Abstract

The paper describes the principal geomechanical approaches to ensuring stability and integrity in mining salt deposits. The various dimensioning methods usually applied are subjected to a comparative analysis. Geomechanical discontinuum models are identified as essential physical models for examining how the collapse of working fields in potash mining areas can occur. A visco-elasto-plastic material model with strain softening, dilatancy and creep is used to describe the time-dependent softening behaviour of the salt pillars. The pillar stability critically depends on the shear conditions of the bedding planes to the overlying and underlying beds. Therefore, a shear model is introduced, describing interface properties, i.e. velocity-dependent adhesive friction with shear displacement-dependent softening for the bedding planes and discontinuities in the contact zone of the pillars with the surrounding salt rocks. As an outcome, the fundamental mechanical and hydraulic conditions that lead to an integrity loss of saliferous barriers are derived. Several examples of worldwide events of flooded salt mines are back-analysed with coupled hydro-mechanical calculations, demonstrating the prominent role of fluid-pressure-driven generation of hydraulic flow paths as a failure mechanism of saliferous barriers.

1. INTRODUCTION

For a long time, the dimensioning of underground openings in salt rocks was primarily based on mining experience. Only in the last century, analytical and numerical calculation methods of geomechanics have been increasingly used. This was not least due to some catastrophic collapses of mining fields (rock bursts) with a strong mining-induced energy release [1], and the loss of potash and rock salt mines by flooding. Both practical experience and geomechanical calculations are essential for an economical and sustainable salt extraction at high recovery rates and complement each other.

The fundamental requirements of safe dimensioning for potash or rock salt mining are the guarantee of stability of the mining system integrity and protection of the hydraulic protection layers or geological barriers.

For the collapse of mining fields insufficient pillar dimensioning and the brittle fracture behaviour of the mined rock salt played a particularly crucial role [2]. The tendency of brittle fracture decreases from carnallitite, hard salt, trona, sylvinite to rock salt. Therefore, rock bursts occurred primarily in potash mines where carnallitite was mined. In sylvinite and rock salt mines few rock bursts are known worldwide, only if extremely high recovery rate and, accordingly, very slender pillars were realised. The analysis of in situ collapses provides a basis to check dimensioning approaches and to derive empirical relationships for the necessary ratio of pillar width to pillar height (slenderness ratio), which is required ensure the viability and stability of pillars in salt rocks.

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