HomeMy WebLinkAbout977100414 Geotech Assessments
HANFORD
TACOMA
SHANNON WILSON, ANC. FAIRBANKS
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS ANCFloaace
SAINT LOUIS
BOSTON
May 16, 1996
Mr. Frank Vane
405 Willow Street
Port Townsend, Washington 98368
RE: GEOTECHNICAL REPORT, LOT 18 COMBS PLACE, PORT TOWNSENI~
WASHINGTON
Dear Mr. Vane:
This letter summarizes our observations, conclusions, and recommendations for the
proposed residential development on the above referenced lot near Port Townsend,
Washington. In a letter to Mr. Vane dated Apri19, 1996, Jefferson County requested that a
geotechnical study and recommendations be developed regarding the stability of the slopes
and proposed development on this lot. Therefore, the purpose of our work was to evaluate
the stability of the site and provide design and construction recommendations for the
planned residence to reduce the risk of slope instability. Our work was conducted in
accordance with our proposal dated April 16, 1996.
SITE DESCRIPTION
The lot is located on the east side of Discovery Bay, approximately 3 miles southwest of
Fort Townsend on the north end of Combs Place (Figure 1). The lot is situated on the
upper portion of a 20- to 40-degree slope that rises from sea level at Discovery Bay on the
west, up to an elevation of approximately 500 feet to the east. The elevation of the
property, relative to sea level, is between 300 and 380 feet.
The lot shape, dimensions, existing and proposed topography, and the locations of the
planned house and carport at the site are shown on Figure 2. The elevations indicated on
the contours on this figure are based on an arbitrary project datum and are not relative to
400 NORTH 34TH STREET • SUITE 100
P.O. BOX 300303 W 7194-02
SEATTLE, WASHINGTON 98103
206.632.8020 FAX 206.633.6777
TDD: 1.800.833.6388
Mr. Frank Vane ~~~~ ~'~~~~~, ~~G•
May 16, 1996
Page 2
sea level. Naturally occurring slopes across the site generally are between 2 horizontal to 1
vertical (2H:1V) and 1.7H:1V. An approximately 15 feet deep depression was excavated in
the central portion of the site at the location of the planned residence, and a trench was
excavated across the western portion of the property for placement of the existing storm
sewer (see Figure 2). The excavated. slopes are typically no steeper than about 0.7H:1V.
Vegetation across the site typically consists of Madrona (up to 2.5 feet in diameter) and fir
trees (up to 1 foot in diameter live trees, 3 feet diameter stumps) with an undergrowth that
includes sa1a1, Oregon Grape, and grass. This assemblage of plants is indicative of
relatively dry, well-drained, near-surface soil conditions. In addition, the growth positions
of some fir trees are indicative of soil creep. Soil creep occurs on nearly all slopes and is
the imperceptibly slow, downslope movement of soils under the effects of gravity.
The proposed residence shown on Figure 2has awedge-shaped footprint, with a maximum
length (east-west) of approximately 32 feet and a maximum width (north-south) of about 26
feet. We understand that the house will be a three-story structure, including a basement
that steps up to the east and daylights out to the west. We understand from Ms. Jean
Anderson, your architect, that the floor elevations have not yet been determined and will be
selected, in part, on the anticipated foundation depths. A carport is also planned at the east
end of the property at the end of Combs Place.
Final grades shown on Figure 2 indicate that a level yard area at elevation 194 feet is
planned for the back (east) side of the house. A rockery, up to 6 feet high, will be
constructed on the east and north sides of the yard to step up to the existing ground surface
beyond. The ground surface along the south side of the house will be graded to about
2H:1V into the existing topography. The slope along the north side of the house will also
be graded at about 2H:1 V parallel to the slope, but will be at an elevation below the
existing ground surface farther north to allow for construction of a small bridge to the house
main entrance.
W-7194-02
Mr. Frank Vane S~A1~NG~ ~~~~~~, ~~C•
May 16, 1996
Page 3
GEOLOGIC AND SUBSURFACE CONDITIONS
Geologic maps of the area indicate that the hillside (from top to bottom) is capped by
Vashon Lodgement fiill, underlain Vashon Advance Outwash, in turn underlain by
undifferentiated stratified pre-Vashon sediment. The oldest sediments at the base of the hill
are the pre-Vashon sediments. These may range from clay to sand and gravel and are
mapped about halfway up the hillside where they are overlain by Vashon Advance Outwash.
