Civil Engineering Reference
In-Depth Information
destabilizing forces can be included in the effective stress slope stability analysis, and this
approach is termed the
pseudostatic method
(see Sec. 9.2.5).
Analysis for Subsoil Profiles Consisting of Cohesionless and Cohesive Soil.
For
earthquake analysis where both cohesionless soil and cohesive soil must be considered,
either a total stress analysis or an effective stress analysis could be performed. As indi-
cated above, usually the effective shear strength parameters are known for the cohesion-
less soil. Thus subsoil profiles having layers of sand and clay are often analyzed using an
effective stress analysis (
c
and
) with an estimation of the earthquake-induced pore
water pressures.
If the sand layers will liquefy during the anticipated earthquake, then a total stress analy-
sis could be performed using the undrained shear strength
s
u
for the clay and assuming the
undrained shear strength of the liquefied sand layer is equal to zero (
s
u
0). Bearing capac-
ity or slope stability analyses using total stress parameters can then be performed so that the
circular or planar slip surface passes through or along the liquefied sand layer.
Summary of Shear Strength for Geotechnical Earthquake Engineering.
Table 5.4 pre-
sents a summary of the soil type versus type of analysis and shear strength that should be
used for earthquake analyses.
5.5.2
Cyclic Triaxial Test
The cyclic triaxial test has been used extensively in the study of soil subjected to simulated
earthquake loading. For example, the cyclic triaxial test has been used for research studies
on the liquefaction behavior of soil. The laboratory test procedures are as follows (ASTM
D 5311):
1.
A cylindrical soil specimen is placed in the triaxial apparatus and sealed in a watertight
rubber membrane (see Fig. 5.16).
2.
A backpressure is used to saturate the soil specimen.
3.
An isotropic effective confining pressure is applied to the soil specimen, and the soil
specimen is allowed to equilibrate under this effective stress. Tubing, such as shown in
Fig. 5.16, allows for the flow of water during saturation and equilibration as well as the
measurement of pore water pressure during the test.
4.
Following saturation and equilibration at the effective confining pressure, the valve to
the drainage measurement system is shut, and the soil specimen is subjected to an
undrained loading. To simulate the earthquake loading, a constant-amplitude sinu-
soidally varying axial load (i.e., cyclic axial load) is applied to the top of the speci-
men. The cyclic axial load simulates the change in shear stress induced by the
earthquake.
5.
During testing, the cyclic axial load, specimen axial deformation, and pore water pres-
sure in the soil specimen are recorded. For the testing of loose sand specimens, the
cyclic axial loading often causes an increase in the pore water pressure in the soil speci-
men, which results in a decrease in the effective stress and an increase in the axial
deformation.
The cyclic triaxial test is a very complicated test, it requires special laboratory equip-
ment, and there are many factors the affect the results (Townsend 1978, Mulilis et al. 1978).
Actual laboratory test data from the cyclic triaxial test are presented in Sec. 6.2.
