Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 579

11.9.1 Background on Strain Rates for Triaxial
Testing
Prior to the 1960s, unsaturated soils were tested in shear
using similar procedures and equipment to those used for
saturated soils. Coarse ceramic disks were placed next to
the soil specimens and the strain rates were relatively high.
In the early 1960s, the test procedures and equipment used
for testing unsaturated soils were modified. A high-air-entry
ceramic disk was placed at the bottom of the soil speci-
men in order to measure or control the pore-water pressure
independently from the pore-air pressure. Several empirical
procedures were proposed for estimating the strain rate for
testing unsaturated soils (Bishop et al., 1960; Lumb, 1966;
Ruddock, 1966).
Laboratory testing equipment was also modified to accom-
modate the independent control of the pore-air and pore-
water pressures in unsaturated soils. The pore-air pressure
was controlled or measured through the use of a coarse
ceramic disk generally placed on the top of the soil spec-
imen. The control or measurement of the pore-water pres-
sure was made possible through the use of a high-air-entry
ceramic disk. The disk was sealed onto the base pedestal.
The modified laboratory testing apparatuses allowed the use
of the axis translation technique (Hilf, 1956). The axis trans-
lation technique continues to be used for the shear strength
testing of unsaturated soils.
Shear strength tests on unsaturated soils have generally
been performed at relatively slow rates in order to ensure
equalization or dissipation of induced pore pressures. Trial-
and-error procedures have been used by several researchers
to assess the appropriate strain rate. Donald (1961)
recommended that the effect of strain rate on the maximum
deviator stress be used as a criterion in assessing an

(a)

(b)

Figure 11.79 Failure envelope obtained from direct shear tests
on glacial till specimen No. GT-16-N5: (a) failure envelope on τ
versus u a −

u w plane; (b) slope of failure envelope (i.e., φ b )for
various levels of applied suction (from Gan and Fredlund, 1988).

pore pressures throughout the specimen during undrained
shear. The selected strain rate during drained shear must
ensure dissipation of induced pore pressures. The estima-
tion of the strain rate for testing soils can be made partly on
the basis of experimental evidence and partly on the basis
of pore pressure dissipation theory.

Figure 11.80 u w plane obtained from several direct shear

tests on compacted glacial till specimens (from Gan et al., 1988).

Failure envelopes on τ versus u a −