7'he outwash was deposited by meltwaters in front of the Vashon Ice sheet as it advanced
from the north toward the region during the Late Pleistocene. Outwash deposits are
typically sands with variable amounts of gravel and silt. The advance outwash is mapped
from the midpoint of the slope up to near the crest of the hill. As the glacial ice sheet
continued to advance and cover the azea, it deposited Vashon Lodgement Till directly
beneath the ice. Till is typically anon-sorted mixture of clay, silt, sand, and gravel with
scattered cobbles and boulders. Till deposits are mapped at the top of the hill. The ice
sheet that covered this area is estimated to have been up to 4,000 feet thick, and because of
the great weight of the ice, the underlying sediments (i.e., Vashon Lodgement Till,
Advance Outwash, and pre-Vashon sediment) were averconsolidated to a very dense or hazd
state.
Since the retreat of the ice sheet approximately 13,000 years ago, the upper few feet of
these very dense/hazd soils have weathered and loosened to form a thin rind of Colluvium
and topsoil. Colluvium is weathered soil that has reached its present location due to the
forces of water and gravity. The topsoil and Colluvium aze not as dense nor as strong as
the underlying deposits from which they were weathered. Consequently, the topsoil and
Colluvium on the hillsides have a tendency to creep or slide down the slope. Geologic
hazard maps indicate that sliding has occurred on most of this hillside at some time.
A geologic reconnaissance of the site confirms the presence of very dense Vashon Advance
Outwash beneath the site, overlain by Colluvium and topsoil. The reconnaissance included
logging the subsurface soils exposed in the excavations across the site and in one hand-dug
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 4
~ iV09V ~WIi.SO~, lf~C.
test pit. The follawing table presents a brief description of the typical subsurface profile
observed in the excavations and test pit.
0 to 1 I Loose to medium dense, silty, gravelly SAND with scattered cobbles and
boulders; moist to dry (topsoiUcolluvium)
1 to 2 1/2 Medium dense to dense, trace to slightly silty, gravelly SAND with
scattered cobbles and boulders; moist to dry (weathered Vachon Advance
Outwash)
2 1/2 to +5 Very dense, sandy GRAVEL/gravelly SAND with scattered cobbles and
boulders; moist to dry (unweathered Vachon Advance Outwash)
Tv~ro areas of likely fill were observed and are noted on Figure 2. Fill in the east end of the
lot may be up to 3 feet thick and is part of a cut/fill pad at the end. of Combs Place. Fill is
also likely present near the center of the site beneath the west side of the proposed
residence. Based on the existing contour data, this fill may be 4 to 6 feet thick.
Groundwater was not observed in the test pit or any of the excavations across the site.
Springs, seeps, or vegetation indicated of near surface groundwater conditions were not
observed on the site.
CONCLUSIONS AND RECO1VIlVi1+:NDATIONS
Slope Stability
Based on our observations and experience with similaz sites, it is our opinion that the risk
of deep-seated slope instability, or sliding of the very dense soils beneath the site, is
relatively low However, the near-surface topsoil/colluvium and weathered advance
outwash are susceptible to sliding. As previously indicated in this letter, the surficial soils
are apparently creeping down the slope. The sliding indicated on geologic hazard maps of
the area likely occurs primarily in these relatively loose surficial soils. Sliding of these
W 7194-02
Mr. Frank vane
May 16, 1996
Page 5
~ ~#ON V1/Il_SC~~I.6~~.
soils may result from over steepening of the hillside (e.g., erosion at the base of the hill due
to wave action, excavation cuts into the hillside), Overloading the surficial soils (e.g.,
placing fill or foundations on the loose soil), and saturation of the loose soils that increase
the weight and reduces the strength of the soil (e.g., heavy rains, broken water, or sewer
lines).
It is our opinion that the proposed residence may be built on the site as indicated on
Figure 2 with minimal impact on the stability of the slope, if the recommendations
contained in this letter are implemented. In addition, it is our opinion that if the
foundations for the house are located within the very dense outwash beneath the site in
accordance with our following recommendations, the risk presented to the structure by
sliding and creeping of the near-surface soils can be greatly reduced. Please note that there
is some risk of future instability 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.
Engineering Recommendations
Foundations
We recommend that the residence and car port be founded on spread footings
bearing at least 2 feet below the top of the very dense outwash. Based on the typical
subsurface conditions observed at the site, this will require that the base of the footings be
located approximately 4 1/2 feet below the existing, naturally occurring ground surface.
