Experimental and numerical study of failure behavior and mechanism of coal under dynamic compressive loads

A comprehensive understanding of the failure behavior and mechanism of coal is a prerequisite for dealing with dynamic problems in mining space.In this study,the failure behavior and mechanism of coal under uniaxial dynamic compressive loads were experimentally and numerically investigated.The experiments were conducted using a split Hopkinson pressure bar(SHPB)system.The results indicated that the typical failure of coal is lateral and axial at lower loading rates and totally smashed at higher loading rates.The further fractography analysis of lateral and axial fracture fragments indicated that the coal failure under dynamic compressive load is caused by tensile brittle fracture.In addition,the typical failure modes of coal under dynamic load were numerically reproduced.The numerical results indicated that the axial fracture is caused directly by the incident compressive stress wave and the lateral fracture is caused by the tensile stress wave reflected from the interface between coal specimen and transmitted bar.Potential application was further conducted to interpret dynamic problems in underground coal mine and it manifested that the lateral and axial fractures of coal constitute the parallel cracks in the coal mass under roof fall and blasting in mining space.

Application of deep borehole blasting on fully mechanized hard top-coal pre-splitting and gas extraction in the special thick seam

In order to solve the problems of top-coal inadequate destruction and large amounts of gas emission in mining extra thick and hard coal seam,this study investigated the pre-splitting for deep borehole blasting and gas pre-draining technologies on top coal.The mechanism of the technologies was systematically expounded based on hard top-coal cracks development obtained by numerical simulation and theoretical analysis.The results show that explosive blasting in the hard rock results in a large number of cracks and large displacement in the rock mass due to the effect of explosion stress.Meanwhile,the thick top-coal caves,and desorbing gas flows along the cracks improve gas extraction.Finally,the pre-splitting for deep borehole blasting and gas pre-draining technologies was applied in No.3802 working face of Shui Liandong Coal Mine,which increases monthly output in the face to 67.34 kt and the drained gas concentration to 86.2%.The drained gas average concentration from each borehole reaches 40%,and the effect is remarkable.

On the excavation-induced stress drop in damaged coal considering a coupled yield and failure criterion

Investigating the stress drop of abutment pressure is the key to a deep quantitative analysis of the discontinuous stress redistribution under mining.In the present study,uniaxial and triaxial compression tests are carried out separately to determine the bulk and shear moduli,the cohesion,and the internal friction angle of the coal samples.By extending the meaning of Mohr’s circle referring to yield stress instead of the maximum principal stress,a yield line is introduced to illustrate the stress drop of Mohr’s circle referring to yield stress instead of the maximum principal stress at the elastoplastic boundary.Furthermore,a theoretical solution of the stress drop as a function of the damage is proposed to investigate how the abutment pressure differs considering the yield line and failure line.In addition,applying the stress drop at the yield line in non-pillar mining,top coal mining,and protective coal mining shows that the damage has a nonlinearly positive influence on the stress drop.The results shows that the bulk modulus and internal friction angle have a more sensitive influence on the stress drop than do the shear modulus and cohesion.Finally,the stress drop is divided into a discontinuous stress drop at the yield line and a structural stress drop at the failure line.The stress drop is effective in describing the discontinuous stress redistribution and shows a clear difference in the movement direction of Mohr’s circle considering the unloading pressure.

Anisotropy of crack initiation strength and damage strength of coal reservoirs

The crack volume strain method and acoustic emission(AE)method are used to analyze the anisotropy of the crack initiation strength,damage strength,the failure mode and the AE characteristics of coal reservoir.The results show that coal reservoirs show obvious anisotropic characteristics in compressive strength,cracking initiation strength and damage strength.The compressive strength of coal reservoirs decreases with the increase of bedding angle,but the reservoirs with bedding angles of 450 and 900 differ little in compressive strength.The crack initiation strength and damage strength decrease first and then increase with the increase of bedding angle.The crack initiation strength and damage strength are the highest,at the bedding angle of 0°,moderate at the bedding angle of 90°,and lowest at the bedding angle of 45°.When the bedding angle is 0°,the failure of the coal reservoirs is mainly steady propagation of large-scale fractures.When the bedding angle is 45°,one type of failure is caused by steady propagation of small-scale fractures,and the other type of failure is due to a sudden instability of large-scale fractures.When the bedding angle is 90°,the failure is mainly demonstrated by a sudden-instability of small-scale fractures.Compared with the cumulative count method of the AE,the cumulative energy method is more suitable for determining crack initiation strength and damage strength of coal reservoirs.

