HomeMy WebLinkAbout950101319 Geotech Assessment,*
GEOTECHNICAL REPORT
1131 THORNDYKE ROAD
PORT LUDLOW, WASHINGTON
PREPARED FOR
FRANKIE & RICHARD CARNEY
BY
GEOTECHNICAL TESTING LABORATORY
OLYMPIA, WASHINGTON
JULY 28, 2006
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GEOTECHNICAL TESTING LABORATORY
CONTACT INFORMATION
PREPARER INFORMATION
GTL PROJECT NUMBER: 06-2634-06
ADDRESS: 10011 BLOMBERG STREET SOUTHWEST
OLYMPIA, WASHINGTON 98512
TELEPHONE: (360) 754-4612
FACSIMILE: (360) 754-4848
EMAIL ADDRESS: GEOTESTLAB@COMCAST.NET
CLIENT INFORMATION
CLIENT: FRANKIE & RICHARD CARNEY
TELEPHONE: (360) 437-9925
MAILING ADDRESS: 1131 THORNDYKE ROAD
PORT LUDLOW, WASHINGTON 98365
SITE ADDRESS: 1131 THORNDYKE ROAD
PORT LUDLOW, WASHINGTON 98365
PARCEL NUMBER: 950101319
GPS LOCATION: N47° 50.226' W122° 41.521'
10011 Blomberg Street SW, Olympia, WA 98512 2
Phone #: (360) 754-4612 Fax #: (360) 754-4848
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GEOTECHNICAL TESTING LABORATORY
SCOPE OF UNDERSTANDING
FRANKIE & RICHARD CARNEY
1131 THORNDYKE ROAD
PORT LUDLOW, WASHINGTON 98365
RE: GEOTECHNICAL REPORT
1131 THORNDYKE ROAD
PORT LUDLOW, WASHINGTON 98365
PARCEL No. 950101319
N47° 50.226' W122° 41.521'
As requested, we have conducted a soils exploration, foundation evaluation, and slope stability analysis for the
above-mentioned parcel. The results of this investigation, together with our recommendations, are to be found in
the following report. We have provided three copies for your review and distribution.
Data has been carefully analyzed to determine soils bearing capacities, footing embedment depths, and setback
distances. The results of the exploration and analysis indicate that conventional spread and continuous wall
footings appear to be the most suitable type of foundation for the support of the proposed structure. Some
variability was encountered in comparing the soil profiles of the site. Net allowable soil pressures, embedment
depth, and total expected settlements have been presented for the site later in the report.
We appreciate this opportunity to be of service to you and we look forward to working with you. in the future. If
you have any questions concerning the above items, the procedures used, or if we can be of any further assistance,
please call us at the phone number listed below.
Respectfully Submitted,
GEOTECHNICAL TESTING LABORATORY
N~ ~~
Harold Parks, L.G., L.E.G.
Senior Engineering Geologist
10011 Blomberg Street SW, Olympia, WA 98512
Phone #: (360) 754-4612 Fax #: (360) 754-4848
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GEOTECHNICAL TESTING LABORATORY
TABLE OF CONTENTS
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CONTACT INFORMATION ..................................................................................................................................... 2
SCOPE OF UNDERSTANDING ............................................................................................................................. .. 3
TABLE OF CONTENTS .......................................................................................................................................... .. 4
INTRODUCTION ..................................................................................................................................................... .. 5
SITE CONDITIONS ................................................................................................................................................. .. 6
SURFACE CONDITIONS .......................................................................................................................................... .. 6
SITE GEOLOGY ...................................................................................................................................................... .. 6
SITE SOILS ............................................................................................................................................................. .. 8
SUBSURFACE EXPLORATIONS ............................................................................................................................... .. 8
SUBSURFACE CONDITIONS .................................................................................................................................... .. 9
SLOPE STABILITY .................................................................................................................................................. .. 9
CONCLUSIONS AND RECOMMENDATIONS .................................................................................................... 10
GENERAL ............................................................................................................................................................... 10
LANDSLIDE -EROSION HAZARD AREAS ................................................................................................................... 10
CLASSIFICATION .................................................................................................................................................... 10
SLOPE STABILITY .....................................................................:............................................................................ 11
BUILDING SETBACK .............................................................................................................................................. 12
SEISMIC -LIQUEFACTION HAZARD ...................................................................................................................... 14
EROSION CONTROL ............................................................................................................................................... 14
EARTHWORK ......................................................................................................................................................... 15
SITE PREPARATION ................................................................................................................................................ 15
STRUCTURAL FILL ................................................................................................................................................. 16
SUITABILITY OF ONSITE SOILS AS FILL ................................................................................................................. l6
CUT AND FILL SLOPES ........................................................................................................................................... 16
FOUNDATION SUPPORT ......................................................................................................................................... 17
FLOOR SLAB SUPPORT .......................................................................................................................................... l 8
RETAINING WALLS ................................................................................................................................................ 18
SLOPE INCLINATION: EQUIVALENT FLUID PRESSURE ........................................................................................... 18
RETAINING WALL ALTERNATIVES ........................................................................................................................ 19
SITE DRAINAGE ..................................................................................................................................................... 19
SEPTIC IMPACT ...................................................................................................................................................... 19
LIMITATIONS ......................................................................................................................................................... 20
FIGURE 1 VICINITY MAP ..................................................................................................................................... 21
10011 Blomberg Street SW, Olympia, WA 98512 4
Phone #: (360) 754-4612 Fax #: (360) 754-4848
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GEOTECHNICAL TESTING LABORATORY
INTRODUCTION
This report summarizes the results of our geotechnical consulting services for the two additions to the existing
structure located along the west shore of the Hood Canal in Jefferson County, Washington. The site (0.42 acres)
is located 5.2 miles west of Port Gamble, Washington. The location of the site is shown relative to the
surrounding area on the Vicinity Map, Figure 1.
