HomeMy WebLinkAboutGeotech Review for SHR2004-00011 - SHR Reports - 4/9/2004 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: April 91h, 2004
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Diane Marcus-Jones, DCD - Planner
FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WO#PLG-04
SUBJ: Geo-Tech Report Review NAME: Holl SFR g 6380 Grapeview Loop Rd
SHR2004-00011
Diane,
The geotechnical report prepared for the proposed Jack& Jacqueline Holl Single Family
Residence (SFR)to be constructed at 6380 E Grapeview Loop Road, Allyn,has been received
and reviewed by Public Works.
The report appears to satisfactorily address County requirements for geotechnical reporting. The
report's author indicates that, in his opinion,the proposed development of the property will not
cause stability problems for either the subject property or neighboring properties. From the
contents of the report, I recommend accepting the report as satisfying the County's
requirement(s) for stability investigation and geotechnical reporting. Recommendations
contained in the report should be incorporated into the site's development and made conditions
for permit issuance. The report indicates that the proposed location of the future residence
complies with a 40' setback recommendation contained in the report.
Adequate erosion and sediment control features need to be implemented during land disturbing
activities to protect neighboring properties and State waters from adverse stormwater runoff
impacts. The migration or release of silty water or mud from the applicant's property will be
considered a violation of County and State water quality protection regulations.
In summary,the geotechnical report appears acceptable, and the proposed residential
development of the property should be allowed to proceed, subject to the engineer's
recommendations.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
S ncerely,
A tan A. Tahj
File: H:\WP\GEO\Reviews\Holl.doc
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901,
Geotechnical Report
6380 E. Grapeview Loop Rd.
Parcel #122322400240
Allyn,Washington
Prepared for
Jack Hall
Manhattan, Kansas
by
Geotechnical Testing Laboratory
Olympia, Washington
February 25, 2004
' GEOTECHNICAL TESTING LABORATORY
' .TACK AND JACQUELINE ROLL
1700 WESTBANK WAY
MANHATTAN, KS 66503
Re: Geotechnical Report
6380 East Grapeview Loop Road
Allyn, Washington
Parcel 122322400240
N47021.311' W 122°49.861'
INTRODUCTION
This report summarizes the results of our geotechnical consulting services for the proposed single-family residence
to be located at 6380 Grapeview Loop Road in Mason County, Washington. The site is located approximately 2.0
miles south of Allyn, Washington. The location of the site is shown relative to the surrounding area on the Vicinity
Map, shown on Figure 1.
J
Proposed Building Area
w
Our understanding of the project is based on our discussions with you and our explorations and review of the site. A
septic system is currently located onsite; drinking water is provided by an onsite well. Stormwater runoff from the
site, roof and hard surfaces will be collected and controlled. The general layout of the site is shown on the Site Plan,
Figure 2.
We further understand that minor grading will be required at the site to reach design grade. In general, grading will
consist of the excavation of the foundation material.
The site is situated on a moderate topographic ridge sloping north, east, and south from the proposed building
location. Eberhart's Cove bounds the site on the east and southeast. The steepest slope measured on the site was in
excess of 100 percent along Eberhart's Cove. Therefore, Mason County requires that a geotechnical report be
prepared in accordance with the Critical Areas Ordinance.
L 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 Mason
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:
' 10011 Blomberg Street SW, Olympia, WA 98512 1
Phone#: (360) 754-4612 Fax#: (360) 754-4848
1
GEOTECHMCAL TESTING LABORATORY
1. Review the available geologic, hydrogeologie, and geotechnical data for the site area.
2. Conduct a geologic reconnaissance of the site area.
3. Investigate shallow subsurface conditions at the site by observing the exposed soil.
4. Evaluate the landslide and erosion hazards at the site per the Mason County Critical Areas Ordinance
regulations.
