HomeMy WebLinkAbout701204002 Geotech AssessmentGeologic Slope Stability Evaluation
Alberti Property Off Broad Spit Road
on Dabob Bay
Quiicene, Washington
June 2003
At 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 with due consideration
to the best #~terests of the public, our clients, and our employees.
Submitted To:
Mr. John Alberti
1100 University Street, Apt. 14-D
Seattle, Washington 98101
By:
Shannon & Wilson, Inc.
400 N 34th Street, Suite 100
Seattle, Washington 98103
21-1-09885-001
111
SHANNON 5WILSON, INC.
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
SEATTLE
RICHLAND
FAIRBANKS
ANCHORAGE
DENVER
SAINT LOUIS
BOSTON
June 4, 2003
Mr. John Alberti
1100 University Street, Apt. 14-D
Seattle, WA 98101
GEOLOGIC SLOPE STABILITY EVALUATION, ALBERTI PROPERTY
OFF BROAD SPIT ROAD ON DABOB BAY, QUILCENE, WASHINGTON
Dear Mr. Alberti:
This letter summarizes our observations, conclusions, and recommendations regarding slope
stability and development of your property (parcel No. 701204002 and 701204010) referenced
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
slope 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, 20.03.... Preliminary Observations and
conclusions were provided to Mr. Alberti and Tillman Engineering orally upon completion of the
site visits.
SITE DESCRIPTION
The site is located on Dabob Bayon 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 topography across the site rises from sea level at Dabob, to about 260 feet to the west and
includes the following.
400 NORTH 34TH STREET · SUITE 100
RO. BOX 300303
SEATTLE, WASHINGTON 98103
206,632.8020 FAX 206.695,6777
TDD: 1,800.833.6388
21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 2
SHANNON &WILSON, INC.
· A beach.
· An 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
southwest at about 45 to 60 degrees.
· A ravine that is incised into the waterfront bank at about 38 degrees (overall height of
about 80 feet).
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).
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
to 35 degrees (locally may be as steep as 38 degrees).
The approximate locations of these features are shown on Figure 2.
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
slide 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.
Vegetation on the waterfront bank includes small-diameter ('typically l'~oot or smaller in
diameter) fir and cedar trees with lesser numbers of alder, maple, and madrona. Undergrowth
includes 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
ravine 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
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
groundwater seeps were observed in some of the gravel lenses.
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
21-1-o9885-0Ol-L1/wp/lkd 21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 3
SHANNON ,~WlLSON. INC.
of sword ferns and scattered salal. Up to 3-foot-diameter trunks of fir trees were also observed
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
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
that appeared to cross the slope in the vicinity of the scarp.
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,
the 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.
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
includes 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
location of recent slide scarp observed on the~cut portion of lhe 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
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
gravelly 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
on Figure 2. The size and location of the proposed residences indicated on Figure 2 are
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.
21-X -09885-001 -LI/wp/Lkd 21 - 1-09885 -001
Mr. John Alberti
June 4,2003
Page 4
SHANNON &WILSON, INC.
GEOLOGIC CONDITIONS
Published 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
for 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.
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.
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. N~) 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 RECOMMENDATIONS
Slope Stability
Geologic hazard maps indicate that recent slope movements have occurred along the waterfront
bank and portions of the upper slope on the northeast comer 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
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
21 - 1-09885-001-LI/wp/lkd 21-1-09885-001
Mr.-John Alberti
June 4, 2003
Page 5
SHANNON &WILSON, INC.
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
slopes on which the underlying glacial soils may be relatively stable. With enough time,
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
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
opinion, slope movement on the waterfront would likely consist of movement of shallow
topsoil/colluvium.
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
should 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
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.
While 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
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
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
hillsides, which the owner must be prepared to accept. Such instability could occur because of
future water line breaks/leaks, uncontrolled drainage, unwise de,2elopment in adjacent areas, or
other actions or events on a slope that may cause sliding. The following piovides further
discussion of risk reduction measures that may be effective at this site. Provided that the risk
reduction measures discussed in this letter are implemented, it is our opinion that the proposed
development will not adversely impact the stability of adjacent properties.
