HomeMy WebLinkAbout901084004 Geotech Assessment W-7891-01
IEFFERSON couNtY
PERMIT CENTER
Geotechnical Report
Discovery.. Bay' Long Plat
Jefferson~County, Washington
September 1997
Mr. Michael G. Ritter
27707 10~h AVenue'S.E.
Kent, Washington 98031
SHANNON bWiLSON, INC.
GEOTECHNICA'L AND ENVIRONMENTAL CONSULTANTS
400 N. 34th St. · Suite 100
P.O. Box 300303
Seattle, Washington 98103
206 · 632 [] 8020
LOG ITEM
Maximum slope behind rockery is
3H to 1V for a horizontal distance
8" Compacted Native Soil
equal to the height of the rockery.
(Impervious Surface Layer) 3
11
Stable Excavation Slope in
Dense Native Soil
.. (Contractor's Respo'nsibility)
1 Openings Chinked
...... · Clean, well-graded sand & gravel or
· .'.'.'.'.'.'-'.'-'.'-'.' crushed rock, 2" max. size, 40 to 60%
· ~'i ~i ~ ..... gravel, less than 5.% fines (passing #200
....... Compact in 6" lifts with min. of 4 coverages
· - ..-...... by hand-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
12" Min. together.
'"~ i~'i" 4" Diameter Slotted ABS Pipe
Bedded in washed 3/8"pea gravel (6" cover
around pipe), sloped to drain and connected
.H/3 Min. Width by tightline to storm drain outfall. No fabric
for Base Rock around pipe.
All loose to medium dense soil at rockery foundation
should be overexcavated down to dense or hard soil
and replaced with compacted backfill 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.
Not to Scale
OCT ~ 0 199"(
JEFFERSON COUNTY
PERMIT CENTER
MINIMUM WEIGHT OF ROCK Discovery Bay Long Plat
Jefferson County, Washington
Portion of wall below 6 feet, 2400 lb.
("6-man") rock. Portion of wall above
6 feet, 1600 lb. ("4-man") rock. TYPICAL ROCKERY DETAIL
September 1997 W-7891-01
SHANNON & WILSON, INC. FIG. 3
Geotechnical and Environmental Consultants
Away From
Structure Wall JEFFER$ .
~ PERMIT CENTE~
Pavement or 10"to Drainage Sand &
Impervious Soil Gravel or Washed
Pea Gravel
Backfill Meeting Gradation 18"__
Requirements for Structural Fill Damp Proofing
(See Note 2)
Weep Holes Vapor
Excavation Slope (See Note 1) Barrier
Contractor's
Responsibility
6" Min. Cover of Pea Gravel ~ o ~ ~ ~ 18" Min. ~)".
(6" Min. on Sides of Pipe) I; %~ -
4" Min.
Subdrain Pipe Washed Pea
2" to 4" Gravel
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. Use of 2"
diameter weep holes as shown is one applicable
% Passing method.
Sieve Size by Weight
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
based on minus 3/4" portion) passing No. 200 sieve
1/4" 75 to 100
(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.
4. All backfill should be placed in layers not exceeding
4" loose thickness and densely compacted. Beneath
SUBDRAIN PIPE paved or sidewalk areas, compact to at least 95%
modified Proctor maximum density (ASTM: D1557-70,
4" minimum diameter perforated or slotted pipe; Method C). Otherwise compact to 92%'minimum.
tight joints; sloped to drain (6"/100' min. slope);
provide clean-outs.
Perforated pipe holes (3/16" to 1/4" dia.)to be Discovery Bay Long Plat
in lower half of the pipe with lower quarter Jefferson Coutny, Washington
segment unperforated for water flow.
Slotted pipe to have 1/8" maximum width slots. SUBDRAINAGE & BACKFILLING
September 1997 W-7891-01
SHANNON & WILSON, INC. FIG. 4
Geotechnical and Environmental Consultants
Discovery," 3
, II
/ ~ ~
Bay /
PROJECT
LOCATION
Map taken from 1:24,000 USGS topographic r a;p~ . u-~ 3tember 1997 W-7891-01
of Port Townsend South, WA quadrangle, datl ~
~G o~ch~l ~d Envimn~n~l C~s~n~
~. GO, ZO. l ON
-,.J~
FIG. 2
SHJ~NON bWILSON, INC.
