Characteristic And Model Study On Rock Creep Damage Under Freeze-thaw Cycles

With the continuous increase in the number of rock mass projects such as slopes,roadbeds,and tunnels in cold regions of our country,the long-term stability of rock mass projects under the effects of freezing and thawing has become an important issue.The study of rock creep damage characteristics under freeze-thaw cycles is the basis for long-term stability evaluation of rock mass engineering,and has important theoretical value and engineering significance.This study mainly focuses on the instantaneous and creep damage characteristics and models of intact and bedding rocks under freeze-thaw cycles.Using a combination of laboratory tests,theoretical analysis,and damage detection,the instantaneous and creep damage characteristics of intact rock and bedding rock were studied.On this basis,for intact rock and bedding rock,an instantaneous meso-statistical damage model considering freeze-thaw cycles and a creep damage model considering the coupling effect of multiple factors were established respectively.Furtherly,based on the plastic yield criterion throughout the ubiquitous-joint and the coupling damage variables of freeze-thaw and load,a three-dimensional creep damage model of bedding rock that can be used for numerical calculation was established.Finally,the secondary development was carried out based on FLAC^3D,and the stability analysis of freeze-thaw layered rock slope is carried out combined with the strength reduction method.The specific research content is as follows:（1）Instantaneous mechanical properties and damage model study on rock under freeze-thaw cycles.The instantaneous mechanical properties of bedding rock（slate）and intact rock（sandstone）under freeze-thaw cycles have been studied through experiments.The effects of freeze-thaw cycles and bedding angle on the mechanical properties of bedding slate and the effects of freeze-thaw cycles on the permeability of intact sandstone in the whole stress-strain process were revealed.The test results show that the peak stress and elastic modulus of slate decrease with the increase of freeze-thaw cycles,but the peak strain increases,and the peak stress and elastic modulus change anisotropic with the bedding angle;Under the action of freezing and thawing,the initial permeability,minimum permeability and maximum permeability of intact sandstone increase with the increase of freeze-thaw cycles.The permeability change of intact sandstone is closely related to its internal fracture expansion.In the process of full stress-strain failure,the permeability decreases first and then stabilizes,and finally increases suddenly after rock failure.On the basis of the above experiments,the Weibull distribution function considering the number of freeze-thaw cycles and different strength criteria were introduced,and the meso-statistical damage models of freeze-thaw effects was established for bedding and intact rocks.The research shows that the model introduced with the improved Hoek-Brown criterion describing the characteristics of anisotropic strength can reflect the influence of freeze-thaw cycles and bedding angles on the mechanical properties of the bedding slate.The model introduced with the Drucker-Prager criterion can reasonably reflect the effects of freeze-thaw cycles and the loading process on the fracture damage and permeability of intact sandstone.The calculated data of the model are in good agreement with the experimental results,indicating that the established meso-statistical damage model can well reveal the mechanism of the change of macro-mechanical properties caused by the meso-fracture of bedding and intact rocks.（2）Experiments and model study on intact rock creep under low temperature freezing and thawing.Through the creep tests of intact rock（gneiss）under freeze-thaw cycles,the influence of the number of freeze-thaw cycles on the elastic deformation,creep deformation,long-term strength,and failure mode of the gneiss was analyzed.With the increase of freeze-thaw cycles,the elastic deformation and creep deformation of gneiss gradually increases,while the failure stress,creep duration,and long-term strength gradually decrease.Through scanning electron microscopy（SEM）and nuclear magnetic resonance（NMR）tests,it was found that as the number of freeze-thaw cycles increases,the mineral particles in the gneiss become loose,and the peak and peak area of the T2 spectrum curve increases significantly,indicating that the micropores and microcracks of rock increase.Based on the Nishihara model,considering the influence of freeze-thaw cycles on creep parameters,load damage variables in the acceleration stage were introduced,a five-component creep damage model of gneiss with variable parameters considering freeze-thaw cycles was established.Comparing the calculation results of the model with the experimental data shows that the model can accurately describe the characteristics of the three stages of decelerated creep,steady-state creep,and accelerated creep of gneiss under freeze-thaw cycles.To further understand the influence mechanism of frozen water ice phase transition on rock creep characteristics,a creep test of red sandstone with different temperature and water content under low-temperature conditions（T<0℃）was carried out.The tests showed that the failure stress and long-term strength of red sandstone decrease with the increase of water content at room temperature（20℃）.But under low temperature（-5℃,-10℃,and-20℃）conditions,the failure stress and long-term strength of red sandstone increase with the increase of water content.Besides,the resistivity change law of frozen red sandstone during the creep tests was analyzed,and the influence mechanism of the ice-water phase change on the creep characteristics of the red sandstone during the freezing process was analyzed by NMR.（3）Experiments and model study on bedding rock creep under freeze-thaw cycles.Based on the study of intact rock creep characteristics,the study of bedding rock creep tests and models under freeze-thaw cycles was carried out.Based on the experimental data,the influence of freeze-thaw cycles and bedding angle on the creep deformation,creep strain rate and failure mode of slate was analyzed.In the triaxial compression creep tests of the bedding slate,the long-term strength of the slate decreases first and then increases with the increase of the bedding angle.The maximum and minimum long-term strength appear at 90°and 30°,respectively,indicating that the bedding slate presents anisotropic characteristics with the change of the bedding angle.In the triaxial compression creep tests of freeze-thaw bedding slate,the instantaneous elastic deformation,creep deformation,axial failure strain,axial initial creep rate,and the steady-state creep rate of the slate at the same bedding angle all increase as the number of freeze-thaw cycles increases,but it was opposite for the failure stress.Based on the five-component creep damage model of gneiss with variable parameters,the soft components of fractional calculus were introduced to form an improved Kelvin body and Binham body,and the coupled damage variables of freeze-thaw damage,bedding damage,and load damage were considered,a six-component creep damage model of bedding slate reflecting the effects of freeze-thaw,bedding and load was established.Comparing the calculated value of the model with the experimental value shows that the model can reasonably reflect the influence of freeze-thaw cycles and bedding angles on the creep characteristics of slate,and can well reflect the characteristics of the acceleration stage of slate creep.（4）Creep damage model development of bedding rock considering freeze-thaw cycles and analysis of slope stability.According to the above-mentioned multi-factor coupling damage idea,the coupling damage factor was introduced and based on the plastic yield criterion throughout the ubiquitous-joint,an improved three-dimensional power-ubiquitous creep damage model（Power-Ubiquitous Freeze-thaw Load Damage Model,PUFLDM）was established.Based on FLAC^3D for secondary development,the model program was written,and the rationality of the model was verified through experimental data.Further combining with the strength reduction method,a long-term stability analysis method of freeze-thaw bedding rock slopes was established,and through engineering examples,the damage distribution and safety factors of slopes caused by freeze-thaw cycles,bedding angles and creep time were analyzed.Calculations show that as the number of freeze-thaw cycles and creep time increase,the potential sliding surface of the rock slope continues to expand and the safety factor gradually decreases.The shape of the potential sliding surface of rock slope presented a broken line,and the specific shape was affected by bedding angle.The calculation results show the effectiveness and rationality of the method.