Civil Engineering Reference
In-Depth Information
14.4.2 Triaxial Compression Test
The triaxial compression test is intended to determine the deformability parameters and
shear strength of intact rock. A representative sample comprises at least three cylindri-
cal specimens with height-to-diameter ratios of 2 to 3 prepared from drill core samples
as described in Section 14.4.1, and the natural water content of the sample must be
preserved until preparation of the specimens (DGEG 1979b, ISRM 1983).
The test apparatus consists of a sufficiently stiff testing machine with a measuring de-
vice for the axial force F according to accepted national requirements and a cell for
applying the confining stress p by means of a liquid such as oil. In addition, a system to
keep the confining stress constant and devices to measure the confining stress and the
axial strain of the specimen are required. The specimen is placed into the cell between
the lower and upper platen of the testing machine. To avoid direct contact with the pres-
sure liquid, the specimen is normally jacketed with a flexible and tight membrane (Fig.
14.7). To be able to compensate potential deviations from plane parallelism at least the
upper platen must be spherically seated (DGEG 1979b, ISRM 1983).
Figure 14.7
Test apparatus for triaxial compression tests (ISRM 1983)
The testing machine must allow the specimen to be triaxially loaded with an axial stress
σ
1
= F/A at constant strain rate and a constant confining stress p =
σ
3
. Also,
axial loading with constant stress rate must be possible (DGEG 1979b, ISRM 1983).
During loading, the axial strain
σ
2
=
ε
a
of the specimen is recorded continuously. If in ad-
dition to
ε
a
the tangential strain
ε
t
is also measured and isotropic deformability is as-
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