TABLE OF CONTENTS
Page
1.0 INTRODUCTION ................................................................................................ 1
2.0 SITE AND PROJECT DESCRIPTION ................................................................. 1
3.0 EXPLORATIONS ................................................................................................ 2
4.0 SUBSURFACE CONDITIONS ............................................................................ 3
5.0 CONCLUSIONS AND RECOMMENDATIONS 4
5.1 Slope Stability and Project Impact .............................................................. 4
5.2 Excavations and Site Grading ..................................................................... 4
5.3 Pavement Recommendations ....... , .............................................................. 6
5.4 Wet Weather Earthwork ............................................................................ 7
5.5 Drainage and Foundation Backfill .............................................................. 8
5.6 Foundations ............................................................................................... 8
5.7 Floor Slab Support ..................................................................................... 9
5.8 Lateral Earth Pressures and Retaining Structures ..................................... 10
5.9 Erosion Control ....................................................................................... 10
5.10 Construction Monitoring and Plans Review .............................................. 11
5.11 Additional Consideration .......................................................................... 11
6.0 LIMITATIONS ................................................................................................... 12
LIST OF FIGURES
Figure No.
1 Vicinity Map
2 Site and Exploration Plan
3 Typical Rookery Detail
4 Subdrainage & Backfilling
LIST OF APPENDICIES
APPENDIX A SUBSURFACE EXPLORATIONS
APPENDIX B IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL
REPORT
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SHANNON ~WlLSON, INC.
GEOTECHNICAL REPORT
DISCOVERY BAYLONGPLAT [~ ~ ~ ~ ~ ~ ~ ~.
JEFFERSON COUNTY, WASHINGTON /~~ - -00T ~ 0 ~
~ · JEFFERSON COUN~
1.0 ~TRODUCTION / PERMIT CENTER
T~s geotec~c~ repoA presents our ~eld explorations, conclusions, and
reco~endations for the proposed long plat near Discove~ Bay, on the east side of State
Aoute 20 ~i~re 1). Our work was conducted in general accord~ce ~th our proposal
dated May 19, 1997. The pu~ose of our work is to provide geotec~cal engineering
assessments and reco~endations for site development in accordance with ~efferson
County Critical ~eas Ordin~ce ~idelines.
Our geotec~c~ evaluations are based on a site reco~aissance performed on July 11,
1997, subsurface exploration completed on and prior to July 11, 1997, and geolo~ and
soil maps (including "Geologic Map of the Ol~pic Pe~nsula, Was~n¢on" USGS Map
1-994, by Tabor ~d Cady, 1978; "Geolo~c Map of No~heastem Jefferson Countf'
Was~n~on State Di~sio~ of Geolo~ OF 76-21, by Gayer, 1976; and "Soil Su~ey of
Jefferson Coun~ ~e~ Was~n~on" USDA Soil Conse~ation Se~ice, by McCrea~ and
~les, 1975). Our understanding of the proposed subdivision layout and development is
based on site plus by Clark Land O~ce, dated May 8, 1996.
2.0 SITE AND PROJECT DESCRIPTION
As indicated on Figure 1, the site is located on the east side of Discovery Bay and State
Route 20 and approximately two thousand feet north of Anderson Lake Road. At
present, it is proposed to divide the 82-acre site into 14 lots. The site and proposed lot
dimensions and layout are shown on Figure 2. Existing and proposed road and utility
easements are also indicated on this figure. The road easements typically follow existing
dirt roads. We understand that some cuts and fills will be required to improve these roads.
However, current plans do not call for the construction of retaining walls.
The ground surface across the site slopes down to the west and southwest. The elevation
difference between the east and west sides of the site is approximately 400 feet. The slope
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SHANNON bWlLSON, INC.
of the ground surface across the eastern three quarters of the site is typically no steeper
than 10 horizontal to 1 vertical (10H: 1V). The slope of the ground surface on the western
quarter of the site is typically no steeper than 3H: 1V.
The site has been logged over the years. As a consequence, the vegetation across the site
consists of coniferous and deciduous trees (e.g., cedar, Douglas fir, maple, and alder) of
various heights and diameters (up to about 3 feet in diameter). Undergrowth on the
eastern half of the site includes species indicative of well-drained soil conditions, including
salal and Oregon grape. On the western half of the site, the undergrowth also includes
sword ferns and nettles.
Surface water (e.g., creeks or ponds) were not observed at the time of our site visit except
in some of the previously excavated test pits. We understand that others will address the
occurrence of surface water and wetlands, as these are not included in our scope of work.
3.0 EXPLORATIONS
An engineering geologist from our firm conducted geologic site reconnaissances on July
11, 1997. As part of this reconnaissance, Mr. Ritter excavated six test pits using a track
excavator. The geotechnical test pits were designated TP-101, TP-102, TP-SA, TP-20A,
TP-21A, and TP-36A. Their locations are shown on Figure 2. The location of each test
pit was determined from the location of existing septic disposal test pits that had been
previously located on site plans and from existing site features on the site plan. The test
pit locations were selected in the field to obtain subsurface information across the site in
the different topographic features, in areas where differing soil conditions might be
expected to occur, and in areas where septic test pits had not been excavated or been
excavated to only shallow depths.
