| With the continuous promotion of the national "the Belt and Road" initiative,amounts of infrastructure construction is or will be carried out in the marginal area around the Qinghai-Tibet plateau and the Tianshan mountains in Xinjiang.The rock mass located in alpine region is frozen for a long time.These mechanical characteristics of frozen rock mass and the damage expansion and failure behavior under impact dynamic load are their key factors to determine the safety of rock mass engineering construction in alpine region.There are lots of initial defects such as pores and cracks in the rock mass,which leads to the complexity of rock mass structure.In addition,the diversity of environmental factors and construction disturbance leads to the unclear answers to the key problems of static and dynamic mechanical properties,mechanical constitutive relationship,damage and failure mechanism of frozen rock mass,which seriously restricts the optimal design and safe construction of rock engineering in cold regions.In this dissertation,some intact sandstone samples and fractured sandstone ones are selected as the research objects,and the mechanical properties,freezing strengthening or weakening effect and main control mechanism,impact compression and splitting failure characteristics of frozen intact sandstone and frozen fractured sandstone are studied by means of laboratory test,theoretical analysis and numerical simulation.The variation of strength and deformation characteristics of frozen intact sandstone and fractured sandstone with freezing temperature and fracture dip angle is analyzed,and the macro-meso mechanism of freezing strengthening or weakening effect is also revealed.The damage and fracture characteristics of frozen fractured sandstone under impact load are studied,and the internal stress balance and deformation characteristics of frozen fractured sandstone under impact compression and splitting load are studied by numerical simulation.Finally,based on the macro-meso damage theory of particle reinforcement theory,a constitutive model considering the initial macro-meso damage is established,and the main factors affecting the dynamic failure of frozen fractured sandstone are analyzed.Some main conclusions are as follows:(1)The uniaxial compressive strength,tensile strength and elastic modulus of frozen intact sandstone are negatively related with freezing temperature,but the variation rate is significantly different in different temperature ranges.There are free water,capillary water and adsorbed water in the sample at room temperature.With the decrease of freezing temperature,the unfrozen water content decreases rapidly at first and then slowly.When the temperature drops from 0? to-4?,the unfrozen water content decreases rapidly,and the ice content in the pores increases rapidly.The support of ice on the sandstone skeleton makes its strength increase rapidly.The uniaxial compressive strength is mainly affected by the thickness of unfrozen water film and the frost heaving.(2)The compressive strength and elastic modulus of frozen fractured sandstone decrease first and then increase with the increase of fracture dip angle.When the fracture dip angle is 30°,the compressive strength of frozen fractured sandstone is the minimum,and the time to reach the acoustic emission peak ringing count is the latest.The fracture initiation angle of frozen fractured sandstone decreases with the increase of fracture dip angle,and the fracture initiation stress and time decrease first and then increase with the increase of fracture dip angle.Freezing has a significant strengthening effect on fractured sandstone.With the increase of fracture dip angle,the strengthening effect becomes weaker.The strengthening effect of freezing mainly comes from the strengthening effect of meso scale and macro scale.The strengthening effect of ice in meso pores includes the support,cementation and filling of ice in pores.The strengthening effect of ice in macro fractures includes the support of fractures,the cementation of ice rock interface and the alleviation of stress concentration effect at fracture ends.(3)The dynamic compressive strengths of frozen fractured sandstone increase with the decrease of temperature.The growth rates of dynamic compressive strength of fractured sandstone samples are small between 0? and-8?.Yet,the dynamic compressive strengths of frozen fractured sandstone samples first decrease and then increase with the increase of fracture dip angle.Except 0?,the strength inflection point appears at 45°at other temperatures.Under the impact load,regardless of the dip angle of the crack,the first failure is the crack ice,and then the rock matrix is damaged.The 0,15°and 30°samples are basically intact,and only a small part of the end face is damaged.The 45°,60°,75° and 90°samples have macro cracks through the sample,and most of the cracks propagate and run through along the tip of the initial crack,and the cracks are stable There are tensile failure parallel to the compressive stress direction and shear failure at a certain angle with the compressive stress,which belong to mixed failure mode.(4)The dynamic splitting strengths of frozen fractured sandstone increase with the decrease of temperature,which are approximately exponential.The dynamic fracturing failure modes of frozen fractured sandstone with different dip angles have common characteristics and significant differences.The common characteristics include two features:?the crack initiation occurs near the initial crack end of the loading side,and all of them are tensile cracks;?During the failure process of the sample,the crack ice and the rock on both sides are de-bonding.The main differences are listed as follows:?when the inclination angle is small,the failure of the specimen is controlled by the extension of the tensile crack,which is perpendicular to the loading direction;when the inclination angle is large,the tensile crack and shear crack control the failure of the specimen together.?When the dip angle is small,the fracture ice rock interface is tensile de-bonding failure,which occurs at the initial stage of loading;when the dip angle is large,the fracture ice rock interface is shear failure,which occurs before the overall failure of the sample.Freezing effect has a significant strengthening effect on the dynamic splitting strength of fractured sandstone.(5)Based on the test results,the fractured sandstone is considered as a composite damage material with both macro-cracks and micro-defects,and the dynamic constitutive model of frozen fractured sandstone considering micro damage is established based on the basic theory of particle reinforced micro-macro damage combination model.Meanwhile,considering the influence of macro defects,the dynamic constitutive model equation of frozen fractured sandstone is proposed.The effect of the constitutive model is verified by comparing the test curves of frozen fractured sandstone with different freezing temperatures and fracture dip angles with the results of the constitutive model.Finally,the influence characteristics of fracture dip angles and freezing temperatures on the mechanical indexes of frozen fractured sandstone are explored:? with the increase of fracture angle,the dynamic compressive strength shows a certain difference,which is related to the gravity migration and precipitation of unfrozen water.?With the decrease of freezing temperature,the dynamic compressive strength shows an overall growth trend,and it will enter the end stage according to the strength increases rapidly after full freezing stage. |
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