Study On Creep Mechanical Properties And Stability Of The Frozen Wall Of Cretaceous Sandstone Shaft During Thawing Process

With the further increase of China’s demand for coal resources,the national energy strategy is gradually transferred to Western China.Due to the differences between the eastern and western regions,the mines in the western region usually need to pass through thick soft rock formations.Freezing method is widely used with its unique advantages.After the excavation of the shaft is completed,the frozen wall will be in a thawed state for a long time.Under the long-term load,the frozen wall may produce large deformation,resulting in instability and rupture of the well wall.This paper addresses the key issues of the creep mechanical properties and the stability of the frozen wall during the thawing process of the frozen wall soft rock in the western region of China.The Cretaceous sandstone in the deep soft rock stratum is selected as the research object based on the freezing project of the vertical shaft of Xinzhuang Coal Mine in Gansu Province.The creep mechanical properties and damage of Cretaceous sandstone during thawing are studied by combining theoretical analysis,laboratory test,model test and numerical simulation.The stability of the frozen wall of the vertical shaft under long-term loading is evaluated.The main research contents and conclusions are as follows:(1)The reasons for the poor stability of Cretaceous formation water are revealed by in-depth analysis of the macro-structural and micro-structural characteristics and physical parameters of the Cretaceous sandstone.By measuring the thermophysical parameters of the Cretaceous sandstone under thawing conditions,the heat transfer law of the sandstone during the thawing process was systematically mastered.By carrying out uniaxial and triaxial compression tests under the thawing conditions of Cretaceous sandstone,the changing law of mechanical properties of Cretaceous sandstone during thawing process was systematically analyzed.The occurrence state of internal water in Cretaceous sandstone after low temperature treatment was explored by nuclear magnetic resonance technology.By analyzing the relationship between sandstone failure and unfrozen water,a binary medium constitutive model of Cretaceous sandstone is established.By analyzing the relationship between sandstone failure and unfrozen water,a binary medium constitutive model of Cretaceous sandstone is established.(2)The corresponding creep strain-time curve and strain rate-time curve were obtained by carrying out the creep test during the thawing process of the Cretaceous sandstone.The study found that the creep test under the thawing condition of Cretaceous sandstone showed obvious attenuation creep stage,steady creep stage and accelerated creep stage.Transient deformation occurs during the decay creep stage.After a short period of time,it enters the steady-state creep stage,where the creep rate is low and the sandstone deformation is small.With the increase of temperature,the creep behavior of Cretaceous sandstone has an obvious acceleration stage,where the deformation of sandstone is larger and the rate is faster.The long-term strength value of the sandstone is finally determined through a variety of methods to verify each other.(3)A creep damage model of Cretaceous sandstone during thawing is established.By analyzing the creep test results of the Cretaceous sandstone during thawing,it is found that the creep failure of the sandstone is closely related to the unfrozen water content.The unfrozen water content of Cretaceous sandstones increases gradually during the thawing process.And there is pressure melting phenomenon in the sandstone creep process.Under the dual action of temperature and pressure,the unfrozen water content in the sandstone increases.Both are positively correlated with the change of unfrozen water content,but it is not a simple linear relationship.Therefore,the fractional-order theoretical zone was introduced to establish the corresponding creep damage model,which revealed the creep failure law of Cretaceous sandstone under long-term load during the thawing process.(4)The indoor similarity model test and numerical simulation analysis were carried out to evaluate the stability of the frozen wall of Cretaceous sandstone.The similarity ratio of the model test is determined according to the actual situation of the project and the laboratory test conditions.The theoretical values of several physical and mechanical parameters are determined by applying similarity theory,such as internal friction angle,gravity,strain,stress,elastic modulus,cohesion and acting force.The composition of the material is determined according to the principle of selecting similar materials.The ratio of similar materials was obtained by orthogonal test.The Cretaceous Luohe Formation medium-grained sandstone in the vertical shaft of Xinzhuang Coal Mine is used as the simulation object.According to the similarity ratio,the vertical shaft model is made.Determine the size of the attached load according to the stratum parameters.Combined with data acquisition system,information processing system and environmental system,the creep deformation of similar materials during thawing is tested.The secondary development of the creep damage equation was carried out based on ABAQUS software.The thawing process of the frozen wall is simulated by numerical calculation method.The stability of the frozen wall of vertical shaft under long-term load is evaluated.