Experimental Study On Mechanical Properties And Crack Propagation Of Thermal Damaged Rock

Natural rocks are affected by temperature fields and subjected to high temperature problems when experiencing complex geological tectonic effects such as volcanic eruptions and plate motions.As engineering rock mass lie in a certain geological environment,in many rock engineering,such as large-scale hydropower project,underground nuclear waste deep disposal,deep oil and gas exploration,carbon dioxide geological storage,etc.,high temperature or thermal damage as a geological condition has significantly effects on the physical and mechanical properties of engineering rock mass.For example,in the enhanced geothermal system(EGS)developed by the hot dry rock(HDR),the study of the thermal cracking mechanism is the foundation of reservoir construction,and the strength and deformation properties of rocks under thermal damage conditions are the key scientific problems in the development and utilization of the HDR.Therefore,in-depth study of the physical and mechanical properties of rock under high temperature is the basis for ensuring the safety and long-term stability of rock engineering.The macroscopic mechanical properties of the thermal damaged rock are closely accosicted with the microscopic crack propagation.Only in-depth mastering the mechanisms of thermal cracking and crack propagation in compression can accurately assess the macroscopic behaviors of the thermal damaged rock.At present,the study of thermal damaged rock is mainly based on the macroscopic laboratory tests,and the research on the mechanism of microscopic thermal cracking is not sufficient.Besides,experimental studies are often limited in consideration of factors other than temperature and the effects of multiple factors coupling.In this paper,the mechanical properties of thermal damaged rock are the main research topic.The compression tests with acoustic emission(AE)system monitoring are carried out under different thermal damage conditions(high temperature or high temperature cycle),different confining pressure,different rock type(sandstone,marble and granite)and different sample size.Afterwards,the constitutive models of the microscopic crack propagation in compression process are established,and the mechanism of thermal cracking of rock is analyzed from the microscopic view.The main contents and results of this paper are as follows:(1)Compression tests for three types of rocks after thermal damage are carried out and the AE of the deformation process is monitored.The results show that the peak strength and the Young's modulus of marble and granite gradually decrease with the increase of heating temperature,while the peak strength and Young's modulus of sandstone increase first and then decrease.The brittle failure of the rock is weakened by thermal damage,but the failure pattern does not change significantly.Meanwhile,the non-linear characteristic of the crack closure stage becomes obvious.Through the comparative analysis of the mechanical properties of three types of rocks after thermal damage,it is found that thermal damage has the strongest effect on marble,followed by granite and sandstone.The AE activity of the rock corresponds well with the progressive fracture process,and AE hit curves of thermal damaged rocks arrive at one wave crest in the crack closure phase.Based on the sensitivity of AE to crack propagation,the damage variables are defined by the AE parameter.By introducing the method of nonlinear crack closure phase,a new damage constitutive model based on the AE parameter is then proposed.The model is well fitted to the thermal damaged rock test data,reflecting every mutational point of stress-strain curves.(2)A series of triaxial compression tests for thermal damaged marble under different confining pressures are conducted.From the results analysis,high temperature causes thermal cracking,while confining pressure inhibits cracks to propagate in the compression process.The deformation and failure characteristics of marble are transition from brittleness to ductility caused by the increasing confining pressure.The peak strength,peak strain and Young's modulus gradually increase with the increase of the confining pressure,and yet,the growth rate of the peak strength gradually slows down.When the Mohr-Coulomb criterion is used to describe the above trend,the rock strength in the low confining zone is overestimated.By considering the variation law of the internal friction angle and the confining pressure,a nonlinear Mohr-Coulomb peak strength model is established.The model is validated by the experimental data of this paper and previous studies.Due to the fact that less constitutive model considers the residual strength of the rock,the logistic function of the damage evolution equation is introduced,and the strain softening damage constitutive model considering the crack closure effect and post-peak residual strength is established.(3)In order to study whether the traditional size effect model is suitable for the thermal damaged rock,uniaxial compression tests for marbles with different sample sizes and different thermal damage levels are carried out,accompanied by AE monitoring.It is found that the peak strength of thermal damaged rock decreases faster with the increase of sample size than that of undamaged rock.The increase of the temperature or the sample size causes the initial nonlinear phase in stress-strain curves to become longer,and has no obvious effect on failure modes.In addition,b-value decreases gradually during the deformation process,and increases with the increasing temperature.The spatial distribution of AE events is in a considerable agreement with the macroscopic fracture surface.The stress-strain relationships of thermal damaged marbles with different sample sizes are well described by the AE model and the strain softening model.(4)In view of the fact that the cyclical heating method can be used to prepare fractured rock mass in a laboratory environment,the effects of cyclical heating on the mechanical properties of marble and granite are experimentally studied.It can be seen from the results that cyclical heating makes the P-wave velocity,the peak strength and the Young's modulus of the two rocks continue to decrease,while the peak strain and crack density parameters continue to increase.Besides,the brittle deformation characteristics are weakened with the increasing number of thermal cycles.The AE characteristics of the rock show phasic behavior,and cyclical heating makes the AE in the initial compression phase more active.The models proposed in this paper fit the constitutive relation of cyclical heated rocks well.(5)The X-ray diffraction analysis of granites with different thermal cycles is carried out,which proves that the change of mineral composition is not the main reason for the change of macroscopic mechanical properties.Through optical microscopy of cyclical heated granite and marble,it is shown that the mineral composition,cementation type,particle size and shape all affect the developmental form and propagation path of cracks.Based on the quantitative analysis of the crack,it is found that the number,length and linear crack density of the grain boundary cracks and the intragranular cracks per unit area increase with the increasing number of thermal cycles,and the anisotropy features of the thermally induced microcracks are not obvious.