Damage Properties Of Coal And Rock And Instability Mechanism Of Rib Pillar-Roof Under Cyclic And Impact Loading

Limited by geological conditions,mining benefits and other factors,the total amount of coal retained under the end slope of the open-pit coal mines is up to 10 billion tons in China.The abandonment of this part of coal not only causes a huge waste of resources,but also leaves behind major safety hazards such as coal seam spontaneous combustion.As an effective way to mining the retained coal resources,the highwall mining techniques has been successfully applied in open-pit mines such as Trabula,Jinzhengtai and Antaibao,with significant economic benefits.The stability of the overlying rock slope is controlled by retaining the rib pillars between the mining openings when the highwall miner is used to mine the coal trapped in the end slope.However,under the joint action of the strong dynamic load generated by the large heavy mining haul trucks driving on the end slope and the impact dynamic load generated by the large-scale open-pit blasting,the rock mass damage of the rib pillar and the end slope will gradually accumulate until the instability occurs,resulting in a major safety accident.Therefore,it is urgent to conduct in-depth research on the damage evolution of coal and rock under the joint action of large mining haul trucks cyclic loading and blasting impact loading and the instability mechanism of rib pillar-roof system.In view of this,this dissertation firstly carried out the research on the propagation characteristics of the large mining haul truck dynamic load and blasting load in the rock mass of end slope,and obtained the propagation laws of truck dynamic load and blasting load.Then the uniaxial cyclic loading and unloading tests of coal and rock were carried out,and a new calculation method of coal and rock damage variables under constant amplitude cyclic loading was proposed.The influence law of cycle times and upper limit of cyclic stress on coal and rock damage variables was studied.Secondly,Hopkinson cyclic impact tests of coal and rock considering initial damage effect are carried out,and a new calculation method of coal and rock damage variables under constant amplitude cyclic impact loading was proposed.The influence law of impact times and incident energy on coal and rock damage variables was studied.Finally,based on the bearing characteristics of rib pillar-roof system under the mining condition of highwall miner,the mechanical model of rib pillar-roof system was established,and the comprehensive criterion of system stability and the design method of the end wall mining parameters was proposed.The main innovative research results obtained in this dissertation are as follows:(1)The damage effect of large mining hual trucks and blasting on coal and rock was considered for the first time,and a numerical calculation model for the study of the propagation law of truck dynamic load and blasting impact load was established.The propagation characteristics of the effective stress of dynamic load sharply attenuated with the increase of surface depth,and the strain rate of coal and rock sharply attenuated with the increase of blasting center distance under the impact load were revealed.The mechanism of coal and rock slope subjected to constant amplitude cyclic loading and constant amplitude cyclic impact loading was explained,which provides a theoretical basis for coal and rock constant amplitude cyclic loading and unloading test and constant amplitude cyclic impact test.(2)The uniaxial cyclic loading and unloading test of coal and rock was carried out.From the perspective of energy dissipation,considering the influence of cycle times and upper limit of cyclic stress,a calculation method of rock damage variable in quasistatic constant amplitude cyclic loading test was proposed for the first time.The coal and rock damage variable showed a power function increasing trend with the increase of cycle times,and showed a quadratic function increasing trend with the increase of upper limit of cyclic stress.The compressive strength deterioration range of coal and rock obtained from the test has a good correspondence with the damage variable value calculated theoretically,which verifies the correctness and rationality of the proposed calculation method of damage variable.Meanwhile,the failure mode of red sandstone under constant amplitude cyclic loading is mainly tensile splitting,accompanied by shear failure,and that of anthracite is mainly tensile splitting.(3)The constant amplitude cyclic impact test of damaged coal and rock was carried out.Combined with the deterioration law of dynamic elastic modulus,and considering the influences of incident energy and impact times,a calculation method of rock damage variable in the constant amplitude multiple impact test was proposed for the first time.The coal and rock damage variable showed an exponential function type increasing trend with the increase of incident energy and impact times.Constant amplitude impact loading would aggravate the deterioration of peak dynamic compressive stress and dynamic elastic modulus of coal and rock,and reduce the bearing capacity of coal and rock.Besides,the failure mode of coal and rock has a trend of changing from tensile to shear failure with the increase of incident energy in macro and meso scale,and the average particle size of coal and rock has a nonlinear decreasing law with the increase of accumulated dissipated energy.(4)A mechanical model of rib pillar-roof system under the combine action of overburden static loading,cycle and impact loading was established.On the basis of the traditional cusp catastrophe model of rock instability,combined with the damage variables defined in the indoor constant amplitude cyclic loading and unloading test and constant amplitude cyclic impact test,a comprehensive criterion of system stability under dynamic and static load conditions was proposed for the first time.The calculation methods of ultimate stress,width of plastic zone and system deflection curve of “variable foundation coefficient rib pillar-constant coefficient roof” system were obtained,which provides a new calculation method for the design of mining parameters of end wall retained coal in open-pit mines,and improves the theoretical system for the parameter design of the rib pillar in the open-pit coal mine.There are 202 figures,65 tables and 205 references in this dissertation.