A series of Indiana limestone, 100 mm cubic rock samples has been tested to failure under biaxial loading condition. The horizontal-to-vertical load ratio was varied from zero (uniaxial load) to 75% of the uniaxial compressive strength of the rock cube, and the loading rate was varied from 0.4 to 10.24 kN/s. Test results are reported including failure loads, ó1-ó2 failure stress data and comparison with Mohr Coulomb and Drucker-Prager failure envelopes. Observed failure mechanisms and measured shear failure angles of the failed cubes are also recorded. It is observed that the biaxial failure mechanism is initiated by rock spalling at the free face of the rock sample. Spalling begins at an early stage of loading and progresses with loading. Nevertheless, it is the out-of-plane shear failure that accounts for the ultimate failure of the rock sample. It is found that the biaxial strength can be as much as 1.93 times the uniaxial compressive strength for the range of confinement stress levels tested.


The strength and behavior of rock under biaxial compressive loading condition is of both fundamental and practical significance. In both uniaxial and Hoek?s cell triaxial compression tests, ó2=ó3 (=0 in uniaxial), whereas in biaxial compression, ó1>ó2 and ó3=0. The latter is the state of stress of any unconfined boundary, albeit roof or floor, of an underground mine/tunnel opening. This paper presents the laboratory procedure and test results of an experimental investigation on the biaxial behavior of a series of limestone cubes. The objective of the laboratory study is to simulate the high stress conditions commonly encountered at the boundary of single underground mine openings. The specific objectives of the test program are:

a) To derive the ó1-ó2 failure strength curves

b) To evaluate the effect of the loading rate on the ultimate strength and failure mechanism

c) To examine the failure mode of samples in the context of biaxial loading, and to compare it to high biaxial stress regime failures observed at the periphery of underground mine openings.

Numerous laboratory studies of rock failure under biaxial compression were carried out in the past. These are reviewed below.

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