Meaningfulness of the Stable Range of Factor of Safety for Deep Pits – A Numerical Modelling Assessment

Abstract

Limit Equilibrium (LE) methods have been used as the main tool to assess rock mass failure mechanisms at inter-ramp and overall slope scales in open pit mines. Rock mass shear strength parameters are determined by reducing the intact strength using Geological Strength Index (GSI), Damage factor (D), and a material constant (mi) to account for discontinuity properties when using Hoek-Brown strength criterion. Rock mass shear strength parameters have been used as typical input parameters for LE methods to calculate Factor of Safety (FoS) as ratio of resisting to driving forces. The calculated FoS are compared with design acceptance criteria to evaluate the stability of the pit wall. FoS of 1.3-1.5 is typical industry accepted minimum value for overall slope scale mechanisms. LE methods and calculated FoS does not provide any information about the stress induced rock fracturing and complex internal deformations happening in the deep open pits. The distributed rock stress behind the highwall in deep pits can be high enough to initiate stress induced rock fracturing for low to medium strength rocks. In this research, numerical modelling simulation using FLAC was used to assess the stress induced fracturing states for three typical cross section with depth of 250 m, 400 m, and 600 m. The input parameters including GSI values were derived using a constant mi to achieve shear strength parameters that results in FoS ≈ 1.5 using LE methods. The GSI based formulations are then used to estimate the deformation modulus required for the numerical modellings. Then numerical modelling was used to assess the stress induced fracturing and stability of the selected cross sections.