HomeMy WebLinkAboutReconnaissance Level Survey - GEO General - 8/18/1997 Bradley-Noble Geotechnical Services
A Division of The Bradley Group, Inc.
2401 Bristol Court SW, Olympia, WA 98502 PO Box 12267, Olympia, WA 98508-2267
Phone 360-357-7883 FAX 360-754-4240
18 August 1997
Mark & Joan Corley
P.O. Box 549
Shelton, Washington 985840-0549
Subject: Evaluation of slope stability, erosional
processes and recommended minimum
setback from the slope for your new
residence to be constructed on lots 6A &
7A of the survey of Tract A of SP No.
1359, Mason County Tax Parcels 32105 76
90071 and 32105 76 90061 respectively.
Dear Mr. & Mrs . Corley:
As requested, we have conducted a reconnaissance level
survey of the above property to determine if mass
wasting events are probable on the slopes . This
letter will present our observations and opinions
based on this site survey and on our research.
The project site in the area of proposed construction
is located on the Vashon recessional outwash plain.
In this upland area, the surface undulates gently. To
the northwest of your proposed building site, the
slope steepens into a north trending valley. This
valley was created during the retreat of the Vashon
glacier when flows of melt water were high. Cutting
of the valley slopes occurred during this time, about
14,000 years ago. Since the retreat of the glacier,
the climate in the area has moderated, and fluvial
erosional processes are not as active. The Vashon
recessional outwash plain is incised by a number of
these north trending valleys along the Hood Canal in
this area.
Soil exposures at your site show a relatively thin
layer of Vashon recessional outwash soils of sand and
gravels . Underlying the outwash unit is the very
dense and highly overconsolidated Vashon subglacial
till. Subglacial till is deposited at the base of the
advancing continental ice sheet, and is a mixture of
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silt, sand, and gravel that is over-ridden by the ice
sheet, thought to have been about one-half to one mile
thick in this area. The grain size distribution and
overconsolidation of the soil by loading by the ice
sheet has created a soil unit that has many
engineering properties similar to those of lean
concrete. It can stand steeply with apparent
stability and slow erosion.
The soil sequence for the slope into the valley is
expected to consist of subglacial till, Vashon advance
outwash, and the Skokomish Gravels in the lower area
of the slope. All these soil units exhibit properties
of overconsolidation from loading by the Vashon ice
mass . Deep slope failures are unlikely to develop
because of this overconsolidation and cementing of
these soils . Typical slope failures are shallow
debris flows caused by the sliding of the weathered
soil zone on the relatively unweathered soils . The
usual triggering events are saturation or freeze-thaw
cycles. We do not consider your upland area to be at
risk from these mass wasting events. These events are
usually random in occurrence with renewed apparent
slope stability and slow erosion rates returning
between events . The volume of material in these
events is usually small, less than 100 cubic meters at
any time, and they are expected to occur only on the
steeper valley slopes. In our research, we found no
indication of ancient or recent landslides being
mapped in this area.
We understand that your new residence will have a
daylight basement with two stories above to take
advantage of the territorial view the property
offers . We expect that excavation for the daylight
basement will encounter the subglacial till, which
offers excellent support for standard spread
footings . It is for all practical purposes
impermeable. Attention to site perimeter drainage and :
under slab drainage will be required to ensure dry
daylight basement slabs .
From the break in slope into the valley and Ridgecrest
Drive, the site slopes away from the break in slope.
Your site drainage should direct all surface storm
water towards Ridgecrest Drive for disposal. This
slope to your lot will also influence the setback from
the break in slope. We recommend 'a minimum setback
from the average top of the ridge line of 50 feet.
r
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With the slope to the lot, the effective setback of
the footings is increased because of the geometry of
the valley slope and the slope between the top of the
valley slope and the building site. We expect that
there will be a horizontal distance between the
footings and the face of the slope of about 70 feet,
depending on the finish elevation of the footings. We
have no objection to the placement of decks or other
low value structures between the residence and the top
of the valley.
Construction adjacent to slopes always has the risk of
future loss resulting from erosional or mass wasting
processes . This risk is reduced by increasing the
horizontal distance from the top of the slope to the
structure. Using the recommended setback presented in
this report, it is our opinion that the risk at this
site is slight.
Estimating future erosional rates is not a precise
science. They may be affected by seismic events,
climatic changes, or tectonic forces . As long as you
are aware that these risks do exist and that they
cannot be accurately predicted by the geological
sciences, we have no objection to the proposed
construction at its recommended setback in
relationship to the slope. We expect that at this
indicated location, you should realize at least 50
years of economic use before erosion or mass wasting
events at the present observed rates would adversely
affect foundation soil support for the residence.
If you have any questions, or if we may be of
additional service to you, please contact us at our
Olympia office.
Cordially, pp
BRADLEY-NOBLE GEOTECHNICAL SERVICES
C [
David C. St ong
Engineering Geologist