Analysis of viscoelastic behaviour of rock salt using hydraulic cylinder test

Abstract

One of the main challenges in the design of a salt mine using the well-known room and pillar method is reliable prediction of the viscoelastic behaviour of rock salt. Part of the problem lies in the fact that the test specimens in the laboratory do not behave in the same way as observed at the mine under in situ conditions. Moreover, in situ strain measurements by means of convergence technique or extensometers in a specific mine cannot easily be generalised and used in other locations, due to their different depth and extraction ratios. In this paper, a new in situ testing technique, based on the hydraulic cylinder test (HCT), is proposed to analyse viscoelastic behaviour of salt rock for salt mine design. This technique enables performing in situ tests of deformation over time under in situ conditions of confinement and temperature of the mine. This test can be considered as an intermediate scale test that bridges the gap between creep data obtained from strain measurements at the mine site by using convergence technique and strain gauges, and the creep data obtained from laboratory tests. To demonstrate the suitability of the proposed technique, a series of tests were completed in a room-and-pillar halite mine located in the tertiary basin of the River Ebro in Spain. The pillars have a height of 6 m and a square section with sides of 20 m. Displacement-time curves obtained in HCT tests, varying the speed of load application, get a good adjustment with regard to the curves of deformation characteristics of these materials.