Civil Engineering Reference
In-Depth Information
pore water fluid. In some cases, the cohesive soil may even be dry and desiccated. The cap-
illary tension tends to hold together the soil particles and provide additional shear strength
to the soil. For a total stress analysis, the undrained shear strength
s
u
of the cohesive soil
could be determined from unconfined compression tests or vane shear tests. As an alterna-
tive, total stress parameters (
c
and
) could be determined from triaxial tests (e.g., ASTM
D 2850 and ASTM D 4767).
Because of the negative pore water pressures, a future increase in water content would
tend to decrease the undrained shear strength
s
u
of partially saturated cohesive soil above
the groundwater table. Thus a possible change in water content in the future should be con-
sidered. In addition, a triaxial test performed on a partially saturated cohesive soil often has
a stress-strain curve that exhibits a peak shear strength which then reduces to an ultimate
value. If there is a significant drop-off in shear strength with strain, it may be prudent to use
the ultimate value in earthquake analyses.
2.
Cohesive soil below the groundwater table having low sensitivity:
The
sensitivity
S
t
of a cohesive soil is defined as the undrained shear strength of an undisturbed soil speci-
men divided by the undrained shear strength of a completely remolded soil specimen. The
sensitivity thus represents the loss of undrained shear strength as a cohesive soil specimen
is remolded. An earthquake also tends to shear a cohesive soil back and forth, much as the
remolding process does. For cohesive soil having low sensitivity (
S
t
4), the reduction in
the undrained shear strength during the earthquake should be small.
3.
Cohesive soil below the groundwater table having a high sensitivity:
For highly sen-
sitive and quick clays (
S
t
8), there could be a significant shear strength loss during the earth-
quake shaking. An example was the Turnagain Heights landslide discussed in Sec. 3.5.2.
The stress-strain curve from a triaxial test performed on a highly sensitive or quick clay
often exhibits a peak shear strength that develops at a low vertical strain, followed by a dra-
matic drop-off in strength with continued straining of the soil specimen. The analysis needs
to include the estimated reduction in undrained shear strength due to the earthquake shak-
ing. In general, the most critical conditions exist when the highly sensitive or quick clay is
subjected to a high static shear stress (such as the Turnagain Heights landslide). If, during
the earthquake, the sum of the static shear stress and the seismic-induced shear stress
exceeds the undrained shear strength of the soil, then a significant reduction in shear
strength is expected to occur.
Cohesive soils having a medium sensitivity (4
S
t
8) would tend to be an interme-
diate case.
4.
Drained residual shear strength
r
for cohesive soil:
As indicated above, the
earthquake analyses for cohesive soil will often be performed using a total stress analysis
(that is,
s
u
from unconfined compression tests and vane shear tests, or
c
and
from triax-
ial tests).
An exception is cohesive slopes that have been subjected to a significant amount of
shear deformation. For example, the stability analysis of ancient landslides, slopes in
overconsolidated fissured clays, and slopes in fissured shales will often be based on the
drained residual shear strength of the failure surface (Bjerrum 1967, Skempton and
Hutchinson 1969, Skempton 1985, Hawkins and Privett 1985, Ehlig 1992). When the sta-
bility of such a slope is to be evaluated for earthquake shaking, then the drained residual
shear strength
r
should be used in the analysis. The drained residual shear strength can
be determined from laboratory tests by using the torsional ring shear or direct shear appa-
ratus (Day 2001a).
In order to perform the effective stress analysis, the pore water pressures are usually
assumed to be unchanged during the earthquake shaking. The slope or landslide mass will
also be subjected to additional destabilizing forces due to the earthquake shaking. These
