Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 587

procedure. For multistage consolidated drained triaxial tests,
the net confining pressure σ 3 −

deformation, particularly during the early stages of loading.
Difficulties related to excessive deformation in multistage
triaxial testing have been reported by several researchers
(Kenney and Watson, 1961; Wong, 1978; Ho and Fredlund,
1982b). When the specimen undergoes excessive deforma-
tion during the early stage of loading, the specimen will
tend to undergo soil structure breakdown, and the strength
may be reduced for subsequent stages of testing. The shear
strength measured at successive stages may tend toward a
reduced ultimate strength value. Excessive strain accumula-
tion can be a problem in multistage testing, particularly in
cases where the soil structure is sensitive to disturbance.

u a is usually maintained con-
stant while the matric suction u a − u w is varied from one

stage to the next. Figure 11.91 illustrates a multistage triaxial
test with three stages. Each stage corresponds to a different
matric suction value. A multistage consolidated drained tri-
axial test can also be performed by maintaining the matric
suction constant and varying the net confining pressure.
Each stage of the multistage test should be terminated when
failure is imminent. Near-failure conditions can generally
be determined by observing a plot of deviator stress versus
strain. The soil specimen should not be subjected to excessive