| North China type coalfield in our country under the threat of bottom water disasters of coal resources reserves of 57 billion tons,and is on the north China type coal field of the shallow coal mining,deep or group of coal mining underwater inrush threat increasingly prominent,how to implement under the threat of confined water safety mining of coal resources is one of the major problems urgently to be solved in the coal industry in our country.Therefore,this paper takes the preact grouting control mechanism of confined fissure aquifer of coal seam floor as the research object,adopts laboratory test,numerical simulation,similarity simulation,theoretical derivation,statistical analysis,and other research methods and means.The damage and water inrush mechanism of load-unloading stress path of coal seam mining floor,diffusion law of grouting grout in the horizontal hole of the confined aquifer in karst fissure,fracture initiation mechanism of split grouting in floor limestone,control factors,and methods of advance grouting grout diffusion in karst fissure aquifer are studied,and the following main conclusions are obtained:(1)Based on the macroscopic numerical simulation calculation,microscopic laboratory test,and theoretical disturbance degree analysis,this paper reveals the characteristics of the state transformation of the principal stress of the floor,the hysteresis effect,and the abrupt change of the instability in the process of coal mining,and establishes the theoretical damage criterion of water inrush under the combined action of the principal stress transformation of the mining-unloading floor and water pressure.The maximum principal stress of floor rock stratum changes from horizontal direction to vertical direction after mining loading,and from vertical direction to horizontal direction after unloading.In the mining process,the curve of the difference between the maximum and minimum principal stresses shows the variation characteristics from "double peak" to "single peak" and then to "flat roof".The fluctuation amplitude of the difference along the coal seam decreases with the increase of depth,and the difference increases near the coal wall and below the goaf;In the unloading test,the release of circumferential constraint directly promotes the development of sample expansion.There is an obvious damage accumulation process after pressure relief,resulting in the instability lag effect of the sample.The stress drop occurs when the sample is unstable,and the drop speed decreases with the increase of pore pressure.It reveals the lag effect and catastrophe characteristics of damage and instability under the loading-unloading stress path of the mining floor from the mesoscopic view;The mining stress disturbance intensity index(k)of floor rock stratum based on the product of mining stress disturbance coefficient(k3d),fracture generation discrimination index(kw)underwater pressure and rock mass instability discrimination index(ki)after mining is established theoretically,and the mining floor water inrush criterion of k>1 is proposed.(2)Based on the theoretical model construction and analysis of radial diffusion of grout in confined inclined fractures,the variation law of grout diffusion of grouting in horizontal holes in confined inclined fractures is revealed,which is symmetrical along the center perpendicular line from the left to right,and asymmetric above and below the borehole.In the pressure bearing inclined crack,the self-weight of slurry plays the role of resistance and power in the crack above and below the horizontal grouting hole respectively,and the hydrostatic pressure plays the role of resistance;Considering the factors such as slurry self-weight,fracture inclination,hydrostatic pressure,slurry viscosity and radial diffusion,the stress model of grouting slurry diffusion in horizontal hole of karst fracture confined aquifer is constructed,and the trace control equations of Newtonian fluid and viscous time-varying fluid slurry radial diffusion in horizontal hole of confined inclined fracture are established;Under different grouting pressure,crack inclination(except horizontal)and opening,and slurry water-cement ratio,the shape of grouting slurry diffusion trace in horizontal hole of pressure bearing inclined crack is an approximate elliptical shape,which is symmetrical left and right along the central vertical line and asymmetric along the upper and lower cracks of horizontal hole,and the central position of the upper crack diffusion trace is slightly concave.At the same time,the maximum and minimum values of slurry diffusion distance are at the radial angle of 0° and 180° respectively.The range of slurry diffusion trace increases with the increase of grouting pressure,crack opening,and slurry water-cement ratio and the increased range of lower cracks is greater than that of upper cracks.The range of slurry diffusion trace increases and decreases with the increase of fracture inclination in the lower and upper fractures,respectively.(3)Based on particle element numerical simulation of split grouting and physical model test of similar materials under true triaxial stress,the initiation mechanism of prefabricated cracks and weak surface split grouting cracks under different influencing factors was systematically revealed.Under the conditions of different water-cement ratio,grout,in-situ stress level,weak surface and crack inclination and opening,the variation law of fracturing grouting initiation pressure:the fracturing pressure of prefabricated weak surface and crack decreases with the increase of grout water-cement ratio,the difference between the maximum and minimum principal stress,weak surface and crack opening,and increases with the increase of the angle between the weak surface and crack and maximum principal stress;The initiation pressure under the condition of the weak surface decreases with the decrease of the strength ratio of weak surface to the matrix.When the strength of the weak surface is high,the initiation pressure is greater than that under the condition of fracture.Under the same working conditions,the secondary initiation pressure is greater than the primary initiation pressure,and the secondary initiation pressure decreases with the decrease of the minimum principal stress,but the reduction range of the second initiation pressure is higher than that of the primary initiation pressure;Variation law of crack extension of splitting grouting:when the grouting hole is in the middle or both ends of the weak surface,the crack starts at the place with low strength of the weak surface around the drilling hole,and then extends along the direction of maximum principal stress.When the grouting hole is in the middle of the crack,the crack starts at the tip of the prefabricated crack and then extends along the direction of the maximum principal stress.The crack initiation crack propagation path is based on the weak surface,and the crack is asymmetric on the left and right.(4)Based on microscopic CT scans，laboratory tests,statistical analysis,and numerical calculation methods of slurry water-cement ratio and end material selection,grouting pressure,horizontal hole spacing,and layout position,pressure holding time,such as study,reveals the coal floor karst fissure aquifer advanced grout diffusion controlling factors,grout diffusion control method is proposed.The control factors and method system of grouting slurry diffusion in the leading area of the karst fissure aquifer in the coal seam floor are established.It includes the macroscopic and mesoscopic determination method of grouting material particle size selection based on the combination of macroscopic and vertical permeability development characteristics of limestone and mesoscopic void quantitative parameters.It includes the determination method of cement-based slurry strength based on the Cracking Criterion under the action of confined water on the floor is proposed.It includes the determination method of grouting end pressure,pressure stabilizing time,and drilling spacing based on the factors such as the slurry diffusion distance of inclined single crack of the horizontal hole under different conditions,the rated working capacity of grouting equipment,and the rock mass cracking pressure.It includes the layout principle of the drilling direction based on the horizontal drilling track perpendicular to the direction of the maximum principal stress and parallels to the direction of the minimum principal stress.It includes the slurry diffusion control method based on adjusting and controlling the water-cement ratio of slurry,grouting pressure,and grouting time to meet the standards of grouting end pressure and pressure stabilization time. |
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