Based on this estimated excavation depth, footing elevations along the north side of the
residence may be between elevations 177.5 feet (west) and 184.5 feet (east). Along the east
side of the residence, footing depths may range between elevations 183.5 feet (north) and
189 feet (south). Footing elevations along the south side may be between elevations 170.5
feet (west) and 189 feet (east).
W 7194-02
Mr. Frank Vane Si-~~I~! ~W1L.SOiV,11~C.
May 16, 1996
Page 6
The footing depth on the west side of the residence will likely be greater than the
anticipated 4 1/2 feet for the remainder of the structure. The west footing must extend
through the fill believed to be present and an additional 2 feet below the top of the very
dense outwash. The existing ground surface in this area is approximately at elevation 175
feet. If 6 feet of fill is present over 2 1/2 feet of preexisting topsoiUcolluvium and
weathered advance outwash, the elevation of the top of the very dense outwash would be at
166.5 feet. 1'he base of the footing would be located 2 feet below the very dense outwash,
or at an elevation of 164.5 feet. If only 4 1/2 feet of fill/colluvium/topsoil/weathered
outwash is present, the base of the footing would be located at an elevation of 168 feet,
allowing for the recommended 2 feet embedment- into the very dense soils.
At the carport, footing depths in the cut on the east side may be as shallow as 3 feet
(elevation 213 feet) while on the west side, footing depths may be up to 7 1/2 feet
(elevation 206.5 feet) to penetrate through filUtopsoiUcolluvium/weathered outwash and 2
feet of very dense advance outwash. To minimize excavation along the west side of the
carport, isolated footings may be excavated or drilled.
The maximum footing elevations for the structures previously discussed are
estimates. Actual footing depths and elevations will depend on the subsurface conditions
and elevations found during construction. We, therefore, recommend that an experienced
engineer or geologist observe the foundation excavations and evaluate whether the
recommended footing embedment of 2 feet into the very dense soils has been achieved. In
addition, the contractor should expect some variation in the planned versus actual footing
depths and the owner should anticipate some potential additional expenditures to achieve an
adequately embedded foundation.
Footings bearing in very dense advance outwash on this slope may be designed for
an allowable soil bearing capacity of 4,000 pounds per square foot (psf). This allowable
capacity may be increased by one-third under seismic loading conditions. We recommend
that continuous footings have an 18-inch minimum width; column (individual) footings
should have a 24-inch minimum width. These minimum footing widths sometimes govern
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 7
~4yAl~8(V~N ~Vi!!LS®h9, ~~VG.
footing design. If footings are located at different elevations, we recommend that the
horizontal distance between them be at least 1.5 times the elevation difference. Where
adjoining continuous footings are at different elevations, the upper footing should be stepped
down to the lower footing.
Floor Slab Support
Floor slabs for the residence and carport may be constructed as slabs-on-grade
bearing on medium dense or more competent native soil or on structural fill (placed. on
medium dense or more competent soil) compacted to at least 95 percent of its. modified
Proctor maximum density. Slab-on-grade subgrades should be observed by a qualified
geologist or engineer to evaluate whether the subgrade is medium dense or denser soil.
Care should be taken to compact any localized backfills, such as footing or utility
excavations, to the same degree as that specified for structural fill. Additional
recommendations regarding compaction of fill are provided in the "Excavations and Site
Grading" section of this letter.
We recommend that a capillary break be placed beneath the basement floor slab. A
4-inch-thick (minimum) layer of washed pea gravel placed over the prepared subgrade, as
shown in Figure 3, is recommended to provide this break. The capillary break should be
hydraulically connected to the perimeter footing drain on the west (downslope) side of the
house. As illustrated in Figure 3, the use of 2-inch-diameter weep holes is one method for
providing a hydraulic connection. A vapor barrier, consisting of a plastic sheet should be
placed directly over the pea gravel. Additional drainage recommendations are presented in
the "Drainage" section of this letter.
Lateral Earth Pressures
The lateral earth pressures acting on a wall depend on the amount the wall can yield
and the slope of the backfill. 1~pically, walls that can yield at the top an amount equal to
0.001 times the wall height can be designed for active earth pressures. Walls that yield less
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 8
SHIN®1V ~W1LSOh1. lNC.
than this amount should be designed for at-rest earth pressures. Active earth pressures may
be estimated by using an equivalent fluid weight of 35 pounds per cubic foot (pcf) for
horizontal backfill; at-rest pressures can be estimated using and equivalent fluid weight of
55 pcf for level ground conditions. Basement walls are generally considered rigid and are
designed using at-rest earth pressures. However, the structural engineer can best make this
determination based on anticipated deflections.