深部软岩巷道变形破坏机制及联合支护技术（英文）

针对深部软岩巷道围岩长期流变大变形破坏问题,以中国枣泉煤矿运输巷道工程为例,利用现场变形监测和声波测试技术,分析巷道软弱围岩长期变形趋势及破坏范围;利用数值模拟方法,建立能够反映工程地质状况及初始设计方案的三维有限元模型,并以现场监测变形数据为基础,反演获取巷道围岩力学参数和流变参数。针对围岩破坏特征,提出"U型钢支架+围岩锚固注浆"联合支护技术,并利用有限元模型对其支护效果进行数值模拟,结合现场监测变形数据分析,验证了支护方法的有效性。

Breaking mechanism and control technology of sandstone straight roof in thin bedrock stope

The key problem to be solved urgently is how to avoid the occurrence of support break-off and water inrush in the stoping of sandstone straight roof under the action of load transfer in unconsolidated aquifer.For this reason,taking the thin bedrock 1602(3)working face of Huainan(the middle part of Anhui Province)Panyi Coal Mine as the engineering background,this study establishes the stope mining model by using the discrete element UDEC software and the mathematics mechanical model of the support load,and analyzes the reason of support crushing and decides to re-mining the working face by using the compulsive roof caving method.It is concluded that when the working face of sandstone straight roof is broken,the"voussoir beam"structure cannot be formed and acts on the support in the form of cantilever beam,but only when it falls to the high key stratum can the"voussoir beam"structure be formed and at this point,at this time,the bracket bears the weight of the rock layer in the range from the fractured sandstone layer to the lower critical layer.The working resistance of the support increases with the increase of the thickness and the breaking length of straight sandstone roof.When the breaking length of the roof reaches a certain extreme value,the support crushing accidents will occur.Managing roof with compulsive roof caving method can reduce the intensity of rock pressure in the stope,and the working face can be safely stoped,which provides a certain reference for similar conditions.

Similar simulation device for unloading effect of deep roadway excavation and its application

The unloading effect of the excavation of deep roadways has been considerably studied, but most research methods have been limited to numerical simulations and field measurements. Only a few have adopted experimental methods for similar simulations. On the basis of the theory of mechanics,the testing system is designed considering initial geostress and dynamic unloading. The system includes an impact unloading gear and in-situ stress loading equipment, and a designed three-link structure and the impact hammer can effectively realize the dynamic excavation of roadways.Meanwhile, a cyclic excavation similar simulation experiment on a deep roadway is conducted in a laboratory. The testing system and the relevant monitoring facilities are utilized, and the unloading effect inside the surrounding rock under the cyclic dynamic excavation is studied. Results show that the cyclic dynamic excavation causes significant unloading only in the nearby rock mass, and the unloading indicators show nonlinear changes.Moreover, when the lateral pressure coefficient is 1.2,the damage is concentrated on both roadsides due to the excavation unloading. Meanwhile, the damage gradually decays as the span increases.

Experimental Study on Seepage Characteristics of a Soil-Rock Mixture in a Fault Zone

A mixture of fault gouge and rubble taken out from a fault zone is used to prepare a S-RM(Soil-Rock Mixture)sample with rock block proportions of 20%,30%,40%,50%,60%and 70%,respectively.A GDS triaxial test system is used accordingly to measure the seepage characteristics of such samples under different loading and unloading confining pressures in order to determine the variation law of the permeability coefficient.The test results show that:(1)The permeability coefficient of the S-RM samples decreases as the pressure increases,and the decrease rate of this coefficient in the initial stage of confining pressure loading is obviously higher than in the semi-late period;(2)The permeability coefficient at different confining pressure levels presents a common trend as the rock block proportion is increased,i.e.,it decreases first then it increases(the permeability coefficient of the sample with rock block proportion 40%being the smallest,70%the largest);(3)In the stage of confining pressure unloading,the recovery degree of the permeability coefficient grows with the increase of rock block proportion(the recovery rate of S-RM sample with rock block proportion 70%reaches 50.2%);(4)In the stage of confining pressure loading and unloading,the sensitivity of the permeability coefficient to the rock block proportion displays the inverse“Z”variation rule(when rock block proportion reaches 60%,the sensitivity is highest);(5)In the stage of confining pressure loading,the relationship between the permeability coefficient and confining pressure can be described by an exponential relationship.

