HomeMy WebLinkAboutBLD2012-00129 Geotechnical Report cm ENGINEERING
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• ° 301 East 6th Street,Suite 1
° t :i f_ Port Angeles.Washington 98362
INCORPORATED
C E O V E (360)417-0501
fax(360)417-0514
E-mail:zenovic@olvmpus.net
APR 2 4 2012
JEFFERSON COUNTY
DEPT.OF COMMUNITY DEVELOPMENT
May 20. 2009
Franz and Janice Gruber
102 East Fir Street
Port Ludlow, WA 98365-8773
Re: Property on East Fir Street, Port Ludlow
Par. No. 821-233-001
Dear Mr. and Mrs. Gruber:
Zenovic&Associates, Inc. presents herein the results of our geotechnical review of the
referenced property, located on the west side of Hood Canal, east of Port Ludlow,
Jefferson County. The property is situated along the shoreline of Hood Canal at the east
end of East Fir Street. Access to the site is from East Fir Street, off of Paradise Bay
Road, approximately two miles north of Highway 104 at the west end of the Hood Canal
Bridge.
We understand that your intent is to construct a single-family residence on the subject
property. A portion of the site is mapped as an Unstable—Recent Slide slope on the
Jefferson County Shoreline Map. Slopes designated as recent slides are based on
evaluations completed in the 1970s. Subsequent sliding has not been documented by
the State and is subject to our review and conclusions.
PURPOSE AND SCOPE OF SERVICES
The purpose of the geotechnical review is to provide information regarding the soil and
slope stability in order to develop the site in a manner that will not induce soil erosion or
endanger on-site or adjacent off-site properties, consistent with the requirements of the
Jefferson County Unified Development Code (JCC Chapter 18.22). The scope of the
review included a field inspection of the site and determination of site and adjacent
property slopes using a hand-held clinometer. No topographic mapping services have
been provided by this office.
Research of available data included a review of aerial photos as provided on the
Jefferson County website(1994, 2000, 2006), the Soil Survey of Jefferson County Area,
Washington (USDA/SCS, August, 1975), the Washington State Coastal Zone Atlas, ano
Jefferson County Unified Development Code.
Page 2
Mr. and Mrs. Franz Gruber—Property on East Fir Street, Port Ludlow
May 20, 2009
REGIONAL GEOLOGIC AND SOIL CONDTIONS
According to the Soil Survey of Jefferson County Area, General Soil Map, a significant
area comprised of the Alderwood-Sinclair soil association extends south and east from
Discovery Bay to the Admiralty Inlet/Hood Canal shoreline, encompassing the majority of
the easterly portion of Jefferson County, including the Dabob and Coyle peninsulas. The
subject site is located in the easterly portion of the mapped soil association. The
Alderwood-Sinclair soils are typically moderately well-drained, dominantly strongly
sloping to steep, gravelly soils underlain by glacial till. Inspection of the soil maps of the
site indicate that the property lies within an area of Alderwood gravelly sandy loam, 0 to
15 percent slopes, that extends throughout the developed area along Paradise Bay
Road.
The Alderwood soils are derived from weathered glacial till. These soils were deposited
by the ice sheet during the Vashon stade glaciation. The Vashon Glacial Till consists of
a poorly sorted mixture of rock fragments deposited directly by the Vashon-age ice
sheet. Finer portions of the soil consist of clay, silt, and sand. These fine particles
comprise a moderately to highly compact matrix in which the coarser components
(gravel, cobbles, and boulders)are firmly imbedded.
SITE OBSERVATIONS
An engineer from Zenovic&Associates, Inc. conducted a site visit on April 21, 2009 to
observe the site slopes, vegetation, surface and immediate subsurface hydrology, and
evaluate in-situ soils for density.
Existing Slopes
The property has a gently sloping upland area, with a gradient of approximately 2 to 3
percent downhill from west to east, terminating at a near vertical marine bluff of
approximately 50 feet in height. The toe of the bluff is situated on the shoreline of Hood
Canal. The bluff has a relatively consistent slope with a gradient of in excess of 150%.
The upper portion of the bluff is a near vertical formation while the lower portion is less
steep. The material creating the less steep slope at the base of he bluff is the colluvial
material that has eroded from the bluff face due to wind erosion and the natural freeze-
thaw cycle that loosens the bluff soils.
