Laboratory studies on the deformability and strength of intact rocks generally involve the uniaxial compression test, the standard triaxial compression test and direct or indirect tension tests. If sl, s2, s3 are principal stresses (s1 > s2 > s3) with compression taken as positive and tension taken as negative, the applied stress field for each one of these tests can be represented as shown in Fig. 1. However, for most in-situ conditions, such as those that exist at any point around an underground excavation, the stress field is truly three-dimensional or multiaxial (s1 ¿s2 ¿s3) and none of the tests shown in Fig. I can be used to predict the mechanical behavior of the rock. There are special cases for which one of the three principal stresses vanishes. Biaxial loading of rocks in-situ takes place, for instance, when rocks deform and fail in plane stress conditions. This can occur in a free unloaded surface of a rock structure such as in the wall of an underground excavation or a rock slope. Parallel to the free surface, the stress field components can be both compressive (s1>s2 >0), both tensile (0 >s2>s3)), or mixed, one component being compressive and the other one tensile (sl >0>s3 ). The purpose of this paper is to present the results of 25 true biaxial tests that have been conducted on cubical specimens of Indiana Limestone using the multiaxial cell available at the University of Colorado at Boulder. The tests include uniaxial compression and tension, biaxial compression and combined biaxial compression-tension. Flexible fluid cushions and brush bearing platens were used for compressive and tensile loading respectively in order to minimize loading end effects on the test specimens. The paper begins with a review of previous investigations on the multiaxial behavior of rock. Then, test results are presented in terms of biaxial strength characteristics of the limestone. Finally, these characteristics are compared to those derived from conventional tests such as those shown in Fig. 1.
The behavior of rocks under unequal stresses (biaxial or triaxial) has been investigated by testing (i) hollow cylinders of rock under different boundary loading conditions such as axial load combined with inner and/or outer confining pressures or axial load combined with torsion and confining pressure, and, (ii) shaped specimens ("dog bone" specimens). A review of the test procedures and a summary of test results can be found in Jaeger and Cook (1976). These tests were used to study the behavior of rocks in uniaxial and biaxial compression and in combined compression-tension but suffered several major disadvantages. The first one is that the stress distributions within hollow cylinder specimens must be assessed from the theory of linear elasticity and are non-uniform unless thin- walled hollow cylinders are used. The non-homogeneous stress distribution leads to the second disadvantage which is the difficulty in defining exactly the stress condition at failure. Problems associated with testing shaped specimens concern specimen preparation, stress concentration at the specimen ends and loading conditions.