21-1-09885-001-Ll/wp/lkd 21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 6
SHANNON ~WlLSON. INC.
Measures to Reduce the Risk Posed by Slope Movement
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 obseryed indications of past slope instability on the
upper slope (other than those that appear to be human-induced), and th~ 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
21-1-09885-001-Ll/wp/lkd 21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 7
SHANNON &WILSON, INC.
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.
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.
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, an6~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 opinion that the backsh0re area near
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.
discharge 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
water 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
2~-]-09885-00t-t.]/wpaka 21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 8
SHANNON &WlLSON. INC.
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.
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 per6ent
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
21-1-09885-001-Ll/wpflkd 2 1-1-098 8 5-00 1
Mr. John Alberti
June 4, 2003
Page 9
SHANNON 5WILSON, INC.
proposed development will 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 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 3A-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 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 dram 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.
21-1-09885-0Ol-L1/wp/lkd 21 - 1-098 85-001
Mr. John Alberti
June 4, 2003
Page 10
SHANNON <5WILSON, INC.
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
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.
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, durin..g 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
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.
21-l-09885-001-Ll/wp/lkd 21-1-09885-001
Mr. John Alberti
June 4, 2003
Page 11
SHANNON ~WlLSON, INC.
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 and others 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, conclusions or recommendations contained in this
letter.
Sincerely,
SHANNON & WILSON, INC.
· ° ' 8
William J. Perkins, L.E.G.
Senior Principal Engineering Geologist
WJP:JW/wjp
Enclosures:
Figure 1 - Vicinity Map
Figure 2 - Site Plan
Figure 3 - Generalized Subsurface Profile A-A'
Figure 4 - Subdrainage and Backfilling
Figure 5 - Typical Rockery Detail
Important Information About Your Geotechnical Report
21-1-09885-001 -LI/wp/lkd ' 21-1-09885-001
ect
Washington
0 1/2 1
I ~ I ~ I I
Scale in Miles
NOTE
Map adapted from 1:24,000 USGS topographic
map of Quilcene, WA quadrangle, dated 1953.
PROJECT
LOCATION
Alberti Property
Quilcene, Washington
VICINITY MAP
May 2003
SHANNON & WILSON, INC.
Geotechnical and Environmental Consulta,,'~
21-1-09885-001
IFIG.
File: I:~)raffing~211~09885-001~1-1-09885-001 fig 2.dwg
?
/
-CD
Date: 05-28-2003 Author:. CNT
I
o
,-I
ITl
Z
('3
File: I:~Draffing~211~09885-001~1-1-09885-001 fig 3.dwg Date: 05-28-2003 Author:. CNT
Elevation in Feet
Elevation in Feet
Pavement or 10" to 15"
Impervious Soil
Backfill Meeting Gradation
Requirements for Structural Fill
(See Note 2)
Excavation Slope
Contractor's Responsibility
6" Min. Cover of Pea Gravel
(6" Min. on Sides of Pipe)
Subdrain Pipe
Sloped to Drain
Away from
Structure
18"
Min.
Wall
e Sand &
Gravel or Washed
Pea Gravel
Proofing
Weep Holes
(See Note 1)
Floor Slab
O
-2" to4"
Washed
Pea Gravel
Not to Scale
4" Min
Vapor Barrier
MATERIALS
Drainage Sand & Gravel with
the Following Specifications:
% Passing
Sieve Size by Weight
1-1/2" 100
3/4" 90 to 100
1/4" 75 to 100
No. 8 65 to 92
No. 30 20 to 65
No. 50 5 to 20
No. 100 0 to 2
(by wet sieving) (non-plastic)
SUBDRAIN PIPE
4" minimum diameter perforated or slotted pipe;
fight joints; sloped to drain (6"/100' min. slope);
provide clean-outs.
