Geological Controls on Strainbursting in Deep Mining Drifts

Abstract

Strainbursts are sudden dynamic failures of rock that pose a serious hazard to mine workers and mine production. The current state of research is limited, and it is not possible to identify when, where, and particulars about how a strainburst occurs. This limits the tools available to engineers for identifying hazardous conditions and designing risk mitigation strategies. The knowledge gap is addressed by identifying strainburst mechanisms based on local geological characteristics and improving the understanding of the underlying rock behaviour. Understanding the mechanisms leading to strainbursting provides leverage for the design of hazard identification and risk mitigation strategies.

Two distinct underlying sources of localized instability are identified, with the common theme that local geology causes local instability. First, shear localization on smooth planar discontinuities or plastic shear strain in weak narrow veins, can cause mixed compression – extension loading. Second, the local loading system stiffness (LSS) is softened by shear displacement on discontinuities. The local LSS close to the discontinuities is not a constant and can soften suddenly to trigger time-delayed strainbursting.

Nonlinear elasticity is identified as an important factor when investigating brittle rock failure. This behaviour is implemented in FLAC3D using a nonlinear, rotating, transversely isotropic, elastic constitutive model, and shown to differentiate high and low potential for mixed compression – extension strainbursting. Nonlinear elasticity demonstrates that the linear-elastic Kirsch equation overestimates tensile stress around a circular opening in an anisotropic stress field. Linear-elastic fracture propagation models overestimate Mode I crack propagation and underestimate shear stress at the tip of a sliding crack.

This model is further applied to continuum plasticity models for brittle rock. In the Damage Initiation Spalling Limit model (DISL), the spalling limit depends on the degree of nonlinearity. As nonlinearity increases, the confinement required to suppress spalling is reduced, reducing potential for brittle failure. The Cohesion Weakening, Friction Strengthening model (CWFS), is updated to include a normal stress dependent Mode II fracture toughness, confinement dependent plastic limits, and time dependence. Nonlinear elasticity also increases friction mobilized during shear failure. These effects improve the realistic prediction of localized brittle rock failure during notch formation around underground excavations.