The Two Zones of Floor Failure and its Control via a ‘Dual Key Layer’ Approach

In the Jiaozuo mining area of the North China coal field, there are multiple water-rich aquifers close to the coal seam. To prevent water inrush accidents, the subfloor strata needs to be reinforced by grouting before mining. According to how the mechanics and hydrogeological properties of the key floor rock strata changed after grouting, the coal seam floor was divided into two zones: a ‘water conductive fracture zone’ and a ‘mechanically strong damaged zone’. A composite model of a key structurally stable water-control floor layer and a key infiltration-water damage-control floor layer was established after grouting of the uppermost (L8) subfloor aquifer. Microseismic (MS) technology was used to analyze the characteristics of the two zones and the water control capability of the two key layers in faulted and non-faulted regions of the experimental working face, which revealed that the grouted L8 limestone served as a key mechanical barrier layer. The change in the elastic modulus of the rock mass before and after grouting was measured by the borehole ultrasonic method, which showed that the elastic modulus of the rock mass increased by 40–852% after grouting. Based on FLAC3D numerical simulation and field measurements, grouting reduced the failure depth of the floor by 51%. Seven factors that influence the floor water control ‘dual key layers’ were proposed, which were quantified using the analytic hierarchy process (AHP) method. We found that fault activation and water-filled faults were the main factors affecting the water control capability of the dual key layer. The results of the research will contribute to the early warning, prevention, and evaluation of floor water inrushes in mines with similar hydrogeological conditions.