Our understanding of the project is based on our discussions with you and our explorations and review of the site.
We understand that the parcel is to be further developed by adding two additions to the existing structure. The
two additions will be constructed at a distance (setback) that is greater from the steep eastern slope than the
existing structure. The site is accessed by a driveway from Thorndyke Road. In general, grading will consist of
the excavation of the foundation and footings.
The site slopes predominantly east towards the Hood Canal from a central north-south ridgeline. The steepest
slope measured onsite was in excess of 70 percent. Therefore, Jefferson County requires that a geotechnical
report be prepared in accordance with the Critical Areas Ordinance.
The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for providing
geotechnical recommendations and design criteria for the project and. to satisfy the requirements of the Jefferson
County Critical Areas Ordinance. Geotechnical Testing Laboratory is therefore providing geologic and
hydrogeologic services for the project. Specifically, our scope of services for this project will include the
following:
1. Review the available geologic, hydrogeologic, and geotechnical data for the site area.
2. Conduct a geologic reconnaissance of the site area and surrounding vicinity.
3. Investigate shallow subsurface conditions at the site by observing the exposed soil and reviewing
published well logs.
4. Evaluate the landslide and erosion hazards at the site per the Jefferson County Critical Areas Ordinance
regulations.
5. Provide geotechnical recommendations for site grading including site preparation, subgrade preparation,
fill placement criteria (including hillside grading), temporary and permanent cut and fill slopes, and
drainage and erosion control measures.
10011 Blomberg Street SW, Olympia, WA 98512
Phone #: (360) 754-46]2 Fax #: (360) 754-4848
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SURFACE CONDITIONS
SITE CONDITIONS
The proposed building site is located in an area of moderate residential development in the Puget Sound glacial
lowland overlooking the Hood Canal. The site has predominantly eastern exposure. We conducted a
reconnaissance of the site area on June 30, 2006. Site elevations range approximately from 280 to over 300 feet.
The additions will be constructed to the north and south of the existing structure.
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Exposed soil due to geologic processes was not observed onsite. The site is vegetated with awell-mixed variety
of plants common to the Northwest. The vegetation includes fir, cedar, madrone, and alder trees as well as sword
fern, bracken fern, blackberry, salal, huckleberry, ivy, rhododendron, and grasses. Vegetated areas appear to be
undisturbed.
The general topography of the site area indicates that drainage is to the east and southwest from the proposed
building locations. Groundwater was not encountered or observed onsite. Evidence of surface flow was not
observed onsite. Slope instability was not observed near the proposed building locations. Sloughing and raveling
were not observed onsite. Evidence of surface water (ponding) was not observed onsite.
SITE GEOLOGY
The site is generally situated within the Puget Sound glacial upland. The existing topography, as well as the
surficial and shallow subsurface soils in the area, are the result of the most recent Vachon stade (stage) of the
Fraser glaciation that occurred between about 9,000 and 11,000 years ago, and. weathering and erosion that have
occurred since. A description of the surficial soils is included in the "Site Soils" section of this, report. In general,
the soils are composed of glacially derived material.
The Geologic Map of Washington -Northwest Quadrant (2002) has mapped the site geology as glacial till
deposits (Qgt) of continental glacial origin. The report reads:
Till -Unsorted, unstratified, highly compacted mixture of clay, silt, sand, gravel, and boulders
deposited by glacial ice; may contain interbedded stratified sand, silt, and gravel. Includes part
of the Pashon Drift undivided.
