Research On Dynamic Characteristics And Deformation Properties Of Expansive Soil Modified By Weathered Sand

To promote the development of China's highways,railways and other infrastructure rapidly,large areas of expansive soil must be considered.The expansive soil is a kind of high plastic clay with dispersed particles.It is susceptible to changes in ambient temperature and humidity,which causes undesirable phenomena such as expansion and contraction and crack development.It cannot be used directly for roadbed filling.This paper uses weathered sand to improve expansive soil,and studies the dynamic characteristics,dynamic deformation characteristics and cumulative deformation characteristics of modified expansive soil.The main research ideas and results are as follows:Through a series of dynamic triaxial tests,the variation law of dynamic elastic modulus and damping ratio of expansive soil modified by weathered sand was studied under different sand content and different confining pressures.The results show that:With the increase of sand content,the dynamic elastic modulus and the maximum dynamic elastic modulus of the modified expansive soil decrease first and then increase,and the critical sand content is 15%;when the sand content v>25%,continue to increase the sand content reduced impact of E_d?E_dmax;The damping ratio increases first and then decreases with the increase of sand content.The damping ratio reaches the maximum when the sand content is 15%.The modified expansive soil dynamic elastic modulus increases with increasing confining pressure,and the damping ratio decreases with increasing confining pressure.Under the same confining pressure,as the soil deformation develops,the damping ratio response is faster than the dynamic modulus.Initially,the critical sand content of the modified expansive soil is 15%,and the optimal sand content is 25%.Under the optimal sand content,the bearing capacity and stiffness of the improved expansive soil are improved.In order to study the dynamic strength characteristics of modified expansive soil,a series of dynamic tests were carried out using single-grade loading.The test results show that:With the increase of sand content,the dynamic strength of the modified expansive soil decreases first and then increases.The critical sand content is 15%.When the sand content v?25%,the dynamic strength of the improved expansive soil is higher than that of the plain expansive soil.At the same content of sand,the dynamic strength of the improved expansive soil increases with increasing confining pressures,and the decay rate of the dynamic strength decreases with increasing confining pressure.There is a good exponential relationship between the dynamic strength and the cyclic vibration times.When the sand content v?15%,the reduction of dynamic cohesion plays a leading role in reducing the dynamic strength of the modified expansive soil.When the sand content v>15%,the increase of dynamic friction angle plays a leading role in improving the dynamic strength of the effect of improving expansive soil.In order to study the dynamic deformation characteristics of the modified expansive soil,a series of dynamic triaxial tests were carried out under different sand contents,consolidation stress ratios and load frequencies.The test results show that: the development of the improved soil dynamic backbone curves goes through 3stages-elastic stage,weak elastic stage and rapid accumulation of plastic strain stage.The effects of sand contents?consolidation stress ratio and loading frequency on the development of dynamic deformation are mainly manifested in weak elastic stage and rapid accumulation of plastic strain stage.As the amplitude of the dynamic stress increases,the slope K of the hysteresis curve increases first and then decreases,while the area S?center offset d and residual strain are all increased,indicating that the elastic properties and stiffness of the improved soil are reduced,the plastic strain and the degree of soil structure damage are increased,and the hysteresis curve in the late stage of step loading shows an asymmetrical ellipse shape with a wide upper part and a narrow lower part.Under the same conditions,compared with the consolidation stress ratio,increasing the load frequency has a greater impact on the dynamic deformation characteristics of modified expansive soil.In order to study the cumulative plastic deformation characteristics of the modified expansive soil under long-term cyclic loading,the modified expansive soil with the optimal amount of sand as previously proposed was used as the research object,and a series of dynamic triaxial tests were carried out using single-grade loading.The results showed that:Under different dynamic stress amplitudes,the relationship between cumulative plastic strain and cyclic vibration times of modified expansive soil can be divided into three types:"Stable,Critical,Destructive".When the dynamic load is less than the critical value,the modified expansive soil practically reaches a plastic stability state in the early stage of cyclic loading.Based on the empirical relationship between the"Stable,Critical"cumulative plastic strain and the loading cycle,propose a fitting formula for the relationship between residual strain and cyclic vibration times.The relationship between residual strain and cumulative plastic strain is mainly affected by the dynamic stress amplitude,When the magnitude of the dynamic stress applied to the soil is less than the critical value,the relationship curves between the two is exponential,and when the magnitude of the dynamic stress is greater than or equal to the critical value,the relationship curve between the two is linear.Under the action of low dynamic stress amplitudes,the single-grade and step loading methods have no significant effect on the cumulative plastic deformation of the modified expansive soil,and the morphological characteristics of the hysteresis curve are basically consistent.With the magnitude of the dynamic stress increases,the dynamic stress-strain hysteresis curve of the modified expansive soil under the condition of graded loading appears relatively convergent.