Self-healing effect of water-conducting fractures due to water-rock interactions in undermined rock strata and its mechanisms

In underground coal mining regions with damaged overlying strata, under the combined effects of long-term water-rock interactions (WRIs) and mining-induced stress, the permeability of water-conducting fractures (WCFs) decreases significantly due to the self-healing of fractures. Such self-healing helps to reduce the range of fractured water-conducting zone in the overlying strata and prevents the loss of groundwater. Understanding the self-healing mechanism is therefore of great significance for the scientific evaluation of mining damage to aquifers and their self-restoration capacity. It was found, according to the WRI experiments, that the self-healing mechanism involves the following three physical and chemical processes: (1) the sealing of fractures due to the swelling of hydrophilic clay minerals and subsequent blockage of the fracture space due to the argillisation and slaking of clay minerals; (2) the dissolution and corrosion of fracture surfaces due to the flow of groundwater, which leads to reduced surface roughness in the fractures, thereby allowing the fracture walls to seal more closely under the action of mining-induced stress; and (3) ion exchange between the anions and cations present in groundwater and in the rock minerals, including aluminosilicates or carbonates, which leads to the production of secondary minerals or crystalline precipitates that eventually fill or block the fractures. The long-term cumulative effect of the compaction and blockage of WCFs is that their permeability gradually decreases, thereby demonstrating the self-healing mechanism. The results can provide a theoretical reference for water preservation and ecological management of the water-deficient mining areas in northwest China.