Study On The Damage And Deterioration Mechanism Of Anhydrite Rock Under Freeze-thaw Cycles

With the development of the social economy and the deep utilization of natural resources by human beings,many geotechnical engineering constructions are emerging in cold regions.Anhydrite rock is an evaporite widely distributed in the crust,which is distributed in Northwest,North,Northeast,and Southwest China.The anhydrite layer located in the cold regions is affected by the freeze-thaw cycles,causing a series of safety problems such as the decline of rock mass bearing capacity,the expansion of primary pores and cracks,etc.,that are not conducive to project construction and operation.For this reason,this paper takes the"damage and deterioration mechanism of anhydrite rock under freeze-thaw cycles"as the research topic.Based on laboratory tests,theoretical model derivation,and numerical simulation analysis.The damage and deterioration laws and of anhydrite rock under freeze-thaw cycles were revealed from mesoscopic,microscopic and macroscopic scales.Based on the characteristics of acoustic emission,the failure precursor signal of freeze-thawed anhydrite under loading was initially proposed.Combined with the maximum tensile strain criterion,the damage constitutive model of freeze-thawed rock was established.The discrete element numerical simulation method was used to discuss the effects of freeze-thaw cycles and prefabricated fractures on the crack propagation,mechanical behavior,and stress field distribution of anhydrite rock.The main research contents and achievements of this paper are as follows:（1）The damage evolution laws of meso and micro-structures of anhydrite rock under freeze-thaw action were analyzed by non-destructive testing methods of nuclear magnetic resonance and electron microscope scanning.The results show that with the increase of freeze-thaw cycles,the micropores（r≤0.1μm）,PT-I pore throats（0～0.18）),fractal dimensions of pores（D_P）and pore throats(D_PT)in anhydrite rock decrease exponentially.However,the mesopores（0.1μm<r<1μm）,macropores（r≥1μm）,PT-II pore throats（0.1～4μm）and porosity increase exponentially.The porosity of anhydrite under freeze-thaw action decreases exponentially with the increase of micropores,PT-I pore throat and fractal dimension of pore structure.However,the porosity increases exponentially with the mesopores,macropores and PT-II pore throats.From the analysis of degree of correlation between meso and micro-structures,macropores,PT-I pore throats,and pore throat fractal dimensions have the most significant influence on the porosity of freeze-thawed anhydrite rock.In addition,under freeze-thaw cycles,the mineral particles on the surface of anhydrite are severely exfoliated,and the roughness and pore area increase significantly.The frost heave force in the cracks during the freezing process and the volume expansion and dissolution of mineral particles during the thawing process jointly promote the damage of the meso and micro-structures of the anhydrite rock under freeze-thaw cycles.（2）The macroscopic damage and deterioration law and mechanisms of anhydrite rock under freeze-thaw cycles were revealed through macro-physical and mechanical laboratory tests.The relationships between the meso,micro-structure,and macro-mechanical parameters were discussed.The damage and deterioration mechanism of freeze-thawed anhydrite was revealed from multiple scales.The study shows that with the increase of the freeze-thaw processes,the main mechanical parameters of anhydrite rock（such as compressive strength,tensile strength,elastic modulus,and cohesion）decrease exponentially,while the mass loss rate and ductility deformation characteristics gradually increase.Moreover,the internal friction angle fluctuates only in the range of4.04°.The tensile elastic modulus of anhydrite rock is the most sensitive to the freeze-thaw environment,its decay rate is the largest（0.96%,λ=-0.00964）,and its half-life value is the shortest.(N_1/2=72 cycles).Anhydrite rock’s compressive and tensile strengths decrease exponentially as the macroscopic damage variable calculated based on the elastic modulus increases.From analyzing the degree of correlation between the meso,micro-structure,and macro-mechanical parameters of anhydrite rock,the macropores,PT-II pore throats,and pore throat fractal dimension have the most obvious influence on its macro-mechanical parameters.It is proposed that water-rock expansion and softening effects play an essential role in the freeze-thaw damage of anhydrite rock.（3）Based on the uniaxial compression tests and acoustic emission monitoring system,freeze-thawed anhydrite rock’s acoustic emission（AE）characteristics under loading were obtained.The crack propagation mechanism was analyzed,and the failure precursor signal of freeze-thawed anhydrite rock under unidirectional compressive stress was initially proposed.The research shows that with the increase of freeze-thaw cycles,the acoustic emission count and energy proportion of anhydrite rock decrease exponentially in stages I and II and increase exponentially in stage III.Moreover,the number of data points of the dominant frequency and intermediate frequency band（IF:100～250 k Hz）signal proportion is reduced,while the low-frequency band（LF:0～100k Hz）signal proportion increases.Under the action of loading,the micro-crack propagation mode of freeze-thawed anhydrite rock is dominated by tensile cracks.With the rise in freeze-thaw processes,the total crack events and micro-scale crack events gradually decrease,while the macro-scale crack events increase progressively.When the dominant frequency signal forms a vertical band-like distribution,the AE event rate F（τ）continues to grow suddenly,or the dynamic b value is lower than 0.71,indicating that the sample is about to undergo macroscopic fracture.（4）Based on the triaxial compressive test data of anhydrite rock under freeze-thaw cycles,the statistical damage mechanics theory was introduced,and a damage constitutive model of freeze-thawed rock was established which could reflect the post-peak stress dropping rate and residual strength.The physical meaning of the model parameters was analyzed,and the rationality and applicability of the model were verified.The results show that the evolution process of the total damage variables of freeze-thawed rock can be divided into the initial damage stage,accelerated damage stage,and complete damage stage.With the increase of freeze-thaw processes,the damage growth rate and the peak value of the total damage evolution rate decrease exponentially,while the total damage variable in the initial damage stage and critical damage strain gradually increase.The model parameters m,f_0,and n represent rock brittle deformation characteristics,ductility deformation characteristics,and post-peak stress dropping rate,respectively.Compared with the model in the literature,the proposed model can effectively reflect the post-peak stress dropping rate and residual strength characteristics of freeze-thawed rocks and has the advantages of a minor error and strong applicability.（5）In the particle flow program PFC2D,numerical models close to the actual mineral mass percentage were constructed.The mechanical behavior,crack propagation,and stress field distribution of intact and fractured freeze-thawed anhydrite models were analyzed.The research shows that under the action of loading,the proportion of tensile cracks in freeze-thawed anhydrite rock is the highest,followed by tensile-shear cracks and compressive-shear cracks.With the increase of freeze-thaw cycles,the proportion of tensile cracks increases exponentially,while the proportions of tensile-shear and compressive-shear cracks decrease exponentially.With the increase of the fracture inclination angle,the proportion of tensile cracks decrease exponentially,while mechanical strength,tensile-shear cracks,and compressive-shear cracks proportions increase exponentially.The samples with fractures（α=0°,30°and 60°）mainly distribute compressive stress and high shear stress accumulation areas at both ends of the fracture.With the increase of the fracture inclination angle,the extreme value of tensile stress and the extreme value of compressive stress at the peak point of the fractured samples decrease exponentially,while the extreme value of shear stress increases exponentially.With the increase of freeze-thaw cycles,the extreme values of maximum principal stress and shear stress of intact and fractured samples decrease exponentially.In addition,the degree of influence of fractures on the mechanical strength,crack propagation mechanism and stress field distribution of the samples decreases with the increase of the inclination angle of the fractures.