fill should be excluded in the setback
determination). In our opinion, the larger setback from the east and southeast facing upper
slopes is prudent given the observed recent movement on the southeast facing upper slope and on
the waterfront bank and ravine below these upper slopes. In order to achieve the recommended
building setbacks, the building foundations will need to extend to a depth, such that the
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Mr. John Alberti SHANNON FiWIL SON,INC.
June 4, 2003
Page 7
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horizontal distance between the footing and face of the slope is a minimum of 20 and 50 feet for
the north-northwest and east-southeast facing slopes, respectively. These minimum building
setbacks are illustrated on Figure 3. Based on the building dimensions and locations provided by
Mr. Alberti and the topographic mapped prepared by Tillman Engineering, the building
foundations should extend at least to an elevation of 114 feet (Tillman Engineering arbitrary site
datum). We note that at this elevation, it is anticipated that the foundation subgrades would
consist of very dense or hard glacially overridden soils, which would provide adequate bearing
for spread footing foundations to support the residence.
111
Septic Drain Field Location
The septic drain field should be located as far as practical from the waterfront bank and
ravine and on a flatter portion of the upper slope to reduce the risk of slope movements. While at
the site,we reviewed with Mr. Alberti and Mr. Eric Page of Tillman Engineering these
requirements and potential drain field locations, and identified the location indicated on Figure 2
as meeting these requirements.
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Drainage
IIn 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
Ifrom impermeable surfaces on the property (e.g., roof, decks, patios, and driveways) and directed
in a tightline to a suitable discharge point. Upon reviewing the site conditions and various
options for discharge (including upland discharge) it is our bpinion that the backshore area near
111 the mouth of the ravine (see Figure 2) would provide a suitable discharge point without
significantly impacting the stability of the slopes on the site or increasing the surface water
Ndischarge or sedimentation to adjacent properties beyond pre-development conditions. The
discharge point should be constructed to allow dispersion of the water and dissipation of energy
to reduce the potential for erosion. Discharge onto an 18-inch-thick mat of 6-inch minus quarry
spalls, extending horizontally 6 feet in the direction of flow, would be one method to provide
Iwater dispersion and energy dissipation.
In addition to surface drainage, we recommend that footing drains be installed around the
perimeter of the building 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
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Mr. John Alberti SHANNON&WI SON INC.
June 4, 2003
Page 8
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washed, 3/8-inch pea gravel. Typical installation details for these drains are shown in Figure 4.
Figure 4 also includes subdrainage and foundation wall backfill recommendations. On-site soils
would not be suitable for use as drainage sand and gravel. Note that the perimeter subdrain
invert should be located at least 18 inches below the lowest adjacent grade. Roof or other drains
should not be connected to the footing subdrains. The discharge from footing drains should be
routed by means of a tightline to a suitable discharge point as previously discussed. All outside
grades should slope away from the residence.
Based on our understanding of the limited, single-residence development of this property,
it is our opinion that the anticipated discharge of roof and footing drains as outlined above will
not significantly affect the pre-development drainage conditions on the adjacent properties.
Impermeable surface around the residence (e.g., paved drives) should be minimized to
reduce potential changes in the existing site drainage characteristics and impacts on adjacent
sites.
Slope Above Existing Access Road
As previously described in this letter, the side-cast fill recently placed on the top of the
ridge have apparently contributed to the recent slope movement observed above the access road
to the building site. Without their removal, future slope movements should be expected on both
sides of the ridge where the side-cast fill has been placed. The side-cast fills above the dirt
access road could be removed to reduce the potential for slope movements affecting the access.
or The risk could also be reduced and erosion protection of the existing cut slope enhanced by
construction of a rockery along the toe of the cut. A typical rockery detail appropriate for this
site is provided on Figure 5.
Erosion Hazard
We note that the according to published U.S. Department of Agriculture (USDA) soil maps,
surficial soils on the site are classified as Beausite-Alderwood complex on 30 to 50 percent
slopes. The USDA maps indicate that these soils have a severe erosion hazard. To reduce the
potential for soil erosion and associated hazards, the following wet weather earthwork
recommendations are presented. Provided that these wet weather earthwork recommendations
and prudent construction practices are used, it is anticipated that the future earthwork for the
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Mr. John Alberti �s _N¢l+ ;.ft ' c
June 4, 2003
Page 9
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proposed development will not significantly affect soil erosion and associated hazards on the
site.
Wet Weather Earthwork
111
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 silty soils are 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 passing the No. 200 mesh sieve, based on wet-sieving the minus 3/4-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
11 to prevent ponding of water.
