HomeMy WebLinkAbout821151014 Geotech Assessment , , '"-"- o,
GEO$CIENOE$ IN(~. , . '~
Post Office Box 6966
Bellevue, WA gSOOS-Og66
Telephone [206) 8~7-3297
Focslmlle ¢206) 881-864 ~
Doug Tourville Job Number 3091
3906 Southwest Ida December 14, 1993
Seattle, WA 98136
Subject: Geotechnicai Engineering Reconnaissance
"Tourville" Paradise Bay Property
Jefferson County, Washington -
Reference: "Tourville" #93-1482
Topography of Portion of Government Lot 4
Sec. 15, T28N, R1E, W.M.
Topographic Map prepared by ADA Engineering
Sealed by Herbert A. Armstrong on 10125193
Dear Mr. Tourville:
This report presents the results of our Geotechnical Engineering Reconnaissance for the new
residence to be constructed in the downslope area of your Paradise Bay lot in Jefferson County,
Washington. Plate 1 presents the guidelines in the use of this report.
The purpose of our services was to develop geotechnical engineering recommendations for the
design of the new home. The proposed lightly-loaded single-family residence will have a
daylight basement, and be built near the toe of the slope (Reference 1). It is anticipated that
maxhnum cuts on the order of eight feet below existing grades will be required. The scope of
our services included walking the property, selecting a test site, and then logging one backhoe
test pit near the base of the slope in the general vicinity of the proposed structure.
One test pit was excavated with a robber-tired backhoe near the 24" Maple at the toe of the slope
(See Reference 1). The test pit was excavated to an approximate depth of 7.0 feet below
existing grades. The undersigned professional engineer logged and sampled the test pit during
excavation.
Although there may very well be subsurface variations and conditions not readily apparent from
the surface, we expect the following subsurface interpretation to be essentially correct in the
building area:
The existing ground surface near the toe of the slope is immediately underlain
with three to five feet of loose gravelly sand with variable amounts of silt. Under
this surfieial layer of loose soil, a dense silty sand was encountered in out test pit.
No groundwater seepage was noted during excavation, however, mst staining was
observed in the upper looser materials.
TO UR VILLE/ PARADISE BAY LOT Job Number 3091
Dedernber 14, 1993 Page 2
Conclusions and Recommendations
General
The following general geotechnical conclusions can be drawn from our field and .laboratory test
1) The proposed residence can be supported on conventional foundations bearing on the
medium-dense native soils below the loose sufficial soils or on compacted structural fill
built up frown competent native soils. These soils, generally encountered within 3 feet of
the existing grades, are capable of providing 2,000 pounds per square foot bearing to the
structure.
2) The maximum temporary cut slope for the sandy soils is 1.5:1 (Horizontal:Vertical).
3) Footing drains are required, and must discharge to an appropriate discharge facility.
We recommend the basement walls be waterproof coated and backfilled with free-
draining soil.
4) Soils with a high percentage of silt and/or clay (>15%) are moisture sensitive and
difficult to impossible to utilize as structural fill due to the presence of the fine-grained
soils. Some of the soil on this site appears to be moisture-sensitive. Contractors should
anticipate the need to import "clean" granular fill soils if earthwork takes place during
inclement weather.
Foundations
The proposed residence may be supported on conventional continuous and spread footings
beating on the medium-dense native sandy soils or on structural fill built up from competent
native soils. All foundations should bear at a minimum depth of eighteen (18) inches below the
adjacent outside finish grade. A bearing pressure of two thousand (2000) pounds per square foot
(psf) may be assumed for foundations beating as described above.
If higher beating pressures are necessary, please contact this office. Bearing pressures may be
increased by one-third to accommodate short term wind and seismic loads. To provide
protection against shear failure, we recommend continuous and spread footings have minhnmn
widths of sixteen (16) inches and twenty-four (24) inches, respectively.
We estimate that foundations bearing on the medium-dense sandy soils will settle one-half inch.
Lateral loads such as wind and seismic forces are accommodated .by friction between the
foundation elements and the bearing soils and/or by passive earth pressure against the
foundations. However, passive earth pressure is only available if structural fill is used to backfill
against the foundation, or the foundation is poured against the existing soil. A coefficient of 0.40
may be used between the foundation elements and supporting soils. The passive resistance of
undisturbed native soils and structural fill may be taken to be an equivalent fluid having the
density of three hundred (300) pounds per cubic foot (pcf).
TOURVILLE,/ PARADISE BAY LOT Job Number 3091
December 14, 1993 Page 3
Slabs-on-Grade
Reinforced slab-on-grade floors are anticipated for the new residence. Slabs-on-grade ~nay be
supported on a ~ninitnutn of four inches of free-draining sand or gravel placed above competent
soils. All loose soft zones should be repaired prior to placement of the subslab fill. A vapor
barrier such as a sheet of a 6-mil plastic me~nbrane should be placed beneath the slab.
Any fill placed below floor slabs should meet the requirements for structural fill given below in
the "General Earthwork and Structural Fill" section of this report.
Excavations and Slopes
Te~nporary and permanent excavations and slopes for this project ~nust meet all applicable
government safety regulations. Temporary cuts to a depth of four feet ~nay be attempted vertical,
although caving of the slopes is expected. Excavation slopes greater than four feet in depth
should be cut no steeper than 1.5:1 (H:V) frown the top of the slope to the excavation botto~n.
Flatter slopes may be required depending upon local variations in soil conditions.