Test pit depths ranged from 5 ½ to 8 feet. Soil exposed in the test pits was logged, and
relative densities or consistencies were estimated in the field by our engineering geologist.
Representative samples were collected in the field and returned to our laboratory for
further visual classification. The test pit logs are presented in Appendix A.
The site reconnaissance also included logging existing, open test pits previously excavated
for septic design. The locations of the septic test pits that were logged by our engineering
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SHANNON &WILSON, INC.
geologist are shown on Figure 2. The geotechnical descriptions and properties of the soils
exposed in the septic test pits are provided in Appendix A.
4.0 SUBSURFACE CONDITIONS
Geologic maps of the area indicate that the western portion of the site is underlain by
Quimper Sandstone of Durham, while the remainder of the site is underlain primarily by
Vashon lodgment till. The test pits confirmed the presence of both the sandstone and till
beneath the site. Septic test pits 1 and 3 and 50 to 58, located along the west side of the
property, (i.e., west half of lots 1, 2, 3, 4, and 5) encountered sandstone at depth, while
the remaining test pits farther east typically encountered Vashon lodgment till at depth.
In septic test pits I and 3 and 50 to 58, at depths of 3 to 4 feet, the test pits encountered a
very low to low strength, medium to highly weathered, friable sandstone, which is
probably the Tertiary Quimper Sandstone indicated on geologic maps to underlie the
western edge of the site. Directly above the sandstone to within approximately 1 to I ½
feet of the ground surface, a dense, brown, silty, fine sand was encountered and appears to
residual soil from complete weathering of the underlying sandstone. Topsoil was
encountered immediately above the dense, silty, fine sand and extends up to the ground
surface. The topsoil typically consists of a fine, sandy silt with scattered gravel and
cobbles.
The remainder of the test pits typically terminated in Vashon lodgment till. Till is a poorly
sorted mixture of clay, silt, sand, and gravel with scattered cobbles and boulders deposited
by a glacier. The till encountered in the test pits typically ranged from a gravelly, clayey,
sandy silt to a silty, gravelly sand with scattered cobbles and boulders. Lodgment till is
deposited at the base ora glacier and is subsequently overridden by the advancing glacier.
The Vashon Ice sheet that deposited the lodgment till is estimated to have been up to
3,000 feet thick in the area, and as a result, the lodgment till and Underlying pre-Vashon
soils were over consolidated to a very dense or hard state due to the great weight of the
ice. The consistency of the till encountered in the test pits was typically very dense at
about 2 to 4 feet below the ground surface. Within 2 to 4 feet'°f the ground surface, the
till has typically weathered to a medium dense to dense state. Typically 1 to 2 feet of
loose to medium dense, slightly gravelly, sandy, silty topsoil was encountered above the
weathered till. In test pits TP-SA, TP-20A, and TP-21A, a 1- to 2-foot-thick layer of
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SHANNON bWIL$ON, lNG.
gravelly, silty sand to slightly silty, sandy gravel was encountered above the weathered
lodgment till. This relatively thin layer of sediment between the topsoil and weathered
lodgment till is loose to medium dense and is probably ablation till. Ablation till resulted
from the deposition of sediment entrained in the ice or on the surface of the ice as the
glacier began to melt and, consequently, has not been glacially overridden.
Perched groundwater was observed in test pits TP-102 and TP-SA. The groundwater in
these test pits was perched at the top of the weathered till (2 to 3 feet below the ground
surface).
5.0 CONCLUSIONS AND RECOMMENDATIONS
5.1 Slope Stability and Project Impact
Based on the subsurface conditions observed at the site and on our experience with similar
subsurface conditions in the region, it is our opinion that the existing slopes are relatively
stable, and provided that the recommendations contained in this report are implemented,
the impact of the proposed subdivision on the stability of the site and adjacent slopes will
be small.
Please note that there is some risk of future instability on all hillsides, which the owner
must be prepared to accept. Such instability could occur because of future water
breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other
actions or events on a slope that may cause sliding.
5.2 Excavations and Site Grading
It is our opinion that permanent excavations into the lodgment till and sandstone at the site
will generally be stable at slopes of about IH: iV. However, permanent slopes cut this
steeply will ravel. It has been our experience that a slope cut to 1.5H: 1V in glacial till
would not ravel and would maintain vegetation. Permanent slopes cut steeper than
1.5H: 1V in the lodgment till or very Iow strength sandstone should be protected with
rockeries, and structures located above these cuts should be located so that their
foundations are behind an imaginary 1.5H: 1V line drawn from the base of the rockery,
back toward the structure. We recommend that rockeries be no taller than 8 feet. Figure 3
provides additional detail and recommendations for rockery construction. Fill slopes and
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SHANNON ~WlLSON, INC.
permanent excavations made into other soils at the site (i.e., topsoil, ablation till,
completely weathered sandstone) should be sloped 2H: ]¥ or flatter.
