Experimental Study On Dynamic Characteristics Of Expansive Soil Under Cyclic Loading

Expansive soil is widely distributed in China,and many projects need to pass through the sections where expansive soil is distributed.Scholars have carried out a lot of experimental research on the composition and structural characteristics,shear strength characteristics,swelling shrinkage mechanism,improvement methods of expansive soil,and achieved fruitful results.However,it is necessary to have a comprehensive and in-depth understanding of its dynamic characteristics under cyclic loading.Therefore,it has important practical value and scientific significance to study the dynamic characteristics of expansive soil under cyclic loading.In this paper,the dynamic characteristic parameters of Hefei expansive soil under variable amplitude graded cyclic load and the dynamic stiffness and cumulative plastic strain under constant amplitude cyclic load are studied.The main work and conclusions are as follows:(1)Combined with the measured basic physical indexes of expansive soil and the engineering characteristics of expansive soil,this paper mainly considers the influence of overconsolidation ratio,confining pressure and water content on the dynamic characteristic parameters of Hefei expansive soil.The test results show that:the backbone curve of expansive soil presents a linear to nonlinear development process with the increase of dynamic strain;the dynamic elastic modulus first decreases rapidly and then gradually slows down until it tends to be stable with the increase of dynamic strain;the dynamic elastic modulus of expansive soil increases with the decrease of water content or the increase of confining pressure and over consolidation ratio;the damping ratio of Hefei expansive soil ranges from 0.065 to0.165 Reducing water content or increasing confining pressure and overconsolidation ratio will increase the damping ratio of expansive soil.(2)Based on the research background of subway traffic load,this paper mainly studies the dynamic stiffness of Hefei expansive soil with over consolidation ratio,dynamic stress and cycle times,and establishes the dynamic stiffness change model of Hefei expansive soil through the analysis of test data.The test results show that the dynamic stiffness of expansive soil is hardening under the action of 20 k Pa and 30 k Pa dynamic stress,and softening under the action of 40 k Pa and 50 k Pa dynamic stress.For the stiffness hardening type,the hardening index increases slowly with the increase of the number of cycles;for the stiffness softening type,the softening index decays rapidly at the initial stage of the cyclic load,and with the increase of the number of cycles,the attenuation of the softening index slows down and gradually tends to be stable.Under the same test conditions,the larger the dynamic stress is,the smaller the stiffness hardening and softening index of expansive soil is,and the larger the over consolidation ratio is,the larger the stiffness hardening and softening index of expansive soil is.(3)Based on the research background of subway traffic load,this paper mainly studies the variation of cumulative plastic strain of Hefei expansive soil with over consolidation ratio,dynamic stress,cycle times and drainage conditions,and establishes the cumulative plastic strain model of Hefei expansive soil through the analysis of test data.The test results show that: at the initial stage of cyclic loading,the axial cumulative strain of expansive soil increases sharply with the increase of the number of cycles.After the number of cycles exceeds 1000,the growth rate of the cumulative plastic strain gradually slows down.After the number of cycles exceeds4000,the growth rate of the cumulative plastic strain further slows down,but the cumulative plastic strain is still increasing.Under the long-term subway load,the relationship curve between the cumulative plastic strain and the number of cycles of expansive soil presents three stages: "rapid growth stage-slow growth stage-gradually stable stage".