Water inflow law of the first working face based on water pre-draining from roof

In order to establish water control technology system in Inner Mongolia-Shaanxi contiguous area, we carried out some researches(e.g. overburden failure law, hydrogeological conditions, prediction of water inflow and pre-drainage of roof water) in Nalinhe No.2 coal mine. The results showed that three methods(i.e. coring, drilling fluid leakage observation and borehole color TV detection) were used to explore water-conducting fracture zone(103.23 m), and the ratio of the height of the fractured zone to the mining height was 18.8. Water-conducting fracture zone could connect three aquifers, and the Zhiluo Formation bottom aquifer was the main aquifer threatening safety mining. Water inflow and pressure of borehole were 92.0-136.0 m³/h and 4.0-5.6 MPa respectively. There were obvious characteristics with large amount of inflow, high pressure and uneven distribution. The static storage capacity and dynamic supply capacity were calculated with dynamic and static reserve combination method, and the values were 2.596×10⁶ m³ and 417.6 m³/h respectively. Subsection method of roof water pre-draining was used to discharge static storage capacity. There was a positive correlation between water inflow in goaf and mining interval in the whole working face during mining process. With the periodic lag collapse of roof, the water-conducting fracture zone also developed periodically to the highest point. Water inflow in goaf increased stepwise too. The total inflow of pre-draining was 4.235×10⁶ m³, and water inflow of working face goaf was 5.313×10⁶ m³. The error was 4.2% between actual drainage(622.8 m³/h) and calculated drainage(596.9 m³/h) in the first coal mining face. Accurate calculation of the inflow and upfront roof water pre-draining were key steps for roof water safety of the first coal mining face, and the technology could be used in other coal mines of Inner Mongolia-Shaanxi contiguous area.