Hydro-mechanical coupling in the shallow crust – Insight from groundwater level and satellite radar imagery in a mining area

Abstract

The hydro-mechanical coupling in the shallow crust influences both geological processes such as earthquakes and landslides and anthropogenic processes such as induced seismicity and mining-produced subsidence. Yet there have been few direct field observation to illustrate how the hydraulic and the mechanical processes are coupled. In this study we use continuous water level data from wells and ground deformation data over a large area (>71 km2) of active coal mining in NW China to examine hydro-mechanical coupling in the shallow crust. We present detailed analysis of the tidal response of groundwater and surface deformation from radar remote sensing in an actively and intensely mined area, and we show that the phase shift of the tidal response of water level may respond significantly to mining disturbances when the excavation workface was many hundred meters to >1 km away, whereas significant ground deformation and water-level drops occurred only when the workface came much closer to the wells. Water level and tidal response recovered after the workface moved away but the subsidence is permanent. We suggest that the permeability of the mined crust may be controlled by narrow conductive fractures that open and close in response to deviatoric stresses, while large ground deformation and water-level drops may be controlled by larger fractures and faulting. One possibility is the release of the deviatoric stresses in the wall rocks by faulting, may have allowed the conductive fractures to close and the water level and its tidal response to recover after the workface moved away. Another possible mechanism is the mobilization of precipitates in clogged fractures by dynamic waves associated with mining-induced seismicity, which may change the permeability of fractures. More studies are warranted for a better understanding of the hydro-mechanical coupling during longwall mining.