The stability and potential building hazard arising from cavities cut into weak Triassic sandstone are a special feature of the urban environment in the city of Nottingham, UK. Numerical modelling analyses using FLAC (Itasca, 1995), have been applied to determine the behaviour of the rock arches above these old mine workings and man-made voids. The modelling has been calibrated against results of a full-scale destructive test of a cavity roof under applied load; initial strength parameters of the sandstone were derived from laboratory testing. Physical modelling of the cavities has provided a range of data that relates cavity dimensions to failure parameters; these results have been correlated to the full-scale situation by the computer modelling. These computer modelling analyses have been combined with the field observations and geotechnical testing. These provide guidelines for assessing the integrity of the sandstone that remains in place over the voids, and which may have to be loaded by the construction of new building foundations.
The stability problems and potential hazards posed by existing caves in the weak Triassic sandstone underlying Nottingham city centre, have been highlighted elsewhere, (Waltham, 1992, 1993, 1996: Roodbaraky et al., 1994). Previous studies have used plaster scale models and Finite Element models (FEM), in order to attempt to simulate failure mechanisms and ultimate failure loads. These have been validated by a single full-scale test, where an expendable cavity roof in central Nottingham was loaded to failure. From this testing it should be possible to model other cavities beneath the city centre with a view to providing guidelines applicable to future constructions in the vicinity. Although the scale models accurately simulate the failure mechanisms associated with the roof beam failure above cavities of varying geometry, more confidence in the modelled failure loads was necessary.