To account for sloping backfill behind the wall we recommend that 0.75 pcf be
added to the active pressure equivalent fluid weight for each degree of upward inclination of
the backslope above the wall (this is valid up to 30-degree inclinations; pressures for
inclinations greater than 30 degrees will require further calculations). For at-rest
conditions, 1.25 pcf should be added to the equivalent fluid weight for horizontal baclcfill
for each degree of upward inclination.
Lateral forces due to active or at-rest earth pressures of fill behind the walls will be
resisted by passive earth pressure developed in the soils in front of the wall and by friction
against the base. In our opinion, passive earth pressures in backfill could be estimated
using an allowable equivalent fluid weight of 500 pcf. We recommend that a coefficient of
friction of 0.5 be used between cast-in-place concrete and soil. These values assume that
the structures extend at least 24 inches below the lowest adjacent grade and the backfill
around the structure is a compacted granular fill, and no groundwater is present
The recommend active, at-rest, and passive pressures assume the walls are drained
so that hydrostatic pressures cannot develop. Recommendations for wall drainage and
backfilling are presented on Figure 3.
Excavations and Site Grading
It has been our experience that a slope cut to 2H:1V in the very dense soils at the
site will not ravel and will maintain vegetation. Excavations in these soils will ravel if cut
steeper than about 2H:1 V. Therefore, we recommend that permanent slopes cut steeper
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 9
2H:1V into the very dense site soils be protected with rockeries. We recommend that
rockeries be no taller than 8 feet. Figure 4 provides additional details and recommendations
for a typical rockery construction. Fill slopes should be sloped 2H:1V or flatter.
During construction, we recommend that the stability of the excavation slopes be
made the responsibility of the contractor, as they will be the one most familiaz with
conditions exposed in the excavation and will be at the site on a fuli-time basis. The
contractor should be responsible for following all current and applicable safety regulations
regarding excavations, shoring, etc.. The contractor should also be responsible for the
control of all ground or surface water wherever encountered on the project.
All fill and/or backfill beneath pavements, slabs-on-grade, and other areas where
settlements are to be minimized, should be structural fill compacted to a dense, unyielding
state, and to at least 95 percent of its Modified Proctor maximum dry density (American
Society for Testing and Materials (ASTM): D 1557-70, Method C or D). In areas where
moderate settlements can be accepted, such as in non-structural landscape areas, the
compaction requirement could be reduced to a dense, unyielding condition and to at least 92
percent of the Modified Proctor maximum dry density. We recommend that the thickness
of filUbackfill layers before compaction not exceed 8 inches for heavy compaction
equipment or 4 inches for hand-operated mechanical compactors.
The nonorganic portion of the on-site soils (topsoil) can be used for filUbackfill if
suitably compacted as previously recommended. In general, these site soils are relatively
free draining and should be workable under most conditions. In heavy, or continuous rains,
they may eventually become sufficiently wet to make them difficult to work and compact.
If earthwork is planned during the rainy season or in wet conditions, it may be necessary to
use imported, clean, granular fill rather than the on-site soils. Additionally, exposing the
site soils to construction activity when wet will substantially increase their erodability.
If imported soil is needed for fill/backfill, it should consist of clean, well-graded
sand and gravel. It should contain not more than 5 percent fines (soil passing the No. 200
W-7194-02
Mr. Frank Vane
May 16, 1996
Page 10
sieve, based on wet-sieving the minus 3/4-inch fraction). The fines should be non-plastic.
It should have a maximum particle size of 3 inches, should be free of organic matter, and
have a moisture content at or slightly below its optimum for compaction.
Drainage
We recommend that footing subdrains be installed along the outside perimeter of the
residence and on the upslope side of continuous interior footings. Footing subdrains should
consist of perforated or slotted, 4-inch-diameter, plastic pipe bedded in washed 3/8-inch pea
gravel. Typical installation of these drains is shown in Figure 3. Figure 3 also contains
subdrainage and foundation wall backfill recommendations. Other than the topsoil, most of
the on-site soils will 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 or at the bottom of the footing, whichever is deeper. We also recommend that
subdrains be installed behind rockeries, as indicated on Figure 4. A drainage geotextile
should not be used around the subdrain pipe for either the footing or rockery drains.