10011 Blomberg Street SW, Olympia, WA 98512 6
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
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GEOTECHNICAL TESTING LABORATORY
The Geologic Map of Washington -Northwest Quadrant (2002) has mapped the geology offsite to the
east as glacial and nonglacial undifferentiated surficial deposits (Qguc). The report reads:
Undifferentiated surficial deposits -Clay, silt, sand, gravel, till, diamicton, and peat; unit
shown where steep slopes preclude more detailed delineation at map scale; includes pre-
Holocene deposits and Holocene alluvium on landslide deposits found along steep slopes of
narrow stream valleys on the northern Olympic Peninsula; also includes poorly exposed
sediments at the base of the Vashon Drift whose stratigraphic assignment is undetermined.
Includes part of the Double Bluff, Possession, and Everson Glaciomarine Drifts, part of the
Vashon Drift undivided, part of the Whidbey Formation, and part of the Olympia beds
(Armstrong and others, 1965).
The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (1993) by Yount
Minard, and Dembroff has mapped the site geology as Vashon Till deposits (Qvt). The report reads:
Vashon Till (Late Pleistocene) -Light to dark gray, nonsorted, nonstratified mixture of clay, silt,
sand and gravel up to boulder-size (diamicton). Very stiff and impermeable, often resulting in
poorly drained bogs developing on the unit in relatively flat areas. Clast lithology diverse with
many rock types derived from outside the quadrangle. Contains minor amount of interbedded
stratified sand and gravel, sometimes separating the till into multiple sheets. Mantles uplands
throughout the quadrangle and drapes discontinuously down valley walls as well. Encountered
in subsurface of the Duwamish and Snohomish River Valleys, underlying as much as 30 m of
recessional outwash deposits. Generally overlies advanced outwash deposits (Qva), but
sometimes rests unconformably on older glacial and nonglacial deposits (Dethier and others,
1982). Usually 1 to 2 m thick, but locally can be as much as 25 m thick.
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10011 Blomberg Street SW, Olympia, WA 98512 ~
Phone #: (360) 754-4612 Fax #: (360) 754-4848
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SITE SOILS
GEOTECHNICAL TESTING LABORATORY
The Soil Survey of Jefferson County, USDA Soil Conservation Service (1975) has mapped the site soils as a
Alderwood gravelly sandy loam, 0 to 15 percent slopes (A1C). The survey reads,
The Alderwood series consists of moderately well drained soils that have a very slowly
permeable cemented layer at a depth of 20 to 40 inches. These soils formed in glacial till under a
forest of mixed coniferous and broadleaved vegetation.
In a representative profile a thin layer of organic litter covers the surface. The top 1 inch of the
soil is very dark grayish-brown gravelly fine sandy loam. Below this, to a depth of 12 inches, is
dark yellowish-brown gravelly sandy loam. Beneath this layer, and continuing to a depth of 30
inches, is gravelly sandy loam that is brown in the upper part and dark grayish brown and
prominently mottled in the lower part. The next layer is a dark grayish-brown cemented layer
that formed in very compact glacial till. Rounded pebbles, cobblestones, and stones are on the
surface and throughout the profile. This nearly level to rolling soil is on glacial terraces. In most
places, the slope is 5 to 10 percent.
Small areas of sandy, very gravelly, or cobbly soils are included with this soil in mapping. This
soil is moderately well drained. Permeability above the cemented layer is moderately rapid.
Roots penetrate to the cemented layer and flatten out on top of it. A perched water table is above
the cemented layer during the winter months. This soil holds 2 to 4 inches of water available for
plants. Runoff is slow to medium, and the hazard of water erosion is slight to moderate.
The Soil Survey of Jefferson County, USDA Soil Conservation Service (1975) has mapped the site soils offsite to
the east as a Cassolary sandy loam, 30 to 50 percent slopes (CfE). The survey reads,
The Cassolary series consists of well-drained soils on upland terraces. Slopes range from 0 to 50
percent. These soils formed in reworked glacial and marine sediments.
In a representative profile in a wooded area, a thin layer of organic litter covers the surface. The
upper 23 inches of the soil is sandy loam. To a depth of 3 inches it is dark gray, between depths
of 3 and I S inches it is dark brown, and between depths of 1 S and 23 inches it is dark grayish
brown. Below this is grayish-brown silt loam that extends to a depth of 27 inches. Beneath this,
and extending to a depth of 33 inches, is grayish-brown silty clay loam. The next layer is light
olive-brown fine sandy- loam that extends to a depth of 49 inches. It is underlain to a depth of 60
inches by very dark grayish-brown medium sand. This nearly level to rolling soil is on upland
terraces. In most places slopes range from 5 to 10 percent.
This soil is well drained. Permeability is moderately slow. Roots penetrate to a depth of more
than 60 inches. This soil holds about 7 to 9 inches of water available for plants. Runoff is slow to
medium, and the hazard of water erosion is slight to moderate.
