HomeMy WebLinkAboutGeoTech Assessment for BLD2004-00274 - BLD Engineering / Geo-tech Reports - 5/20/2004 STAr T7 BE KEPT IN THE
M cc MASON COUNTY PARCEL FILE
N
N Y PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shehon,Washington 98584
DATE: May 20'', 2004
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Tammi Clark,DCD -Planner
FROM: Alan A. Tahja,P/W-Co. Hydr. Engr- WO# PLG-04
SUBJ: Geo-Tech Assessment Review NAME: Matteson SFR addition k Union
BLD2004-00274
Tammi,
The geotechnical assessment prepared for the proposed Cliff Matteson Single Family Residence
(SFR) addition E 281 Windstar Road,Union,has been received and reviewed by Public Works.
'The assessment appears to dispel the need for further geotechnical investigation,conforms to
standards of engineering principles and practices for this time and place, and appears to conform
to the minimum requirements of DCD for geotechnical assessment reporting. The evaluation
raises no issues or concerns, and appears acceptable for the level of reporting required.
Recommendations contained in the report should be incorporated into the site's development.
The escape of silt or mud from the subject property constitutes a violation of both State and
County stormwater quality protection regulations, and could subject violators to fines and
penalties.
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.
Si cerely,
v
Alan A. Tahj a
File:H.IWPIGEOIAssessmentslMatteson.doc
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Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
PO Box 12267, Olympia WA 98508-2267
Phone 360-357-7883 9 FAX 360-867-9307
10 May 2004 RECEIVED
MAY 17 2004
Mr. Cliff Matteson "CCD - PLANNING
23107 47th Avenue West
Mount Lake Terrace, Washington 98043
Subject: Geotechnical assessment of the slope
stabilitv and erosion potential to your
proposed actalrion LU LIM,' c i -L_L1-Ly
home at E. 281 Windstar Road, Tax Parcel
Number 32235-75-0026, south shore of
Hood Canal area, Mason County,
Washington.
Dear Mr. Matteson:
As requested, we visited the above property in order
to develop an opinion on slope stability and erosion
potential at the site of the proposed addition and to
evaluate their probable long-term influence on the
proposed construction of an addition to the existing
mobile home. Our scope of work consisted of a site
visit, review of available geologic information, and
preparation of this report based on our observations,
research, and discussion with you about the proposed
construction.
The site is located in the upland area above the south
shore of Hood Canal. Access to the site is gained off
of State Highway 106 by a narrow road that leads up
the slope of the former fjord to the upland area. In
the upland area, the relief is typical of the Vashon
recessional outwash plain with a gently undulating
surface. At your lot, we found the surface to have a
slight rise from the road to the proposed building
area. In the area of the proposed addition we found
the general site to be of low relief. Walking to the
north of the existing mobile home, we found the
surface to be well vegetated for a distance that
varied from 45 to 60 feet to the north from the north
side of the mobile home before the top of slope was
encountered. The slope visible appears to be well
vegetated, trees growing vertically, and no evidence
of recent mass wasting process were noted.
04051101
Page 1 of 3 ,
1
04051101
Page 2 of 3
The location of the proposed addition was marked by
stakes and string lines on the surface. This addition
will be isolated from the existing mobile home. The
addition will be located in the general location of
the southwest corner area of the mobile home. The
addition will not extend north of the existing north
wall line of the mobile home.
The slope to the north is moderate to steep down into
the Hood Canal. This slope was created after the
retreat of the Vasho a ff j rd
now occupied by Hood Canal. The valley slopes vary in
apparent stability. In this area, the soils forming
the slope are mapped as Vashon advance outwash (Qva)
of gravels and sands . The upland area where
construction is proposed is mapped as Vashon
subglacial till (Qvt) . In this area we find a thin
layer of outwash sands and gravels (Qvr) over the
subglacial till . Soils on the slope have limited
outcrops other than in areas of road cuts . Erosion of
the slope since the retreat of the glacier have
mantled the slopes with colluvium. Geology of the
area is mapped in Plate 1, Water-Supply Bulletin 29,
Geology of Southeastern Mason County, Washington by
John B. Noble and Dee Molenarr. Based on soil
exposures, we agree with this general mapping.
Relative Slope Stability of the Southern Hood Canal
Area, Washington by Mackey Smith and R.J. Carson,
published by the Washington Department of Natural
Resources, Division of Geology and Earth Resources
1977, map I-853-F presents an opinion on slope
stability. We find the slope to the north between
this upland area and Hood Canal Lo be classified as a
Class 3, areas inferred to be unstable because of
slopes, generally greater than 15%, which are
underlain by weak, unstable material in which old or
recent active landslides have occurred. Includes
areas of sand and gravel on top of impermeable silt
and clay, mostly along steep valley sides . We are in
general agreement with this opinion.
Slope failures in this area are typically shallow
events on the steep slopes, usually due to
overstepping of the toe support by grading or by
directing concentrated flows of water over the slope.
Concentrated flows can create areas of localized
saturation and induce slope instability.
04051101
Page 3 of 3
Visual examination of the slope to the north of this
site did not indicate recent mass wasting events . We
do expect that the typical mass wasting event that is
likely to develop on this slope will involve the
active soil zone, the zone of colluvium and topsoil
that can become oversteepened and saturated and slide
on the relatively unweathered soils at depth. If one
of these events were to develop at the top of slope at
this property, a sudden loss of top of slope on the
nrder of ten feet could occur. After the event, we
expect that natural reveg�ian u " L-LB 1-ope and
slump scarp will occur and apparent stability return
to the site. These events are random in occurrence
with apparent stability and slow erosion returning
between events .
At the present setback of the exiting mobile home from
top of slope, we do not consider that it would be
adversely affected by one of these events . With the
proposed addition located even further from top of
slope, its being adversely affected is even more
unlikely during its economical useful lifetime of at
least the next 50 years . Good storm water disposal
practices will be required to prevent concentrated
flows of storm water from being directed to the slope.
Based on our field observations and research, it is
our opinion that the addition may be constructed at
the location indicated with no significant risk of
future loss or damage at the observed erosion rates
and process for at least the next 50 years . We agree
with the planner that a geotechnical assessment is
appropriate for this site and proposed construction.
We do not consider the preparation of a geotechnical
report would significantly change our opinion on
stability and erosional processes at this site.
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 `e oEVVash;�
David C. Strong, L.E.G.
<�. -my' l'•�Z .
1129
Posed Geo�o
DAVID C. STRONG