5. Provide geotechnical recommendations for site grading, including site preparation, subgrade preparation, fill
placement criteria(including hillside grading), suitability of on-site soils for use as structural fill,temporary
and permanent cut and fill slopes, drainage and erosion control measures.
6. Provide recommendations and design criteria for the structural foundation and floor slab support, including
allowable bearing capacity, subgrade modulus, lateral resistance values and estimates of settlement.
' SITE CONDITIONS
SURFACE CONDITIONS
The proposed building site is located in an area of sparse residential development in the Puget Sound glacial upland.
The site is situated on a slight ridge sloping to the north and to the southeast. Eberhart's Cove and a coastal bluff
t are to the southeast of the proposed building area. We conducted a reconnaissance of the site area on February 9,
2004. Onsite elevations range from approximately zero to over 58 feet. The general layout of the site is shown on
the Site Plan, Figure 2.
The site has vegetation indigenous to the northwest. The vegetation includes fir, madrone, and cedar trees as well as
Scot's broom, salal, ivy, huckleberry, sword ferns, and grasses.
d
r.
At the time of the site visit, we observed no evidence of surface erosion. No evidence of deep-seated slope instability
was observed at the time of our investigation. The general topography of the site area indicates that surface
drainage flows toward the north and southeast. No evidence of surface water flow was observed in the site area at
the time of our reconnaissance.
SITE GEOLOGY
' The site is generally situated within the lower 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 Vashon stade of the Fraser
' 10011 Blomberg Street SW, Olympia, WA 98512 2
Phone#: (360) 754-4612 Fax#: (360)754-4848
GEOTECHNICAL TESTING LABORATORY
glaciation that occurred between about 10,000 and 12,000 years ago, and weathering and erosion that has occurred
since. A description of the surficial soils is included in the"Site Soils" section of this report.
In general, the soils on the site are predominantly represented by Vashon glacial till deposits.
SITE SOILS
The Soil Survey of Mason County, USDA Soil Conservation
Service(1960) has mapped the site soils as Alderwood gravelly
sandy loam, 5-15 percent slopes (Ab). The Alderwood soils
typically formed from glacial advance outwash. They are described �` ,-, • •
as havinggood natural drainage. Typically, there is no occurrence
g g
of a high water table. Internal drainage is described as medium. An
erosion hazard may exist if the vegetation is removed; otherwise, the
soil has a low erosion hazard in its present condition. Cementation
is usually present. We observed no active erosion or slope
disturbance in the site area during our reconnaissance. The soils are
not considered hydric.
The Coastal Zone Atlas, Volume 9, Mason County(MA-12)maps the site as Vashon glacial till The till is a
h tough, dense material of low permeability. It provides excellent foundation support. The site slope stability is
described as"stable" at the building site and"intermediate" along the coast.
SUBSURFACE EXPLORATIONS
Subsurface conditions at the site were evaluated by observing the exposed foundation material and the exposed
coastal bluff. At the building location, partially cemented/consolidated gravelly sandy loam was found to a depth of
50 feet and beyond.
SUBSURFACE CONDITIONS
In general, undisturbed cemented/consolidated gravelly sandy loam was observed throughout both the site and the
' proposed building location. Groundwater seepage was not observed onsite. Seasonally perched groundwater
' 10011 Blomberg Street SW, Olympia, WA 98512 3
Phone#: (360) 754-4612 Fax#: (360)754-4848
1
' GEOTECHNICAL TESTING LABORATORY
1 conditions may not be expected during periods of extended wet weather due to the coarse gravelly sandy material
present and the topographic location of the site.
SLOPE STAIIILITv
Slopes in excess of 100 percent were observed onsite. Since
slopes of 40 percent or greater with 10 feet or more of vertical
relief occur on portions of the site, Mason County requires that
a geologic hazards 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 subsurface undisturbed native soils of the site
consist of a mixture of variable amounts of gravels, sand, and
silt. These soil materials are in a dense condition except where
they have been disturbed by weathering activity. These soils are generally stable relative to deep-seated failure. No
evidence of deep-seated landslide activity or significant erosion was observed at the site at the time of our
investigation. Near the existing coastal building and southeast of the proposed building location, a coastal slump
has developed(shown above). The coastal slump is not considered deep-seated and no further evidence of slumping
was observed onsite.
