HomeMy WebLinkAboutSlope Stability Investigation for BLD2001-00365 - BLD Engineering / Geo-tech Reports - 5/20/2001 � ^ Q. 61,12-ool CrD365-
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Slope Stability Investigation
� 311 NE Cutlass Court
Belfair, WA
Prepared for
• Stephen Johnson,Inc.
by
Geotechnical Testing Lab
Olympia, WA
' _ May 20, 2001
GEOTECB NICAL
TESTI
NG LABORATORY
STEPHEN JOHNSON,INC.
P.O. BOX 488
BELFAIR, WA 98528
Re: Slope stability&setback analysis
311 NE Cutlass Court
Belfair,WA
Gentlemen:
As per your request, we have conducted a slope stability investigation for a home located on the above referenced
property. The result of this investigation, together with our recommendations, are to be found in the following report.
The site is located in Mason County, Washington.
SUBSURFACE EXPLORATION PROGRAM
The subsurface exploration program consisted of a site reconnaissance and a review of information from our files of
similar work performed in this area. The slope was observed by our geologist, who studied the soils exposed at the
site. The soil criteria shown on the slope profile were confirmed on the geological maps.
SITE CONDITIONS
The proposed home is to be located at the highest point of the lot on a leveled area to the west of the main access
street. No excessive erosion or deep seated instability was observed. Under the surface, the soil was found to be a
dense sandy gravel. The vegetation consists of grasses and trees indigenous to the northwest. The slope is bare of
vegetation except for a few Fir trees.
GEOLOGIC CONDITIONS
Thd geology of the area as mapped by the Coastal Zone Atlas of Mason County Vol. 9 is Vashon Till (Qvt)
characterized by Pleistocene silt deposits of sands,silty sands, and pebbly sands commonly bedded. It may contain
large erratics with gravel lenses. The lower slopes are composed of a Vashon recessional outwash(Qvr). These are
the youngest member of the Vashon Drift, consisting mainly of sandy and gravely meltwater or ablation deposits
which have not been compacted by the continental ice sheet.
NThe native soils can be considered to be at or near their natural angle of repose, and the factor of safety for most of
the slope is near unity in the unsaturated condition.
HYDROLOGIC CONDITIONS
The lot slopes to the west and all surface runoff discharges to well established drainage ways that ultimately run
south and to the Hood Canal. Runoff from the roof and/or parking areas will not pose significant problems due to
the high porosity of the native soils.
SLOPE STABILILTY MODELING& ANALYSIS
The site was modeled using a slope program and we have included a profile of the model later in this report. The
native soils were modeled using 124 Ibs/ft3 unit weight with an assumed phi angle of 33' in their cemented state.
10011 Blomberg Street SW,Olympia, WA 98512
Phone#:(206)754-4612 Fax#:(206)754-4848
GEOTECHNICAL
TESTING LABORATORY
We have calculated the slope and have found no indication of deep seated instability there.
Slope 1 is a profile of the natural slope to the west. Sectional failure may occur under saturated conditions. The
slope stabYlity is mapped by the Coastal Zone Atlas of Mason County Vol. 9 as "stable". Sloughing in this area
will not impact the home because of the distance from the home to the western slope.
Our study also indicates that the slope is stable.
IMPACT ANALYSIS
We do not anticipate that the proposed project will have any off-site impact.
HISTORY OF LANDSLIDE ACTIVITY
Landslide activity has not occurred historically on the subject property. However occasional sloughing does occur
during periods of heavy precipitation along the North Shore Road over a mile from this site and under different
hydrologic and geologic conditions.
OPINION OF POTENTIAL STABILITY
Potential for instability is minimal. Potential for minor erosion is probable since all vegetation has been removed
from the site. To correct this annoyance, the bare ground should be seeded with grass or straw.
HILLSIDE FOUNDATIONS (DAYLIGHT DIMENSION)
Structures built on sloping sites offer a number of potential problems. Fig. 1 illustrates a basic consideration for
what is called the"daylight"dimension, which is the horizontal distance from the bottom of the footing to the
adjacent ground surface. Many building codes require a minimum distance for this dimension in order to assure
some safety against the pushing out of the footing in the downslope direction. Logically, the limit for this distance
depends on the type of soil and the angle of the slope of the ground surface.
For love slope angles, a reasonable daylight dimension is assured simply because of the usual requirements for the
minimum depth of the bottom of the footing below the surface in a vertical direction. As the slope increases, the
daylight dimension is less assured and should be limited to some minimum distance. A setback of 8 feet from the
horizontal intersect of the slope will provide the critical exposure necessary. (see Fig. 1)
A common problem with hillside construction is that shown in Fig. 2, where recontouring of the construction site
results in a portion of the building being placed on some significant depth of fill. If possible, all of the footings
should be carried down to a depth below the fill. If structural fill is used and placed with reasonable compaction, it
may be possible to rest the footings in the fill, provided that this is done only with inspection and supervision by a
qualified soils consultant.