SUBSURFACE EXPLORATIONS
Subsurface conditions at the site were evaluated by observing the exposed building site soil, reviewing available
well logs, and logging a test pit. Depth to competent soil is approximately 6 inches throughout the proposed
building locations. Groundwater was not observed or encountered to a depth beyond the scope of this report.
1001 l Blomberg Street SW, Olympia, WA 98512
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
SUBSURFACE CONDITIONS
In general, undisturbed dense gravelly sandy loam was observed throughout the proposed building locations.
Based on the site topography and the nature of the near-surface soil, seasonally perched groundwater conditions
should not be expected during periods of extended wet weather.
SLOPE STABILITY
The Coastal Zone Atlas, Volume 11, Jefferson County (JE-11) maps the site as Unstable.
Unstable (U) slopes are considered unstable because of geology, groundwater, slope and/or
erosional factors. They include areas of landslides and talus too small or obscure to be
individually mapped.
Since slopes in excess of 70 percent were observed
onsite, Jefferson County requires that a geotechnical
report be completed according to the Critical Areas
Ordinance.
The near-surface soils are in a dense to very dense
condition except at the ground surface. The surficial
soils are generally in a medium dense condition.
In general, the undisturbed native soils of the site consist
of a mixture of variable amounts of sand, silt, and gravel.
These soil materials are in a dense condition except
where they have been disturbed by weathering activity.
These soils are generally of intermediate stability relative
to deep-seated failure.
Weathering, erosion, and the resultant sloughing and
shallow landsliding are natural processes that can affect steep slope areas. Instability of this nature is typically
confined to the upper weathered or disturbed zone, which has been disturbed and has a lower strength.
Significant weathering typically occurs in the upper 2 to 3 feet and is the result of oxidation, root penetration,
wet/dry cycles, and freeze/thaw cycles. Erosion in steep slope areas such as this can be reduced by encouraging
vegetation and discouraging runoff from the steep slope. Erosion control recommendations for the sloping areas
are provided in the "Erosion Control" section of this report.
10011 Blomberg Street SW, Olympia, WA 98512 g
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GENERAL
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GEOTECHNICAL TESTING LABORATORY
CONCLUSIONS AND RECOMMENDATIONS
Based on the results of our site reconnaissance, subsurface observations, and our experience in the area, it is our
opinion that the site is suitable for the proposed remodel. The building location slopes are stable relative to deep-
seated instability and will not be affected by the proposed structure if our recommendations are respected. The
proposed additions will not undermine adjacent structures or slopes. Proper drainage control measures will
reduce or eliminate the potential for erosion in this area and improve slope stability. The hazards of the landslide
area can be overcome in such a manner as to prevent harm to property and public health and safety, and the
project will cause no significant environmental impact.
' In general, the gravelly loamy sandy soils observed at the site may
be suitable for use as structural fill material. Saturated soil
conditions may be associated with these soils during or following
' extended periods of rainfall. However, to reduce grading time and
construction costs, we recommend that earthwork be undertaken
during favorable weather conditions.
Conventional construction equipment may be utilized for work at
the site. Conventional spread footings may be utilized at the site
for support of the structure. A vapor barrier is recommended for all
slabs-on-grade.
Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed
single-family residence are presented below.
LANDSLIDE -EROSION HAZARD AREAS
' CLASSIFICATION
The Jefferson County Critical Areas Ordinance (3.6.7) defines a landslide hazard. area as:
' Areas potentially subject to mass movement due to a combination of geologic, topographic and
hydrologic factors including:
A. Areas of historic failures or potentially unstable slopes, such as:
1. areas described and mapped as having severe or very severe building limitations for dwellings
without basements within the United States Department of Agriculture/ Soil Conservation
Service Soil Survey for Jefferson County;
2. areas described and mapped as recent or old landslides or slopes of unstable materials within
the Washington State Department of Ecology Coastal Zone Atlas of Jefferson County; and,
3. areas described and mapped as areas of poor natural stability, former landslides and recent
landslides by the Washington State Department of Natural Resources, Division of Geology and
Earth Resources.
B. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, or
undercutting by wave action; and
C. Areas with any indications of earth movement, such as:
1. rockslides;
' 2. earthflows;
3. mudflows; and,
4. landslides.
10011 Blomberg Street SW, Olympia, WA 98512 10
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
Slopes in excess of 70 percent were observed onsite. Since the site area is mapped as "unstable" by The Coastal
' Zone Atlas, Volume 11, Jefferson County (JE-11), the site meets the requirement of a landslide hazard area and a
geotechnical report is to be completed according to the Critical Areas Ordinance.
