Mechanism of collapse sinkholes induced by solution mining of salt formations and measures for prediction and prevention

Abstract

In salt mining, the salt caverns formed by solution mining may lead to collapse sinkhole disasters. Predicting and preventing this type of sinkholes is a real and urgent problem. In this paper, three collapse sinkholes at the Dongxing Salt Mine were taken as examples to investigate the collapse mechanism of overlying strata above salt caverns induced by solution mining. Geophysical exploration was firstly carried out, with results showing that the overlying strata dropped down layer by layer; the prediction measures concluded that whether the anhydrite roof is destroyed or not can be treated as a marker for the identification of a salt cavity anomaly. Then, a mechanical model for the anhydrite roof was established. According to the model, major factors that may affect the stability of the anhydrite roof were identified and qualitatively analyzed, including the pressure decrease of the brine, the strength decrease of the rock in the roof under the immersion of brine, and the increase of horizontal stress in the roof. Numerical simulation was carried out to further analyze the collapse process and the influencing factors. Some prevention technologies were drawn as follows: (1) Mining of salt formations should be prohibited within the impact scope of the weak structural zone; (2) As regards the protective layer, a certain thickness of salt formation above the salt cavity should not be mined to prevent the dissolution upward; (3) An oil pad should be adopted to reduce or even avoid the contact of brine and roof; (4) Appropriate brine pressure should be maintained to improve the stability of the salt cavity when it is abandoned. The above prediction and prevention measures are not only applicable for halite but also suitable for other minerals obtained by solution mining, such as natural soda, Glauber’s salt, and others.