Civil Engineering Reference
In-Depth Information
utilized for earthquake engineering analyses. These guidelines for the selection of the
undrained shear strength
s
u
as applied to bearing capacity analyses are as follows:
1.
Cohesive soil above the groundwater table:
Often the cohesive soil above the
groundwater table will have negative pore water pressures due to capillary tension of the
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 to provide additional shear
strength to the soil. For the total stress analysis, the undrained shear strength
s
u
of the cohe-
sive soil could be determined from unconfined compression tests or vane shear tests.
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, an unconfined compression 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 the bearing capacity analysis.
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. Thus the undrained
shear strength from the unconfined compression test or vane shear tests could be used in
the bearing capacity analysis (for field vane tests, consider a possible reduction in shear
strength due to strain rate and anisotropy effects, see Table 7.13 in Day 2012).
3.
Cohesive soil below the groundwater table having a high sensitivity:
For highly
sensitive and quick clays (
S
t
8), the earthquake-induced ground shaking will tend to
shear the soil back and forth, much as the remolding process does. For these types of soils,
there could be a significant shear strength loss during the earthquake shaking.
The stress-strain curve from an unconfined compression test performed on a highly sen -
sitive or quick clay often exhibits a peak shear strength that develops at a low vertical strain,
followed by a dramatic drop-off in strength with continued straining of the soil specimen.
An example of this type of stress-strain curve is shown in Fig. 8.12. The analysis will need
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 high bearing pressure acting on the soil).
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) tend to be an intermediate case.
Some of the other factors that may need to be considered in the bearing capacity analy-
sis are as follows:
1.
Earthquake parameters:
The nature of the design earthquake, such as the peak ground
acceleration
a
max
and earthquake magnitude, is a factor. The higher the peak ground
acceleration and the higher the magnitude of the earthquake, the greater the tendency
for the cohesive soil to be strained and remolded by the earthquake shaking.
2.
Soil behavior:
As mentioned above, the important soil properties for the bearing
capacity analysis are the undrained shear strength
s
u
,
sensitivity
S
t
,
maximum past pres-
sure
vm
, and the stress-strain behavior of the soil (e.g., Fig. 8.12).
