Geology Reference
In-Depth Information
22
CHAPTER TWENTY-TWO
Triaxial Compression
Test
(Referenced Document: ASTM D 2850)
Triaxial Compression
Test
The triaxial compression test is carried out in a manner somewhat sim-
ilar to the unconfined compression test (Chapter 21) in that a cylindri-
cal soil specimen is subjected to a vertical (axial) load. The major
difference is that, unlike unconfined compression tests, where there is
no confining lateral pressure, triaxial tests are performed on cylindrical
soil specimens encased in rubber membranes with confining lateral
pressure present. The magnitude of the lateral pressure can be chosen
and is made possible by placing a specimen within a pressure chamber
(see Figures 22-1 and 22-2), into which water or air is then pumped.
The soil specimen in the chamber under a chosen lateral pressure is sub-
jected to an increasing axial load until the specimen fails. The procedure
is then repeated on additional specimens at other confining pressures.
The lateral pressure, or chamber pressure, which is applied to the
ends of the specimen as well as its sides, is called the
minor principal
stress
. The externally applied axial load divided by the cross-sectional
area of the test specimen is the
unit axial load
. The minor principal
stress plus the unit axial load is called the
major principal stress
.
Triaxial compression test results are analyzed by plotting Mohr cir-
cles for the stress conditions of each specimen when failure occurs. By
evaluating the plotted Mohr circles, the soil's shear strength parameters
(cohesion
c
and angle of internal friction
INTRODUCTION
) can be determined. These
parameters are used to evaluate the shear strength of soil.
337
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