Stability evaluation of room-and-pillar rock salt mines by using a flat jack technique - A case study

Room-and-pillar mining is commonly employed for the extraction of rock salt in underground mines. Pillar stress is a major concern in these mines as it is directly related to stability and mineral recovery. In this study a flat jack method was used to measure pillar stresses in three underground rock salt mines in Pakistan. The field work included determination of in-situ stress, in-situ elastic modulus, recording of field variables (pillar length and width, height and width of opening, opening width to height ratio, extraction ratio, and overburden height) and collection of salt block samples. The geomechanical properties of rock salt (uniaxial compressive strength, Young's modulus, Brazilian tensile strength, and density) were also determined to estimate overburden stress, pillar strength, and factor of safety (both estimated and actual). It was found that the measured pillar stresses are proportional to the overburden stress values, with their magnitude ranging from 6.05 MPa to 11.97 Mpa, and the pillars were found to be stable. Regression analysis was performed to develop statistical models for in-situ stress and in-situ elastic modulus. Finally a quick guideline chart was developed to determine the suitable length of pillar for a given span and required level of safety. Keywords: flat jack, room-and-pillar mining, in-situ stress, factor of safety, regression analysis.