Evaluation and Remediation of an Abandoned Gypsum Mine

The remediation measures taken for an abandoned gypsum mine in the Italian Alps are presented. The mine consists of four levels, excavated by the room-and-pillar method in an evaporitic formation. The two lower levels were flooded by water coming from adjacent strata after mining ended about 30 years ago. The dissolution process affecting the submerged pillars can jeopardize the long term stability of the entire mine. An overview of the problem is given, and experimental results of dissolution tests are shown. An analytical model developed to simulate the laboratory data is applied to determine the time to failure of the mine. It is demonstrated that the expected collapse time is relatively short. Remedial measures consisting of chamber filling with an appropriate mixture of cement, water, and silty clay are proposed. A theoretical analysis of their effects in delaying time to failure and minimizing its consequences is presented. The analysis allowed the optimal mechanical characteristics of the filling mixture to be determined. The results of a full-scale field test to check the validity of the remedial action are highlighted.