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Geotechnical News • September 2014
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GEOTECHNICAL INSTRUMENTATION NEWS
of the aluminum plate. As the plate
was heated, the wax softened, and the
simulated landslide displaced along
the failure surface. A grid pattern was
painted on both sides of the model to
allow any internal deformation to be
more easily identified. As the landslide
moved along the failure surface, a
well defined zone of reverse shearing
developed through the bluff consistent
with the inclinometer results. The
combination of the finite element and
the physical models confirmed the
validity of the inclinometer results and
the interpreted configuration of the
basal rupture surface. That knowl-
edge facilitated the development of
remedial measures to stabilize the
landslide. Those measures have been
effective to date.
Landslide movement induced by
expansive soils
The second case history involves
movement of a landslide that occurred
after the construction of a large gravity
buttress. The presence of the recently
active landslide was identified during
the pre-grading investigation of a resi-
dential development in Clayton, Cali-
fornia. The majority of the landslide
was removed during grading, however
a portion of the slide debris was left
in place behind a gravity buttress that
was designed to provide a factor of
safety in excess of 1.5 with respect to
gross stability. An
aerial photograph
of the landslide area is provided as
Figure 5, while a stratigraphic cross
section though the site is provided as
Figure 6. Up to approximately 100 feet
of fill was placed above the landslide
debris that remained in place. The fill
consisted predominately of moder-
ately to highly expansive clay and
clayey silt derived from the Martinez
and Domengine Formations. Shortly
after the residential development was
completed, a series of cracks formed
within streets and other improvements
parallel to the top of the descending
slope. Inclinometers were installed
within the development and monitored
for several years. Up to approximately
three inches of lateral movement was
Figure 3. Portion of landslide finite element model.
Figure 2. Cross section through Big Rock Mesa Landslide.
Figure 4. Physical model of landslide.
Figure 5. Aerial photograph of Keller Ridge Landslide
area – Clayton, California.