Permanent cut and fill slopes should not exceed 2:1 (H:V). Water should not be allowed to flow
uncontrolled over the top of any slope. All permanently exposed slopes should be seeded with
appropriate vegetation to reduce erosion and improve stability of the surficial soil.
Site Drainage
The site should be graded so that surface water is directed away frown the construction area.
Water should not be allowed to stand in any area where the foundations, pavements, or slabs are
to be constructed. Final site grades adjacent to the structure should be sloped at least two percent
(2%) away from the structure for a minimmn distance of ten feet.
·
Roof and surface water drains should not be connected to footing drains and should discharge to
an appropriate discharge facility. Footing drains for the foundation walls of the residence should
consist of a slotted four-inch-diameter PVC pipe bedded in, and covered with a minimum of six
inches of drain rock. A non-woven geotextile fabric (Mirafi 140N, Supac 4NP or equivalent)
should be wrapped around the outside of the drain rock. The PVC pipe should be sloped to
drain. The invert of the pipe should be below the lower floor slab. Foundation wall backfill and
other general wall design and construction reco~nmendations are given below.
TOURVILLE/ PARADISE BAY LOT Job Number 3091
December 14, 1993 Page 4
Base~nent Walls
Basement walls must be designed to support the lateral earth pressures which abutting soils will
impose. The following reco~runendations are for basement walls twelve feet high:
Design
Parmneter Value
Active Earth Pressure* 35 pcf
Passive Earth Pressure 400 per
Unit Weight of Soil 125 pcf
Coefficient of Friction 0.40
*The active earth pressure recormnended assmnes the wall can deflect at least
0.002 times the wall height. If this assumption is incorrect, which is often the
ease for basement walls, a uniform lateral pressure of one hundred psf should be
added to the active earth pressure.
The values recommended above are ulthnate values, and should be reduced by an appropriate
safety factor. As a guideline, we reemnmend a minhnmn factor of safety of 1.5 for overturning
and sliding. The resultant force frmn the soil (neglecting the passive pressure force) can be
calculated by taking mo~nents about the toes of the. wall. The resultant force should be pass
through the middle third of the footing.
The above design values do not include hydrostatic pressures behind the walls and assume that
no surcharge slopes or loads are placed above, or near the wails. Design values can also be
exceeded if heavy construction equipment is allowed within a prism defined by a 1:1 (H:V) line
extending up to the soil surface frown the back end of the retaining wall footing. If any of these
conditions exist, then the above design values should be aug~nented by appropriate additional
pressures. If walls higher than twelve feet are required for this project, please contact our office
for supplemental recommendations.
The foundation walls should be backfilled with clean compacted free-draining backf'fll. This will
prevent the buildup of hydrostatic pressures. The wall backfill should contain no more than five
percent silt or clay, no organics, and no cobbles greater in diameter than four inches. The
percentage of sand (From the #4 sieve to the #200 sieve) should range from 25 to 75 percent. If
the on-site soils are used as backfill, a drainage composite such as Mimdrain 6000 or equivalent
may be required on the foundation walls. During compaction of wall backfill, care should be
taken to not damage the wall. The top one to two feet of wall backfill should consist of a
relatively impermeable soil or topsoil.
All below-grade walls on the structure which enclose living areas should be waterproofed.
TO UR VILLE/ PARADISE BAY LOT Job Number 3091
December 14, 1993 Page 5
General Earthwork and Structural Fill
Site construction should begin by stripping and clearing the building areas of vegetation, roots,
and any other deleterious lnaterial. Stripped lnatedals should be stockpiled or removed from the
site.
Structural fill is defined as any fill placed below structures, including pavements, where the fill
soils would need to support loads without unacceptable deflections or shearing.-. Structural fill
should be placed above unyielding site soils in maxhnum eight-inch-thick loose lifts and
compacted to 95 percent of Modified Proctor (ASTM D-1557). Moisture-sensitive soils
typically have a significant percentage of fine-grained material. The on-site soil is somewhat
silty, and is moisture-sensitive. These soils will be very difficult to place and compact as
structural fill if these soils are ~nuch wetter than the opthnum moisture content at the time of
compaction. During wet weather or under wet conditions, structural fill should consist of a
granular soil having less that five percent silt or clay 0neasured on that portion which passes the
3/4-inch sieve).
In general, we recomtnend the earthwork portion of this project be conducted during the
normally drier smmner ~nonths.
Closure
It is recolmnended we be retained to review the final develop~nent plans to verify site specific
subsurface requirements are met and our reco~mnendations have been accurately interpreted in
the plans.
It is also recommended that we be retained to provide professional geotechnical consultation,
and observation services during design and construction. This allows us to: 1) confirm that
design conforms to specific subsurface requirements; 2) confmn that subsurface conditions
during construction are consistent with those indicated by this report; 3) evaluate whether
earthwork and foundation construction activities conform to the intent of the contract
specifications and plans, and; 4) provide reco~nmendations for design changes in the event of
changed conditions. While on the site during construction, we will not direct or supervise the
contractor or the work, nor we will be responsible for maintaining or providing for safety during
construction activities. Also, we will not be responsible for dimensional measurements during
construction.
TO UR VILLE/ PARADISE BAY LOT Job Number 3091
December 14, 1993 Page 6
It has been a pleasure providing you with our professional services. If there are any questions,
please call ~ne directly at (206) 867-3297.
Sincerely:
DODDS Geosciences Inc.
~rk K. D~, ~E.
P~sident
~D:wd
Enclosure: 2 Plates