| After long-term mining,shallow coal resources are increasingly exhausted,the depth of coal mining is increasing year by year,and the geological environment is becoming more complex.In the whole service cycle of the mining roadway,the surrounding rock of the roadway is damaged to varying degrees under the influence of the primary mining,resulting in changes in the deformation,failure mode and strength of the surrounding rock of the roadway.Under the secondary mining,the cracks of the surrounding rock of the damaged roadway were developed and the amount of deformation is large.Effectively controlling the deformation of this kind of roadway has become an urgent problem to be solved.In this thesis,the research methods of laboratory test,theoretical analysis,numerical simulation,and field test are comprehensively used to effectively control the deformation of damaged surrounding rock.The evolution law and fracture mechanism of triaxial and true triaxial fractures of different damaged coal bodies are studied from the macro and micro point of view,and the true triaxial stress-strain constitutive model of different damaged coal bodies is established.Combined with the geological conditions of Dafosi hole,The influence of bolt support parameters on the distribution of surrounding rock cracks between bolts is deeply analyzed.(1)The energy method is used to define the initial damage variable of coal samples,and the damage evolution law of coal samples under cyclic loading and unloading is studied.According to this law,coal samples with different initial damage degrees are made.It is found that with the increase of the initial damage degree,the elastic modulus and triaxial compressive strength decrease,and the post peak curve of coal sample gradually changes from the brittleness at the time of falling to the ductility of falling in stages and slowly falling.(2)Based on the transverse strain method and conventional triaxial compression test,the characteristic stress of coal crack with different initial damage degree under different confining pressure is studied.It is found that the crack initiation threshold of coal sample is between 0.11 ～ 0.31 and the crack damage threshold is between 0.7 ～0.88,which increases nonlinearly with the increase of confining pressure.With the increase of the initial damage degree of coal sample,the crack initiation stress increases linearly,with the change range of 9% ～ 54%,and the crack damage stress decreases linearly,with the decrease range of 21% ～ 50%.(3)Based on the volume strain method and true triaxial compression test,the characteristic stress changes of coal samples with different initial damage degrees under different intermediate principal stresses are analyzed.It is found that the crack initiation stress changes nonlinearly with the increase of intermediate principal stress,ranging from 8.9mpa to 22.1MPa.When the confining pressure is between 5MPa and15 MPa,the crack initiation stress increases gradually because it is within this rangeσ2inhibit the crack initiation to a certain extent.When the intermediate principal stress reaches 20 MPa,the crack initiation stress begins to decreaseσ2 after exceeding the specified value,tensile stress will be generated in the sample,which promotes the initiation of crack.With the increase of damage degree,the crack initiation stress shows a nonlinear decreasing trend,and the crack damage stress first increases and then decreases.(4)Based on the theoretical knowledge of meso-statistical damage,assuming that the micro element strength of coal sample obeys Weibull probability distribution,the damage evolution equation of coal sample with initial damage is established.Drucker Prager strength criterion is introduced to measure the micro element strength.According to the crack damage effect of coal sample,considering the two stages before and after crack initiation,a statistical damage constitutive model which can accurately describe the coal samples with different damage degrees under true triaxial conditions is established.According to the geometric characteristics of the peak point of the stress-strain curve,the theoretical expression of the parameters of the damage constitutive model is deduced.The parameters obtained in the laboratory are substituted into the constitutive model to obtain the theoretical stress-strain curve of coal samples with different damage degrees under different confining pressures.The curve is in good agreement with the test curve,which shows that the constitutive model can better reflect the stress-strain relationship of coal samples with different damage degrees under true triaxial conditions.(5)Based on the particle discrete element method,the true triaxial compression failure characteristics of coal samples with different initial damage degrees under different maximum principal stresses are revealed from the meso-point of view.According to the machine learning algorithm,a neural network is established to obtain the micromechanical parameters of the particle flow numerical model.It is found that with the increase of intermediate principal stress,the difference of rose diagram of relative frequency distribution of model fracture tendency gradually increases,and the maximum contact force of force chain decreases from 2.15 k N to 1.93 k N.When the intermediate principal stress increases from 5MPa to 15 MPa,the energy storage limit increases gradually,while when the intermediate principal stress increases to 20 MPa,the energy storage limit decreases,but it is still higher than that when the intermediate principal stress is 15 MPa.With the increase of the initial damage degree of coal samples,the probability of fracture distribution inclined between 170～190 ° and 350 ～10° increases from 0 to 5.2%,and the energy storage limit of coal samples decreases continuously.(6)Based on the geological conditions of Dafosi mine,a numerical model considering the rise line of coal seam floor is established,and the deformation,displacement and stress distribution of roadway surrounding rock with different floor elevation and strike under the influence of different mining are studied respectively.Through the continuous discrete coupling numerical model,the crack arrest effect of bolt is analyzed from the engineering scale.Increasing the bolt density can inhibit the formation of shear cracks and tension cracks,but the control of tension cracks is more obvious.Increasing the bolt length is helpful to control the production of tension cracks.Increasing the bolt preload can reduce the total number of cracks and have a greater impact on the formation of shear cracks.The research scheme based on the above theory effectively controls the deformation of roadway surrounding rock affected by multiple mining.There are 113 pictures,25 tables and 210 references. |
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