HomeMy WebLinkAboutBLD2001-00013 GEO TECH •
23,2000
parcel#942300023, Jefferson County, WA
finical Report
s to the beach below. I witnessed several of these slides in March 1999. Due to the loss of
drop
soil at the top of the silt/clay slope, the upper slope has lost lateral support an numerou
s
headwall scarps have formed as the slope failures have progressed up the slope. The top most
portion of the slope consists of the uppermost headwall scarp, and recent failures are apparent at
several locations.
No ground water seeps were observed during our site visit in August 2000. However, numerous
wetland plants were observed at the top of the silt/clay slope. Most of the upper bluff slope is
covered with a stand of mature second growth Douglas fir with an understory of bracken fern,
grasses, evergreen huckleberry, and salal. Numerous Douglas fir trees are tilted indicating that
recent movement has taken place on portions of the slope. We also observed a few trees with
pronounced bends indicating older past movements have occurred as well.
CONCLUSIONS AND RECOMMENDATIONS
Based on our visual inspection of the subject property and vicinity, and our assessment of the
landslide area, we conclude that a residence can be located on the property such that landsliding
will not threaten the proposed residence within the expected life of the structure (75 years) as
long as our recommendations are followed. We recommend a set back distance of 150 feet from
the top of the steep slope. It is our opinion from an engineering geology standpoint that this
distance will be adequate to allow for expected bluff failures.
The presence of mature Douglas fir trees and the fact that even the lower slope immediately above
the beach was well vegetated as recently as 1994, indicates that the active sliding that took place
two winters ago is probably not typical. However, the upper part of the steep slope is potentially
very unstable and will be subject to future very al go
slides.
The lower steep slope is underlain by very compact silt/clay. Because of the compact condition of
this soil, it is very resistant to erosion. Given the distance of the proposed residence from the
shoreline, we do not anticipate that shoreline erosion poses a threat to a residence set back 150
feet from the top of the steep bluff.
A deep-seated failure of the silt/clay unit took place approximately a half of a mile northeast of the
subject property in 1998. The failure mechanism is a result of lack of lateral pressure to the east
creating a structural weakness within the compacted unit. These types of slides tend to move
episodically. Trees on this slide indicated that little or no movementsihad t en place included uplifted
rato g
period of time. However, indications of a deep-seated slide at the
silt/clay layers on the tide flats and topographic features. We did not observe any topographic
File:8.9.00
Stratum Group
> o
August 23, 2000
Tax Parcel#942300023, Jefferson County, WA
Geotechnical Report
features indicating deep-seated failures within the silt/clay unit in the vicinity of the subject
property, and did not observe any uplifted silt/clay layers on the tide flats along the bluff. We
conclude that there is a low risk of a deep-seated failure within the silt/clay unit at the subject
property.
Soils on the property are very well drained, and therefore we do not anticipate discharging of
footing drains will be necessary. Roof drainage and any storm water should be discharged into the
grouted near the building.
We recommend a 50-foot native vegetation buffer consisting of tress and low brush at the top of
the bluff The top of the bluff should be disturbed as little as possible, and we recommend that no
grading be performed on the steep slopes of the bluff face. Trimming of tree branches for view
purposes will not have any negative impacts to the steep slope. However, no trimming debris
should be left on slopes steeper than 30 degrees as this material may become saturated and slide
down the slope.
We do not anticipate that the development of the subject property will cause any negative impacts
on the stability of the slopes in the vicinity of the subject property as long as our recommendations
are followed.
Site grading soils or debris, landscape debris, or any other material should not be disposed of over
the steep slope on the property or placed at the top of the slope.
We do not anticipate that the septic drainfield will have any adverse affects on the stability of the
steep slopes on the subject property or elsewhere. We recommend that the drain field be located
at least 100 feet from the top of the steep slope.
File:8.9.00
Stratum Group