| The first phase of water supply project in northern Xinjiang crossed the expansive soil area,that is a cross-basin water diversion project mainly based on open channels.Some of open channels cross the expansive soil area,it has been multiple sliding failure phenomenon since running.At present,scholars at home and abroad have many research results on the mechanical properties of expansive soil in open channels,but the experimental conditions studied are mostly drying-wetting cycles.They believed that the change of water content is the main factor causing the deformation and failure of canal slopes.Considering the influence of freezing-thawing cycles on its mechanical properties,there are few studies.The research on the mechanical properties of expansive soil in open channel projects in cold and arid areas needs to be further explored.Therefore,on the basis of the existing research results,in order to further reveal the failure mechanism of the expansive soil slope of the open channel the first phase of water supply project in northern Xinjiang,the indoor direct shear test,compression test,permeability test,triaxial test and microscopic scanning electron microscope test of the expansive soil under normal temperature and drying-wetting-freezing-thawing cycles of soil are carried out separately,the results of mechanical properties soil are investigated and their affecting mechanism are discussed.Based on the existing non-linear elastic E-B constitutive model,the deformation law of expansive soil under drying-wetting-freezing-thawing cycles is improved.The relevant conclusions of the experimental study are as follows:（1）Experiment on mechanical properties of expansive soil under normal temperature:（1）Direct shear test:The cohesion is decreases with the increase of water content.The internal friction angle decreases slowly when it is less than the optimal water content,and then decreases significantly.Due to dry density increasing,the cohesion increases more significantly than the internal friction angle.（2）Compression test:According to the analysis result,the stable void ratio decreases trend with increasing water content.With increasing dry density,the initial void ratio decreases,and the stable void ratio tends to be constant.With increasing consolidation pressure,the structure type of soil gradually evolves to turbulence and laminar flow structure from flocculation structure,the effect of particle aggregation is obvious,the number and size of pores decreases,and the compressibility of expansive soil decreases,all above are established based on result of scanning electron microscopy experimental test（SEM）.（3）Permeability test:The permeability of expansive soil is strong under low consolidation pressure,and it is small under high pressure（200～1 600k Pa）with an order of 10-6～10-8.The permeability coefficient is positively correlated with the void ratio,which can be expressed in the form of power function.The results of scanning electron microscopy（SEM）show that the loose accumulation structure changes into a closely combined laminar flow structure with increasing consolidation pressure,and the area of pores decreases,which provides the conditions for the permeability coefficient decreases significantly.（2）Experiment on mechanical properties of expansive soil under drying-wetting-freezing-thawing cycles:（1）Direct shear test:With the increase of drying-wetting-freezing-thawing cycles,the fracture rate of expansive soil increases and the shear strength decreases gradually.Among them,the change of cohesion is more significant,and the internal friction angle only has a slight fluctuation.Cohesion is exponentially positively correlated with the number of cycles and fracture rate.（2）Compression test:The relationship between fine microscopic index and compression index under cyclic action was established.With the increase of drying-wetting-freezing-thawing cycles,the overall compressibility of expansive soil increases,and its compression curve can be divided into quasi elastic and quasi plastic segments.With the increase of the number of cycles,the rebound index showed a trend of stable fluctuation.The compression index was positively correlated with the number of cycles,and was linearly correlated with the fine-micro cracks.Drying-wetting-freezing-thawing cycles action slightly weakens soil anisotropy and forms a loose temporary structure in the form of aggregates-pores-filled particles.When the soil sample is subjected to vertical pressure,the pore spacing of expansive soil decreases and the compressibility is large.When the pressure exceeds the consolidation yield stress,the roundness of soil aggregate decreases,the polar angular frequency increases and the compressibility gradually stabilizes.（3）Permeability test:The variation of permeability coefficient in the cycle process is divided into three stages:slow,rapid and stable.The permeability coefficient changed varies greatly in the fifth cycle,and gradually stabilized after 7 cycles,which was positively correlated with the number of cycles and surface fracture rate.The grey correlation degree between permeability coefficient and microscopic parameters is greater than 0.65,and microscopic porosity is the main influencing factor.Under cycles action,the microscopic pores develop obviously and form a new seepage channel.The permeability coefficient is linearly and positively correlated with the microscopic porosity.（3）Triaxial test of expansive soil and improvement of nonlinear elastic E-B model:（1）Triaxial test of expansive soil at room temperature:The shear strength of triaxial consolidated drained test is greater than that of undrained test.（2）Triaxial test under drying-wetting-freezing-thawing cycles:The strength index of expansive soil under cyclic conditions decreases rapidly at the beginning of the cycle,and then tends to be stable.The structural damage effect of cycle on expansive soil mainly occurs in the first cycle.The crack development under cyclic action gradually develops from both ends of the triaxial specimen with more severe cyclic action to the middle.When it develops to a certain depth,it will produce convergence deflection,and finally become the longitudinal penetrating failure surface throughout the specimen.（3）Fine-micro test:When the expansive soil is not cycles,the expansive soil structure is more rational,and there is a strong cementation between the particles.The weakening of the cementation caused by the cyclic action is the main reason for the decrease of the cohesion of the sample.Under the axial load,the failure mode gradually changes from tensile failure to shear failure.Based on the triaxial test data,the Duncan-Chang model was established.It was found that the parameters of the Duncan-Chang model fluctuated in the early stage of the cycle,and there was a good correlation between the parameters and the number of drying-wetting-freezing-thawing cycles in the later stage of the cycle.The parameters of Duncan-Chang model fluctuated at the beginning of the cycle,and there was a good correlation between the parameters and the number of drying-wetting-freezing-thawing cycles at the later stage of the cycle.The Duncan-Chang E-B model can better reflect the stress-strain relationship of expansive soil under drying-wetting-freezing-thawing cycles,but it is insufficient to predict the volume change relationship. |
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