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. Evidence of surficial erosion was not observed at the site. Only minor sloughing
' and raveling was observed onsite at the time of our investigative visit(see photo above).
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 concentrated runoff on the steep slope. Erosion control recommendations for the
sloping areas are provided in the"Building Setback"and"Erosion Control" sections of this report.
CONCLUSIONS AND RECOMMENDATIONS
' GENERAL
Based on the results of our site reconnaissance and subsurface observations, and our experience in the area, it is our
opinion that the site is suitable for the proposed single-family residence.
' The slopes are stable relative to deep-seated instability and will not be affected by the proposed single-family
residence. Proper drainage control measures will reduce or eliminate the potential for erosion in this area and
' improve slope stability.
If our analysis and recommendations are followed, we do not anticipate any on site or off site impact from the
' proposed construction. It is our conclusion that potential landslide hazards from the Iandsfide area can be overcome
so as not to cause harm to property, public health and safety, or the environment.
t 10011 Blomberg Street SW, Olympia, WA 48512 4
Phone#:(360)754-4612 Fax#: (360)754-4848
1
GEOTECHMCAL TESTING LABORATORY
In general, the site soils are suitable for use as structural fill material. Saturated soil conditions are not expected
with these soils during or following extended periods of rainfall, but we recommend that earthwork be undertaken
during favorable weather conditions to reduce grading time and construction costs.
Structure to be Removed and
Proposed Building Location
Conventional construction equipment may be utilized for work at the site. Conventional spread footings may be
utilized for support of the structure. We do recommend that roof and footing drains be installed. Footing and roof
drains may not be connected. A vapor barrier is recommended for all slab-on-grades.
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 Mason County Critical Areas Ordinance(17.01.100)defines a landslide hazard area as one containing slopes
equal to or greater than 40 percent with more than a 10-foot vertical relief. The site slopes are in excess of 100
percent and the vertical relief is in excess of 10 feet. Based on this, this site does meet the technical criteria of a
landslide hazard.
The Relative Slope Stability of the Southern Hood Canal Area, Washington, (1977)describes the site area as Class
' 1. Class 1 is expressed as,
Areas believed to be stable. Slopes generally less than 15 percent, but may be greater locally
in areas too small to be shown at the map scale. Largely comprises rolling uplands
underlain by very stable material such as young glacial till, mantled in places by a thin layer
of.sandy gravel or other permeable material; also includes flood plains, deltas, alluvial fans,
and some beach deposits. Class 1 areas immediately adjacent to steep slopes of class 3 areas
may be threatened by potential landsliding. Normal, proper engineering practices generally
' are adequate to insure stability in these areas.
The Mason County Critical Areas Ordinance(17.01.104)defines an erosion hazard area as:
' Areas in Mason County underlain by soils which are subject to severe erosion when
disturbed. Such soils include, but are not limited to, those for which potential for erosion
10011 Blomberg Street SW, Olympia, WA 98512 5
Phone#: (360)754-4612 Fax#: (360)754-4848
1
1 GEOTECIINICAL TESTING LABORATORY
' is identified in the Soil Survey of Mason County, USDA Soil Conservation Service, 1960,
or any subsequent revisions or additions to this .source. These soils include, but are not
limited to, any occurrence of River Wash ("Ra') or Coastal Beaches ("Cg") and the
following when they occur on slopes 15% or steeper:
a. Alderwood gravelly sandy loam ("Ac"and "Ad')
b. Cloquallum silt loam ("Cd')
c. Harstine gravelly.sandy loam ("Hb')
d. Kitsap.silt loam ("Kc')
The soils at the site are mapped as Alderwood gravelly sandy loam(Ab). This site does not meet the technical
criteria of an erosion hazard area. With proper drainage control and re-vegetation, erosion hazard concerns may be
overcome.
