HomeMy WebLinkAboutGeo-Tech Assessment Review for RLC99-00103 - BLD Engineering / Geo-tech Reports - 8/17/1999 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: August 17, 1999
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Shandra O'Haleck, DCD - Planner
FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. DCD WO# 9963
SUBJ: Geo-Tech Assessment Review NAME: Frederick Mulligan
RLC99-0103
Shandra,
A geotechnical assessment prepared for the proposed Frederick Mulligan residential construction
has been received and reviewed by Public Works, and determined to adequately address the
requirements pertaining to a geotechnical assessment.
The assessment appears to dispel the need for additional geotechnical investigation, but does
contain recommendations pertaining to surface and subsurface drainage of the site.
Stormwater should be disposed of as recommended by the geologist in a sheet flow fashion.
Collected stormwater should not be allowed to be disposed of onto steep slopes. With conditions
pertaining to stormwater runoff control,this project should be allowed to proceed.
Please feel free to contact me at County extension 461 if you have any questions pertaining to
these comments, or this department's review.
Si rely,
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An A. Tahj a
File: H:\WP\GEO\REVIEWS\MULLIGAN.IDC
4
Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
2401 Bristol Court SW, Olympia, WA 98502 Phone 360-357-7883
PO Box 12267, Olympia WA 98508-2267 FAX 360-754-4240
e-mail: bradleynoble@iuno.com
13 August 1999
Mrs . Frederick C. Mulligan
2735 South Bay Road N.E.
Olympia, Washington 98506
Subject: Geotechnical assessment of your property
at 60 East Effie Prosser Road, Mason
County Tax Parcel Number 32136-14-90060,
as being a landslide or erosion hazard.
Dear Mrs . Mulligan:
As requested, we visited the above site in order to
observe slope conditions and soils that would affect
your proposed construction of a foundation for a
manufactured home on this lot. At the time of our
visit, the building pad had been cut at the east side
of the lot. The ground surface slopes gently from
east to west on this property. Slope angles are in
the 15 to 20 percent range with the slopes being
steeper to the west away from the building area.
The project site is located on the Vashon recessional
outwash plain. Soils at this site as exposed in the
building pad and road cuts in the area are typical for
the Vashon recessional outwash plain in this area. We
find a thin, poorly developed topsoil overlying
fluvially deposited soils of interbedded sands and
gravels . These are the outwash deposits which in this
area typically vary from two to four feet in
thickness .
Underlying the outwash deposits, we find the very
dense and highly overconsolidated Vashon subglacial
till. Subglacial till is a mix of silt, sand, and
gravels that was deposited at the base of the
advancing Vashon continental ice sheet. The grain
size distribution and consolidation caused by the
loading of the overriding ice sheet, thought to have
been about one-half to one mile thick in this area,
has created a soil unit that has many engineering
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properties similar to those of lean concrete.
Examination of the slopes on this and adjacent
properties did not reveal any land forms associated
with mass wasting events. We observed that the trees
growing on the slopes are vertical, which suggests
that soil movement is not occurring.
The subglacial till is for all practical purposes
impermeable. We expect that during periods of heavy
or prolonged rainfall, flows of ground water develop
on the surface of the subglacial till. Saturation of
the overlying permeable outwash soils may extend to
the surface for short durations during heavy rainfall
events . To ensure that your crawl space area remains
dry, attention to perimeter drainage around the
building pad should be addressed in your site
development plans . Maintaining positive slope away
from the building pad or the construction of
interceptor drains should be included in the plans .
We also recommend that your roof rain leaders be
connected to a tightline disposal system with the
storm water carried to the west for infiltration below
the drainfield area. Dissipation of the collected
water by use of perforated pipe following the contours
is acceptable. We do not recommend that point
discharge of the water occur onto the slope. Point
discharge can create areas of localized saturation and
induce localized accelerated erosion of the slope.
Based on our field observations, research, and
understanding of the local geology and erosional
process, we do not consider this site prone to mass
wasting events or accelerated erosion by the proposed
construction. Addressing the control of storm water
flow on the surface and on the surface of the
subglacial till to ensure a dry crawl space should be
included in your plans.
If you have any questions, or if we may be of
additional service to you, please contact us at our
Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL SERVICES
David C. trong
Engineering Geologist
GARY YANDO,DIRECTOR
P��N.STATFo �
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o A Ou DEPARTMENT OF COMMUNITY DEVELOPMENT
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o PLANNING - SOLID WASTE - UTILITIES
N Y y BLDG. I 9411 N. 5TH ST. • P.O. BOX 578
of 1864 SHELTON,WA 98584 • (360) 427-9670
July 20, 1999
Welsh/Mulligan
2735 South Bay Road NE
Olympia, WA 98506
RE: Resource and Critical Areas Review for Parcel Number 32136-14-90060; 60 East Effie
Prosser Road, Shelton.
Dear Mr. Mulligan and Ms. Welsh:
Due to steepness of the slope on your property(15-30%) and the classification in a Mason
County Critical Area (landslide hazards) a geo-assessment of your site will be required prior to
permit approval. I have included the main points that should be included below:
• A discussion of geological conditions in the immediate vicinity of the project, and a
discussion of specific soil types at the project site. Terminology should be consistent with
U.S. Geological maps of Mason County.
• A discussion of ground water conditions at the site, including depth to water and quantity
of surface seepage.
• The approximate depth to hard or dense component soil, e.g. glacial till or outwash sand.
• A discussion of landslide activity in the area (from"Relative Slope Stability of the
Southern Hood Canal Area, Washington" by Smith and Carson, 1977 or from records on
file with Mason County Community Development).
• An opinion on potential for landslide activity in light of the proposed development.
This project should retain a vegetative buffer on the slope and be revegetated in the disturbed
areas as soon as possible due to the hazards that can be created by development in a sloped area.
Enclosed please find information on slope erosion control and the applicable chapter of Mason
County's Resource Ordinance that addresses landslide hazard areas. If you have any questions,
please contact me at (360)427-9670, ext. 295.
Sincerely,
Shandra O'Haleck, Senior Planner
Department of Community Development