Study On Dynamic Mechanical Property And Constitutive Relationship Of Sandstone Containing Weakly Filled Joints With Various Angles

As a kind of weak structural plane,filled joints exist extensively in geological structure.Research shows that many collapse accidents are caused by ignoring the dramatic changes of lithology near filled joints with various angles.In this paper,theoretical analysis and laboratory tests were utilized to study the static and dynamic mechanical characteristics,energy dissipation characteristics of sandstone containing weakly filled joints with various angles.The static and dynamic mechanical parameters of jointed sandstone were obtained.The fracture evolution mechanism and the law of stress wave propagation were analyzed.The dynamic damage constitutive model of weakly filled jointed rock mass was established,providing analytical and theoretical bases for the stability maintenance and disaster assessment of jointed rock mass engineering.The main research contents and conclusions are as follows.(1)The failure mode,dynamic stress-strain relationship,dynamic strength,dynamic peak strain,energy dissipation characteristics,failure mode,typical failure process,crack convergence type and fracture micro mechanism of sandstone containing weakly filled joints with different angles in uniaxial tests were systematically analyzed,and the mechanical effect of joint occurrence was deduced.The results show that with the increase of joint angle,the dynamic compressive strength and peak strain of sandstone specimens showed a U-shaped trend,and the minimum value occured at 45°.The specimen with a joint angle of 45° had the largest energy reflection coefficient and the smallest absorbed energy density.The ultimate failure modes of the jointed specimens could be divided into splitting tensile failure,tension-shear composite failure and shear failure.With the increase of joint angle,the failure sequence gradually changed from weakly filled joint to rock matrix.The stress bimodal phenomenon of specimen with a joint angle of 45° was higher than that of specimen with a joint angle of 60°.Four kinds of crack convergence modes in the dynamic fracture process of jointed specimens were captured and summarized with the help of a high-speed camera.The crack convergence types of jointed specimens under static load were compared and analyzed.The micro fracture morphology could reveal the macro fracture behavior of jointed sandstone.(2)The effects of joint angle and strain rate on dynamic mechanical properties and energy evolution of sandstone under three-dimensional stress state were studied.The results showed that the rebound strain phenomenon was obvious and the growth rate of stress changed in phased variations.Dynamic peak strain was inversely proportional to joint angle under three different strain rates.Dynamic compressive strength,elastic deformation modulus,and plastic deformation modulus were in similar variable tendencies with incremental joint angles.Moreover,the sensitivity of plastic deformation modulus to joint angle was obviously inferior to that of elastic deformation modulus when joint angle increased from 0° to 45°.Furthermore,both elastic deformation modulus and plastic deformation modulus were independent of strain rate,which was contrary to dynamic compressive strength and dynamic peak strain.Additionally,absorption energy release rate was introduced and defined to describe energy release and conversion characteristics of joint specimens.The variation trend of energy reflection coefficient was completely opposite to that of energy transmission coefficient and absorbed energy release rate.Absorbed energy density was linearly decreased with incremental joint angle and was increased with the increase of strain rate.(3)The coupled dynamic-static loading tests were carried out to study the influence of confining pressure and joint angle on the dynamic mechanical properties and energy evolution characteristics of sandstone specimens.The results showed that the greater the confining pressure was,the greater the slope of the plastic section of the stress-strain curve was,and the more obvious the brittle failure characteristics were.Under different confining pressures,strain rebound phenomenon was observed in the stress-strain curves of jointed specimens.The larger the confining pressure was,the more serious the strain rebound phenomenon was.The strain rebound phenomenon was obvious for the specimens with larger joint angles.The compressive strength of jointed sandstone increased with the increase of confining pressure.Under different confining pressures,the dynamic strength of jointed specimens showed a V-shaped variation trend.The mechanical effect of joint occurrence under three-dimensional stress state could well explain the test variation trend of dynamic compressive strength.The confining pressure had a more significant effect on restraining the separation along the cementation surface and limiting the deformation of sandstone specimens with larger joint angles,which could greatly improve the dynamic compressive strength and reduce the dynamic peak strain.The pre axial compression made the specimens with larger joint angles easier to fail along the cementation planes,and the brittle failure characteristics were more obvious;the axial compression was conducive to the compressive deformation performance of the weakly filled joints with smaller angles,leading to the obvious plastic failure characteristics of the jointed specimens.The sandstone specimens with larger joint angles were unfavorable to rock breaking under confining pressures,and the specimens after loading could still store considerable elastic energy,which might trigger secondary rockburst in practical engineering.The energy reflection coefficient decreased with the increase of confining pressure,while the energy transmission coefficient and absorbed energy density increased with the increase of confining.pressure.