Research On Dynamic Characteristics Of Saturated Expansive Soil Under Cyclic Loading

Expansive soil is not only widely distributed in various regions of our country,but also related reports and researches on expansive soil exist in many countries and regions around the world.With the development of economy and the advancement of world civilization,the structures will inevitably involve problems in the area of expansive soil.With the dynamic loads brought by high-speed railways,spontaneous earthquakes,and waves,the impact on the foundation of the structure cannot be ignored,and important hidden dangers are buried for safety.In view of this,this article takes the expansive soil in Hefei area as the research object and conducts a series of dynamic triaxial tests to study and analyze the dynamic characteristics of expansive soil under cyclic loading.First,a multi-stage cyclic loading test was carried out using the British GDS true-dynamic triaxial instrument to study the dynamic stress-strain relationship of different confining pressures,consolidation ratios,and vibration frequencies.The results show that:at small amplitude,Kondner model can describe the dynamic elastic modulus and dynamic strain reciprocal relation;with increasing confining pressure,consolidation ratio,vibration frequency,the initial dynamic modulus of elasticity and the elastic modulus all increase.Under the same conditions,the dynamic elastic modulus decreases greatly with the increase of dynamic strain and tends to be stable.The dynamic elastic modulus ratio changes little and has not obvious tendency under different confining pressure,consolidation ratio,and vibration frequency.And the change of the damping ratio is more complicated and related to the accumulated strain.Secondly,Based on the study of dynamic stress-strain characteristics,the characteristics of dynamic strain are quantitatively and qualitatively analyzed,and the morphological characteristics of the hysteretic curve are in-depth analyzed.The morphological characteristics of the hysteretic curve include:the degree of inclination k,the size of the area S,the degree of closure ε_p,and the center distance of the curve d.The research results show that the dynamic stress-strain backbone curve of expansive soil presents a hyperbolic shape as a whole,and the k value of the hysteretic curve decreases rapidly with the increase of dynamic stress and then tends to be stable.It increases with the increase of confining pressure,consolidation ratio,and vibration frequency.The larger the k value,the stronger the anti-deformation ability of the soil.In the early stage of cyclic loading,when the dynamic stress amplitude is small,the hysteresis curve area S,center distance d,and residual strainε_pchange little with the increase of dynamic strain.In the later stage of loading,the dynamic stress amplitude increases rapidly.And it decreases with the increase of confining pressure,consolidation ratio and vibration frequency.Finally,a single-stage cyclic loading test was carried out on the basis of the multi-stage cyclic loading test to analyze the dynamic strength test of the improved expansive soil with different confining pressures and comparison of different admixtures.Experimental analysis shows that the dynamic strength of expansive soil increases with the increase of confining pressure.Under the same confining pressure,the dynamic strength of expansive soil improved by weathered sand is the largest.The dynamic strength index can be obtained by quantitative analysis,Under the same failure vibration frequency,the weathered sand modified expansive soil has the largest dynamic cohesion and the dynamic internal friction angle changes little.