Underground potash mine design based on rock mechanics principles and measurements

Abstract

The design criteria for an underground potash mine are established on the basis of rock mechanics principles and measurements taken at the Cleveland Potash Limited Boulby mine in North Yorkshire, thgland. The requirements of the mine design are that a maximum and efficient extraction ratio is achieved, stable and safe working conditions exist within the mines and no damaging subsidence occurs on the surface. In particular, the workings at Boulby are at a depth of 1100 metres below the surface and are overlain by water bearing Bunter sandstone strata, 120 metres above the potash horizon. Consequently, the major problem of the mine design concerns minimizing the subsidence-induced strains at the base of the Bunter sandstone in order to prevent a possible water inrush into the mine. The various design parameters to meet these requirements are established on the basis of theoretical, laboratory and tin-situs analyses. A study of Canadian potash mining practice is included to provide valuable design information. The theoretical work consists of both face element and finite element numerical techniques. These are initially elastic solutions to provide comparisons of initial stress conditions around different mine panel geometries. A face element technique is developed to analyse mining panels where the length is considerably greater than the panel width. Large scale underground instrumentation provides detailed information on stress conditions, time-dependent deformations around mine roadways and strain distributions within pillars. This information, together with surface subsidence measurements and laboratory determined rheological properties, is used to provide more realistic boundary conditions for the numerical modelling of the mining layouts. The combination of the theoretical analyses and the measured data provide a set of design criteria for an optimum mining layout which satisfies the above requirements.