Impact of the intermediate stress component in a plastic potential function on rock mass stability around a sequentially excavated large cavern

Although plastic potential functions considering an intermediate principal stress component are widely employed for elasto-plastic analyses, its effect on excavation-induced stress re-distribution is not adequately examined and verified based on in-situ stress measurements. The present study addressed this issue by not only examining the analytical solutions of elasto-plastic models but also conducting a case study based on large cavern excavation and the continual measurement of the in-situ stress at a great depth. The analytical solutions indicate a clear difference in volumetric plastic strain increment between different plastic potential functions with the same yield function. Specifically, when σ2/σ1 exceeds 0.6, the Drucker–Prager (DP) plastic potential function produces an increase in volumetric strain rate larger than that of the Mohr–Coulomb (MC) potential function, hence suggesting the increase in confining stress in the surrounding rock mass. For the case study, the numerical analyses of the large cavern excavation with the different potential functions demonstrated that both the potential functions yield comparable excavation-induced stress change at the stress monitoring point, however the failure state transition of the surrounding rock mass is non-identical. When the MC potential function is employed, the surrounding rock mass continuously undergoes failure throughout the excavation stages, whilst for the model with the DP potential function, unloading takes place due to the volumetric expansion of failed rock masses associated with the intermediate principal stress component, as the excavation stage proceeds. This was found to be more consistent with field observations. Thus, we concluded that the intermediate principal stress in a plastic potential function plays a vital role in the evolution of volumetric plastic strain, thereby exerting an influence on the overall stability of rock mass structures, especially for a fractured rock mass and when the intermediate stress is relatively large, compared to the minor principal stress value.

https://www.sciencedirect.com/science/article/pii/S1365160918311043