Investigation On Mechanical Properties And Failure Mechanism Of Cracked Coal-rock-like Combined Body Under Uniaxial Compression

In underground mining,coal-rock combined engineering body is common,such as driving of underground semi-coal rock roadway.A new force system is formed between roof and floor and coal seam in the process of coal face advancing,and a new surrounding rock bearing structure is formed between coal pillar and roof strata in room and pillar mining.The coal seam is not a single occurrence in the stratum,but coexists with the roof and floor.In other words,underground engineering is usually carried out in stratified strata with different lithology and stress states.In this case,it is more reasonable to regard strata as a composite composed of various media with different bonding modes and different bonding thicknesses.Because the roof with hard lithology is difficult to collapse naturally,and large goaf will be formed,which may cause rock-burst,roof falling and other disasters.Therefore,a large number of pre-cracks need to be formed on the roof through hydraulic fracturing or blasting to weaken the roof strength,which remove the roof pressure,and make the roof collapse naturally.The effectiveness of these pressure relief techniques depends largely on the geometry,size and location of the pre-existing cracks.In addition,the formation process of coal in underground special and subsequent complex geological movement for a long time,which can lead to the raw coal contain a lot of natural joints,fractures and other natural defects,these defects have a great impact on the stability of engineering structure,so the combination of coal and rock fracture mechanics behavior and failure mechanism is very important also very meaningful.Based on the above problems,laboratory tests and numerical simulation studies were carried out in this paper,mainly from the strength deformation characteristics,acoustic emission response characteristics,deformation and failure mechanism and energy evolution law.On the basis of verifying the validity of the numerical model and the feasibility of the numerical calculation method,the failure mechanism is explored,analyzed and verified at the micro level.The mechanical behavior of fractured coalrock composite materials under different fracture sizes,different fracture positions and different coal-rock strength ratios was studied.The main conclusions were as follows:(1)The number of AE events,deformation and failure process and energy evolution curve have good consistency with the change of uniaxial compression load.The AE events of the intact specimens are more stable than those of the cracked specimens.The cumulative AE counts curve has several peak points corresponding to the phenomenon of local stress drop.The crack angle has a significant effect on its peak strength.The smaller the crack angle is,the greater the influence on the peak strength of specimen is.By curve fitting,the peak strength,elastic modulus and prefabricated crack angle show quadratic and linear relations respectively.(2)The crack angle affects the failure and crack propagation modes of coal-rock combination specimens.When the crack angle is lower than or equal to 60°,the failure mode is the joint failure of coal and rock mass at the interface.When the crack angle is higher than or equal to 75°,the failure mode is the main failure mode of coal body.Generally speaking,the propagation mode of crack is anti-N type failure.Based on the ultimate tensile load of coal and rock near the interface,the criterion is proposed,which can well explain the failure phenomenon observed in the test.(3)Based on the validity of the microscopic parameters of coal and rock monomer and the feasibility of the numerical simulation method.The numerical study of the uniaxial compression combination is carried out,and the failure mechanism of coal and rock interface is analyzed from the microscopic point of view.According to the displacement trend vector diagram of particles,the phenomenon that crack initiation and propagation can penetrate the interface of some coal and rock mass is explained,and the tensile failure mode of rock mass after penetration is also explained.The reason of crack arrest at the interface of some coal and rock mass is also analyzed,which further verifies the correctness of the theoretical analysis of interface failure mechanism.(4)For the coal and rock combination samples with different crack length,with the increase of crack length,the peak strength,elastic modulus and peak strain increase.In addition,the combination of the extent of damage of more intense with the increase of crack angle.And the stressstrain curve with crack angle change difference has significantly increased.For the coal and rock combination specimens with different crack positions,the peak strength will increase with the increase of the prefabricated crack angle.When the position of the crack located on the side of coal or rock mass,the variation trend of its strength with the crack angle is basically the same,but the overall strength and peak strain are smaller in coal.For coal and rock combination specimens with different coal-rock strength ratios,when the strength of coal increases,the prefabricated cracks have little influence on the strength of part of coal,and the strength of the whole is gradually close to that of coal.In addition,the failure mode of the assemblages is basically the same,but the failure degree is increasing.