| With shallow coal resources being fully exploited,coal development will continue to move deeper into the earth.Coal resources mining of kilometers depth will gradually become the new normal of resource development.However,there are more and more dynamic disturbances caused by large-scale mining in deep coal such as fierce roof weighting and high-energy mine earthquake,and the surrounding rock affected by high intensity stress concentration will form the strong stress unloading effect after coal seam mining,the strong disturbance characteristics induced by confining aquifer pressure increases along with the increasing of geostress levels have become more and more prominent.Especially under the condition of deep complicated hydrogeological conditions,due to the in-situ stress and confining aquifer pressure increasing,the effective stress of the deep rock mass structure has increased,which drives the primary cracks and joints to be damaged,extended in the coal rock body,fractured or cut-through.It can easily lead to fierce weighting,floor water inrush of high confining aquifer pressure and other high-energy,mass frequent engineering disasters.Therefore,in view of the major problems caused by high stress,great overburden,confining aquifer pressure,and strong disturbance in deep coal mining,it can provide strong support for obtaining deep coal resources in China and has great significance in guiding deep coal mining safety production and preventing major engineering accidents by in-depth research on the fracture mechanism and its model of mining-induced floor rock mass under the deep in-situ stress environment and engineering disturbances and providing control technologies of safe,high-efficiency,and low-ecological damage mining.In this dissertation,many methods such as theoretical analysis,numerical simulation,laboratory tests and field measurements were applied.Taking the strong disturbance action in deep mining process as the main line,floor disturbance failure caused by the instability of main roof in deep coal mining was systematically analyzed.Based on the elastic-plastic mechanics and unloading rock mechanics theories,it gave the detailed exploration of failure patterns of floor rock mass,crack extension and fracture mechanism,the strong disturbance risk of unloading damage and the engineering scale effect.Main conclusions and innovative results are as follows:?1?Failure behavior and failure patterns of floor rock mass under strong disturbance in deep coal mining?1?Based on underground pressure characteristics of floor rock mass failure behavior under strong disturbance in deep coal mining,it gives that fierce weighting of main roof lead to the non-uniform failure behavior of floor rock mass.Meanwhile,fracture behavior of floor rock mass in deep coal mining was obtained combining with the on-site microseism monitoring.During the dynamic disturbance,the roof and floor of the goaf formed an unloading-dominant fracture zone mainly appearing as the pressure arch axis;The energy of majority of microseismic events in the roof and floor at the side of entitative coal seam was lower than 5×103J,but it was mainly high-energy microseism events in the goaf side which was much higher than 5×103J;the total number of high-energy microseism events was a positive correlation between roof and floor.?2?Combining with the dynamic load disturbance action of the main roof structural instability,floor failure patterns in deep coal mining were divided into loading failure mode and unloading rebound failure mode.Based on the plastic hinge theory,loading failure models of the main roof initial instability,the cantilever beam and masonry beam structure were established respectively according to the forms of the main roof structural instability.Loading failure action of main roof structure instability on the floor of coal rib and touching the gangues zone was discussed using the energy principle.And the distance between the main roof beam end at the goaf side and coal rib were obtained.Because of dynamic disturbance,the stress at the floor of the coal rib and touching gangues zone in the main roof beam end will form coal rib effect zone and touching gangues effect zone.According to the floor pressure arch axis,the unloading and rebound effect of the floor stress was defined based on the floor pressure arch axis,and the floor unloading rebound failure model of main roof structural instability was established.Due to the dynamic disturbance,the disturbance stress of coal rib and touching gangues points at the main roof beam end increases,and it is continuously transmitted to the deep floor,which cause the pressure arch and the unloading rebound zone of the floor changing.Then the unloading stress and the unloading failure depth increase,the floor deformation increases,the fracture degree of the crack is more serious.?3?With the elastic theory,the disturbance stress on the coal rib and touching gangues zone and the maximum failure depth of floor were derived.The relationship between the main roof weighting and floor failure depth was given.The results show that:The longer the roof weighting interval is,the greater the disturbance stress of the coal rib and touching gangues zone of floor by the dynamic disturbance action;the floor stress increases nonlinearly by the compression deformation of immediate roof and gangues,with weighting interval increasing,the floor failure depth first increases and then tends to be stable;the fierce periodic roof weighting causes the floor failure periodically.?2?Extension and fracture mechanism and model of floor cracks in deep mining?1?According to the stress floor distribution and the numerical simulation of 3DEC software,the floor vertical stress formed obvious coal rib effect zone,unloading zone and touching gangues effect zone under instability disturbance of the main roof.The extension and fracture model of floor cracks was established,and the detailed partitions of floor stress and crack extension and fracture were provided.The corresponding crack extension tapes were given according to partition stress value.Then it carried out engineering verifications by the triaxial seepage test and ground radar detection.?2?In the coal rib effect zone,the deformation and extension of floor cracks were divided into different stages according to the magnitude of the vertical stress?1.And the critical stress of frictional sliding,self-similar extension,and unsteady extended deformation of the cracks were given,which are?1?0?,?1?c?and?1?cc?respectively.When?1<?1?0?,the floor rock mass is in the elastic stage;when?1?0?<?1<?1?c?,the primary cracks come into frictional sliding deformation;when?1?c?<?1<?1?cc?,cracks expend self-similar;when?1?cc?