Because the test pits typically did not penetrate more than a few feet into the sandstone, it
is possible that the strength of the sandstone could increase or decrease with depth and
allow steeper or require flatter cuts at depth than recommended in the previous paragraph.
For plarming purposes, the slopes recommend in the previous paragraph should be used.
HoweYer, if relatively deep cuts will be made into the sandstone (e.g., greater than about 8
to 12 feet), the final constructed slope could be steeper or flatter than recommended. We
recommend that a qualified geologist or geotechnical engineer or their representative be
present to evaluate the strength of the rock and the steepness of the slope for relatively
deep cuts.
During construction, we recommend that the stability of the excavation slopes be made the
responsibility of the Contractor, since he/she will be the one most familiar with conditions
exposed in the excavation and will be at the site on a full-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 1557-91, Method C or D). In areas where
moderate settlements can be accepted, such as in nonstructural 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 fill/backfill 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 and sandstone can be used for fill/backfill if
suitably compacted as previously recommended. Due to the low-strength and friable
nature of the sandstone, it is anticipated that once it is excavated, it will break down into a
slightly silty to silty, fine, sandy, soil-like material. Because of the relatively high silt
content and natural water content, most of the site soils (including the broken sandstone
material) are moisture sensitive, making them difficult to work with and to compact when
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SH/~NNON ~WILSON, lNG.
wet. The natural moisture content of these soils is often sufficiently high to require some
drying/aeration before compacting. If earthwork is planned during the rainy season or in
wet conditions, it will likely be necessary to use imported, clean, granular fill rather than
the on-site soils.
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 sieve,
based on wet sieving the minus-3/4-inch fraction). The fines should be nonplasfic. 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.
5.3 Pavement Recommendations
In order to provide frost protection for pavements, we recommend that a total of at least
12 inches of pavement, base course, and/or granular subbase be provided between the
native site subgrade and the top of the pavement. Four inches of crushed rock should be
placed immediately beneath the asphalt. The remainder of the base course and granular
subbase should consist of clean, pit-run sand and gravel, well-graded crushed rock, or a
blend of commercial rock products, and contain not more than 3 percent material finer
than 0.02 mm. Normally, soil containing no more than 5 percent fines will meet this
criteria.
Base and subbase materials should be thoroughly compacted to achieve a dense and
unyielding surface, to at least 98 percent of its Modified Proctor maximum density.
Pavement subgrades should consist of medium dense to very dense or very stiffto hard,
native soil or compacted structural fill. All loose, sof[, or disturbed soil and all soil
containing organics should be removed from beneath areas to be paved. In general, this
will require 1 to 2 1/2 feet of stripping depth. We recommend that prepared pavement
subgrades be proof-rolled with a loaded dump truck or scraper prior to placement of base
and subbase materials. Sott, loose, or spongy materials identified during the proof-rolling
should be removed and replaced with cleaner and/or properly moisture-conditioned
materials.
We recommend that ditches be constructed on the uphill sides of all roads that cut across
the slope. The bottom of the ditch should be 3 feet below the top of the pavement. 'Water
collected in the ditches should be conveyed to the storm water detention facility.
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5.4 Wet Weather Earthwork
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 most
advisable to schedule earthwork during the normal dry weather months of June through
mid-October. Earthwork performed during the wet weather months will generally prove
more costly.
The soils at the site generally contain sufficient silt and plastic fines to produce a cohesive,
unstable mixture when wet. Such soils are highly susceptible to changes in water content,
and they become muddy, unstable, and difficult or impossible to proof-roll and compact if
their moisture content significantly exceeds the optimum. The condition of exposed till
may soften rapidly when exposed to moisture and construction activity.
It should also be noted that particularly during the wet weather months, groundwater
levels would be highest within the relatively loose top soil and ablation till, which overlie
the lodgment till and sandstone. Such groundwater could seep into site excavations and
would need to be intercepted by drainage ditches, trench drains, or otherwise removed. It
is our experience that the presence of standing water upon an exposed till or the
completely weathered sandstone-derived soil observed at the site, along with construction
activity, will result in disturbance and softening of the exposed subgrade. This could lead
to deeper excavations than possibly anticipated.
The following recommendations are applicable if earthwork is to be accomplished in wet
weather or in wet conditions:
a. Earthwork should be accomplished in small sections to minimize exposure to wet
weather. If there is to be traffic over the exposed subgrade, the subgrade should
be protected with a compacted layer (generally 8 inches or more) of clean sand and
gravel or crushed rock. The size or type of equipment may have to be limited to
prevent soil disturbance.
b. Fill material should consist of clean, granular soil, of which not more than 5
percent by dry weight passes the No. 200 mesh sieve, based on wet sieving the
fraction passing the 3/4-inch sieve. The fines should be nonplastic. Such soil
would need to be imported to the site.
c. The ground surface in the construction area should be sloped and sealed with a
smooth-drum roller to promote the rapid runoff of precipitation, to prevent surface
water from flowing into excavations and to prevent ponding of water.
