HomeMy WebLinkAboutEvaluation of Slope Stability and Impact - BLD Engineering / Geo-tech Reports - 5/1/2002 Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
PO Box 12267, Olympia WA 98508-2267
Phone 360-357-7883 • FAX 360 867-9307
1 May 2002 7MAY
it R irrMr. Mike Huson -
P.O. Box 34 PLAV6NIN1
Tahuya, Washington 98588
Subject: Addendum to our 22 April 2002 report for
your proposed timber harvest on Mason
County Tax Parcels 123302400000 and
123302400010.
Dear Mr. Huson:
The Mason County Planner reviewing our report had two
questions that we did not specifically address. 1)
Depth to the impervious layer. We noted in our report
that the Vashon recessional outwash sands and gravels
(Qvr) were deposited onto the Vashon subglacial till
(Qvt) . Soil exposures in road cuts in the area
indicate that in this location the recessional outwash
soils are at least six to ten feet thick. We did not
observe the subglacial till cropping out in the
exposures. To determine the actual thickness of the
recessional outwash soils, work beyond the scope of an
assessment would be required, either exploration with
test pits or drilling. With the harvest work being
completed on the surface, we do not expect that the
impervious layer would be exposed or have any effect
on the harvest work.
2) During periods of heavy or prolonged rainfall, flow
of ground water may develop on the surface of the
subglacial till. These flows are seasonal and feed n
the lakes that have formed in the depressions of the ljJ`
subglacial , till in this area. The actual year around
water table used for domestic water source would be
found below the subglacial . till, probably 60 to 80
feet below the present ground : surface. The timber
harvest would not have an effect on this water table.
The subglacial till forms an impervious layer between
the recessional soils that the timber harvest will be ��
conducted on and the soils in the zone of the year
around water table.
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With the 200 foot no-cut buffer from Lake Larson and
if the recommendations of the report are included in
the timber harvest plaii, we do not expect that the
proposed harvest would have any water quality issues
with this lake.
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
ke
David C. Strong
Engineering Geologist
0
Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
PO Box 12267, Olympia WA 98508-2267
Phone 360-357-7883 9FAX 360 867-93WECEIVED
22 April 2002 API z 6 2002
Mr. Mike Huson MASON CO.PLANNING DEPT.
P.O. Box 34
Tahuya, Washington 98588
Subject: Evaluation of slope stability and impact
of the proposed tree thinning operation
on Lots 1 through 4 in the Short P,at of
Mason County Tax Parcels 1233 4 0000
and 123302400010.
Dear Mr. Huson:
As requested, we met with you at the above property in
order to evaluate the impact of: the proposed timber
thinning on slope stability issues. Within the scope
of this work, we are also presenting recommendations
for work to reduce surface erosion after completion of
the harvest.
We understand that the harvest will result in the
removal of about 50% of the merchantable timber on
these properties. The harvest will utilize a rubber
tired skidder to move the logs to the landing area.
The landing area will be at the north end of the
existing access road to . the property. Walking the
property, we found numerous existing or overgrown skid
roads crossing the property. From our discussion, we
u-iders' tand that these existing roads will be used for
the skidding operation and that no new significant
road construction is proposed to allow for the
harvest. A 200-foot, no-cut buffer zone will be
observed from the extreme high water mark of Larson
Lake.
This buffer area contains the major slope of the
property with slopes in . the maximum range of 10
degrees or about 18% slope. Trot 1 and the western
areas of Lots 2 through 4 are of low relief and less
than 9 degrees or about 15% slope.
The existing access road to the properties, after the
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completion of the harvest, will be upgraded as the
access road to the properties as part of the
residential development. We expect that a gravel
wearing course will create the roadway. As part of
this development work,, construction of drainage
ditches, crowning of the road, and construction of
energy dissipaters in the ditches will be included in
the work.
Site geology controls slope stability and surface
erosion in this area. The property is located on the
Vashon recessional outwash plain. Soils found in this
area are sands and sandy gravels'. These soils were
fluvially deposited during the retreat of the Vashon
continental glacier from the Puget Sound Lowland,
about 11,000 years ago.
These recessional soils were deposited on the Vashon
subglacial till unit. The subglacial till is a highly
overconsolidated soil unit with many engineering
properties similar to those of lean concrete.
Subglacial till is a mixture of silt, sand, and small
gravel that was deposited at the toe of the advancing
ice sheet. This soil was overridden and consolidated
by an ice mass thought to have been about one-half to
one mile thick in this area. The surface of the
subglacial till undulates. We find in this area many
small lakes that have formed in these undulations,
such as Larson Lake which occupies part of this
property.
Slope stability is controlled by the angle of internal
friction of the soils and their drainage
characteristics. In sands in the firm to dense range,
such as these soils, we expect that the angle of
internal friction would be in the 32 to 35 degree
range. With the slope angles significantly less than
the angle of internal friction, slope stability is
inferred. The outwash . soils in this area have good
internal drainage characteristics. We do not expect
that long-term saturation of the soils on the slopes
would develop and that water would not be a
destabilizing concern at this site.
The Relative Slope Stability of the Southern Hood
Canal Area, Washington by Mackey Smith and R. J.
Carson, Washington Division of Geology and Earth
Resources 1977, indicates this area as being a Class 1
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that is generally considered stable with slopes less
than 15% with small areas of slope greater, which at
the scale of the map cannot be shown. This area is
not located adjacent to a Class 3 area with weaker
underlying soils. We concur with the opinion that
this area is stable.
After completion of the skidding operation, we do
recommend that the skid roads be rough leveled to fill
ruts. If growing conditions allow, broadcasting of
grass seed to aid in the development of surface
vegetation is recommended. This will reduce the
possibility of silt-laden water developing. Where
skid roads traverse down slopes, then in addition to
the rough leveling and seeding, we also recommend
placement of water bars to prevent erosion by
concentrated surface flow of storm water.
Based on our site visit, our understanding of the
proposed harvest methods, and our research into the
area geology and slope : stability history, we do not
expect that the harvest will have any adverse effect
on slope stability issues. Control of silt-laden
water from disturbed surfaces must be included in the
restoration work to ,protect Lake Larson and the buffer
area.
If you have any questions, or if we may be of
additional service to you on this project, please
contact us at our Olympia office.
Cordially,
BRADLEY-NOBLE GEOTECHNICAL. SERVICES
David C. trong
Engineering Geologist