'
► Earthwork should be accomplished in small sections to reduce exposure to wet
conditions. If there is to be vehicular traffic 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.
► 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)
experienced in wet weather earthwork, to determine that all unsuitable materials are
removed and suitable compaction is achieved.
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Mr. John Alberti SHANNON {y`'.`:` 'h6,INC
June 4, 2003
Page 10
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.
Construction Observation
1111
With respect to implementing the risk reduction measures outlined in this letter, we recommend
that a geotechnical engineer/engineering geologist or their representative observe geotechnically
related construction, including rockery subgrades, drainage installation, and building footing
subgrade preparation once they are excavated. The building footing locations should be
observed to determine if foundation depths provide the minimum horizontal setbacks outlined in
this letter and to provide recommendations for additional excavation as needed.
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LIMITATIONS
The conclusions in this letter are based on site conditions visually observed during our site
reconnaissance and inferred from published geologic, soils, 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.
111
10 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
express or implied.
This letter was prepared for the use of Mr. Alberti 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 visits and discussion of geologic conditions included in this letter.
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Mr. John Alberti SHANNON ''=�''rl#._SON,INC.
1111 June 4, 2003
Page 11
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Please note that the scope of our services did not include any environmental assessments or
IIevaluation 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.
if
Shannon &Wilson has prepared the attached, "Important Information About Your Geotechnical
Report,"to assist you and others in understanding the use and limitations of our report.
Or
We appreciate the opportunity to provide geologic services to you, and are available to answer
IIany questions regarding our observations,conclusions or recommendations contained in this
letter.
11,
Sincerely,
IISHANNON & WILSON, INC.
a��of Wash;
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William Jose•h Perkins
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William J. Perkins, L.E.G.
IISenior Principal Engineering Geologist
WJP:JW/wjp
IEnclosures: Figure 1 —Vicinity Map
Figure 2—Site Plan
II Figure 3 —Generalized Subsurface Profile A-A'
Figure 4—Subdrainage and Backfilling
Figure 5 —Typical Rockery Detail
IIImportant Information About Your Geotechnical Report
I
21-1-09885-001-LI/wp/Ikd 21-1-09885-001
10
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I--I I I I I Alberti Property
Scale in Miles Quilcene, Washington
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NOTE VICINITY MAP
I Map adapted from 1:24,000 USGS topographic
map of Quilcene, WA quadrangle, dated 1953. May 2003 21-1-09885-001
a
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m SHANNON&WILSON, INC. FIG. 1
ii Geotechnical and Environmental Consultants
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354'
FOUND REBAR AND CAP
/ 505..5'SET BY'WRIGHT'LS j 4233 Approximate Stormwater
r 10090' �' Discharge Location on
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Approximate Location dto '`�
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scarps, SITE PLAN
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o son: Inc.,
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SHANNON WILSON, INC. FIG. 2
m Geotechnical and Environmental Consultants
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Quilcene, Washington
GENERALIZED SUBSURFACE
PROFILE A-A'
o May 2003 21-1-09885-001
im Ci i3LO( atiAN.NON!vim L.sOC.NonslNc. FIG. 3
I
• •
I
I. �.,,-----Wall
Sloped to Drain
Away from
1 Structure
Drainage Sand&
Pavement or 10"to 15" . o o Gravel or Washed
Impervious Soil Pea Gravel
0 0
18" o°f Damp Proofing
Backfill Meeting Gradation Min G. a
Requirements for Structural Fill o o
(See Note 2) °°. —Weep Holes
�° (See Note 1) Vapor Barrier
° Floor Slab
Excavation Slope .•o
Contractor's Responsibility o ;`I_ o . ° •o• ,) o •IL o° a ;o o b o\
o' o ,00 , o ' o , °o°,p° a o18"Min.00" ono o
0
— - I j -
6"Min.Cover of Pea Gravel �/
(6"Min. on Sides of Pipe) 2"to 4" Washed 4"Min—
Pea Gravel
Subdrain Pipe
Not to Scale
MATERIALS NOTES
Drainage Sand &Gravel with 1. Drainage gravel beneath floor slab should be
the Following Specifications: hydraulically connected to subdrain pipe on the
down-slope side of the structure only. Use of 2"dia.
Passing weep holes as shown is one applicable method.
