Effects Of Drying-Wetting Cycles On Deformation Characteristics And Microstructural Damage Mechanism Of Granite Residual Soil Under Cyclic Loading

The interlayer of coarse-fine particles between subgrade structure layer and foundation caused by the long-term loading of moving trains played an important role in bearing and transferring the loading of trains.For the granite residual soil(GRS)areas,the drying-wetting cycles(D-W cycles)further accelerated the formation of coarse-fine interlayer in the subgrade.The study of deformation characteristics and structural damage of granite residual soil under the impact of D-W cycles and long-term dynamic loading could provide theoretical guidance for further accelerating the construction of railway subgrade in GRS areas and ensuring the safe operation of railway transportation.Therefore,through the dynamic triaxial test and microstructure study of the GRS with different fine particle content subjected to D-W cycles,the deformation and structural damage laws of GRS were systematically analyzed,the results were as follows:(1)The types of permanent deformation developed from plastic shakedown to plastic creep type under the influence of D-W cycles.Fine particles had three functions that lubrication,densification and asphyxiation on permanent deformation.The prediction models of permanent deformation with dynamic loading were quadratic functions in the log-log curve.The dynamic elastic modulus of granite residual soil was used to measure the elastic deformation,that has an obvious turning point under the effect of D-W cycles.The dynamic elastic modulus was affected by fine particles,but its logarithmic curve with loading number conformed to linear function.The damping ratio gradually decreased with the increase of loading times,decreasing rate was large in the early stage,tended to zero gently in the late stage.D-W cycles and particle composition variations had little effect on the damping ratio.(2)All parameters of the hysteretic curves showed that when the loading times exceeded a certain threshold,the variable of change rates decreased.The increase of DW cycles and fine particle content did not cause the continuous increase or decrease of the single variable.According to analyzing the relationship between energy dissipation and damage variables,it could found that:At the early stage,because of the compaction influenced,the energy mainly converted to elastic-plastic strain.The intergranular fracture damage and friction energy dissipation also increased.In the late stage,because of the compaction increasing slowly,the elastic-plastic strain increased barely,but the structural damage of soil kept increasing linearly due to particle breakage.The energy dissipation was mainly recoverable elastic energy and structure damage energy dissipation.(3)The structure changes of GRS subjected to D-W cycles were as follows:the dispersion of clay minerals and the loss of cementing materials in wetting process,and the agglomeration of clay minerals during drying process.However,with the increase of D-W cycles number,the hydrophilic ability of clay minerals decreased lead to the soil structure gradually loosens.The fine particle composition mainly affected the soil structure change at the early stage of D-W cycles.The main mechanism was that the soil particles were differently enveloped by clay minerals and the distribution patterns lead to the differences in the humidification process and drying process.