Research On Pore Structure Evolution And Damage Mechanism Of Freeze-thaw Rock Mass In Cold Region

With the deepening implementation of “the Belt and Road” initiative,numerous engineering construction activities are or will be carried out in the western high-cold and high-altitude regions in China.The seasonal and diurnal temperature changes over a long time lead to the damage and destruction of rock due to freeze-thaw cycles,which is prejudicial to the security and stability of rock engineering in cold regions.Therefore,it is necessary to study the macroscopic mechanical behavior,microscopic pore structure evolution and damage mechanism of freeze-thaw rock masses in cold regions.In this paper,typical sandstone in seasonal permafrost areas is selected as the research object,and the mechanical properties,acoustic emission characteristics,crack evolution process,and microstructure damage evolution of intact sandstone,rock-like material with three-dimensional built-in cracks and sandstone with penetrating cracks under freeze-thaw cycle action are studied by means of laboratory mechanical tests,nondestructive testing technology and theoretical analysis.The influences of soaking conditions,crack morphology and loading mode on the strength,deformation,failure mode and crack development progress of sandstone under freeze-thaw cycles are analysed.The damage constitutive model of sandstone for different soaking conditions and freeze-thaw cycles during uniaxial compression,the microscopic damage model considering pore distribution under different freeze-thaw cycles and the damage constitutive equation under the coupling effect of freeze-thaw and cyclic loading and unloading are established,revealing the damage evolution mechanism of freeze-thaw rock masses in cold regions.The freezing characteristics of pore water and the evolution of unfrozen water content for the saturated sandstone are analysed based on real-time nuclear magnetic resonance(NMR)technology under the action of freeze-thaw cycles.The relationship between freeze-thaw damage and pore distribution,porosity and temperature are further discussed and determined through the theoretical calculation and analysis of frost heaving strain.Finally,an elasto-plastic model is proposed to predict the frost heaving deformation by considering the pore size and distribution,matrix yield stress,unfrozen water film thickness,disjoining pressure,and influence factor of water saturation during the process of freeze-thaw cycles,and the model is verified by experiments.The main conclusions are as follows:(1)The uniaxial compressive strength,tensile strength and elastic modulus of intact sandstone are negatively correlated with the number of freeze-thaw cycles.Brazilian splitting test displacement,peak strain,wave velocity,porosity,acoustic emission count and damage variables are positively correlated with the number of freeze-thaw cycles.Based on digital image correlation(DIC)technology,the crack initiation and propagation evolution mechanism of sandstone under freeze-thaw cycles is revealed.The pores and cracks in the Brazilian split sample increased as the freeze-thaw cycles increased,the strain difference between the end and the middle decreased gradually,and the location of crack initiation changed from two ends to the middle.However,the local strain band of the intact uniaxial compression sample is generated from one end,gradually widens and moves to the other end of the sample,and finally forms an oblique or vertical local strain band in the middle of the sample.Based on the damage variable defined by the principal strain obtained by DIC technology,the damage evolution process of sandstone can be divided into three stages: slow or negative growth stage,steady growth stage and rapid growth stage.The damage variable can better describe the deformation and damage evolution law of the sandstone.(2)The uniaxial compressive strength,elastic modulus and failure mode of semi-immersion in water solution intact sandstone are more seriously damaged than by anhydrous immersion by under the freezing-thawing cycle.When the sample was subjected to 10～50 freeze-thaw cycles under anhydrous immersion conditions,the high distinct AE counts were distributed discretely after the initial crack closure stage,the prepeak stage and the postpeak stage.When the freeze-thaw cycle is 0,few AE counts are concentrated near the peak strength stage.When the freeze-thaw cycles are from 0 to 50,the highly distinct AE counts are concentrated in the prepeak stages and postpeak stages,and almost no AE counts are discretely near the crack closure stage and elastic deformation stage for the sample subjected to semiimmersion in water solution.Kernel density estimation(KDE)results show that the tensile crack has a higher AF(acoustic emission count/duration)value and a lower RA(rise time/amplitude)value under the lower freeze-thaw cycles,and the obvious shear/mixed cracking phenomenon occurs after the higher freeze-thaw treatment.The K-means clustering method has advantages in the classification of AE cracking events.The b value increased with the increase in the number of freeze-thaw cycles,indicating that the degree of internal damage of the sample increased with the increase in the number of freeze-thaw cycles,and the damage degree of the anhydrous immersion sample was greater than that of the semi-immersion sample in water solutions.There are four macroscopic failure modes of uniaxial compression: splitting failure,single shear failure,conical failure and multicrack failure.The number of cracks increased with the increase in the number of freeze-thaw cycles.Scanning electron microscopy(SEM)indicated that the freeze-thaw action caused new cracks in the inner part of the sample,and the damage of the anhydrous immersion samples was more serious than that of the sample subjected to semi-immersion in water solution.A continuous damage constitutive model is established to quantitatively analyse and describe the damage evolution law of the sample subjected to different freeze-thaw cycles under different immersion conditions during uniaxial compression,which is in good agreement with the prepeak and postpeak stages of the axial stress-strain relationship.