Sieve Size by Weight
2. Imported structural fill should consist of well-graded
granular soil with not more than 5`)/0 fines (by weight
a 1 1/2" 100 based on minus 3/4" portion)passing No.200 sieve(by
N 3/4" 90 to 100
1/4" 75 to 100
0 wet sieving)with no plastic fines.
a No.30 20 to 65 3. Backfill within 18"of wall should be compacted with
M No.50 5 to 20 hand-operated equipment. Heavy equipment should
N No. 100 0 to 2 not be used for backfill, as such equipment operated
8 (by wet sieving) (non-plastic) near the wall could increase lateral earth pressures
o and possibly damage the wall.
s
o 4. All backfill should be placed in layers not exceeding 4"
loose thickness and densely compacted. Beneath
paved or sidewalk areas, compact to at least 95%
Modified Proctor maximum dry density(ASTM: D1557,
SUBDRAIN PIPE Method C). Otherwise compact to 92% minimum.
0
9
4"minimum diameter perforated or slotted pipe;
o tight joints;sloped to drain(6"/100'min.slope); Alberti Property
provide clean-outs.
Quilcene, Washington
o Perforated pipe holes(3/16"to 1/4"dia.)to be in
co lower half of the pipe with lower quarter segment
cn
unperforated for water flow. SUBDRAINAGE AND BACKFILLING
Slotted pipe to have 1/8"maximum width slots.
May 2003 21-1-09885-001
FE
SHANNON&WILSON, INC. FIG. 4
m Geotechnical and Environmental Consultants
U-
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Max. Slope
II 11 2
16"Min.Width
II
H- for Top Rock
8"Compacted Native Soil
, (Impervious Surface Layer)
'~ ,. Stable Excavation Slope
�,� • ."in Dense Native Soil
• .. • . �• ;` (Contractor's Responsibility)
111 1
..•' r. - •- Opening Chinked with
H = 10'Max. 4 `'.. ' ` ::.''• 2 to 4-inch Quarry Spalls
Dense to Very Dense
•' •
• Undisturbed Native Soil
III '/�- Backfill
•!ijk," • Clean,well-graded sand and gravel or
Apr.. " •. crushed rock,2-inch maximum size,40 to
` .° t 60%gravel, less than 5%fines(passing
.. / `, .-. .'.. #200 sieve). Fines shall be non-plastic.
—• • ' • � Compact in 4"lifts with minimum of 4
IIIDense to 12"Min. " • ` •i:•:: coverages by hand-operated tamper.
Very Dense • ; Compact to at least 92%of Modified
+ •• • ° ° iiii;:i.eh;,:... Proctor maximum dry density(ASTM
Native Soil �
111
6" Min. _�• D-1557). Backfill and rock placement
should be built up together.
. H/3 Min.Width
for Base Rock
II _, 6" Diameter Slotted Pipe
J Bedded in washed 3/8"to No.8 sieve size
t pea gravel(6"cover around pipe),sloped to
III Q,,
All loose to medium dense soil at rockery foundation drain and connected by tightline to storm
should be overexcavated down to dense to very dense drain outfall. No fabric around pipe.
S soil and replaced with compacted backfill as described Maximum slot width is 1/8".
above. The excavation shall be kept free of water. Where slotted drain connects to tightline,
o 0 The prepared foundation shall be evaluated by a construct impervious collar(concrete or clay)
soils engineer prior to placement of rock. to force all water into tightline.
0
I ,
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Not to Scale
I 9'
Lo
03
4 Alberti Property
' MINIMUM WEIGHT OF ROCK QuilCene, Washington
I ,
L. Portion of wall below 6 feet,2400 pound
03
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("6-man")rock. Upper 6 feet of wall, 1600 TYPICAL ROCKERY DETAIL
pound ("4-man")rock. Rock shall be sound
and have a minimum density of 160 pounds
per cubic foot. May 2003 21-1-09885-001
0
SHANNON&WILSON,INC. FIG. 5
6 Geotechnical and Environmental Consultants
• •
111 -1 1 SHANNON &WILSON, INC. Attachment to and part of Report 21-1-09885-001
- Geotechnical and Environmental Consultants
Date: June 4,2003
i 11.111.I I To: Mr.John Alberti
Seattle,Washington
11
I IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVIRONMENTAL
REPORT
I
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 I
a construction contractor or even another civil engineer. Unless indicated otherwise,your consultant prepared your report expressly for you
Iand 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 geotechnicaUenvironmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors.
IDepending 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.
IUnless 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
Ialtered; (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.
ISUBSURFACE CONDITIONS CAN CHANGE.
Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is
Ibased 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.
IConstruction 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 of
any such events,and should be consulted to determine if additional tests are necessary.
MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS.
I
ISite 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
Ireduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect.
t
Page 1 of 2 1/2003
• • II
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
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 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 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 1
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 Practicing in the Geosciences,Silver Spring,Maryland
I
Page 2 of 2 1/2003 I