Study On Coal Seam Floor Failure Rule Under Mining Dynamic Loading Effect And Control Technique Of Water-inrush

Since the shallow coal resources have been exhausted gradually in China, more and more coal mines begin to exploit deep resource. According to the field test, the lagging deepening of floor failure depth induced by the dynamic load of periodically caving roof threatens the deep mining badly under the condition of thin aquiclude, high water pressure and heavy dynamic load disturbance. Therefore, the damage and failure mechanism of floor rock under mining- induced dynamic load and the water inrush prevention technology were analyzed systematically through field test, theoretical analysis, physical analogous simulation, numerical simulation and the method of water inrush hazards prevention. Some innovative achievements were made such as following:(1) The opinion of mining-induced failure depth lagging coal wall secondary deepening rule was firstly detected and putted forward. Through water injection test, floor rock strain detection and microseismic monitoring to investigate the floor failure law, it was found that the mining-induced crack of floor appeared secondary lagging deepening behind coal wall 15 m to 20 m. Moreover, the damage evolution and failure mechanism, mechanical response characteristics, evolution of water inrush and the prevention and control method of mining-induced crack for floor strata were investigated. The result shows that the floor strata rupture and the original defects(such as crack, fault and collapse column) activate and propagate under the dynamic load of periodically caving roof, accounting for the secondary lagging deepening of floor.(2) The theoretical analysis was firstly used to study the damage evolution and failure mechanism of goaf floor under the dynamic loads. We established the response expression of the concentrated and uniform types of stress state of the floor strata and proposed the damage evolution model of the floor with consideration of a dynamic strain rate. Combining the advantages of Bingham and Generalized boydin models, a new damage constitutive B-G model and related the damage evolution model is proposed. Combining uniform strength theory of double shear and B-G damage constitutive mode, a damage yield criterion associated with strength theory of double shear is established for the rock under mining-induced dynamic load. In addition, a failure discriminant for the floor under mining-induced dynamic load is established.(3) The view that the mining-induced floor strata go through four stages was firstly put forward:(1) stress concentration before mining;(2) dilatation and decompression after mining;(3) mining-induced dynamic load impact;(4) goaf filled and balanced. Combined with field test and theoretical analysis, the displacement and stress characteristics of mining- induced floor strata were investigated to demonstrate the four stages through physical analogous simulation. And it shows that the initiation, propagation and deepening of floor strata crack mainly occur in the transition from stage(1) to stage(2) and from stage(2) to stage(3).(4) The lagging failure law of the weak parts of floor under mining-induced dynamic load was investigated thoroughly. The numerical model was built to analyze the forming process of water-conducting channels for floor containing cracks under dynamic load from stress, displacement, plastic zone, fracture seepage velocity, height of pore water pressure, dynamic load magnitude and the water pressure of aquifer. And the failure and water inrush mechanism of floor containing defects under mining-induced dynamic load was figured out.(5) The technology to prevent and control water inrush induced by floor strata lagging failure was proposed. Based on the analysis of water inrush mechanism, considering the characteristic that the floor containing defects damaged more seriously under dynamic load, we proposed the method of grouting reinforcement of floor strata to build the natural-artif icial water-resisting key strata. By geophysical prospecting, drilling, water injection test, rock permeability r esistance test, rock permeability test, rock microstructure test and field test, it proved that grouting reinforcement plays a key role in controlling the development of water-conducting zone and the lagging deepening for floor, and improving the water inrush resistance of floor.This research enriches the existing mining- induced floor failure theory, laiding the foundation of theory and practice to prevent the lagging water inrush of goaf floor and to mine the deep coal seam above aquifer.