HomeMy WebLinkAbout701153022 Geotech Assessment (1997)NT(
N~~REERN TERfTOR~E~, INS,
Engineers ~ Land Surveyors ~ Planners
Construction Coordination ~ Materials Testing
GEOTECHNICAL INVESTIGATION
for
Tract 5 of Dabob View Tracts
in
Section 15, Township 27 North, Range 1 West, W.M.
Jefferson County, Washington
(Parcel #701-153-022)
Prepared for
Mark Walker
Prepared by
Northwestern Territories, Inc.
717 South Peabody
Port Angeles, Washington 98362
August 22, 1997
X17 SOU?ri °EABOCY, ?OR? ANGELES, irJA 9836% (3"00) 452-849 i 1-800-05^'.-5545 FA: 452-848$
Caeotechnical investigation
fior
Tract 5 of Dabab View Tracts
Background
At the request of Mr. Mark Walker, the undersigned conducted areconnaissance level
investigation of the proposed building site on July 29, 1-997. At the time of the
investigation, the building site and access road had been cleared and graded including
the placement of rock rip rap at two locations along the existing driveway.
For purposes of orientation and reference, the Site Pian for the Sewage Disposal System
(approved 6/26/97) is attached.
Purpose and Scope
The purpose of this investigation is to evaluate the overall slope stability and make
setback recommendations for the proposed home. The scope of the investigation is
limited to a visual inspection of the upland site on 7/29/97, review of published
information and examination of a photograph of the marine bluff and shoreline furnished
by Mr. Walker.
Physical Setting
As shown on the attached topographic map (USGS) and photographs, the project site
is typical of the series of. ravines incised into the steep high bluffs along the eastern shore
of Dabob Bay. The site is located near the head of Dabob Bay and is somewhat
sheltered from high winds and associated wave action that typically undercuts marine
bluffs and causes long term bluff recession.
The site is surrounded by forest containing some very large trees estimated to be over
75 years old. The height of the bluff is estimated to be over 200 feet above the shoreline.
However, the building site is located on an intermediate bench area on the west side of
the ravine. A natural drainage way generally follows the easterly property line. The
driveway and building site have been cut into the western side of the ravine.
Soils and SioQe Stabiliit~{
The soils throughout the site are of glacial origin and mapped by the Soil Conservation
Service (SCS) as "Everett gravelly sandy loam, 30 to 50 percent slopes (EvE)." As such
they are reported by the SCS to be "somewhat excessively well drained, gravelly soils."
"Runoff is medium and the hazard of water erosion is moderate."
The photograph of the bluff face reveals a fairly consistent soil profile with no evidence
of major stratifiication or saturated permeable soils perched above less permeable
material. If present, this condition would be a factor in mid to upper level bluff slides. The
apparent absence of this condition, together with the light to moderate wave action at the
toe, indicates a relatively stable marine bluff. Due to its- exposed face and granular
composition, bluff recession due to normal weathering action of wind and rain directly on
the bluff face may be slightly greater than average. However, this form of erosion and
bluff recession is much less significant than groundwater saturation at the face and/or
severe wave action at the toe. Notwithstanding acatastrophic event such as an
earthquake, the marine bluff is considered to be grossly stable.
The east and west slopes of-the ravine reveal evidence of minor "downslope creep". This
is most evident in the larger trees to the east that are tilted. However, this is considered
a "surficial" condition affecting only the upper two to three fleet of topsoil. This condition
can be mitigated with slope stabilization measures such as rock revetment like the owner
is currently installing along the driveway.
Findings
Based on the information presented above, the marine bluff is grossly stable and
long term bluff recession should be minimal.
2. Although there is potential for "surficial" slides within the side slopes of the ravine,
only the west slope will impact the design of the proposed home.
Recommendations
The proposed 100 foot setback from the top of the marine bluff for the house is
considered adequate.
2. A minimum 15 foot setback from the toe of the west slope is recommended.
According to the Septic Permit, the septic system facilities will result in a setback
of 20 feet or more. In addition, the owner plans to provide additional rock
revetment on the west slope which will reduce the potential for surficial slides into
this setback area. - --
2
3. As proposed, the house would be approximately 50 feet or more from the existing
top edge of the drainage way bank. This setback is considered to be more than
adequate from a soil stability perspective. it is reported that there is no "fill" that
has been placed in the proposed "footprint" of the house. Therefore, conventional
foundation construction is recommended.
4. Nominal allowable foundation pressures of 1500 psf infirm, undisturbed native soil
can be increased per Footnote 2 in Table. 18-1-A of the Uniform Building Code
{UBC).
5. Common to any site development plan is proper drainage. Specific drainage and
erosion control measures are speci#ied by County ordinance.
Limitations
This report is based on a visual inspection of the existing site conditions. No scientific
measurements, tests or calculations were performed. The findings and report are limited
to the normal standard of care of the industry for an investigation without benefit of
scientific data and calculations. Unless construction plans are reviewed and the
construction work is inspected by the Engineer, no warranty is made by the Engineer
concerning conformity of the construction to the conclusions and recommendations of
this report.
The sole objective of this investigation has been to diagnose the geotechnical and/or
conditions at the site. The report has been prepared for the exclusive use of Mark Walker
and may be used by others only with expressed written permission of the Engineer. The
report .has not been prepared for use by others or other uses. The observations,
interpretations, and conclusions herein are based on generally accepted professional
engineering and geologic principles and practice. This warranty is in lieu of all others,
either expressed or implied.
Sincerely,
NORTHWESTERN TERRITORIES, INC.
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Photo 4 (Naar NV~I Corner of Building Situ 729/97
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aecember 96~January 97 storms.
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