Experimental Research On Dynamic Characteristics Of Unsaturated Loess Under Cyclic Loading

Loess is widely distributed in northwest China.The groundwater level in this area is deep and the soil is mostly in unsaturated state.Over the years,with the rapid development of basic transportation construction in western China,More and more roads and railways will cross the unsaturated loess region.Under the repeated loading of low frequency,low amplitude and long holding time,the strength and stiffness of soil will decrease,which will lead to engineering accidents such as subgrade settlement and instability.Therefore,to explore the unsaturated loess under cyclic load of the train mechanical characteristics and evolution law is of great significance.Based on GDS unsaturated triaxial test system,taking unsaturated loess as the research object,to carry out the soil water characteristic curve test and different confining pressure,the matrix suction,dynamic load frequency,consolidation ratio under the condition of dynamic deformation and dynamic strength test.The variation of water retention and dynamic characteristics of unsaturated loess is further analyzed.Thus,the failure mechanism of loess subgrade under the action of semi-sinusoidal wave cyclic load is discussed.The research results can provide a theoretical reference for the construction of unsaturated loess subgrade and stability evaluation in northwest China.The main conclusions of this paper are as follows:（1）With the increase of matric suction,the water content of unsaturated loess decreases at different stages.When the matric suction is the same,the dynamic strength of unsaturated loess under different loading modes has a similar trend with the change of consolidation ratio and confining pressure.The dynamic strength measured by step loading is slightly higher than that by single loading.The dynamic strength along with the increase of confining pressure increase,and the confining pressure,the greater the increase.Other conditions are constant,dynamic strength increases with consolidation ratio,frequency,suction increases.Suction on the dynamic strength is greater than the influence of other factors.When the confining pressure is constant,the dynamic cohesion and internal friction Angle of unsaturated loess increase with the increase of consolidation ratio,frequency and matric suction.The greater the factors,the more obvious the increase of dynamic intensity index.The increase of dynamic cohesion and the increase of internal friction Angle are relatively small.Matrix suction has more influence on dynamic strength index than frequency and consolidation ratio.（2）The dynamic elastic modulus of unsaturated loess decreases nonlinearly with the increase of strain.When the strain is less than 0.5%,dynamic modulus of elasticity decreases rapidly.When the strain is greater than 0.5%,the dynamic elastic modulus decreases gradually with the increase of strain until approaching the same number.At the same dynamic strain,as the confining pressure,consolidation ratio and frequency,the increase of the suction,showed different degrees of dynamic elastic modulus increase.With the increase of confining pressure,consolidation ratio and frequency,the initial dynamic elastic modulus increases in different degrees when the matrix suction is constant.Influence degree from high to low in turn as the confining pressure,consolidation ratio and frequency.The modified Hardin-Drnevich model can describe the relationship between 1/E_dandε_dwell.Establish dynamic modulus of unsaturated loess under different conditions than the attenuation expression and the damping ratio expression.Can be well described along with the evolution regularity of dynamic strain.（3）When the matrix suction is fixed,the numerical value of the unsaturated loess damping ratio under different confining pressure,frequency and consolidation ratio conditions is distributed in the range of 0.03-0.2.With the increase of dynamic strain,increasing damping ratio of unsaturated loess under different conditions.Under the same dynamic strain,with the decrease of frequency and confining pressure,the consolidation ratio and suction increase,the damping ratio of soil increases gradually.Along with the confining pressure,consolidation ratio,frequency and increase of the suction of unsaturated loess dynamic backbone curve move up.