Non-elastic deformation of rocks in tension and its influence on stresses

Tensile failure appears widely around underground tunnels and openings. Unfortunately investigations on tensile deformation of rocks are much less popular than those on compression. Test results from quite a few authors have shown that for some rocks, Young's modulus in tension, ET, is much smaller than that in compression, EC. As a consequence, large errors may arise in the stress values obtained from traditional methods for such rocks. Test results were presented on deformation behaviour of several rock types in both direct tension and compression. It was found that for the rocks of which ET << EC, the deformation in tension is nonlinear and plastic though their behaviour in compression is approximately linear and elastic. Two examples are presented to show how errors would appear by using a linear bi-elastic model to a structure where the deformation in tension is plastic. These examples show that plastic behaviour in tension has great influence on stress values and failure. Therefore, further investigations on tensile deformation are necessary to be conducted, for more rocks.