Study On Dynamic Characteristics And Microstructure Of Loess Under Cyclic Dynamic Load

Loess is a typical Quaternary wind-formed sediment,which is widely distributed in China.Loess has developed joints and fissures,and the fine structure is loose and porous,with strong rheological,water-sensitive and wet-set properties,and is highly susceptible to catastrophic changes under external disturbance.The large area of loess is distributed in the northwest of our country.It is the joint area of multiple earthquake zones in our country,where many moderate and strong earthquakes have occurred.At the same time,this area is the key area for the implementation of the major strategy of high-quality development and ecological protection in the Yellow River Basin.Seriously affect the safety of the project.Therefore,it is of great practical significance to study the dynamic disaster-causing characteristics of loess.In this paper,the effect of dynamic loading on the dynamic stress-strain relationship and stiffness weakening characteristics of unsaturated loess,the dynamic stress-strain relationship on saturated loess,the pore water pressure growth law and the liquefaction characteristics of loess are analyzed and studied by indoor cyclic dynamic triaxial tests in the Hefangtai area of Gansu.The microstructure of the tested specimens was observed and analyzed by scanning electron microscopy,and the SEM images were data processed using IPP software to quantitatively analyze the changes in the microstructure of loess under different test conditions,and the following conclusions were drawn:(1)Under the action of cyclic dynamic load,the cumulative strain time curve of unsaturated in-situ loess shows a stable type,and the curve can be fitted by empirical equation usingε_p=aln N+b,which is a good fit;when the number of vibration is the same,the cumulative strain increases with the increase of dynamic stress ratio and water content,and decreases with the increase of frequency;the stiffness weakening index decreases with the increase of dynamic stress ratio,and decreases with the decrease of frequency.The weakening index decreases with the increase of dynamic stress ratio and decreases with the decrease of frequency,and the smaller the weakening index,the higher the degree of stiffness decay.(2)The deformation rate of saturated in-situ loess is gradually accelerated with the application of vibration,and the combination of high amplitude and low frequency dynamic stresses can destroy the specimen in the shortest time.The effective stress of the soil under the action of dynamic stress gradually decreases,and the area enclosed by the stress-strain hysteresis curve gradually increases for each cycle.The dynamic strength and liquefaction resistance decrease with the increase of vibration number,and are more obviously influenced by the magnitude of dynamic load and vibration frequency,and decrease with the decrease of vibration frequency,and decrease with the increase of dynamic stress ratio.(3)The analysis of the parameters of loess particle morphology in microstructure shows that the fractal dimension gradually increases and the abundance increases as the dynamic stress ratio increases and the vibration frequency decreases,and the loess particle morphology becomes more and more fragmented;the anisotropy rate of particles indicates that the degree of orientation arrangement of particles increases when the water content increases;from the changes of the average diameter of the characterization parameters of particle scale features,it is concluded that the increase of dynamic stress and the frequency The average diameter of the particles decreases with increasing dynamic stress and decreasing frequency,while the average diameter of the particles increases with increasing water content.The pores of loess are mainly small pores,and the shape of pores is dominated by long and irregular pores.