107 / 2021-09-30 00:02:04

Quantifying the Influence of Variations in Rock Mass Properties on Stope Stability

Rock mechanics,3D numerical simulation analysis,Stope,failure probability and consequence

Variations in rock mass properties are a common natural occurrence due to the inherent heterogeneity found in geomaterials. It is not uncommon for a geologic formation to pass through multiple structural zones and be logged across two or three rock mass classes. Laboratory results also indicate a range of values for parameters such as the unconfined compressive strength, Young's Modulus of Elasticity, and Poisson's Ratio. Combining these with varying rock mass classes commonly results in a wide distribution of the properties for a geologic unit. It is a standard practice in the mining industry to conduct numerical modelling studies with the average values of rock mass properties being used as input parameters. This approach does not account for the most likely values that would best describe the rock mass, nor can it assess situations where minimum and maximum conditions persist for these properties. Alternatively, a simple parametric study would merely examine the relative influence of different factors on a final stability criterion and provide limited insight into operational safety. Numerical models are usually used for assessing stope sequence alternatives and variations in rock mass properties examined with a classical parametric study may not be able to provide the required insight for these complex scenarios.



In this study, a steeply dipping orebody typically found in the Canadian Shield is modelled with the finite difference code FLAC3D. Laboratory results and core log data obtained from a case study mine are used to determine the input rock mass properties. These are analyzed and categorized into most likely, maximum, and minimum values for the various geologic units present. Two stope sequences using rib pillars – 1-4-7 and 1-5-9 – are simulated with the most likely rock mass properties. Tensile strength and brittle shear ratio (BSR) criteria are used with volumetric analysis to quantitatively determine the more stable approach that will provide increased operational safety. This sequence is then run with variations in the footwall, hanging wall, and orebody properties and their impact on the volume of unstable rock mass is assessed.



It is shown that for operational scenarios in underground mining, simplified parametric studies are unable to render the full range of stability potential for engineering design and safety purposes. This is primarily due to complex interrelationships between rock and rock mass properties, in-situ and induced stresses, and stope sequence alternatives. Volumetric analysis with different stability criteria can be used to assess the safer extraction option, as well as for indicating the impact of variations in rock mass properties on the chosen stope sequence.