Research On Soil Dynamic Slaking Characteristics And Multiphase Coupled Analysis

Railway transportation is essential to ensure economic development and energy security in China.The subgrade is the premier part of the railway;therefore,its performance in the service period is a significate impact on the safety of the track.The subgrade soil,a key ingredient of the subgrade,is constantly subjected to dynamic load and environmental effects,which causes the diseases such as uneven settlement and mud pumping.The diseases waste maintenance labor with high cost,which has high safety risks.The researches show that the coupling of slaking and dynamic load is the significant cause of subgrade soil diseases.Understanding the hydraulic-mechanical character of soil under slaking and dynamic load significantly contributes to revealing mechanism of the disease.Therefore,it is of great significance to study the moisture migration character and the mechanism and sensitive factors of deformation under dynamic slaking to prevent and treat subgrade diseases.Supported by the National Natural Science Foundation of China(51478279),in this paper,dynamic slaking characteristic and moisture migration law of silty clay from railway subgrade are studied by theoretical derivation,equipment development,laboratory test and multiphase coupled analysis.The softening mechanism of subgrade filler is revealed,which provides a new perspective for the improvement of subgrade design and the supportability of existing line.The research contents and conclusions are as follows.(1)In order to determine local resistivity measurement in limited space,the analytical solution of van der Pauw(vd P)arrangement for cylinder shape is derived based on electrostatics theory.Moreover,the application of the vd P resistivity method for the local resistivity measurement of the triaxial sample is extended.The results show that the measurement error is negligible when the length of the electrode and sample is equal,and the measurement error of electrode width is also negligible.The limitation criterion of the flat specimen with the circumferential arrangement is equal to 0.3,defined by the ratio between the height and diameter.Last,local resistivity measurement based on the vd P method is proposed,and the effective region of resistivity is defined clearly,whose length is 0.5 times the cylinder diameter at the electrode center.(2)Based on the proposed local resistivity method,the equipment used in limited space for moisture migration character detection is developed.The circuit parameters of the equipment are determined,and acquisition software is also developed.The flexible electrode sensor for moisture content detection is designed by printed circuit technology with excellent fit,remarkable adaptability,and high durability.The results show that the short-time preload(120k Pa)at the beginning and minimum contact pressure(20 k Pa)throughout the experiment are required to ensure the electrode contact state.The precision of the proposed method is confirmed by laboratory tests.Compared with the oven-drying method,the maximum relative error is within 5%.(3)Based on the moisture migration equipment,the dynamic slaking triaxial tests are carried out to study soil moisture migration,deformation and modulus softening behaviors.The results show that the moisture content decreases with the dynamic stress increase.The distribution of moisture content is non-linear from top to bottom.The speed of the slaking increases with dynamic stress increase.The accelerating moisture migration is caused by the pumping phenomenon in tiny pores under dynamic slaking load.The deformation induced by the dynamic slaking load can be divided into the dynamic slaking stage and the consolidation stage of quasi-saturated soil.Hysteresis loops are oval and closed and whose area changes negligibly.A linear empirical model between moisture content and resilience modulus is proposed and verified with high accuracy.(4)The dynamic numerical simulation under slaking is conducted based on the multiphase coupled finite element method of viscoelastic plastic unsaturated soil.The dynamic slaking deformation and softening mechanism are studied by the effects of dynamic stress amplitude,void ratio and permeability coefficient.The results show that dynamic stress amplitude,void ratio and permeability coefficient significantly influence plastic deformation.The simulation reproduces the distribution saturation difference in each row and reveals that the difference is related to the amplitude of local pore pressure vibration under dynamic load.Moreover,the softening deformation form is reproduced by simulation,and its mechanism is also presented.