Three-dimensional effects of hydraulic conductivity enhancement and desaturation around mined panels

Abstract

Changes in the three-dimensional hydraulic conductivity field that accompany undermining by a longwall panel are evaluated to define the potential for changes in flow patterns and desaturation. Relations linking changes in conductivity with the mining-induced strain field that develops around the advancing face are defined through the assumption of an equivalent porous medium with the strain partition between fractures and matrix apportioned relative to respective deformation constants. Model results are validated against behavior recorded at a well instrumented field site, replicating the complex steady flow pattern that develops around an advancing longwall face. Significant hydraulic conductivity increases are restricted to strata at shallow depth, ahead of the advancing face, and to deep zones behind the face, particularly in the caving zone and abutment shear zones. These very large increases in conductivity at depth, that include the out-of-section conductivity enhancement that develops along the panel rib, exercise primary control on induced water level changes and the potential for saturation. Zones of desaturation are readily defined, with these enabling locations of well vulnerability (to water loss) to be identified. These zones of vulnerability are directly above the panel, with their extent potentially modulated by the presence and the location of flatlying layers of low conductivity. The complex response recorded in the near-face region mandates use of three-dimensional models to adequately represent observed behavior.