(3)The uniaxial compressive strength and elastic modulus of the rock-like material with three-dimensional built-in cracks are positively correlated with the inclination angle of the precast cracks.After 50 freeze-thaw cycles,the uniaxial compressive strength for the 90° fissure inclination angle sample is the lowest.The cracks on the surface of the sample gradually increased with increasing freeze-thaw cycles.After 50 freeze-thaw cycles,massive particles on the sample surface fell off with the surface.The number of acoustic emission events increases suddenly when the stress drops or fluctuates on the axial stress-strain curve of rock-like samples with three-dimensional built-in cracks in the crack closure stage,elastic deformation stage and yield damage stage.The acoustic emission location results show that under the same stress level,the acoustic emission count increases with the increase in the number of freeze-thaw cycles,indicating that the freeze-thaw cycles cause irreversible damage inside the rock.The damage variable is defined based on the number of AE events,revealing that the damage evolution process of the sample could be divided into the initial damage stage,stable damage growth stage,accelerated damage growth stage and postpeak damage stage during uniaxial compression.Based on nuclear magnetic resonance(NMR)technology,the T2 distribution,total spectral area,pore size distribution and other microscopic parameters are obtained,and the pore structure changes under different freeze-thaw cycles are revealed.A continuous damage model considering pore size distribution is established to describe the damage evolution characteristics of samples after freezing-thawing cycles,predicting the uniaxial compressive strength of rock-like materials with three-dimensional built-in cracks after freezing-thawing cycle treatment.(4)The uniaxial compressive strength and elastic modulus of sandstone with penetrating fractures increase with increasing fissure inclination angle,while the uniaxial cyclic loading and unloading strength decreases with increasing number of freeze-thaw cycles.After 50 freeze-thaw cycles,the uniaxial cyclic loading and unloading strength of the 90° fissure inclination angle sample is the highest and that of the 0° fissure inclination angle is the lowest.Based on computed tomography(CT)X-ray scanning,the number of shear cracks increases with increasing freeze-thaw cycles at the same fissure inclination angle.The failure mode of the sample changes from single shear or split failure to conical failure and then to multicrack mixed failure.Under the same freeze-thaw cycle conditions,the failure mode of the sample without freeze-thaw cycle treatment changes to single shear failure,cone failure and split failure with increasing fissure inclination angle.After freezing-thawing cycle treatment,with increasing fissure inclination angle,conical failure and multicrack mixed failure appeared,and the damage area gradually expanded from the end of the sample to the inside.As a result of freezing-thawing and loading and unloading cycles,the cumulative damage strength increases proportionally with increasing fissure inclination angle.The morphological characteristics of the hysteresis curve of the stress-strain curve in the uniaxial loading and unloading process of sandstone with penetrating cracks after freeze-thaw cycles are analysed,and the damage evolution characteristics of sandstone with penetrating cracks under the coupling effects of freeze-thaw cycles and uniaxial cyclic loading and unloading are obtained.Based on NMR technology,the T2 distribution and NMR imaging are detected,revealing the microstructure damage evolution of the sandstone before and after freeze-thaw cycles,the damage factor based on the definition of axial strain is introduced,and the damage constitutive equation is established to quantify the damage evolution of sandstone with penetrating fractures under the coupling action of freeze-thaw cycles and uniaxial cycle loading and unloading.(5)The real-time NMR technique is used to reveal the pore structure distribution,pore water freezing characteristics and unfrozen water content changes of coarse-grained sandstone,medium-grained sandstone and fine-grained sandstone during freeze-thaw cycles.The free water content of the three sandstone samples decreases sharply with decreasing temperature during freeze-thaw cycles,while the bound water in small pores requires a lower temperature to freeze.At-35℃,the unfrozen water content of the three samples is much higher than the theoretical calculation results.The larger the equivalent mean radius is,the faster the water ice conversion rate.The frost heaving effect is mainly caused by free water freezing at the early stage of freezing.The more free water content in the sample,the greater the frost heaving force generated in the sample.With a further decrease in temperature,the bound water content plays a leading role in the change in frost heaving stress.In the range of 0～-10°C,the frost heaving strain increases sharply and gradually flattens with decreasing temperature.Coarse-grained sandstone with high porosity has the maximum frost heaving strain,while fine-grained sandstone with low porosity has the minimum frost heaving strain.The wave velocity and SEM test results of rock samples before and after freeze-thaw cycles show that the freeze-thaw damage of coarse-grained sandstone with large frost heaving strain is more serious than that of the other two kinds of medium-grained sandstone and fine-grained sandstone with frost heaving strain.The larger the porosity is,the larger the frost heave strain,and the more serious the damage.(6)The micromechanical frost heaving strain model based on the Fagerlund function considering the thickness of the unfrozen water film and separation pressure can better predict the frost heaving deformation under freeze-thaw action when the influence factor of water saturationξ=0.5.During the freezing process,the frost heaving stress of pore water increases rapidly,and the freezing rate of pore water decreases after most pore water freezes.The existence of thermal stress and film stress weakens the frost heaving stress.The plastic strain increases gradually with increasing frost heaving stress during the freezing process.The elastic strain recovers,but the residual plastic strain still exists during the thawing process.The model can accurately estimate the residual plastic strain of sandstone under freezing-thawing action.