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The Jefferson County Critical Areas Ordinance (3.6.7) defines an erosion hazard area as:
Areas containing soils or soil complexes described and mapped within the United States
Department of Agriculture/ Soil Conservation Service Soil Survey for Jefferson County as
having a severe or very severe erosion hazard potential.
The Soil Survey of Jefferson County, USDA Soil Conservation Service (1975) describes the site area erosion
potential as slight to moderate.
SLOPE STABILITY
Based on our field observations, explorations, and our experience with the soil types encountered on the property,
we conclude that although portions of the slopes on the lot exceed 70 percent, the site is generally stable relative
to deep-seated failure in its present configuration.
To prevent minor sliding, uncompacted fill material and any underlying vegetation shall be removed in order to
construct the foundation. Excavation and backfilling will occur based on appropriate engineering and earthwork
recommendations found in the following "Earthwork" section. Grading in the building portion of the site should
be conducted in accordance with geotechnical recommendations provided herein.
As previously discussed, weathering, erosion, and the resultant surficial sloughing and landsliding are natural
processes that affect slope areas. Significant weathering typically occurs in the upper 2 to 3 feet and is the result
of oxidation, root penetration., wet/dry cycles, and freeze/thaw cycles. Over-excavation may be necessary to
ensure the removal of deleterious material.
10011 Blomberg Street SW, Olympia, WA 98512 11
Phone #: (360) 754-461.2 Fax #: (360) 754-4848
These processes can be managed and the risk reduced through proper construction of the residence. Erosion
control recommendations in the slope and buffer areas are provided in the "Building Setback" and "Erosion
Control" sections of this report.
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GEOTECHNICAL TESTING LABORATORY
BUILDING SETBACK
Based on our geotechnical evaluation of the site and
our experience in the area, a building setback will be
needed for this lot. A building setback of 15 feet
from the crest of slopes 40% or greater to the bottom
of the footing should otherwise be observed. The
building setback may be measured from the bottom of
the footing to the face of the steep slope in
accordance with the 2003 International Building Code
(IBC). Foundation elements must be constructed in
native material and not fill material.
Peak Shear Stress vs. Normal Stress
Gravelly Sandy Loam
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Slope stability was modeled using the GeoStudio 2004 (version 6.17) in both static and extreme dynamic
' conditions (ca = 0.3). Factors of safety were determined using Bishop's, Janbu, and the Morgenstern-Price
methods. The site was modeled using a gravelly sandy loam. The gravelly sandy loam material was determined
to have a unit weight of 126 pcf, cohesion of 400 psf, and a shear angle (~) of 40°. Under static conditions, the
slope was not susceptible to shallow failure or susceptible to deep-seated failure. Under dynamic loading, the
3,328 computations demonstrated that the slope might be susceptible to surficial raveling but not to large deep-
seated failure. The following figure illustrates the moment factor of safety for slope "A" under the existing
conditions. The figure is the solution of greatest concern and exhibits the need for a building setback of 15 feet
from the face of slopes greater than 40 percent. All foundation elements shall be constructed on native material or
engineered fill material.
' 10011 Blomberg Street SW, Olympia, WA 98512 12
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
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Distance (ft)
As previously discussed, weathering, erosion and the resultant surficial sloughing and shallow landsliding are
natural processes that affect slope areas. Surficial raveling and sloughing were not observed onsite. To manage
and reduce the potential for these natural processes, we recommend the following:
1. No drainage of concentrated surface water or significant sheet flow onto the sloped areas.
10011 Blomberg Street SW, Olympia, WA 98512 13
Phone #: (360) 754-4612 Fax #: (360) 754-4848
GEOTECHNICAL TESTING LABORATORY
According to the Seismic Zone Map of the United States contained in the 2003 International Building Code (IBC),
the project site is located where the maximum spectral response acceleration is 45 percent of gravity (g).
Based on the subsurface conditions observed at the site, we interpret the site conditions to correspond to a seismic
Soil Profile Type C, for Very Dense Soil, as defined by Table 1615.1.1 (IBC). This is based on the range of SPT
(Standard. Penetration Test) blow counts and/or probing with a '/2-inch diameter steel probe rod. The shallow soil
conditions were assumed to be representative for the site conditions beyond the depths explored.
The Liquefaction Susceptibility Map of Jefferson County, Washington by Palmer, Magsino, Poelstra, Bilderback,
Folger, and Niggemann (September 2004) maps the site area as having a very low liquefaction potential. Offsite
to the east is an area mapped as having a low to very low liquefaction potential.
The Site Class Map of Jefferson County, Washington by Palmer, Magsino, Bilderback, Poelstra, Folger, and
Niggemann (September 2004) maps the site area as site class C. Site class C is a very stiff soil or soft rock.