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SHANNON ~WlLSON, iNC.
d. No soil should be left uncompacted and exposed to moisture. A smooth-drum
vibratory roller, or equivalent, should be used to seal the ground surface. Soils
that become too wet for compaction should be removed and replaced with dean
granular soil.
e. Excavation and placement of structural fill material should be observed on a full-
time basis by a geotechnical engineer, or his/her representative, experienced in
wet-weather earthwork, to determine that all unsuitable materials are removed and
suitable compaction and site drainage is achieved.
f. Covering of 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. Bales of straw and/or geotextile silt
fences should be aptly located to control soil movement and erosion.
5.5 Drainage and Foundation Backfill
For buildings that will eventually be constructed on the site, we recommend that footing
drains be installed along the outside perimeter of the structures and on the up slope side of
continuous interior footings. Footing subdrains should consist of slotted, 4-inch-diameter,
plastic pipe bedded in washed 3/8-inch pea gravel. Typical installation details for these
drains is shown in Figure 4. Figure 4 also contains subdrainage and foundation wall
backfill recommendations. On-site soils will 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. A drainage geotextile should not be used around the subdrain
pipe.
Roof drains should not be connected to the footing subdrains. The discharge from footing
drains and roof drains should be routed by means of a tightline to a suitable discharge
point (i.e., road ditches, storm sewers, etc.). All hard surfaces around the structures
should be sloped to catch basins and the collected water disposed of as previously
outlined. All outside grades should be graded to slope away from the structures.
5.6 Foundations
In our opinion, spread footings bearing in the medium dense to dense, weathered till or the
dense, completely weathered, sandstone-derived soil could be designed for 3,000 pounds
per square foot (psf) maximum allowable bearing pressure. Stripping depths to reach
these soils is about 1 to 3 feet, based on the subsurface conditions observed in the test
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SHghl~ION bWILSON, lNG.
pits. Footings bearing in the sandstone or unweathered lodgment till could be designed
for higher pressures and should be evaluated on a case-by-case basis as needed. Footings
bearing within structural fill placed over the bearing soils could be designed for allowable
bearing pressures up to 3,000 psf. Continuous footings should have a minimum width of
18 inches, and column footings should have a minimum width of 24 inches. Minimum
footing widths may govern footing design. Footings should bear at least 24 inches below
the lowest adjacent grade. The beating pressures given above may be increased by one-
third for seismic loading conditions.
If footings are supported by structural fill, this fill should extend beyond the outer edges of
footings a minimum distance equal to the fill thickness below the footing. If adjacent
individual footings are located at different elevations, it is recommended that the
horizontal distance between them be at least 1.5 times the elevation difference between
their bases. Where adjoining continuous footings are at different elevations, the upper
footing should be stepped down to the lower footing.
Foundation subgrades should be evaluated during construction to verify the presence of
competent beating soil and to determine that all soft or loosened, disturbed soils and all
existing topsoil have been removed. This evaluation should be made by a geotechnical
engineer or his/her representative.
Assuming compliance with the recommendations in this report, we expect settlement of
conventional spread footings to be no more than about '/2 inch, with differential
settlements (between adjacent footings or over a 20-foot span of continuous footing) of
approximately ¼ inch, or less. Settlements would occur almost simultaneously with load
application.
5.7 Floor Slab Support
Floors for future structures could be constructed as slabs-on-grade bearing on medium
dense to very dense native soil or on structural fill placed on these soils and compacted to
95 percent of its modified Proctor maximum density. 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.
We recommend that a capillary break be placed beneath the slab. A 4-inch-thick
(minimum) layer of washed pea gravel placed atop floor subgrades, as shown in Figure 14,
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SHANNON ~WILSON. INC.
is one method to provide this break. The capillary break should be hydraulically
connected to perimeter footing drains down slope. As illustrated in Figure 14, 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 gravel.
5.8 Lateral Earth Pressures and Retaining Structures
Basement walls (rigid) of the future structures should be designed for an at-rest equivalent
fluid pressure of 55 pounds per cubic foot (pcf), plus 1 pcffor each degree of upward
inclination of the back-slope above the wall (this is valid up to 20-degree inclinations;
pressures for inclinations greater than 20 degrees will require further calculations).
Cantilevered retaining walls that are not connected to a structure and can yield at the top
an amount equal to at least 0.001 times the wall height could be designed for an active
equivalent fluid pressure of 35 per, plus 1 pound per cubic foot for each degree of upward
inclination of the back-slope above the wall (up to 20 degrees). This active pressure
would apply to basement walls that can yield as indicated above. These pressures assume
the walls are drained so that hydrostatic pressures cannot develop. Recommendations for
wall drainage and backfilling are presented on Figure 4.
