Multi-scale Experimental Study On Water Retention Characteristics And Swelling Characteristics Of Mile Expansive Soil

Natural expansive soil foundation subjected to long-term seasonal rainfall evaporation cycles undergoes the swelling-shrinkage deformation under high-speed rail-way subgrade thus seriously affecting railroad performance.The nature of this engineering problem relies on the water-air transport effects and the deformation responses of the unsaturated natural expansive soil foundation under the joint effect of railway subgrade load and wetting-drying cycles.Aiming at the above problems,this paper took multi-scale experimental research and theoretical analyses as research methods to study the water-air migration mechanism and the law of expansion of unsaturated expansive soil foundations in view of the water-retention characteristics and expansion characteristics of expansive soils.Taking Mile undisturbed expansive soil and compacted expansive soil as the research objects,this paper explores the water retention characteristics and pore distribution characteristics of undisturbed and compacted samples.On the basis of pore distribution data of macroscopic and microscopic level obtained from mercury intrusion porosimetry(MIP),the prediction of the soil-water characteristic curve is developed.The expansion response law of compacted expansive soils in different initial states is studied by expansion tests under constant stress and constant volume.The main research contents and results are as follows:(1)The soil-water characteristics curve(SWCC)of the undisturbed expansive soil and compacted expansive soil within the full suction range was obtained by joint tests of cold mirror dew point water potential meter and a high-range suction meter.It is found that undisturbed expansive soil has a better water retention capacity,and the hysteresis of compacted expansive soil is more obvious than that of undisturbed expansive soil.Using a variety of classic models to fit the soil-water characteristic curve,it is found that the Fredlund&Xing model has a better fitting effect and the derivation of the Fredlund&Xing model can be calculated to obtain the more accurate values than traditional mapping methods.(2)Swelling tests under constant stress were carried out on compacted expansive soils of different initial water content and initial void ratio.It is found that the swelling rate under constant stress of expansive soil increases significantly with the decrease of initial void ratio,and slightly increases with the decrease of water content.A formula for predicting expansion under constant stress rate considering load,void ratio and water content was established.A constant volume expansion test was performed on the compacted expansive soil.It is found that the time history curve showed a bimodal distribution.Also,the law that swelling pressure of compacted expansive soil changes with the initial water content and initial void ratio and the corresponding formula to calculate swelling pressure are obtained.(3)Through MIP and scanning electron microscope(SEM)test,undisturbed and compacted expansive soil under different dry-wet paths were microscopically tested,explaining the result of the interaction between expansive soil and water from a microscopic point of view.Through six kinds of mutual relations to coordinate all aspects of the behaviors of expansive soil and conduct coupling analysis,the corresponding relations of different behaviors of Mile expansive soil at different scales were obtained.SWCC was predicted by way of SWCC model based on the microstructure,and the corresponding prediction formula was raised.