Micro-mechanics-based constitutive model for porous rock: Application to deep tunnel excavation

The stability of underground tunnel excavation and the efficiency of mineral resource extraction are significantly influenced by the damage of porous rocks. The brittle-ductile transition and contraction deformation characteristics of porous rocks under different confining pressures have not been sufficiently considered in traditional rock constitutive models, which constrains the stability assessment of deep engineering projects. This paper presents a micromechanics-based constitutive model for porous rock, focusing on the brittle-to-ductile transition and contraction yielding behavior, which are critical for assessing the stability of deep tunnels. The model treats rock as a representative volume element (RVE) with a dual structure of pores and cracks, applying micro- mechanical homogenization methods to derive the constitutive equations. The proposed model is capable of accurately reflecting the mechanical behavior of porous rocks during brittle-ductile transition and shear-shear contraction under different confining pressures. Moreover, the model's high predictive accuracy is demonstrated by comparing the triaxial compression test results of various rocks with the model predictions, which can accurately describe the stress-strain curves of porous rocks under different confining pressures. Ultimately, the model was applied to the excavation simulation of the Mine-by experimental tunnel at the Canadian URL Underground Research Laboratory, where the simulation outcomes were found to be highly consistent with the observed patterns of rock mass failure and the measured displacement data. The simulation concludes that within burial depths of 200 m to 800 m, volume fracture zones exhibit shear yielding characteristics, while at greater depths, they display contraction yielding characteristics, aligning with the multi-modal failure characteristics of deep-buried tunnel excavations. This research contributes to a better understanding of the mechanical behavior of deep porous rocks and offers a practical basic for the design and risk assessment of deep underground projects.