(4)Under the static and dynamic splitting loadings,the joint angle had a significant effect on the failure mode of the sandstone specimen.In dynamic tests,with the increase of joint angle,the evolution process of the main failure modes of the jointed specimens were as follows:the separation of rock matrix and joint along the cementation surfaces,the violent sliding failure along the cementation planes,the tension-shear composite failure of rock matrix,the Z-shaped tension-shear composite failure of main fracture paths,and the typical Brazilian disc tensile failure.Similar failure modes were observed in the static tests.Only 60°,75° and 90° jointed specimens accorded with the Brazilian disc validity test principle.The brittle failure characteristics from apparent to inapparent could be as follows:joint specimens under static loading,complete specimens under static loading,complete specimens under dynamic loading and joint specimens under dynamic loading.Weakly filled joints significantly reduced the tensile bearing capacity of sandstone,especially for the specimens with lower joint angles.The peak load and peak displacement were proportional to the joint angle and loading rate.Mechanical effect of joint angle under splitting load could well explain the variation trend of peak load.The peak displacement of jointed specimens was not sensitive to loading rate,but it was obviously dependent on the joint angle.The peak displacement of jointed specimen in dynamic tests was far less than that in static tests.In practical engineering,the safety monitoring of rock mass under dynamic tensile load should be strengthened to avoid accidents.(5)In the dynamic Brazilian disk tests,the energy reflection coefficient decreased with the increase of joint angle and loading rate,which was opposite to the variation tendency of energy transmission coefficient.The energy transmission coefficients of 0°,15° and 30° jointed specimens were not sensitive to loading rate and joint angle,while the energy transmission coefficients of 45°,60°,75° and 90° jointed specimens increased with the increase of loading rate and joint angle.The energy reflection coefficient and transmission coefficient of 90° jointed specimens were the closest to those of the intact specimens.The transmission energy showed a "slow-fast-slow-fast"four stages variation feature,and was more sensitive to joint angle than loading rate.Compared with the joint angle,the absorbed energy was more sensitive to loading rate,showing a "fast-slow" two sections characteristics.In addition,the fracture morphology analysis was helpful to explain the macro failure characteristics of the specimens.The interaction of joint angle,loading rate and rock anisotropy was the main reason for different macro failure modes and micro fracture morphology of joint specimens under dynamic tensile load.(6)Based on the dynamic datas of uniaxial state and three-dimensional state,the dynamic constitutive model of jointed sandstone was derived and verified,which could consider the joint angles,strain rate effect and confining pressure effect.The results showed that:the dynamic constitutive model curve could better reflect the test curves,the variation trends of theoretical peak stress were basically consistent with the test results,and the modified model could describe the dynamic characteristics of jointed sandstone under different stress states.The propagation law of scattered stress wave field and the influence of S-wave derived from P-wave were analyzed when P-wave was incident into weakly filled jointed rock mass at any angle.The stress ratio and energy ratio of stress wave after n times of transmission and reflection were derived.(7)Based on the background of deep roadway blasting excavation,the rock mass containing multiple fracture zones with various angles in practical engineering was simplified as weakly filled jointed specimens on laboratory test scale.The static and dynamic mechanical parameters of rock mass were obtained by static and dynamic mechanical tests,the failure mode of rock mass was analyzed,and the blasting parameters were optimized.The bench method and millisecond blasting method were adopted for construction.The blasthole utilization rate was more than 95%,the average single entry was increased by more than 20%,the disturbance to surrounding rock was small,and the roadway was well formed.Figure[81]Table[10]References[249].