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 100 percent, they are generally stable relative to deep-
seated failure in their present configuration. The following figure represents a shear angle for the gravelly sandy
loam. Shear angle and cohesion are variables used to model the site.
Peak Shear Stress vs. Normal Stress
' Gravelly Sandy Loam
3000
43°
2500
1 w
G 2000
m
1500
1 �
m
L
to
Y
10y 1000
' a
500
-+-114 ton
■-112 ton
— -1 ton
0
0 500 1000 1500 2000 2500 3000
' Normal Stress(psf)
' Erosion control recommendations for the slope are provided in the"Building Setback" and"Erosion Control"
sections of this report. These processes can be managed and the risk reduced through proper construction of the
residence.
1
' 10011 Blomberg Street SW, Olympia, WA 98512 6
Phone#:(360)754-4612 Fax#: (360)754-4848
1
GEOTECI- MCAL TESTING LABORATORY
BUILDING SETBACK
Slope stability was modeled using the GEO-SLOPE/W program(version 5.13) 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 geology was modeled using a monolithic layer of gravelly sandy loam. The gravelly sandy loam was
determined to have a unit weight of 130 pcf, cohesion of 200 psf, and a shear angle(�)of 43°. Under static
conditions, the slopes remained stable to deep-seated and shallow failure. Under dynamic loading, the 3328
computations demonstrated that the slope is not susceptible to surficial raveling; large deep-seated failure was not
demonstrated by our model. The following figure represents the dynamic moment F.S. for cross-section A. The
solution represents the factor of safety of greatest concern generated by the model and exhibits the need for a
building setback.
Holl Site
Analysis Method: Morgenstern-Price •�•
' Direction of Slip Movement: Left to Right .
Slip Surface Option: Grid and Radius
Seismic Coefficient: Horizontal and Vertical
l
90
' 80 Proposed •107 '
70 Building
Area
� 60
30 Aldenvood
' D Soil Model:Mohr-Coulomb
L
Unit Weight:130
10 Cohesion:200
Phi:43
' 0
-10
0 20 40 60 80 100 120 140 160 180 200
' Distance (ft)
A building setback from landslide hazard areas is required unless evaluated and reduced by an engineering geologist
or a licensed professional engineer. Based on our geotechnical evaluation of the site and our experience in the area,
a building setback will be needed for this lot. A setback of 40-feet from the crest of the southeastern slope 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 along the shoreline, in accordance with the Unified Building Code. The current
' proposed footprint is compliant with the building setback recommendations.
As previously discussed, weathering, erosion and the resultant surficial sloughing and shallow landsliding are
' natural processes that affect slope areas. To manage and reduce the potential for these natural processes, we
recommend the following:
' 10011 Blomberg Street SW, Olympia, WA 98512 7
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GEOTECHwAL TESTING LABORATORY
1. No filling within the setback zone unless retained by retaining walls or constructed as an engineered fill.
2. Trees may be removed on sloped areas as long as the stumps remain.
SEISMIC—LIQUEFACTION HAZARD
According to the Seismic Zone Map of the United States contained in Figure 16-2 of the 1997 UBC(Uniform
Building Code),the project site is located within Seismic Risk Zone 3.
Based on the subsurface conditions observed at the site,we interpret the site conditions to correspond to a seismic
Soil Profile Type Sc, for very dense soils, as defined by Table 16-J(UBC). This is based on the range of SPT
(Standard Penetration Test)blow counts and/or probing with a %a-inch diameter steel probe rod. The shallow soil
conditions were assumed to be representative of the site conditions beyond the depths explored.
Based on our review of the subsurface conditions, we conclude that the site soils are not susceptible to liquefaction.
