Civil Engineering Reference
In-Depth Information
stress generated during the earthquake must be determined. An advantage of the effective
stress analysis is that it more fundamentally models the shear strength of the soil, because
shear strength is directly related to effective stress. A major disadvantage of the effective
stress analysis is that the pore water pressures must be included in the earthquake analysis.
The accuracy of the pore water pressure is often in doubt because of the many factors which
affect the magnitude of pore water pressure changes, such as the determination of changes
in pore water pressure resulting from changes in earthquake loads. For effective stress analy-
sis, assumptions are frequently required concerning the pore water pressures that will be
generated by the earthquake.
Cohesionless Soil.
These types of soil are nonplastic, and they include such soils as
gravels, sands, and nonplastic silt, such as rock flour. A cohesionless soil develops its
shear strength as a result of the frictional and interlocking resistance between the indi-
vidual soil particles. A cohesionless soil can be held together only by a confining pres-
sure, and it will fall apart when the confining pressure is released. For the earthquake
analysis of cohesionless soil, it is often easier to perform an effective stress analysis, as
discussed below:
1.
Cohesionless soil above the groundwater table:
Often the cohesionless soil above
the groundwater table will have negative pore water pressures due to capillary tension of
pore water fluid. The capillary tension tends to hold together the soil particles and to pro-
vide additional shear strength to the soil. For geotechnical engineering analyses, it is com-
mon to assume that the pore water pressures are equal to zero, which ignores the capillary
tension. This conservative assumption is also utilized for earthquake analyses. Thus the
shear strength of soil above the groundwater table is assumed to be equal to the effective
friction angle
from empirical correlations (such as Figs. 5.12 and 5.14), or it is equal to
the effective friction angle
from drained direct shear tests performed on saturated soil
(ASTM D 3080).
2.
Dense cohesionless soil below the groundwater table:
As discussed in Chap. 6,
dense cohesionless soil tends to dilate during the earthquake shaking. This causes the
excess pore water pressures to become negative, and the shear strength of the soil is
actually momentarily increased. Thus for dense cohesionless soil below the groundwater
table, the shear strength is assumed to be equal to the effective friction angle
from
empirical correlations (such as Figs. 5.12 and 5.14); or it is equal to the effective friction
angle
from drained direct shear tests performed on saturated soil (ASTM D 3080). In
the effective stress analysis, the negative excess pore water pressures are ignored, and the
pore water pressure is assumed to be hydrostatic. Once again, this is a conservative
approach.
3.
Loose cohesionless soil below the groundwater table:
As discussed in Chap. 6,
loose cohesionless soil tends to contract during the earthquake shaking. This causes the
development of pore water pressures, and the shear strength of the soil is decreased. If liq-
uefaction occurs, the shear strength of the soil can be decreased to essentially zero. For any
cohesionless soil that is likely to liquefy during the earthquake, one approach is to assume
that
is equal to zero (i.e., no shear strength).
For those loose cohesionless soils that have a factor of safety against liquefaction
greater than 1.0, the analysis will usually need to take into account the reduction in shear
strength due to the increase in pore water pressure as the soil contracts. One approach is to
use the effective friction angle
from empirical correlations (such as Figs. 5.12 and 5.14)
or the effective friction angle
from drained direct shear tests performed on saturated soil
(ASTM D 3080). In addition, the earthquake-induced pore water pressures must be used in
the effective stress analysis.
