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

Abstract

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.

Stability Analysis of the Exploitation System with Room and Pillar by Analytical Methods

“…In order to characterize the physical state of the salt and its influence on solving the problem of stability and reliability of the geometric elements (pillars, slabs, etc.) of the salt exploitation methods, it is necessary to know the value of the main physical properties, namely: specific gravity, volumetric weight, natural humidity, compactness, and porosity [9,[12][13][14][15]. Importance for the field of geomechanics is the knowledge of the two aspects of the mechanical behavior of rocks, that is, rock strength and rock mass deformability.…”

“…In the case of the reopening of some mining fields within the salt panels and the exploitation in depth with square pillars, the establishment of the optimal parameters of the elements of the exploitation system requires the establishment of the loads that appear in the surrounding salt massif (the natural stress state); the qualitative-quantitative determination of the way of distribution of the secondary stress state in the pillars; evaluating the bearing capacity of the pillars; and establishing their geometric elements [27,[37][38][39]. The way of appearance, distribution, and manifestation of the secondary stress state, as well as its magnitude, is not only a characteristic of the salt massif because it is not dependent only on its quality but is a function of the "complete and inseparable interaction" of a complex of natural, technical, mining, and organizational factors [7,[11][12][13][14][15]22,23,31,36,39,40]. Studies related to the analytical assessment of the stability of the dry rock salt exploitation system with the increase in depth were carried out in the paper [12].…”