The near-surface soils are generally in a dense condition and the static water table is located well below the surface.
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.
Temporary and permanent erosion control measures should be implemented and maintained during construction
and/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. A typical 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.
EARTHWORK
SITE PREPARATION
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 6 to 8 inches will be necessary to
remove the root zone and 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 1 foot 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.
10011 Blomberg Street SW, Olympia, WA 98512 g
Phone#. (360)754-4612 Fax#:(360)754-4848
GEOTECHNICAL TESTING LABORATORY
If structural fill is needed,we recommend that a member of our staff evaluate the exposed subgrade conditions after
removal of vegetation and topsoil stripping is 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.
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 suitability of material for use as structural fill will depend on the gradation and moisture content of the soil. 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 of well-graded sand and gravel with less than 5 percent(by weight)passing the No. 200 sieve
based on that fraction passing the'/,-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.
SUITABILITY OF ON-SITE SOILS AS FILL
On-site soils may be used as structural fill. In general,the native soils(gravelly sandy loam)encountered on the site
should have less than 10 percent fines(material passing the US No. 200 Sieve)and therefore 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.
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 looselmedium dense condition. Temporary slopes of 1 to 1 or flatter may
be used in the unweathered dense to very dense sands and gravels or till.
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.
10011 Blomberg Street SW, Olympia, WA 98512 9
Phone#:(360)754-4612 Fax#: (360)754-4848
GEOTECHNIC.AL TESTING LABORATORY
Subsurface drainage may be required if seepage areas are discovered. Surface drainage should be directed away
from all slope faces. Some minor raveling may occur with time. 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 of 5 percent or more,the footings should be located a minimum
of 2 times the footing width from the slope face(horizontally),and founded in medium dense or denser native soils
or properly prepared structural fill.
We recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings.
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 medium dense to 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
200 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'lz inch or less.
Most of the settlements should occur essentially as loads are being applied. However, disturbance of the foundation
sub-grade during construction could result in larger settlements than predicted.
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 5 percent fines(by weight). The drainage material
should be placed and compacted to an unyielding condition.
A synthetic vapor barrier should 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
Future 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 35 pcf(equivalent
fluid density). This design value assumes a level backslope and drained conditions as described below.
10011 Blomberg Street SW, Olympia, WA 98512 10
Phone#: (360)7544612 Fax#:(360)754-4848
GEOTEcimcAL TESTING LABORATORY
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 Equivalent Fluid Pressure
Horizontal 0% 35 pcf
3H:1V 25% 44 pcf
2H:l V 50% 53 pcf
I H:1 V 75% 61 pcf
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.
We recommend that a non-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 can, with
time, 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.
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. Passive pressure may be determined using an allowable
equivalent fluid density of 35 pcf(pounds per cubic foot). Factors of safety have been applied to these values.
RETAINING WALL ALTERNATIVES
Typically,reinforced-earth 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.
I0011 Blomberg Street SW, Olympia, WA 98512 11
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SITE DRAINAGE
All ground surfaces, pavements and sidewalks should be sloped away from the residences and associated structures.
Surface water runoff should be controlled by a system of curbs, berms, drainage swales and directed away from the
slope.
Onsite irrigation to lawn areas should be closely monitored. We do not expect any adverse affects on the recharge
condition of the groundwater system.
LIMITATIONS
We have prepared this report for the use of Robb's Construction and members of their 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.
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.
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.
Respectfully submitted,
GEOTECIINI�CAL TESTING LABORATORY
Harold Parks, L.G., L.E.G.
Senior Engineering Geologist
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HAROLD PARKS
7"" 3��"
10011 Blomberg Street SW, Olympia, WA 98512 12
Phone#:(360)7544612 Fax#:(360)7544848
GEOTECBMCAL TESTING LABORATORY
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10011 Blomberg Street SW, Olympia, WA 98512
`� Phone#: (360)754-4612 Fax#: (360)754-4848
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