Longwall mining-induced fault reactivation and delayed subsidence ground movement in British coalfields

Abstract

Faults located in areas undergoing mining subsidence during the longwall extraction of coal seams may undergo reactivation. This has been observed and documented throughout the UK (and in other major coalfields around the world) over the past 150 years. Subsidence-induced fault reactivation may cause moderate to severe damage to foundations, houses, buildings, structures and underground services, as well as damage to agricultural land through disruption of drainage and alteration of the gradient. Monitoring of faults, as they are affected by undermining, has resulted in a better understanding of fault reactivation mechanisms and of the various styles of fault reactivation, in different geological and mining settings. The duration of fault reaction is difficult to determine because of the lack of observational data. However, trough subsidence following longwall extraction of coal is rapid, often being completed within weeks to months. This is commonly followed, shortly afterwards, by a period of delayed subsidence known as residual subsidence, which in the British Coal Measures rarely accounts for more than 10% of the total subsidence. In many circumstances, where faults are not present, residual subsidence is complete within 4 months, although several cases have been recorded where subsidence effects were still being observed more than 2 years after mining had finished. Generally, it is accepted that fault reactivation sometimes may extend over the period of residual subsidence. In parts of the abandoned or partially active coalfields in the UK, relatively smaller ground movements have been observed in the vicinity of fault outcrops many years after mining has ceased. The reasons for this are not fully understood. None the less, prolonged periods of fault reactivation may have an important effect on land use and construction. The objectives of this paper are to consider fault reactivation and, in particular, to document examples of post-mining ground movements around fault outcrops and to discuss possible causal mechanisms. Features associated with these movements include increases in elevation of the ground surface and deformation (e.g. subsidence, grabens, scarps or steps, fissures, compression and tension) of the ground surface in the vicinity of faults. These features, in turn, may be associated with groundwater or mine water rebound.