Experimental Study On The Mechanical Response Of Water-saturated Coal Samples Under Coupled Static-dynamic Loading

Rock burst is one of the major disasters in coal mine.It may be caused by the static and static-dynamic loading conditions.During the rock burst,the hypocerter shock stress wave transmitted through rock and coal seams may cause the major damage to the underground structures,facilities and even mine personals.Coal seam water injection is one of the commonly used methods for preventing and controlling the occurrence of rock burst,as the water may reduce the possibility of occurrence of rock burst under static loading condition.On the othe hand,in the deep coal seam,the rock burst is caused by the combination of static and dynmic loading.Under such a loading condition,the stress wave propagation and attenuation,the strength variation of coal and the disappearing characteristics of energy in the water bearing coal seam are not clear.Therefore,the experimental investigation on dynamic response of water-saturated coal samples is conducted using RMT-150 B testing machine and modified SHPB system.With the static and dynamic loadings applied,the dynamic strength and stability,the attenuation of stress wave,the energy disappearing as well as the damage and fracture characteristics of different water-saturated coal samples are tested and studied.The major findings are summarized as follows:(1)Static uniaxial and triaxial compressive tests have been conducted on the coal samples,which are in the natural condition and placed in water for 3 and 7days respectively.It is noted that uniaxial and triaxial compressive strength decrease with the increase of the coal sample in the water.Under the triaxial compressive loading,the post-peak of stress-strain curve of the coal samples placed in the natural and water for 7days presents different features.The brittle character has been detached for the samples in the nature condition,and plastic behavior has been identified for the samples placed in water for 7 days.Also,the statistical damage constitutive model of water-saturated coal samples under uniaxial compression loading is established on the basis of Weibull distribution,and the model verification has been conducted accordingly.(2)Special-shaped punch method has been used to liminate dispersion effect and uniform stress.Also,the size of coal sample is has been determined as 50mm×30mm for dynamic loading tests.For the water-saturated coal sample,the shear wave velocity is basically unchanged,the longitudinal wave velocity increases obviously,and the stress wave attenuation coefficient decreases.Also,one dimensional static and dynamic loading testwas carried out by using the modified SHPB system for the samples placed in the natural,and water for 3 days and 7 days.The stress-strain curve of coal samples between dynamic and static loadhas obvious difference.The dynamic strength of coal samples will decrease with the time placed in water increased,but the modulus of elastic only has a slightly rise.The dynamic strength and modulus of elasticity is higher than the static strength and the modulus of elasticity.The fracture propagation of coal samples may be divided into few stages,including the stage of fracture compression stable,the stage of crack sharp expansion,the stageof crack coal essence according to the influence of axial pre-static load on the development of coal-like crack.The dynamic strength of coal sample increases initially and then decreases afterward.Axial pre-static load is greater than 50%-52% of the static load strength(Critical dead load point),the dynamic strength of one-dimensional static-dynamic combination began to decline.(3)The dynamic strength of axial static load in the elastic range of coal sample increases with the increase of the axial static load under the three dimensional static and dynamic loading tests with the same confining pressure and different axial compression.The dynamic strength of coal samples in water for 7 days is higher than the one in the natural condition,but it is lower under the static loading and one dimensional static-dynamic loading conditions.The dynamic strength and the modulus of elasticity of coal samples placed in the natural and in water for 7days increase with an increasing of confining pressure under the same axial compressionand different confining pressure.The stress-strain curves of coal samples under one dimensional and three dimensional static-dynamic loading are divided into 4 stages: compaction stage,elastic stage,crack growth stage and unloading stage.(4)The stress field evolution characteristicsof crack tip are analyzed,and the crack model of water-saturated open wing under the condition of static and dynamic-static loading was established by using the fracture damage mechanics.The fracture initiation,propagation and stop criterion of coal sample under the action of hydraulic pressure has been proposed.The mathematical expressions of the compressive strength of water-saturated coal sample under the condition of static loading,dynamic and dynamic-static loading conditions were established.The variation mechanisms of the dynamic compressive strength of the water-saturated coal sample and the rock material were also presented.The water-saturated coal sample damage variableunder one-dimensional dynamic-static combined loading was determined.The constitutive model of water-saturated coal sample under the combination of static and dynamic loading was established,and verified.(5)The time for the destruction of natural coal samples under one-dimensional dynamic-static loadingis greater than water-saturated coal samples.The broken particles of coal samples gradually become smaller with the increase of water saturation time.The greater failure instability dissipative energy,the smaller the fragmentation degree,the greater the fractal dimension.The fractal dimension of coal sample in the natural condition is less than the samples placed in water for 3days,7days respectively.The fractal dimension increases with an increasing of the density of energy consumption,which indicates a positive correlation.The grain size decreased with increasing saturation time under same density of energy consumption,which shows a negative correlation.The grain and the density of energy consumption have a great linear relationship.The effect of water on the size of the sample is significant.