Water-inrush mechanism of the thick composite aquifer in the roof of the Hujirt mining area

During the coal mining process in the Hujirt mining area of the Ordos Basin,significant impacts are observed from the aquifers of the Jurassic Zhiluo Formation and the Cretaceous Zhidan Group,with average thicknesses of 150 m and 350 m,respectively. Situated between these aquifers is a relatively impermeable layer from the Anding Formation,averaging about 94 m in thickness. These layers together constitute a thick composite confined aquifer system atop the mining area. This study, taking the Shilawusu Coal Mine as the research object,aims to clarify the previously ambiguous water-inrush mechanism from the thick composite aquifer in the roof of the coal seam mining in the Hugirt mining area. Analyzing the water yield in the goaf of the working face and the response characteristics of the water level of the thick composite aquifer on the roof,combined with the development law of the water conducted zone,three modes of roof water-inrush and filling were identified:lateral replenishment from the lower aquifer,comprehensive replenishment from the composite aquifer,and lateral replenishment mainly from the upper aquifer. This delineates a “ stepwise progression” in the mechanism of water-inrush at the identified working face. Through numerical simulation,the contribution of each aquifer to the underground water-inrush across five stages of working face retreat was calculated. The findings indicate that in the first stage,the Zhiluo Formation aquifer is the primary contributor to water-inrush,accounting for about 87. 3% of the total. In the second stage,the contribution of the Zhidan Group aquifer surpasses 70. 0%. In the third stage,replenishment from the Zhidan Group aquifer decreases gradually,while the contribution of the Zhiluo Formation aquifer increases slowly to 34. 0%. The fourth stage witnesses a sharp increase in groundwater replenishment from the Zhidan Group,constituting 75. 0% of the total water-inrush volume. In the final stage, groundwater replenishment from the Zhidan Group diminishes gradually,with the contribution from the Zhiluo Formation increasing slightly to 29. 3%. The simulation results align closely with empirical observations,affirming the “stepwise progression” water inrush mechanism.