Perforated pipe holes (3/16" to 1/4" dia.)to be in
lower half of the pipe with lower quarter segment
unperforated for water flow,
Slotted pipe to have 1/8" maximum width slots.
'2.
o
NOTES
Drainage gravel beneath floor slab should be
hydraulically connected to subdrain p!pe on the
down-slope side of the structure only. Use of 2" dia.
weep holes as shown is one applicable method.
Imported structural fill should consist of well-graded
granular soil with not more than 5 % fines (by weight
based on minus 3/4" portion) passing No. 200 sieve (by
wet sieving) with no plastic fines.
Backfill within 18" of wall should be compacted with
hand-operated equipment. Heavy equipment should
not be used for backfill, as such equipment operated
near the wall could increase lateral earth pressures
and possibly damage the wall.
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,
Method C). Otherwise compact to 92% minimum.
Alberti Property
Quilcene, Washington
SUBDRAINAGE AND BACKFILLING
May 2003 21-1-09885-001
SHANNON & WILSON, INC. I FIG. 4
Geotec~nical and Environmental Consultants
I
16" Min. Width
~"-for Top Rock
Max. Slope
2
8" Compacted Native Soil
(Impervious Surfaco Layer)
H = 10' Max.
4
Dense to
Very Dense
Native Soil
12" Min.
H/3 Min. Width
~'~ for Base Rock
All loose to medium dense soil at rockery foundation
should be overexcavated down to dense to very dense
soil and replaced with compacted backfill as described
above. The excavation shall be kept free of water.
The prepared foundation shall be evaluated by a
soils engineer prior to placement of rock.
Not to Scale
MINIMUM WEIGHT OF ROCK.
Portion of wall below 6 feet, 2400 pound
("6-man") rock. Upper 6 feet of wall, 1600
pound ("4-man") rock, Rock shall be sound
and have a minimum density of 160 pounds
per cubic foot.
ble Excavation Slope
in Dense Native Soil
(Contractor's Responsibility)
Chinked with
2 to 4-inch Quarry Spalls
Dense to Very Dense
Undisturbed Native Soil
Backfill
Clean, well-graded sand and gravel or
crushed rock, 2-inch maximum size, 40 to
60% gravel, less than 5% fines (passing
#200 sieve). Fines shall be non-plastic.
Compact in 4" Iifts with minimum of 4
coverages by hand-operated tamper.
Compact to at least 92% of Modified
Proctor maximum dry density (ASTM
D-1557). Backfill and rock placement
should be built up together.
· -~6" Dia'meter Slotted Pi.p~.
~-Bedded in washed 3/8 to No,8 sieve size
pea gravel (6" cover around pipe), sloped to
drain and connected by tightline to storm
drain ouffall. No fabric around pipe.
Maximum slot width is 1/8".
Where slotted drain connects to tightline,
construct impervious collar (concrete or clay)
to force all water into tightline.
Alberti Property
Quilcene, Washington
TYPICAL ROCKERY DETAIL'
May 2003 21-1-09885-001
SHANNON & WILSON, INC. I FIG. 5
Geotechnical and Environmental Consultants
SHANNON & WILSON, INC.
Geotechnical and Environmental Consultants
Attachment to and part of Report 21-1-09885-001
Date: June 4, 2003
To: Mr. John Alberti
Seattle, Washington
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL/ENVlRONMENTAL
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 geotechnical/envkonmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors.
Depending on the project, these may include: the general nature of the structure and property involved; its size and configuration; its
historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots;
and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly
problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations.
Unless your consultant indicates otherwise, your' report should not be used: (1) when the nature of the proposed project is changed (for
example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an
unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is
altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for
application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors
which were considered in the development of the report have changed.
SUBSURFACE CONDITIONS CAN CHANGE.
Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is
based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose
adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for
example, groundwater conditions commonly vary seasonally.
Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect
subsurface conditions and, thus, the continuing adequacy ora 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.
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.
Page 1 of 2 1/2003
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 ofa 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
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
Page 2 of 2 1/2003