Soil Conditions
Visual observation of the site indicates that the site soils appear to be consistently
Alderwood soil types as shown on the Soil Survey maps. Inspection of the bluff face
indicates that the depth of the surficial gravelly sand loam is typically deeper than is
typical for the Alderwood soils, extending to depths of up to 60 inches. These more
gravelly soils are less stable than the underlying compact glacial till, so some sloughing
of the upper area of the bluff face is anticipated.
Page 3
Mr. and Mrs. Franz Gruber—Property on East Fir Street, Port Ludlow
May 20, 2009
Vegetation
Site vegetation consists of well maintained lawn throughout the majority of the site. A
small area of the native coniferous forest with a dense understory of brush covers the
westerly third of the property. Two large fir trees(48" diameter) are located
approximately 20 feet from the top of the bluff.
Hydrology and Drainage
The upland area of the site is basically flat as is the tributary area to the west. No
drainageways were observed on the upland area of the property or on the face of the
bank.
International Building Code Site and Soil Categorization
Seismic design parameters for this site include a seismic zone soil profile of Type D in
accordance with Table 1615.1.1 of the International Building Code.
The native surficial site soils can be classified as a sand, silty sand, clayey sand, silty
gravel, and clayey gravel consistent with Class 4 materials as set forth in Table 1804.2
of the International Building Code. Increases for depth and width as set forth in the table
footnotes are not recommended for structures within 75 feet of the top of the bank.
CONCLUSIONS
The bank is shown to be unstable—recent slides in the Washington Coastal Zone Atlas.
This determination is based on review of available aerial photography in the 1970's and
does not reflect the current situation. However, the face of the bluff does show some
minor weakness, so a setback for development is appropriate. The standard bluff
setback of one foot for each foot of height is reasonable to provide adequate protection
of any structures. This setback will be approximately 50 feet for the site; decks and
similar appurtenances may extend into the setback area consistent with Section
18.25.410(j) of the Uniform Development Code.
RECOMMENDATIONS
General
The proposed project shall be constructed in conformance with all requirements of
Jefferson County, including, but not limited to, the International Building Code and
applicable clearing, grading, drainage, and erosion control requirements.
Page 4
Mr. and Mrs. Franz Gruber—Property on East Fir Street, Port Ludlow
May 20, 2009
•
Excavation and Grading
Excavation for the proposed foundation components of any structures on the site shall
be carried to firm, substantial, unyielding subsoil, at a depth of not less than 12 inches
feet below the existing surface grade. Excavation work shall occur not earlier than
March 1, and shall be complete to the permanent condition before October 31.
Drainage and Erosion Control
Drainage facilities shall be provided to direct the runoff from all new impervious surfaces
into a new infiltration system that is located a minimum of 75 feet from the top of the
bank. No runoff from impervious surfaces shall be permitted to flow over the edge of the
bluff.
Slope Maintenance
Proper maintenance of the existing marine bluff is critical to the continued safety of the
site and for minimizing damage to the bluff. No organic material, including grass
clippings and tree or brush trimmings should be thrown over the top of the bluff. These
materials may help hold water on the slope, thereby reducing slope stability.
On-site Sewage Disposal System
The septic tank and pump chamber for the existing on-site sewage disposal system are
located approximately 25 feet from the top of the bank. These tanks and associated
piping may remain in place. The area around these two tanks should be inspected on a
regular basis to make sure that no leakage is occurring. Visual inspection of the lawn
area around the tanks for unusual growth will be adequate.
USE OF THIS REPORT
This report has been prepared at the request of Mr. and Mrs. Franz Gruber for the
purpose of determining the relative stability of a steep bank on the subject property.
This report is specific to this site and the information contained herein is the property of
the Grubers and may not be used without their permission.
The findings set forth in the report are based on a site inspection and review of available
data. The conclusions and recommendations derived from these findings assume that
the site conditions do not vary radically throughout the site. If future excavations and/or
construction activities discover that site conditions are significantly different than those
documented herein, Zenovic&Associates, Inc. must be contacted to verify or modify the
conclusions, as appropriate.
Page 5
Mr. and Mrs. Franz Gruber—Property on East Fir Street, Port Ludlow
May 20, 2009
We appreciate the opportunity to provide this information to you. Please contact this
office if you have any questions regarding the contents.
Sincerely,
Zennvic&Associates, Inc.
/ f:jp8
hen M. Zenovic, P.E. �4 Wits
/ P incipai Engineer ��,� .�.
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Enclosures
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