Offsite to the east is an area mapped as site class C to D. Site class C is a very stiff soil or soft rock and site class
D is a stiff soil.
Based on our review of the subsurface conditions, we conclude that the site soils are only mildly susceptible to
liquefaction.
The near-surface soils are generally in a dense condition and. the static water table is considerably deep. Shaking
of the already dense soil is not apt to produce a denser configuration and subsequently excess pore water pressures
are not likely to be produced.
EROSION CONTROL
It is our opinion that the potential erosion hazard of the site is not a limiting factor for the proposed development.
Removal of natural vegetation should be minimized and limited to the active construction areas. Yard
landscaping around the home is permissible, but understory growth on the slopes should be encouraged as much
as possible as a deterrent to erosion. Trees located on steep slopes may be removed only if the stumps remain to
deter erosion.
10011 Blomberg Street SW, Olympia, WA 98512 14
Phone #: (360) 754-4612 Fax #: (360) 754-4848
SEISMIC -LIQUEFACTION HAZARD
GEOTECHNICAL TESTING LABORATORY
Temporary and permanent erosion control measures should be installed and maintained during construction or as
soon as practical thereafter to limit the additional influx of water to exposed areas and protect potential receiving
waters.
Erosion control measures should include, but not be limited to, silt fences, berms, and swales with ground
cover/protection in exposed areas. Atypical silt fence detail is included on Figure 2. Any re-contouring of the
site will create a need for erosion control measures as listed above. Areas of exposed soil must be seeded as soon
as possible.
SITE PREPARATION
EARTHWORK
All areas to be excavated should be cleared of deleterious matter including any existing structures, debris, duff,
and vegetation. Based on our observations, we estimate that stripping on the order of 8 to 12 inches will be
necessary to remove the remaining surficial soils containing organics. Areas with deeper, unsuitable organics
' should be expected in the vicinity of depressions or heavy vegetation. Stripping depths of up to 18 inches may
occur in these areas. These materials may be stockpiled and later used for erosion control and landscaping.
Materials that cannot be used for landscaping or erosion control should be removed from the project site.
' Where placement of fill material is required, the exposed subgrade areas should be proof-rolled to a firm and
unyielding surface prior to placement of any fill. We recommend that trees be removed with the roots, unless
located on a slope. Excavations for tree stump removal in any building area should be backfilled with structural
fill, compacted to the density requirements described in the "Structural Fill" section of this report.
' If structural fill is needed, we recommend that a member of our staff evaluate the exposed subgrade conditions
after vegetation removal and topsoil stripping are completed.
Any soft, loose, or otherwise unsuitable areas delineated during foundation preparation or probing should be
' compacted, if practical, or over-excavated and replaced with structural fill, based on the recommendations of our
report.
10011 Blomberg Street SW, Olympia, WA 98512 15
Phone #: (360) 754-4612 Fax #: (360) 754-4848
~i
GEOTECHNICAL TESTING LABORATORY
STRUCTURAL FILL
All fill material should be placed as structural fill. The structural fill should be placed in horizontal lifts of
appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least
90 percent of MDD (maximum dry density as determined in accordance with ASTM D-1557) to within 2 feet of
' subgrade and 95 percent MDD in the upper 2 feet.
The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We
recommend that the appropriate lift thickness be evaluated by our field representative during construction.
' The suitabili of material for use as structural fill will de end on the radation and moisture content of the soil.
tY p g
As the amount of fines (material passing No. 200 sieve) increases, soil becomes increasingly sensitive to small
changes in moisture content and adequate compaction becomes more difficult to achieve. During wet weather, we
recommend the use ofwell-graded sand and gravel with less than 7 percent (by weight) passing the No. 200 sieve
based on that fraction passing the 3/4-inch sieve.
' If prolonged dry weather prevails during the earthwork and foundation installation phase of construction, a
somewhat higher (up to 10 percent) fines content will be acceptable.
' Material placed for structural fill should be free of debris, organic matter, trash, and cobbles greater than 6 inches
in diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction.
Onsite soils may be considered for use as structural fill. In general, the native soils (sand, loam, and gravel)
encountered on the site must have less than 10 percent fines (material passing the US No. 200 Sieve) to be
suitable for use as structural fill.
CUT AND FILL SLOPES
' All job site safety issues and precautions are the responsibility of the contractor providing services and/or work.
The following cut/fill slope guidelines are provided for planning purposes.
10011 Blomberg Street SW, Olympia, WA 98512 16
Phone #: (360) 754-4612 Fax #: (360) 754-4848
"~ SUITABILITY OF ONSITE SOILS AS FILL
GEOTECHNICAL TESTING LABORATORY
Temporary cut slopes will likely be necessary during grading operations. As a general guide, temporary slopes of
1.5 to 1 (horizontal to vertical) or flatter may be used for temporary cuts in the upper 3 to 4 feet of the glacially
consolidated soils that are weathered to a loose/medium-dense condition. Temporary slopes of 1 to 1 or flatter
may be used in the unweathered dense to very dense sands and gravel.
