Rock-Mass Movement Monitoring System in Historical Salt Mines, Using the Example of the Bochnia Salt Mine

Abstract

The old workings may still generate mining menace hazard around the Polish historic salt mines of Wieliczka and Bochnia, despite the fact that adequate protection support have been provided along the underground tourist routes. The water and collapse hazards dominate in underground salt mines. Water hazard is manifested by leaks, excessive leaching, or water disasters which may lead to catastrophic flooding, while collapse hazard is marked by cracking and rock erosion. Such processes caused ceiling collapses in previous centuries, followed by sinkholes above shallow workings. Our analysis indicated that salt-mine collapse hazard monitoring is usually reduced to the observation of the rock-mass movement on the scale of both mining area and locally. Mining area movements are observed by taking measurements of the vertical displacements of land-surface and mine levels. Local movements are observed by several methods: measurement of workings’ convergence, shelf and pillar deformation, and rock-layer deformation close to walls and ceilings. The frequency of measurements depends on the assumed accuracy. This paper describes the rock-mass monitoring methods applied in the Polish underground salt mines, with presentation of specific measurement results. Our analysis demonstrated that the monitoring systems used in present-day mining practice are rather reduced to rock-mass movement recording. However, such records cannot be the grounds for determination of the collapse hazard status. First of all, the measurement results should be used for a quantitative description of the rock-mass movement caused by human activity and obtaining the values of the parameters for constitutive laws applied in geomechanical models. Only the knowledge of the phenomena which occur in the rock mass will allow us to define the indicators that are usable to determine the degree of hazard and estimate the threshold values that would suggest the necessity to start implementing proper countermeasures