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Characterization and analysis of discontinuous subsidence associated with block cave mining using advanced numerical modelling and insar deformation monitoring Woo, Kyuseok

Abstract

While block caving presents an economic means to develop lower grade ore deposits, it often leads to significant ground deformations threatening the safety of overlying mine infrastructure. For guidance on relationships between caving depth and surface subsidence, a comprehensive cave mining database was developed and the database clearly shows caving-induced surface deformations tend to be discontinuous and asymmetric due to large movements around the cave being controlled by geologic structures, rock mass heterogeneity and topographic effects. Also shown is that as undercut depth increases, the magnitude and extent of the caved zone on surface decreases. Numerical modelling conducted in a benchmark study testing several different numerical methods (finite-element, distinct-element, FEM-DEM with brittle fracture and 3-D finite-difference) indicates this is only the case for macro deformations and the lateral extent of smaller strain deformations increases as a function of undercut depth, which indicates caution should be taken against relying on existing empirical design charts for estimates of caving-induced subsidence where small strain subsidence is of concern, as the empirical data does not properly extrapolate beyond the macro deformations. In addition,sophisticated 3-D numerical modelling was investigated as a means of predicting the extent and magnitudes of caving-induced surface subsidence. Results from a back analysis of the cave-pit interactions at the Palabora mine were used to constrain the rock mass properties and far-field in-situ stresses derived from field characterization data. The “best fit” set of input properties obtained was then used for forward modelling. Further calibration was performed using high-resolution InSAR monitoring data. The close fit achieved between the predictive 3-D numerical model and InSAR monitoring data demonstrates the significant value of InSAR calibrated 3-D numerical models.Collectively, the results of this research help to further the characterization, assessment and understanding of block-caving subsidence, by addressing existing limitations in the use of empirical and numerical subsidence analysis methods. The limitations and uncertainty arising from mine site data are described, specifically the representation of mine geology, rock mass properties, in-situ stresses and cave propagation, together with means to constrain these inputs and calibrate sophisticated 3-D numerical models through back analysis and integration with InSAR data.

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