Lateral forces would be resisted by passive earth pressure against the buried portions of
structures and by friction against the bottom. In our opinion, passive earth pressures in
backfill could be estimated using an equivalent fluid pressure of 280 per above the
groundwater table (or subdrain) and 140 pcfbelow the water table (or subdrain). These
values assume that the backfill arOund the structure is a compacted granular fill. The
above values include a factor-of-safety of 1.5.
We recommend that a coefficient of friction of 0.5 be used between cast-in-place concrete
and soil. An appropriate factor-of-safety should be used to calculate the resistance to
sliding at the base of footings.
5.9 Erosion Control
Long-term erosion control can be achieved through adequate control and discharge of
surface and subsurface drainage. Following the drainage recommendations contained in
the previous section of this report will mitigate potential long-term erosion. Re-vegetating
cut and fill slopes with grasses will also provide long-term erosion control. An
appropriate grass seed mixture for this area includes:
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SHANNON bWILSON, INC.
· Red rescue 40%
· Colonial bentgrass 10%
· Perennial ryegrass 25%
· Orchard grass 15%
· White Dutch dover 10%
Other seed mixtures may also be appropriate. For slopes of 2H: 1V or greater, hydro-
seeding is recommended. An appropriate hydroseed mixture on a per acre basis may
include:
· 100 pounds of grass seed
· 2,000 pounds of wood fiber mulch
· 250 pounds of 12-24-24 fertilizer
· 40 gallons of liquid soil bonding agent
If the application is done in summer add:
· 80 pounds moisture retention agent
· 500 pounds extra of wood fiber mulch (2,500 pound total)
Summer applications must be irrigated. Seeding should be accomplished by September 15
to provide any erosional control for the following rainy season
In our opinion, erosion at the site during construction can be minimized by implementing
the recommendations in the Wet Weather Earthwork section, and can be .controlled
through the judicious use of fabric silt curtains and/or straw bales. Temporary erosion
control should be the responsibility of the Contractor since he is also responsible for the
excavation, the ditching, the sumps, etc.
5.10 Construction Monitoring and Plans Review
We recommend that we be retained to review portions of plans and specifications
pertaining to earthwork and foundations to determine whether they are consistent with our
recommendations. We also recommend that we be retained to monitor earthwork,
including structural fill placement and compaction and subgrade preparation.
5.11 Additional Consideration
It is likely that test pit excavations may be backfilled prior to construction of buildings,
driveways, or roads on the lots. If such a test pit is located in a proposed building or
W-7891-01
11
SHANNON ~WILSON. INC.
pavement area, the loose material should be removed and replaced with appropriately
compacted structural fill. Alternatively, the excavation could be structurally bridged.
6.0 LIMITATIONS
The 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 from those encountered in the test pits and site reconnaissance. If,
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 and/or Engineer in the design of the
development and 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 test pit logs, site reconnaissances, and discussion of subsurface
conditions included in this report.
Unanticipated conditions are commonly encountered and cannot be fully determined
merely by taking soil samples or 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.
Please note that the scope of our services did not include any investigation for the
presence or absence of wetlands or 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.
W-7891-01
12
SH~qNON aWILSON. INC.
Shannon & Wilson has prepared the attached Appendix B, "Important Information About
Your Geotechnical Report," to assist you and others in understanding the use and
limitations of our report.
SHANNON & WILSON, INC.
William J. Perkins, R.P.G.