RECOMMENDATIONS
All material excavated from the site cannot be dumped on site without adequate subgrade preparation at the spoils
location and proper fencing and runoff controls. All spoils soil must be compacted to 90% MDD (maximum dry
-i density)as per ASTM D-1557 and protected from erosion.
10011 Blomberg Street SW,Olympia, WA 98512
Phone#:(206)7544612 Fax#:(206)7544848
1
-- GEOTECBMCAL
TESTING LABORATORY
The conclusions and recommendations in this report are based on our interpretation of the site conditions as they
- presently exist, anticipated future construction activities, and the expectation that our exploratory efforts adequately
define the subsurface conditions throughout the site. In the event that the scope or location of the project should
change, or subsurface conditions different from those encountered during our study be observed or suspected, we
should be advised. At that time, a review of the changed conditions will be made and alternative or remedial
- recommendations given as requested.
Thank your for this opportunity to be of service to you. If you have any questions concerning this report, please
contact us at(360)7544612.
Respecfully submitted,
- GEOTECIINICA.L TESTING LAB
Harold Parks,
.. Engineering Geologist
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10011 Blomberg Street SW,Olympia, WA 98512
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P.O. Box 488
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c^� TABLE 23-1-K-1—ALLOWABLE SHEAR FOR WIND OR SEISMIC FORCES IN POUNDS PER FOOT FOR WOOD STRUCTURAL PANEL SHEAR
WALLS WITH FRAMING OF DOUGLAS FIR-LARCH OR SOUTHERN PINE1�
N
PANELS APPLIED OVER V6�-.11NCH(13 mm)
PANELS APPLIED DIRECTLY TO FRAMING OR s/a-INCH(16 mm)GYP SOM SHEATHING
MINIMUM MINIMUM NAIL Nail Spacing at Panel Edges(in.) INall Spacing at Panel Edges(In.)
NOMINAL PANEL PENETRATION x 25.4 for mm 7 Nall Slze x 23.4 for mm
THICKNESS IN FRAMING Nall Size (Common
(Inches) (Inches) (Common or 6 4 3 2z or 6 4 3 23
Galvanized Galvanized
PANEL GRADE t x 25.4 for mm Box) x 0.0146 for N/mm Box) x 0.0146 for N/mm
5/16 11/4 6d 200 300 390 510 8d 200 300 390 510
Structural) 3/8 2304 3604 4604 6104
7/16 11/2 8d 2554 3954 5054 6704 10d5 280 430 550 730
15/32 280 430 550 730
15/32 15/8 I0d5 340 510 665 870 — — — — —
_ 5/16 11/4 6d 180 270 350 450 8d 180 270 350 450
C-D,C-C 3/8 200 300 390 510 200 300 390 510
Sheathing,plywood 3 2204 3204 4104 5304
panel siding and other 7/16 1 t/ 8d 2404 3504 4504 5854 10dS 260 380 490 640
grades covered in
U.B.C.Standard 23-2 /3, 260 380 490 640
or 23-3
15/32 15/8 10d5 310 460 600 770 — — — — —
19/32 340 510 665 870
Nail Size Nail Size
(Galvanized (Galvanized
Casing) Casing)
Plywood panel siding 5/16 11/4 6d 140 210 275 360 8d 140 210 275 360
in grades
covered in U.B.C. 3 1 8d 160 240 3l0 410 IOdS 160 240 3I0 410
Standard 23-2 /8 1 /'-
I All panel edges backed with 2-inch(51 mm)nominal or wider framing.Panels installed either horizontally or vertically.Space nails at 6 inches(152 mm)on center along
intermediate framing members for 3/8-inch(9.5 mm)and 7/16-inch(1 I mm)panels installed on studs spaced 24 inches(610 mm)on center and 12 inches(305 mm)on
center for other conditions and panel thicknesses.These values are for short-time loads due to wind or earthquake and must be reduced 25 percent for normal loading.
Allowable shear values for nails in framing members of other species set forth in Table 23-III-FF of Division III shall be calculated for all other grades by multiplying
the shear capacities for nails in Structural I by the following factors:0.82 for species with specific gravity greater than or equal to 0.42 but less than 0.49.and 0.65 for CI
with a specific gravity less than 0.42. Z
2 Where panels are applied on both faces of a wall and nail spacing is less than 6 inches(152 mm)on center on either side,panel joints shall be offset to fall on different framing t7
members or framing shall be 3-inch(76 mm)nominal or thicker and nails on each side shall be staggered. n
317raming at adjoining panel edges shall be 3-inch(76 mm)nominal or wider and nails shall be staggered where nails are spaced 2 inches(51 mm)on center. 0
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