Background In the Yushen mining area, coal mining is gradually shifting toward areas with thick soil layers. This leads to continuous water release from the saturated soils above the hydraulically conductive fracture zones, thereby significantly increasing the burden of mine water drainage. Focusing on the Yushuwan Coal Mine in northern Shaanxi, this study investigated the mechanisms behind mining-induced water release from effective impermeable soil layers and predicted mine water inflow.

Methods Using conventional hydrochemistry, isotopic hydrochemistry, and a high-throughput microbial sequencing method, this study conducted qualitative and quantitative identification of the primary water sources of a mine in the Yushuwan Coal Mine. Through water pressure tests, this study characterized the additional stress field in the overall subsidence zone that hosts effective impermeable soil layers during coal mining. By analyzing the diagram showing effective stress in soil layers, this study examined the characteristics of the additional effective stress in the effective impermeable soil layers after water drainage of bedrock aquifers. Accordingly, this study established a model to calculate consolidation-induced water release from effective impermeable soil layers under dual driving forces of mining and water drainage, thus elucidating the water release process. Additionally, this study proposed a method for predicting direct-indirect mine water inflow and validated its reliability by comparison with four statistical models for water inflow prediction.

Results and Conclusions  Conventional hydrochemistry, isotopic hydrochemistry, and the high-throughput microbial sequencing method determined that the loess layers in the Lishi and Baode formations accounted for 50.71%, 61.08%, and 57.00%, respectively and that bedrock aquifers in the Zhiluo Formation represented 49.29%, 38.92%, and 43.00%, respectively. Among these, the results of the isotopic hydrochemistry and the high-throughput microbial sequencing method proved more reliable. Forces driving the consolidation-induced water release from effective impermeable soil layers consisted of mining-induced additional stress in the overall subsidence zone and the additional effective stress resulting from water drainage of the underlying aquifers. The former was determined at 0.2 MPa, while the latter was affected by the thickness of the impermeable soil layers and the hydraulic head height. Based on one-dimensional consolidation theory, this study revealed that the consolidation-induced water release process can be divided into three stages: slow growth, stable growth, and growth decay. Durations for the consolidations of over 95% of loess in the Lishi Formation and over 95% of laterite in the Baode Formation in the study area were calculated at 329 days and 17833 days, respectively. By integrating the water inflow calculation formula in the big well method and the equation for consolidation-induced water release, this study proposed a comprehensive formula for predicting mine water inflow from direct and indirect water-filling aquifers in mines. It was predicted that the mine water inflow would reach 1215.59 m³/h after 3 years, which approached to the result obtained using the statistical prediction method (1229.40 m³/h). This study can serve as a reference for zonal water inflow prediction and water hazard prevention and control in water-preserved coal mining zones of the sand-soil-matrix type.