Different Stress Paths Lead to Different Failure Envelopes: Impact on Rock Characterisation and Design

The strength of rock is a non-intrinsic property, and this means that numerous parameters influence the strength values. In most laboratory experiments, specimens are free of stress at the start of the tests, and the load is increased systematically until failure occurs. Around excavations, the opposite path occurs, i.e., the rock is in equilibrium under a triaxial stress state and at least one stress component decreases while another component may increase. Hence, the stress paths in classic laboratory experiments are different from the in situ stress paths. In the research presented, a first step was made to evaluate with an open mind the effect of these different stress paths on the failure process and failure envelope. The research was based on distinct element models, allowing the simulation of micro-fracturing of the rock, which is essential to correctly model rock failure. The micro-fracturing when loading rock (from zero or low stress state) until failure was different from the micro-fracturing when unloading rock (from the in situ stress state) until failure. And, hence, by this difference in weakening processes, the failure envelopes were significantly different. The conventional loading resulted in the largest strength and, thus, overestimated the rock strength in comparison to the real in situ behaviour. This finding, after being confirmed by further lab experiments, will have a direct effect on how one characterises rock material and on the design of rock excavations.