Slope stability analysis is a branch of ground engineering science where there are a number of significant uncertainties. Although probabilistic slope stability analysis is an option in most commercial software, the use of this method is not common in practice. Apart from the ability of the probabilistic method to assess the impact of uncertainties on slope stability, it can also be used as a tool to optimise the geotechnical site investigation program. The first-order second-moment approximation of the Taylor series method is one of the probabilistic slope stability methods that determines the relative contribution of uncertainty projected by each component random variable. For a slope with a sequence of different geological units, each unit can be modelled as having several random variables such as cohesion and friction angle. A geological unit whose random variables are responsible for the greatest contribution to the uncertainty in the Factor of Safety (FS) will be the most controlling unit. This characteristic can be used to design geotechnical site investigation programs in order to minimise the uncertainties in these controlling units, which will enhance the Reliability Index of the computed FS.
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.
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