Senior Geologic Engineer
WJP:CAR:TEK/wjp
W7891411.RPT/W7891-1kd/eet
W-7891-01
13
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Key Rev. I 7-12-96
GROUP/GRAPHIC
MAJOR DIVISIONS SYMBOL(~ TYPICAL DESCRIPTION
Io~o~WelI-GradedGravels, Gravel-Sand
Clean Gravels® GW Mixtures, Little or No Fines
Gravels (less than
(more than 50% 5% fines) GP l~e~ Poorly Graded Gravels, Gravel-Sand
ofcoaree~,- Mixtures, Little or No Fines
fraction retained
Coarse-Grained on No. 4 s/eve) Gravels with(~) GM Silty Gravels, Gravel-Sand-Silt Mixtures
Soils (more than Fines (more
50% retained on than 12% fines) Clayey Gravels, Gravel-Sand-Clay
No. 200 sieve) GC ~ ' Mixtures
Well-Graded Sands, Gravelly Sands,
Clean Sands(~ SW Little or No Fines
Sands (/ess than
Poorly Graded Sand, Gravelly Sands,
(50%ofcoarseOr more 5% fines) SP :'.il..]~ ..Little or No Fines
[Use Dual Symbols fraction
for 5 - 12% Fines passes the Sands with(~) SM Silty Sands, Sand-Silt Mixtures
(i.e. GP-GM)](~ No. 4 sieve) Fines(more
than 12% fines) SC Clayey Sands, Sand-Clay Mixtures
Inorganic Silts of Low to Medium
ML Plasticity, Rock Flour, or Clayey Siits
Silts and Clays Inorganic with Slight Plasticity
(liquid limit Inorganic Clays of Low to Medium
less than 50) CL Plasticity, Gravelly Clays, Sandy Clays,
Silty Clays, Lean Clays
Organic Silts and Organic Silty Clays of
Fine-Grained Soils Organic OL Low Plasticity
(50% or more
passes the Inorganic Clays of Medium to High
No. 200 sieve) CH~K'~'J~l.~,~~~ ClayPlasticity' Sandy Fat Clay, Gravelly Fat
Silts and Clays Inorganic Inorganic Silts, Micaceous or
(liquid limit MH Diatomaceous Fine Sands or Silty Soils,
50 or more) Elastic Silt
OH ~//~ Organic Clays of Medium to High
Organic
Plasticity, Organic Silts
Highly Organic Primarily organic matter, dark in Peat, Humus, Swamp Soils with High
Soils color, and organic odor PT Organic Content (See D 4427-92)
NOTES Discovery Bay Long Plat
1. Dual symbols (symbols separated by a hyphen, i.e., Jefferson County, Washington
SP-SM, slightly silty fine SAND) are used for soils
with between 5% and 12% fines or when the liquid
limit and plasticity index values plot in the CL-ML SOIL CLASSIFICATION
area of the plasticity chart. AND LOG KEY
2. Borderline symbols (symbols separated by a slash,
i.e., CL/ML, silty CLAY/clayey SILT; GW/SW, sandy September 1997 W-7891-01
GRAVEL/gravelly SAND) indicated that the soil may
fall into one of two possible basic groups. SHANNON & WILSON, INC. I FIG. A-1
Geotechnical and Environmental ConsultantsI Sheet 2 of 2
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Key Rev. 1 7-12-96
Shannon & Wilson, Inc. (S&W), uses a soil GRAIN SIZE DEFINITIONS
classification system modified from the
Unified Soil Classification (USC) System.
Elements of the USC and other definitions
FINES < #200 (0.08 mm)
are provided on this and the following page.
Soil descriptions are based on visual- SAND*
manual procedures (ASTM D 2488-93) · Fine · #200 - #40 (0.4 mm)
unless otherwise noted. · Medium · #40 - #10 (2 mm)
· Coarse · #10 - #4 (5 mm)
S&W CLASSIFICATION GRAVEL*
OF SOIL CONSTITUENTS · Fine · #4 - 3/4 inch
· Coarse · 3/4 - 3 inches
· MAJOR constituents compose more than 50
percent, by weight, of the soil. Major COBBLES 3 - 12 inches
constituents are capitalized (SAND). BOULDERS > 12 inches
· Minor constituents compose 12 to 50 percent
of the soil and precede the major constituents * Unless otherwise noted, sand and gravel, when present,
(silty SAND). Minor constituents preceded by range from fine to coarse in grain size.
'slightly" compose 5 to 12 percent of the soil
(slightly silty SAND).
RELATIVE DENSITY / CONSISTENCY
· Trace constituents compose 0 to 5 percent of
the soil (slightly silty SAND, trace of gravel).
N, SPT, RELATIVE N, SPT, RELATIVE
BLOWS/FT. DENSITY BLOWS/FT. CONSISTENCY
MOISTURE CONTENT DEFINITIONS
0 - 4 Very loose <2 Very soft
Dry Absence of moisture, dusty, dry to 4 - 10 Loose 2 - 4 Soft
the touch 10 - 30 Medium dense 4 - 8 Medium stiff
30 ~ 50 Dense 8 - 15 Stiff
Moist Damp but no visible water Over 50 Very dense 15 - 30 Very stiff
Over 30 Hard
Wet Visible free water, from below water
table
ABBREVIATIONS WELL AND OTHER SYMBOLS
ATD At Time of Drilling
['~ CemenVConcrete / Asphalt or PVC Cap
Elev.