Cluster modeling of the short-range correlation of acoustically emitted scattering signals

As a widely used measurement technique in rock mechanics,spatial correlation modeling of acoustic emission(AE)scattering signals is attracting increasing focus for describing mechanical behavior quantitatively.Unlike the statistical description of the spatial distribution of randomly generated AE signals,spatial correlation modeling is based mainly on short-range correlation considering the interrelationship of adjacent signals.As a new idea from percolation models,the covering strategy is used to build the most representative cube cluster,which corresponds to the critical scale at peak stress.Its modeling process of critical cube cluster depends strongly on the full connection of the main fracture network,and the corresponding cube for coverage is termed the critical cube.The criticality pertains to not only the transition of local-to-whole connection of the fracture network but also the increasing-to-decreasing transition of the deviatoric stress with an obvious stress drop in the brittle failure of granite.Determining a reasonable critical cube guarantees the best observation scale for investigating the failure process.Besides,the topological connection induces the geometric criticality of three descriptors,namely anisotropy,pore fraction,and specific surface area,which are evaluated separately and effectively.The results show that cluster modeling based on the critical cube is effective and has criticality in both topology and geometry,as well as the triaxial behavior.Furthermore,the critical cube length presents a high confidence probability of being correlated to the mineral particle size.Besides,its pore fraction of cube cluster is influenced strongly by the critical cube length and confining pressure.

Prediction of fracture and dilatancy in granite using acoustic emission signal cloud

The invisibility of fracture network evolution in the rock under triaxial compression seriously restricts the correlation modeling between dilatancy behavior and fracture interconnectivity.The key to solving such a challenge is strongly dependent on the accurate modeling of the spatial correlation in fracture network,which could be indirectly re-constructed by the acoustic emission(AE)signal cloud.Considering the interaction of local fractures,a cube cluster approach is established to describe the spatial correlation.The evolutional cube clusters effectively present the geometric characteristics induced by the increasing dilatancy of fracture.Two descriptors(i.e.three-axis length sum and pore fraction)are introduced to correlate cluster model with dilatancy behavior.Most fitting results support the linear correlation between two descriptors and volumetric strain,which verifies the sensitiveness of the cube cluster model to dilatancy.More importantly,by the statistical analysis of cluster structure,the cluster model shows the potential of calculating fracture angle.Moreover,a comparison between dilatancybased damage and porosity-based damage is made not to prove the best but provide an AE-based prediction of local damage evolution.Finally,four classical models for calculating fracture angle are compared.The deviations prove the huge difficulty of describing the development of the fracture network uniquely dependent on a fracture angle.The proximity of measured angle and cluster-based angle supports the effectiveness of predication by the cube cluster approach.

A Review of Research on Catastrophic Formation and Evolutionary Mechanism of Deep High Stress Rock Mass under Impact Loading

With the exploitation of resources expanded to deep, the geological environment which is “three tenors one disturbance” of rock mass in great depth has been more complex, resulting in some new engineering geological disasters, such as rock burst, pressure bumping, large deformation of surrounding rock, brittle-ductile transition of rock and zonal disintegration of rock mass, which occur frequently in deep underground engineering rock mass. The impact load caused by collision, explosion, extrusion and outburst is the root cause of the dynamic instability of the deep rock mass. What should be emphasized is that high in-situ stress and blasting excavation disturbance complicate disaster developing mechanism of deep underground engineering rock mass and sharply increase the difficulty of controlling disaster. This paper is aimed at the research status and development trend, of which dynamic characteristics of deep high stress rock mass and its damage and failure effect each other under impact, and conduct analysis, in the later stage where I would discuss how to carry out the response law of the deep high-stress rock mass under the impact load and the mechanism of catastrophe developing, which is of great significance to build a model of instability and fracture evolution about deep rock mass under shock disturbance and to maintain its safety and stability.

Research on the pressure variation law and enhancing CBM extraction application effect of CO_(2) phase transition jet coal seam fracturing technology

Due to the limited permeability and high methane content of the majority of China’s coal seams,significant coal mining gas disasters frequently occur.There is an urgent need to artificially improve the permeability of coalbed methane(CBM)reservoirs,enhance the recovery efficiency of CBM and prevent mine gas accidents.As a novel coal rock fracture technology,the CO_(2) phase transition jet(CPTJ)has been widely used due to its advantages of safety and high fragmentation efficiency.In this study,to ascertain the effects of the pressure of CPTJ fracturing,the influence of its jet pressure on cracked coal rock was revealed,and its effect on CBM extraction was clarified.In this research,the law of CPTJ pressure decay with time was investigated using experimental and theoretical methods.Based on the results,the displacement and discrete fracture network law of CPTJ fracturing coal rock under different jet pressure conditions were studied using particle flow code numerical simulation.Finally,field experiments were conducted at the Shamushu coal mine to assess the efficiency of CPTJ in enhancing CBM drainage.The results showed that the pressure of the CPTJ decreased exponentially with time and significantly influenced the number and expansion size of cracks that broke coal rock but not their direction of development.CPTJ technology can effectively increase the number of connected microscopic pores and fractures in CBM reservoirs,strongly increase the CBM drainage flow rate by between 5.2 and 9.8 times,and significantly reduce the CBM drainage decay coefficient by between 73.58%and 88.24%.