Roof or rockery drains should not be connected to flow into the footing subdrains,
nor should roof or footing drains be connected to flow into rockery drains. The discharge
from footing, rockery drains should be routed by means of a tightline to a suitable discharge
point. Water should not be allowed to discharge onto the surface of a slope or into slope
soils near or above the residence. All hard surfaces around the structures should be sloped
to catch basins and the collected water disposed in a controlled manner. Perimeter grades
around structures should be sloped away from the structures.
From the standpoint of minimizing the risk of slope movement, the ideal discharge
point for any water collected from the drains would be on the beach at the base of the
slope. As previously indicated in this letter, the presence of water in the near surface soils
will increase the risk of slope instability. The storm sewer that crosses the site potentially
provides a discharge point. However, the storm sewer presently discharges onto the slope
50 feet west and downhill of the proposed residence. Water collected in the storm sewer
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 11
S}~Ab;i`f®~ ~WILS~N. Ib9C.
system and discharged onto the slope increases the risk of slope instability. Discharging
water from the footing, roof andJor yard drains around the house into the storm sewer
would locally decrease the risk of slope instability close to the residence but would increase
the risk at the storm sewer discharge point.
Ideally, the storm sewer should be routed to the beach and discharge from drains
around the property be routed into the storm sewer. A second best alternative would be to
route the water collected in drains on the property in a smaller, individual tightline to the
beach, and not change the discharge point of the existing storm sewer. A third, Less ideal
alternative would be to route the water from the site into the existing storm sewer and not
change the discharge point of the storm sewer with the understanding that there is some
decrease of slope instability risk in the immediate vicinity of house and an equal increase in
risk at the existing storm sewer discharge point.
Construction Monitoring
As previously outlined in this letter, we recommend that the residence and carport
footings be located at least 2 feet below the top of the very dense outwash to reduce the risk
of movement of these structures. We therefore recommend that we be retained to monitor
the footing excavations and evaluate whether the recommend 2 feet minimum embedment
has been achieved. In addition, we recommend that we be retained to monitor other
earthwork at the site, including, subgrade preparation, structural fill compaction, and
drainage installation.
LIlVIITATIONS
1fie conclusions and recommendations presented in this report are based on site conditions
as they presently exist and assume that the explorations are representative of the subsurface
conditions throughout the site; i.e., the subsurface conditions are not significantly different
than those encountered in the sides of the excavations, test pit and site reconnaissance. If,
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 12
S~N~~N ~ONILS0~,1~1~.
during construction, subsurface conditions different from those encountered in the
explorations are observed or appear to be present, we should be advised at once so that we
can review those conditions and reconsider our recommendations where necessary. If there
is a substantial lapse of time between submission of our report and the start of work at the
site, we recommend that this report be reviewed to determine the applicability of the
conclusions and recommendations, considering the changed conditions and/or elapsed time.
This report was prepared for the use of-the Owner, Architect, and/or Engineer in the design
of the proposed structures. With respect to 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 excavations, test pit, site reconnaissance, and discussion of
subsurface conditions included in this report.
Unanticipated conditions are commonly encountered and cannot be fully determined merely
by making explorations. Such unexpected conditions frequently require that additional
expenditures be made to achieve a properly constructed project. Some contingency fund is
recommended to accommodate such potential extra costs.
The scope of our services did not include any environmental assessment for the presence or
absence of 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 happy to
discuss these with you as 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.
W 7194-02
Mr. Frank Vane
May 16, 1996
Page 13
Si~IN®N ~WiL..SON, INC.
If you have any questions regarding the observations, conclusions, or recommendations
contained in this report, please do not hesitate to call. We appreciate the opportunity to be
of service.
EXPIRES 11% %CJ~~
.~.~...~
Sincerely,
SHANNON & WILSON, INC.
~~PEO F ROFFS
S
~~ OREGON /C2
~ LIAM J. E S ~
N 1650 ~ ~(p~ (P
CFOLOG~`'~
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o~ ~cr; t~E°~
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William 7. Perkins, R.P.G. W.P. Grant, P.E.