Elevation
ft feet ~ Bentonite Grout ~ Cobbles
HSA Hollow Stem Auger
ID Inside Diameter ~ Bentonite Seal
Fill
in inches ~ Slough ~ Ash
lbs pounds
Men. Monument cover ~'1 ["-?.'1 Silica Sand
Bedrock
N Blows for last two 6-inch increments
2" I.D. PVC Screen ~ Gravel
NA ' Not Applicable or Not Available [::.~-.-.'.J (0.010-inch Slot)
OD Outside Diameter
OVA Organic Vapor Analyzer
PID Photoionization Detector
~ Discovery Bay Long
Plat
ppm parts per million I "---~~'~ II Jefferson County, Washington
PVC Polyvinyl Chloride i~l r,.- l~ccll
SS Split Spoon sampler ! · ~ ~ SOIL CLASSIFICATION
SPT Standard Penetration Test ~----~'-~ A.o LOG
USC Unified Soil Classification r~! .'o ~-~l I September 1997 W-7891~01
WLI Water Level Indicator ~,I ~ 1~ ~l [ ~
[ SHANNON & W,LSON.,NC. FIG. A-1
' :~i J~ ~1 I Geotech in cal ~d Environmental Consultants I Sheet 1 of 2
I
SHANNON &WILSON, INC.
APPENDIX A
SUBSURFACE EXPLORATIONS
W-7891-01
SHANNON &WILSON. INC.
APPENDIX A
SUBSURFACE EXPLO1LATIONS
i~FFERSON COUNTY
TABLE PERMIT CENTER
Table No.
A-1 Geotechnical Log of Septic Test Pits
LIST OF FIGURES
Figure No.
A-1 Soil Classification and Log Key
A-2 Log of Test Pit TP- 101
A-3 Log of Test Pit TP- 102
A-4 Log of Test Pit TP-36A
A-5 Log of Test Pit TP-SA
A-6 Log of Test Pit TP-21A
A-7 Log of Test Pit TP-20A
W-7891-01
A-i
sHANNON ~WlLSON. INC.
APPENDIX A
SUBSURFACE EXPLORATIONS
Subsurface conditions across the site were explored by test pits. Six geotechnical test pits were
excavated on July 11, 1997, by Mr. Ritter using a track-mounted excavator. The geotechnical
test pits were designated TP-101, TP-102, TP-gA, TP-20A, TP-21A, and TP-36A. Their
locations are shown on Figure 2 in the main body of the report. The location of each test pit was
determined from the location of existing septic disposal test pits that had been previously located
on site plans and from existing site features on the site plan.
Test pit depths ranged from 5 ½ to 8 feet. Soil exposed in the test pits was logged, and relative
densities or consistencies were estimated in the field by our engineering geologist. Representative
samples were collected in the field and returned to our laboratory for further visual classification.
The logs for test pits TP-101, TP-102, TP-SA, TP-20A, TP-21A and TP-36A are presented in
Figures A-2 through A-7, respectively. A soil classification and'log key are provided in Figure A-
l.
Selected, existing, open test pits previously excavated for septic design were also logged. The
location of the septic test pits that were logged by our engineering geologist are shown on Figure
2. The geotechnical descriptions and properties of the soils exposed in the septic test pits are
provided in Table A-1.
OCT ,~ 0 1997
JEFFERSO.,"t COUNTY
PERMIT CENTER
APP-A-B.RPT.doc/W7891 -lkd/eet
W-7891-01
SHANNON <SWlLSON, INC.
APPENDIX B
IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL REPORT
W-7891-01
Geotechnical and Environmental Consultants Dated: ~ September 30, 1997
To: Mr. Michael G. Ritter
Kent, Washington
Important Information About Your Geotechnical/Environment ! Report
CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIHC CLIENTS.
Consultants prepare reports to meet the specific needs of specific individuala 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.
~ CONSULTANT*'S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS.
A geotechnical/envimnmental 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 recommendation& 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
parlclng 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 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 tho 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 gcotechnical/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. P~sk the consultant to advise if additional tests are desirable before
construction starts; for example, groundwater conditions gommonly 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 indicate& Actual conditions in areas
not sampled may differ from those predicted in your report. While nothing can be done to prevent such situation~ you and your
consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction opera-
lions can be particularly beneficial in this respect.
'J"'"l OC T 0 1997'
i:;'ERMiT CENTFR
Page 2 of'2
A REPORT'S CONCLUSIONS ARE pIIELIMINARY.
The conclusions contained in your consultant's report are preliminary because they must be based on the assumption that condi-
tions revealed through selective exploratory sampling are indicatixe 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 conclusion~ Only the consultant who prepared the report is fully familiar with the background information needed to
determine whether or not tho report's recommendations based on those conclusions are valid and whether or not the contractor is
abiding by apphcable 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.
Cosily problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/envir-
onm~ntal 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 TIlE REPORT.
Final bering 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 architectmal 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 limitatiom~ assumin~ that a contractor was not one of the
specific persons for whom the report was pillared, 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 altematixe 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 coustmction 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 disciplinea This situation has resulted in wholly unwarranted claims being lodged against consultanta 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 recog-
ni~,e 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
questiona
Tho preceding paragraphs are based on information provided by tho
ASFE/Association of Engineering Firms Practicing in the Oeosciences, Silver Spring, Maryland
1/97