The excessive deformations that accompany the brittle fracturing and dilation of a highly stressed rock mass, also referred to as bulking, is a major concern for support design as it can lead to overloading and failure of the support system. Initial understanding of the dilation potential is obtained by performing conventional triaxial compression tests on intact rock. Then, guided by empirical data and engineering experience, the intact rock values are scaled to fit the dilative characteristics of the rock mass in situ. A variety of dilation models (also called flow rules in plasticity theory) have been suggested having the common assumption of s2-independency. Key inputs for these models are largely derived from empirical curve fitting of laboratory data. Most dilation models are formulated using ?, but ? does not have true physical meaning. In this paper, we suggest a new model that uses the Plastic Strain Increments Ratio (PSIR) of (equation), instead of ?. We will show that (equation) is a physically meaningful attribute that provides a mechanistic means to understand and explain the dilative characteristics of a rock mass susceptible to stress-induced fracturing and spalling. Our results suggest that the proposed flow rule is more convenient for modelling rock mass dilation, minimizing sources of parameter uncertainty, and guiding numerical modellers as to how the dilation parameters used affects the modelled rock mass behaviour, tunnel convergence, and displacement-based support design.

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