Evaluation of fully-coupled strata deformation and groundwater flow in response to longwall mining

A finite element poroelastic model is presented to evaluate fully-coupled strata deformation and groundwater flow resulting from underground longwall mining in variably saturated fractured geologic media through an equivalent porous elastic continuum representation. The explicit coupling between the groundwater flow field and the solid skeleton deformation field is achieved by simultaneously solving the governing equations for groundwater flow and solid skeleton deformation. The implicit coupling between these two fields is also accomplished by considering the effects of both unsaturated water flow and solid skeleton deformation on the hydraulic properties such as water content and hydraulic conductivity. In addition to the implicit coupling caused by unsaturated water flow, the other implicit coupling caused by solid skeleton deformation is also important because it may produce hydraulic heterogeneity and anisotropy in a geologic medium affecting the groundwater flow field even when the system is in a fully saturated condition. A physically based constitutive model is proposed to account for the deformation (strain)-dependencies of porosity and saturated hydraulic conductivity of the fractured media. To evaluate the impacts of longwall mining and to understand their hydrogeomechanical nature, the numerical model is applied to a well documented case study with satisfactory results. Although the numerical simulation results presented in this paper will not apply to all underground longwall mines because of differences in site-specific hydrogeological and geomechanical settings, the methodology described herein may find some useful applications in many longwall mining projects.