Experimental Study On Unsaturated Characteristics And Microstructure Of Mudstone On Weak Expansive Foundation Of High-speed Railway

With the rapid development of China's high-speed railway industry,railway construction inevitably passes through expansive soil areas.While expansive soil foundation meets water,it will upward,which seriously affects driving comfort and safety.Taking the expansive mudstone on the foundation of a high-speed railway as research object,the expansive characteristics,unsaturated characteristics and microstructure of mudstone are systematically studied by means of laboratory tests and theoretical analysis.The research results can provide theoretical basis for preventing foundation disasters in expansive soil areas.Meanwhile,it is beneficial to promote the application of unsaturated theory in the research of expansive soil.The central parts and achievements of this thesis are as follows:(1)The basic physical and mechanical properties of mudstone are experimentally studied.Judging from code requirement and items of previous research,previous research might bring the expansion characteristic to a more reasonable level,especially in high-speed railway foundation.(2)The large-scale expansion model experiment and expansion potential experiment are carried out.And the results show that temporal curve of expansion changes in a "step-like" way on the foundation model.The expansion deformation rate is fast in the early stage and slow down in the late stage.The foundation expansion curve meets the first-order attenuation exponential function.And its amount has a negative correlation with the thickness of the overlying soil layer.In the expansion potential experiment,the temporal curve of expansion meets Boxlucas mathematical model.The initial dry density and initial moisture content have a plane relationship with final expansion amount of the sample.(3)The soil-water characteristic curve of mudstone is measured by pressure board method and filter paper method where unsaturated theory is introduced.And the results show that the total suction and matrix suction of mudstone decrease nonlinearly along with the increase of moisture content in single logarithmic coordinate.The higher the dry density of mudstone with same volume moisture content,the greater the total suction and matrix suction.The mudstone soil-water characteristic curve is approximately "S" shape.LangumuirEXT2 mathematical model is used to fit the mudstone soil-water characteristic curve.A single parameter calculation model of the soil-water characteristic curve is obtained.It is found that the classical soil-water characteristic curve models(F-X model,Gardner model and V-G model)all attain high fitting degree to the soil-water characteristic curve of mudstone.The F-X model has the degree of accuracy to the soil-water characteristic curve of mud rock in this experiments.(4)Mercury intrusion porosimetry(MIP)is used to analyze the microstructure of mudstone.Based on the advance and retreat mercury curves,it is found that there are manybottleneck pores in mudstone.The content of bottleneck pores is analyzed.It is found that the pore size distribution curve of mudstone shows obvious three peaks.Therefore,mudstone pores are quantitatively divided into large pores,medium pores and small pores.When compare the fractal characteristics of thermodynamic relation model with the Menger sponge model.The thermodynamic relation model is more suitable for calculating the fractal dimension of test mudstone.Further more,the pore wall of mudstone is rougher and the pore structure is more complex once the dry density of mudstone increased.(5)According to the structural characteristics of mudstone pores,an indirect calculation method of matrix suction is obtained on the base of experiments datas and theoretical derivation.Then the calculated method is modified based on experimental data.The generality of the modified method can also be verified by other literature data.