Anomalous increases in piezometric levels in advance of longwall mining subsidence

The typical initial piezometric response to longwall undermining is a decline in head due to the opening of fractures and bedding planes during early subsidence. However, in studies over two active longwall mines in southern Illinois, temporary rises in head just before subsidence were observed in piezometers constructed in low-permeability units. Although the initial phase of subsidence is considered dilational, these head increases indicate compressional effects that raise pore-water pressures. Possible mechanisms are shear stresses at the leading edge of subsidence or transmission of stress related to dewatering of underlying permeable units.