' These guidelines assume that all surface loads are kept at a minimum distance of at least one-half the depth of the
cut away from the top of the slope and that significant seepage is not present on the slope face. Flatter cut slopes
will be necessary where significant raveling or seepage occurs.
Surface drainage shall be directed away from all slope faces. All slopes should be seeded as soon as practical to
facilitate the development of a protective vegetative cover or otherwise protected.
' FOUNDATION SUPPORT
Where foundation elements are located near slopes between 5 and 30 percent, the footings should be located a
' minimum of 2 times the footing width from the slope face (horizontally), and founded in dense or very dense
native soils or properly prepared structural fill.
We recommend a minimum width for isolated footings and for continuous wall footings to meet IBC 2003
standards. Footings founded as described above can be designed using an allowable soil bearing capacity of 2,000
psf (pounds per square foot) for combined dead and long-term live loads in areas of dense to very dense soils.
The weight of the footing and any overlying backfill may be neglected. The allowable bearing value may be
increased by one-third for transient loads such as those induced by seismic events or wind loads.
' Lateral loads may be resisted by friction on the bases of footings and floor slabs and as passive pressure on the
sides of footings. We recommend that an allowable coefficient of friction of 0.40 be used to calculate friction
' between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent
fluid density of 250 pcf (pounds per cubic foot).
We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch for the
anticipated load conditions, with differential settlements between comparably loaded footings of 'h inch or less.
Most of the settlements should occur essentially as loads are being applied. However, disturbance of the
foundation subgrade during construction could result in larger settlements than predicted.
10011 Blomberg Street SW, Olympia, WA 98512
Phone #: (360) 754-4612 Fax #: (360) 754-4848
0
J
GEOTECHNICAL TESTING LABORATORY
FLOOR SLAB SUPPORT
Slabs-on-grade should be supported on medium dense or dense native soils or on structural fill prepared as
described in the "Structural Fill" section of this report. We recommend that floor slabs be directly underlain by a
minimum 6-inch thickness of coarse sand and/or gravel containing less than 3 percent fines (by weight). The
drainage material should be placed in one lift and compacted to an unyielding condition.
A synthetic vapor barrier may be used for the control of moisture migration through the slab, particularly where
adhesives are used to anchor carpet or the to the slab. A thin layer of sand may be placed over the vapor barrier
and immediately below the slab to protect the liner during steel and/or concrete placement. The lack of a vapor
barrier could result in wet spots on the slab, particularly in storage areas.
RETAINING WALLS
' Retaining walls may be utilized on the sloping portion of the site to retain fill material. The lateral pressures
acting on the subgrade and retaining walls will depend upon the nature and density of the soil behind the wall. It
is also dependent upon the presence or absence of hydrostatic pressure. If the adjacent exterior wall space is
' backfilled with clean granular, well-drained soil (washed rock), the design active pressure may be taken as 0.30
(coefficient). This design value assumes a level backslope and drained conditions as described below.
' Retaining walls located on or near the toe of a slope that extends up behind the wall should be designed for a
lateral pressure, which includes the surcharge effects of the steep slope in proximity to the wall. Although not
expected at this site, the following data is provided for planning purposes.
For an irregular or composite slope, the equivalent slope angle may be determined by extending a line upward
from the toe of the wall at an angle of 1 to 1 (Horizontal to Vertical) to a point where the line intersects the ground
' surface. The surcharge effects may be modeled by increasing the equivalent fluid pressure for flat ground. by the
percentage given in the following table:
SLOPE INCLINATION: EQUIVALENT FLUID PRESSURE
Slope Angle Percent Increase
Horizontal 0%
3H:1 V 25%
2H:1 V 50%
Equivalent Fluid Pressure
35 pcf
44 pcf
53 pcf
61 pcf
1H:1V
75%
' If the walls are greater than 4 feet in height, exclusive of the footing, additional design considerations should be
applied.
' Positive drainage, which controls the development of hydrostatic pressure, can be accomplished by placing a zone
of coarse sand and gravel behind the walls. The granular drainage material should contain less than 5 percent
fines. The drainage zone should extend horizontally at least 18 inches from the back of the wall. The drainage
zone should also extend from the base of the wall to within 1 foot of the top of the wall. The drainage zone
should be compacted. to approximately 90 percent of the MDD. Over-compaction should be avoided as this can
lead to excessive lateral pressures.
