Occurrence of mining-induced open fissures and shear walls in the Permian limestones of County Durham

Subsidence-induced fissures and vertical relative movements in the Permian limestones of County Durham, England, are locally familiar, but poorly documented phenomena. They result from block rotation within the limestone rock mass as stress relief opens previously closed joints and faults in the relatively strong limestone following subsidence in the underlying weak, unconformable Coal Measures strata during and after mining operations. This creates patterns of surface subsidence that are not well described in the subsidence literature and which represent a potential hazard to leisure activities, construction, quarrying and agriculture.

Four main subsidence features dominate the surface topography, each with its own associated unique hazard. These surface features are: large, deep open fissures; small open fissures; shear cliff walls; and fault breccia pipe collapse. All new developments on Permian strata should be the subject of in-depth mining reports carried out with reference to these features. In practice it is possible to predict where all these features may occur. Large, deep open fissures and, where traceable, small open fissures should be dug out, and where development work is being undertaken on ground that may be susceptible to small open fissures impermeable barriers should be considered in the foundation design to mitigate possible gas migration.