Estimation of confined peak strength for highly interlocked jointed rockmasses

Abstract

The determination of rockmass strength for mining has become critically important in recent years due to the increase in the number of projects at depths exceeding 1500 m. The commonly used empirical approaches for the estimation of rockmass strength are primarily based on experiences at shallow depths (< 1500 m) and from observations of rockmass behaviours at low confinement (e.g., tunnel wall failure). Therefore, the application of these techniques for estimating the strength of rockmasses when highly interlocked and confined (e.g., pillar cores) is hypothesized to be flawed. The goal of this research is to develop reliable means of estimating the confined strength of highly interlocked jointed rockmasses. A two-dimensional code based on the Distinct Element Method (DEM) and its embedded Grainbased Model (GBM) is used to simulate the behaviour of a highly interlocked jointed rockmass to better understand its Strength Degradation (SD) from intact rock with increasing confinement. The GBM is first calibrated to the laboratory response of intact and granulated marble. The term "granulated" refers to a heat-treated marble where the cohesion at grain boundaries has been destroyed. The granulated marble represents an analogue for a highly interlocked jointed rockmass. The calibrated GBMs are then used to simulate micro-defected and defected rocks and jointed rockmasses. The results of triaxial test simulations on the calibrated synthetic rockmass specimens are used to develop two semi-empirical approaches. In the first approach, called the SD approach, equations that relate the strength degradation of a jointed rockmass from intact rock to the confinement are developed. The second approach is based on adjusting the strength parameters of the Hoek-Brown failure criterion to extend its applicability to highly interlocked jointed rockmassess. It is demonstrated that these two approaches can be used to estimate the confined strength of such rockmasses in a situation where the unconfined and confined strengths of the intact rock and the unconfined strength of the rockmass are known. The findings of this research provides the foundation for a better characterization of the strength for highly interlocked jointed rockmasses, and increases our understanding of the influence of confinement on rockmass strength.