' A perforated PVC pipe with a minimum diameter of 4 inches should be placed in the drainage zone along the base
of the wall to direct accumulated water to an appropriate discharge location.
10011 Blomberg Street SW, Olympia, WA 98512 lg
Phone #: (360) 754-4612 Fax #: (360) 754-4848
1
GEOTECHNICAL TESTING LABORATORY
We recommend that anon-woven geotextile filter fabric be placed between the drainage material and the
' remaining wall backfill to reduce silt migration into the drainage zone. The infiltration of silt into the drainage
zone, with time, can reduce the permeability of the granular material.
The filter fabric should be placed in such a way that it fully separates the drainage material and the backfill, and
should be extended over the top of the drainage zone.
Typically, block wall systems are more cost effective for long-term walls than the other options. Specific design
criteria for these options can be provided at your request by the block manufacturers.
' SITE DRAINAGE
All ground surfaces, pavements, and sidewalks should be sloped away from the residence and associated
' structures. Surface water runoff should be controlled by a system of curbs, berms, drainage swales, and/or catch
basins and tight-lined to the access street. Footing drains shall be installed for the single-family residence. Roof
drains should not be connected. to the footing drain. For footing drains, the drain invert should be below the
bottom of the footing. Drainage control measures are included on Figure 3. We do not anticipate any adverse
' affects on the recharge condition. of the groundwater system.
SEPTIC IMPACT
' The existing drainfield location is beyond the scope of this report.
10011 Blomberg Street SW, Olympia, WA 98512 19
Phone #: (360) 754-4612 Fax #: (360) 754-4848
Lateral loads may be resisted by friction on the bases of footings and as passive pressure on the sides of footings
and the buried portions of the wall. We recommend that an allowable coefficient of friction of 0.40 be used to
calculate friction between the concrete and the underlying soil.
n
GEOTECHNICAL TESTING LABORATORY
LIMITATIONS
We have prepared this report for Frankie Carney and members of her design team to use in the design of a portion
of this project. The data used in preparing this report, and this report, should be provided to prospective
contractors for their bidding or estimating purposes only. Our report, conclusions, and interpretations are based
on data from others and our site reconnaissance, and should not be construed as a warranty of the subsurface
conditions. This report is quantified as a micro-study and not amacro-study. Geotechnical Testing Laboratory
and its personnel cannot be responsible for unforeseen and widespread geologic events (such as earthquakes,
large-scale faulting, and mass wasting) beyond. the scope of this project.
' Variations in subsurface conditions are possible and may occur with time. A contingency for unanticipated
conditions should be included in the budget and schedule. Sufficient consultation with our firm during
construction should continue to confirm that the conditions encountered are consistent with those indicated by our
' observations, to provide recommendations for design changes should the conditions revealed during the work
differ from those anticipated, and to evaluate whether earthwork and foundation installation activities comply with
our specifications.
' If our analysis and recommendations are followed, we do not anticipate any onsite or offsite impact from the
proposed construction. It is our conclusion that potential landslide hazards can be overcome so as not to cause
' harm to property, public health and safety, or the environment.
The scope of our services does not include services related to
environmental remediation and construction safety precautions.
Our recommendations are not intended to direct the contractor's
methods, techniques, sequences, or procedures, except as
specifically described. in our report for consideration in design.
If there are any changes in the loads, grades, locations,
configurations, or types of facilities to be constructed, the
conclusions and recommendations presented in this report may
not be fully applicable. If such changes are made, we should be
given the opportunity to review our recommendations and provide
written modifications or verifications, as appropriate.
10011 Blomberg Street SW, Olympia, WA 98512 20
Phone #: (360) 754-4612 Fax #: (360) 754-4848
7
u
GEOTECHNICAL TESTING LABORATORY
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Phone #: (360) 754-4612 Fax #: (360) 754-4848
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1/2 INCH MINIMUM DIAMETER STEEL ROD
(STRAP) CLAMPED SECURELY TO PIPE
\ CORRUGATED TIGHTLINE 4 INCH
MINIMUM, 6 INCH SUGGESTED
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GRASS-LINED SWALE SHOULD BE A
MINIMUM ONE FOOT WIDE AT THE
BOTTOM AND ONE FOOT DEEP WITH
A MAXIMUM SLOPE OF 5 PERCENT.
MINIMUM 4 FEET
LEVEL SECTION
GEOTEXTILE FABRIC
Geotechnical Testing Laboratory
Geotechnical Services 10011 a~o~e~ ~. sw FIGURE 3
Q~QC $efNlCes Phone: (3 OA 754-4612
Testing Services Fax: (sso~ 7s4-4x48 Not to scale DRAINAGE DETAILS
TIGHTLINE ANCHORED WITH TWO,
3 FOOT REBAR LENGTHS OR BOLTS.