HomeMy WebLinkAbout021202003 Geotech Assessment
James B. Scott, P.E.
GEOTECHNICAL CONSULTANT
360-293-6044
.FAX 360-293-6044
Email geoscott26~a.comcas#.nex
J. B. SCOTT & ASSOCIATES
August 20, 2005
Kenton and Rosa Larson
455 Winslow Way West.
Bainbridge Island, WA 98110
LC)G !1-E
3
~~~E O
F 4
2005 ~~%
_,
7
RE: Geotechnical inspection of bluff slope conditions on a parcel located in the
NW% of SW '/ of Sec. 20, T. 30 N., R. 1 E.W.M., Marrowstone Island, Jefferson
County, WA
Dear Mr. Doe:
At the request of Ms. Barbara Blowers of Waves Waterfront Properties, Inc., and
with your authorization, a site inspection was made of the bluff slope where you
propose to place a stairway. This site inspection, made in the presence of Ms.
Blowers, was made on August 11, 2005. Photographs were taken to document
field conditions.
In addition to the site inspection, review was made of (1) Coastal Zone Atlas of
Washington, volume 11, Jefferson County, Washington Department of Ecology,
1978; (2) Soil Survey of Jefferson County Area, Washington, USDA Soil
Conservation Service, 1975; (3) Aerial Photograph JEF-0482, 7112/94,
Washington Department of Ecology, 1994; and (4} .the field inspection which
consisted of walking over the shoreline bluff area and inspecting slope stability
conditions of the bluff in addition to surface drainage.
An Engineering Corporation
3601 West S~ Street
Anacortes, WA 98221 f ~ ~ j ~`~~ --
~'°i OEC ~2
PHOTO 1 -AERIAL PHOTO OF PROJECT SITE AREA
i
SUMMARY AND RECOMMENDATIONS - Based on the Coastal Zone Atlas as
shown in Figure 1, the entire lot is underlain by Vashon till
~5 .Z` ~~ z ,;y ~ .~.
~ ~ 9.
> ,` f"
~, ;~~ k
FIGURE '1 -COASTAL ZONE GEOLOGIC MAP OF PROJECT AREA
The presence of Vashon till was confirmed in the field by exposures of till like
material in the upper portion of the bluff slope as shown in Photo 2. This till was
standing stable at an average slope angle of 50%, which is less than normal for
till.
In other locations in the North Puget Sound area, Vashon till exposures at sea
level are rather rare. In this area, tilt is exposed at sea level and because of this,
the entire shore line zone is classed as unstable with recent landslides. Why is
this?
x
~.
J
~, ~ _._
„, tl w
,{ ~ ^ ;
4 o~c -- Z zoa5 ;~v
e
,,. ,
`''
.,..::
FIGURE 2 -COASTAL ZONE MAP OF STABILITY CLASSIFICATION
As shown. in Figure 2, is a map showing slope stability classifications. You will
note that a portion of the shore lines are shown as Urs. That classification
means "unstable zones having recent slope #ailures" are present. Why is this
when many other areas with even steeper slopes of till are classed as stable?
First it will be necessary to explain how the till was formed. The glacial outwash
material that is now Vashon till was subjected to very heavy ice loading by
glaciers in the past. In some areas, this ice was as thick as 6000 feet.
2
~~^q ~~ ~ fit'' ~~
r ~ 0 ~ ':
1
~~ ~ ,,.
°I ~~~ AEEC.s2 2005 ,'rfp
That resulted in a condition known "over consolidation", `which m essence,
resulted from the glacial material being reduced in volume or being ~c~nsolidated.'
This consolidated resulted in a material forming that iS almost like a weak
.concrete having extremely low permeability. Then upon th~~ removal ~of the ice
load, this material, which was like steel springs ready to "rebound" back to the
original volume and density, then started to rebound.
As long as the material is relatively dry, it will rebound at a very stow pace. But
when that till is subjected to wetting and drying cycles along a shoreline along
with tidal changes of sea level, that rebound accelerates and that results in a
very rapid and drastic reduction of the strength of the till: In a matter of weeks or
months, such material will expand and break down to what is what before
consolidation. Therefore, this condition cannot be stopped unless it is protected
from constant wetting and drying. In other words the shoreline till in the general
area will continue to break down and slope failures will be common.
In addition to the rapid break down of the till along the shore line, in the upland
portions of the till formation, the surface or weathered zone has a depth of about
two to three feet, is quite loose, and has very high permeability values.
Therefore, surface water will enter the ground with ease and will then tend to
"perch along the contact with the underlying unweathered till.
Excellent soil andlor formation exposures exist along upper portion of bluff as
shown in Photos 2 and 3. In Photo 2, the red arrow is pointing at an outcrop of
till material standing stable at a slope angle of 60°.
Because of the conditions just discussed, the proposed stairs should be
anchored by a "shear block" type anchor that extends down into unweathered till
material. This will also tend to stop any lateral movement of the stairs that
could result from a strong seismic event. Since little or no surface water runoff
will occur, the placement of a surface drain is not considered as necessary.
However, I would expect that the perched ground water zone overlying the till
3
PHOTO 2 -TOP OF BLUFF SLOPE PHOTO 3 -LOOKING DOWN BLUFF