<?1,cracks instability extend and fracture,partial cracks unload and deform which caused floor rock mass degraded;when dynamic disturbance intensity leads to?1 peak exceeding the ultimate strength of coal and rock,cracks mainly generate compressed and closed deformation,and instability extension cracks are partially closed.?3?In the unloading zone,the extension and fracture of cracks were divided into unloading to the horizontal stress?3,zero or tensile stress stage according to the unloading level of?1.When?1 unloads to?3,cracks reverse slip;when?1 unloads to zero or tensile stress,they will buckling and instability extend or open and cut-through;and the critical stress of them are provide respectively.When the dynamic disturbance intensity increases,the critical stress of crack reverse slip increases,and the increasing of?1 further intensifies the reverse slip deformation of cracks.When?1 unloads to zero or tensile stress,because of the starting point of unloading increases under dynamic disturbance,and after the secondary cracks and reverse slip extension have been experienced,floor cracks will be dominated by buckling and instability extension and opening and cut-through extension.Meanwhile,the greater the dynamic disturbance intensity,the greater the stress at the floor of coal rib,the higher the starting point of unloading,the more stress conditions that can satisfy the instability extension of floor cracks,and the more favorable the crack propagation.?4?Combining with the permeation characteristics of rock mass,the permeation extension effect of high confining aquifer pressure in deep mining was analyzed.In the coal rib effect zone,when?1<?1?cc?,the effective stress?ne increases,and the permeability coefficient K of loading disturbance decreases;when?1>?1?cc?,the instability extension of secondary cracks lead to the number of cracks increasing,then K increases;and when the disturbance intensity increases to the floor stress exceeding the ultimate strength of rock mass,K decreases.But in the unloading zone,the higher the unloading starting point,the larger the effective unloading of the fractured rock mass,the unloading permeability coefficient K?is much greater,and K?will mutation increases when?1 unloads to a certain value.Meanwhile,the greater the disturbance intensity,the deeper the mutation depth of K?,floor inrush water will happen when the mutation depth of K?is greater than the thickness of the aquicludes.?3?Fracture and cut-through pattern of floor cracks under strong disturbance in deep coal mining?1?The extension and fracture evolution of floor cracks in deep coal mining was carried out by planar similar material model.Fracture and cut-through of floor cracks was divided into slant pattern and approximately vertical pattern under the effect of mining and unloading.Under the condition of slant pattern,main cracks fracture aslant,branching cracks extend to the goaf direction until oblique crossing to horizontal stratification of coal strata.Under the condition of approximately vertical pattern,main cracks fracture approximately vertical,branching cracks extend finitely and have little connection to horizontal stratification.?2?With the dynamic disturbance intensity increasing,branching cracks develop,and they connect with primary cracks preferentially,then they extend to the deep constantly until crack cutting through the aquicludes.The fractal dimension of crack network increases with cracks extension degree increasing.The advantage angles of floor cracks developing are from 50°to 85°,which determined by main cracks dip angles.Cracks of approximately vertical pattern develop more focusing on 70-82°intensively,but it is dispersed for slant pattern.And with main cracks dip angles increasing,branching cracks develop fewer and more difficult to cut through horizontal stratification.?4?Unloading damage and fracture model and strong disturbance risk of floor rock mass in deep coal mining?1?The strong disturbance characteristics of floor rock mass unloading damage in deep coal mining were analyzed by using 3DEC software.As mining to the deeper,the unloading starting point and unloading stress of deep rock mass increase,and unloading value is reduced.With unloading value of vertical stress increasing,unloading rate increases nonlinearly;the larger the unloading value,the faster unloading rate.The deeper the mining depth is,the faster the unloading rate increases,and the unloading value of unloading rate suddenly changed is much lower.With vertical stress unloading increasing,rock vertical displacement increases,and eventually leads to the instability mutation.And the lower unloading value of deep rock mass can cause serious damage.?2?Based on the deep rock mass unloading theory,the damage fracture model was built,and the extension and fracture conditions of crack rock mass unloading were deduced.Then unloading damage disturbance partition was presented.According to the unloading value,floor was divided into unloading failure zone,unloading extension zone and unloading permeation zone from top to bottom vertical direction.Combining with the permeation extension effect of confined aquifer pressure,the disturbance risk of cracks damage and fracture were discussed,and corresponding surrounding rock control technologies were presented.The results show that:with unloading value increasing,damage factor increases exponentially;the deeper the mining depth is,the more serious the damage and fracture degree of rock mass is;With crack dip angle increasing,unloading value of crack extension increases;if unloading value of horizontal stress become larger,crack will extend much easier;and the smaller the percentage of horizontal stress to vertical stress is,it is much easier to extend and cut through for floor cracks.?5?Engineering scale effect of floor coal rock damage and fracture and engineering application of grouting reinforcement in deep mining?1?Combining with fracture mechanics theory,engineering scale effect of dynamic disturbance intensity and mining depth was analyzed.It is concluded that:with the disturbance intensity increasing,the peak value of stress concentration increases;as mining depth increasing,stress peak increases,but stress concentration coefficient decreases;crack dip angle,mining disturbance or unloading effect could cause the variation or deflection of the stress field near cracks,which will lead to failure strength and stress concentration of crack rock mass changed consequently.?2?The damage and fracture degree of floor rock mass under different engineering scale were calculated and analyzed by mainly using the displacement maximum and unloading value.With the disturbance intensity increasing,the unloading value of floor vertical stress increases;the deeper the mining depth is,the larger the compressive deformation and unloading upheave of the floor rock mass is,and the more the unloading value is;with the distance of random cracks layer to the top surface of floor increasing,the floor upheave displacement and vertical stress unloading value increases first and decreases subsequently;when the random cracks layer is in the middle deep of floor aquicludes,it is the worst serious for damage and fracture degree of floor rock mass.?3?The grouting reinforcement technology of the combination of directional boreholes and crossing stratums boreholes in the floor were presented,and it could effectively reduces the water inrush disturbance risk of floor rock mass unloading in the filed engineering application. |
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