Environmental Engineering Reference
In-Depth Information
Figure 11.57 Possible stress paths followed during unconfined compression test.
It is quite likely that unconfined compression tests on
unsaturated soils may greatly underestimate the available
shear strength in the field. Unconfined compression tests
are routinely performed for bearing capacity, slope stability,
and other engineering studies. The engineer must understand
that an approximate simulation of the field confining pres-
sure is strongly recommended when performing undrained
laboratory strength tests. The undrained tests should not be
performed as a confined compression test when the soils are
unsaturated.
(a)
11.7 DIRECT SHEAR TESTS
Direct shear tests have one distinct advantage over the tri-
axial test, in that the length of the drainage path associated
with pore-water pressure equalization is considerably shorter
in the direct shear test than in the triaxial test. Consequently,
direct shear strength testing of an unsaturated soil can con-
siderably expedite shear strength testing. The basics of the
test procedure and the interpretation of the test results are
explained in the following sections.
11.7.1 Direct Shear Test Procedure
The consolidated drained direct shear test on an unsatu-
rated soil specimen can be conducted using the modified
direct shear apparatus shown in the cross-sectional view
in Fig. 11.59. The direct shear test apparatus consists of
a split box with a top and bottom portion. The soil speci-
men is sheared by moving the lower portion of the shear box
(b)
Figure 11.58 Use of unconfined compressive strength q u to
approximate undrained shear strength c u for unsaturated soil and
saturated soil: (a) relationship between q u and c u for unsaturated
soil; (b) relationship between q u and c u for saturated soil.
 
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