Study on the floor failure ‘two-zone’ characteristics and the ‘dual key layer’ water control theory in floor grouted mining working face

Abstract

Abstract In Jiaozuo mining area of North China coal field, there are multiple strong water-richness aquifers close to the coal seam. In order to prevent floor water inrush accident, it is need to be reinforced by floor grouting before mining. According to the changes of the mechanics and hydrogeological properties of the floor key rock strata after floor grouting, the coal seam floor is divided into ‘two zones’ of ‘water conduction fracture zone’ and ‘mechanical strength damage zone’. The composite water control model of ‘floor structure stability water control key layer’ and ‘floor damage infiltration water control key layer’ is established after grouting reinforcement of upper hard rock aquifer. Microseismic(MS) technology is used to analyse the characteristics of ‘two zones’ and the water control capability of ‘dual key layers’ in fault regions and no-fault regions respectively of experimental working face. It is revealed that the L8 limestone plays the role of mechanical barrier key layer after grouting. The change of elastic modulus of rock mass before and after grouting is measured by borehole ultrasonic method, which result shows that the elastic modulus of different rock mass increases by 40–852% respectively after grouting. By using the formula method, FLAC3D numerical simulation and field measurement, it is found that the failure depth of the floor after grouting is reduced by 51% compared with that without grouting. Seven influencing factors for floor water control ‘dual key layer’ are proposed, which are quantified by AHP (Analytic Hierarchy Process) method. A scheme of ‘dual key layer’ water control capability evaluation is formed by AHP method and influencing factors evaluation method. It is revealed that fault activation and fault with water-filling are the main factors that affect the failure of water control capability of ‘dual key layer’ and floor water inrush. The results of the research are consistent with the water inrush of the experimental working face, which will contribute to the early warning, prevention and evaluation of floor water inrush in mines with the same hydrogeological conditions.