Strength and GSI of deep rock mass: A GZZ strength criterion- and non-associated flow rule-based constitutive model and its application

Abstract

Accurate mechanical parameters are crucial for deep rock engineering. Traditional two-dimensional strength models often fail to reflect the complex three-dimensional mechanical properties of deep rock mass. This study proposes an elastoplastic constitutive model based on the smooth GZZ strength criterion, incorporating a non-associated flow rule to account for rock dilatancy. The model was numerically implemented and validated against theoretical and experimental results. Applied to a deep-buried tunnel, it determined the scale-dependent uniaxial compressive strength (UCS) and Geological Strength Index (GSI) of the rock mass. Numerical experiments revealed a transition from brittle to ductile failure with increasing confining pressure. Both the strength and GSI increase nonlinearly with confining pressure. The impacts of the plastic flow rule and rock matrix strength criterion on these parameters were quantitatively analyzed. At high confining pressures, neglecting dilatancy leads to their overestimation. Compared to the proposed model, using the Hoek-Brown criterion for the rock matrix fails to capture continuous hardening and yields conservative strength predictions under high confinement. Overall, the proposed model offers an improved tool for predicting rock mass strength and GSI under high confinement, with direct implications for the design and stability assessment of deep underground excavations.

Keywords

Deep rock mass / GZZ strength criterion / Equivalent fractured rock mass / True triaxial test / Digital in-situ testing

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