Geologic Engineer Vice President
WJP:WPG/wJP
Enclosures: Figure 1 -Vicinity Map
Figure 2 -Site Plan
Figure 3 - Subdrainage & Backfilling
Figure 4 - Typical Rockery Detail
Important Information About Your Geotechnical Report
cc: Tom Morello, Tillman Engineering
Jean Anderson
W7194-02.LTR/W 7194-1kd/lkd
W 7194-02
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Sloped to Drain
Away From
Structure Wall
~~
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
Drainage Sand &
0 o Gravel or Washed
° Pea Gravel
18" °
00
Min. ° ' _~ Damp Proofing
o~ ~ Weep Holes Vapor
(See Note 1) Barrier
a Floor Slab
° ° •O e0 O 0 ° 0• 18 Min. o • °
o n ,°
2" to 4"
Not to Scale
MATERIALS
NOTES
4" Min.
Drainage Sand & Gravel with 1. Drainage gravel beneath floor slab should be
the Following S pecifications: hydraulically connected to subdrain pipe along the
Passing vrest (downslope) footing. Use of 2" diameter weep
Sieve Size by Weight holes as shown is one applicable method.
1-1/2" 100 2. Imported structural fill should consist of well-graded
3/4" 90 to 100 granular soil with not more than 5% fines (by weight
1/4" 75 to 100 based on minus 3/4" portion) passing No. 200 sieve
(by wet sieving) with no plastic fines.
No. 8 65 to 92
No. 30 20 to 65 3. Backfill within 18" of wall should be compacted with
No. 50 5 to 20 hand-operated equipment. Heavy equipment should
No. 100 0 to 2 not be used for backfill, as such equipment operated
(by wet sieving) (non-plastic) near the wall could increase lateral earth pressures
and possibly damage the wall.
ci ~onoe u~ n~n~
4" minimum diameter perforated or slotted pipe;
tight joints; sloped to drain (6"/100' min. slope);
provide clean-outs.
Perforated pipe holes (3/16" to 3/8" dia.) to be
in lower half of the pipe with lower quarter
segment unperforated for water flow.
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 density (ASTM: D1557-70,
Method C). Otherwise compact to 92% minimum.
Washed Pea
Gravel
Slatted pipe to have 118" maximum width slots.
Maximum slope behind rockery is
3H to 1 V for a horizontal distance
equal to the height of the rockery.
3
Very t
Dense 12" Min.
Soil
H/3 Min. Width
for Base Rock
4" Diameter Slotted ABS Pipe
Bedded in washed 3/8" pea gravel (6" cover
around pipe), sloped to drain and connected
by tightline to storm drain outfall. No fabric
around pipe.
All loose to dense soil at rockery foundation should
be overexcavated down to very dense soil and
replaced with compacted backfil! as described above.
The excavation shall be kept free of water. The
prepared rockery foundation shall be evaluated by a
soils engineer prior to placement of rock.
MINIMUM WEIGHT OF ROCK
Stable Excavation Slope in
•'• Dense Native Soil
(Contractor's Responsibility)
Openings Chinked
with Quarry Spalls
Backfill
Clean, well-graded sand & gravel or
crushed rock, 2" max. size, 40 to 60
gravel, less than 5% fines (passing #200
sieve). Fines shall be non-plastic.
• Compact in 6" lifts with min. of 4 coverages
~'•• .•••'• ~'•~•'•••.•. byhand-operated tamper. Compact to at
least 92 % of Modified Proctor maximum
dry density (ASTM D-1557-70). Backfill
and rock placement should be built up
together.
Not to Scale
Portion of wall below 6 feet, 2400 Ib.
("6-man") rock. Portion of wall above
6 feet, 1600 Ib. ("4-man") rock.
H=8' Max. 4 I l
~" ~ SHANNON & WILSON, INC.
~ Geotechnical and Environmental Consultants
W-7194-02
Attachment to Report Page 1 of 2
Dates: May 16, 1996
To: Mr. Frank Vane
Port Townsend, Washington
Important Information About Your Geotechnical/Environmental Report
CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC P[JRPOSES 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/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-
specific factors. Depending on the project, these may include: the general nature of the structure and property im~olved; 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 haw 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 discavered on or
near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orienta-
tion of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site.
Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors which were considered
in the development of the report have changed.
SUBSURFACE CONDITIONS CAN CHANGE.
Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental
report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a
report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before
construction starts; for example, groundwater conditions commonly vary seasonally.
Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may
also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant
should be kept apprised 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. Retainigg your consultant to observe subsurface construction opera-
tions can be particularly beneficial in this respect. .
Page 2 of 2
A REPORT'S CONCLUSIONS ARE PRELIIVIINARY.
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 indicatitie 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 LS SUBJECT TO MISINTERPRETATION.
Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/envir-
onmental 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 off ce 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 authorial 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 liabilit}c 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 liabilitiesto 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
1196