Study On The Micro And Dynamic Characteristics Of Loess In Lanzhou Metro Line 1

In this paper,the dynamic triaxial test,scanning electron microscope test,direct shear test and particle flow numerical simulation test are carried out to study the dynamic and micro characteristics of loess at different stations of Lanzhou metro,and the correlation between microstructure parameters and dynamic characteristics parameters of loess is calculated by grey correlation degree principle.The numerical direct shear test model is established by PFC,which reveals the macro dynamic mechanism of loess.The main research work and achievements of this paper are as follows:(1)In view of the complexity of mechanical properties of loess in Lanzhou metro,the mechanical properties of loess in different directions of different stations are analyzed by laboratory dynamic triaxial test.It is found that the vertical strength of loess at Gongxingdun,Qilihe,Yingmentan and Chenguanying stations is greater than that of horizontal.With the increase of dynamic stress,the dynamic strain of loess at different stations increases rapidly and then increases slowly.With the increase of soil depth,the dynamic shear modulus ratio of soil moves upward with the increase of soil depth under different deposition directions and depths;the silty clay shows stronger nonlinear and energy dissipation characteristics at different sampling depths and the same deposition direction.(2)Qualitative and quantitative analysis was made on the microstructure differences of 8 stations in different depositional directions.The microstructure parameters such as long axis,minor axis and azimuth angle were selected for qualitative analysis.It was found that the long axis and horizontal short axis of loess particles were mainly concentrated in the range of 0~20?m,and the lateral square angle was more regular than the vertical.From the quantitative analysis of particle morphology,arrangement and size characteristics,the particle shape characteristic parameters(fractal dimension,shape coefficient and abundance)show that the shape of particles in the vertical direction is more complex;the characterization parameters of particle arrangement(probability entropy,anisotropy rate)reflect that the anisotropy rate of particles in the vertical direction is greater than that in the horizontal direction;the particle size characteristic parameters(apparent void ratio,shape coefficient and abundance)are more complex than those in the horizontal direction The average diameter,particle number and average particle area)indicate that the particles are more dense in the vertical direction.(3)According to the principle of grey correlation degree,the correlation degree and weight between the maximum dynamic elastic modulus of loess and the microscopic parameters in different depositional directions are calculated.In the transverse direction,the abundance is the most sensitive to the dynamic characteristic parameters of loess,and the correlation value between them is 0.809;Vertical direction: the fractal dimension is the most sensitive to the dynamic parameters of loess,and the correlation between them is 0.767;the average diameter of particles presents the lowest weight value in both horizontal and vertical directions,indicating that the influence of average diameter of loess particles on dynamic parameters of loess is relatively reduced.(4)The discrete element software PFC is used to simulate the direct shear test,which is compared with the indoor direct shear test,It is found that the model void ratio decreases first and then remains unchanged with the increase of shear displacement;the coordination number in the shear band increases with the increase of shear displacement;the collimation of force chain between particles is better with the increase of confining pressure;The influence of meso parameters on the shear strength of loess is as follows: stiffness > stiffness ratio > friction coefficient > shear rate > void ratio.The influence of different friction coefficients on the shear strength of loess is compared.It is found that the friction coefficient of particles has a greater influence on the peak strength of stress-strain curve of loess,and obvious dilatancy occurs.The internal friction angle of loess increases with the increase of particle friction coefficient,and the cohesion decreases at first and then increases with the increase of particle friction coefficient.