by Ken Cole and Ian Statham, Arup Geotechnics.
This paper was first pubslished in GE’s March 1992 issue.
In preparing the technical report volume ‘Effects of mines’ for the Review of Mining Instability in Great Britain (Arup Geotechnics 1990), it was noted that general subsidence, also known as trough or areal subsidence (Bruhn et al 1980, Waltham 1989) is regarded as much less frequent above partial extraction workings, than crownhole subsidences. Broad categories of mine collapse condition for partial extraction mines, defined by the state of the mine floor, roof and pillars are presented in Table l.
Isolated examples of general subsidence have been reported within the UK for partial extraction mines of various minerals. For example; general subsidence above coal mines at Bathgate in Scotland (Carter et al 1981), and two examples in south Wales have been noted (Statham et al 1986, Ove Arup 4 Partners, 1991).Ove Arup ttg Partners (1983a 8z b) also reported this type of failure in the West Midlands limestone mines and similar subsidences have been noted above salt mines. However, these reports form a small precentage of the total number of the reported collapses, where the dominant process is apparently crownhole formation.
This paper sets out to examine the mechanisms that can lead to general subsidence and concludes that this process may well have been widespread, and has largely passed unrecorded, as the surface effects are generally much less disruptive than crownholes. A methodology for assessing the potential for general subsidence above abandoned (and working) mines is proposed. The full risk assessment would take into account the consequences of failure.
At a meeting of the Midland Geotechnical Society at Birmingham University on 2 December 1991, Ken Cole described the collapse by crushing of overburden pillars as an additional mechanism to those conventionally invoked to account for general subsidence (punching, shearing, etc.). He referred to the process as one of ‘squatting’, in which the overburden ‘squats down’ver the mineral pillars as a result of failure of pillars in the overburden, where those pillars are the result of extensive roof falls. An essential feature is that the mineral pillars are stronger than the overburden material, and evidence that this is so for many coal seams was demonstrated.