Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 581

1000

800

.

600

For strain rates ε
from 5.3 x 10 − 5
to 3.3 x 10 − 3 %/s

400

200

0

0 8 16 24

Axial strain ( ε y ), %

(a)

Figure 11.82 Direct shear tests on compacted Madrid clayey
sand: (a) horizontal projection of failure envelope onto shear
strength versus σ u a plane; (b) horizontal projection of failure

envelope onto shear strength versus u a −

−

u w plane (after Escario

and Saez, 1986).

11.9.2 Strain Rates for Triaxial Tests
Strain rate can be defined as the rate at which a soil specimen
is axially compressed:

ε f
t f

ε ˙ = (11.32)

where:

˙ ε = strain rate for shearing a specimen in the triaxial

test,

ε f = strain of the soil specimen at failure, and

t f = time required to fail the soil specimen, or “time to

failure.”

The strain at failure, ε f , depends on the soil type and the
stress history of the soil. Table 11.8 presents typical val-
ues of strain at failure, ε f , obtained from numerous triaxial
testing programs on unsaturated soils. The information in
Table 11.8 can be used as a guide when determing a suitable
strain rate

(b)

Figure 11.83 Effect of strain rate for constant-water-content
tests on Dhanauri clay: (a) effect of strain rate on deviator stress;
(b) effect of strain rate on matric suction change (after Satija, 1978).

ε for shear strength testing. ˙