| Loess in China is mainly distributed in the arid and semi-arid areas north of the Kunlun Mountain,Qinling Mountains,and Taishan Mountains.It has a well-developed tubular pore structure and vertical joints.Affected by the stress history and the sedimentary environment,the microstructure parameters such as the shape,size,and direction of the loess particles and pores are quite different.The differences,complexity and uncertainty of these microstructures make it manifest in the diversity of macro mechanics,which has attracted the attention of scholars.At the same time,with the implementation of my country’s "Belt and Road" initiative,more and more projects in the loess area are involved and the scale is gradually expanding.In the construction of roads,dams,bridges,etc.,facing various loess problems such as earthquake subsidence,subsidence,settlement,etc.,seriously threaten the safety of the building.It is particularly necessary and practical to study the microstructure characteristics,macroscopic mechanical properties and mesoscopic deformation mechanism of loess.This article takes the undisturbed Malan loess in Qinghai Xining,Lanzhou New District and Qingyang as the research object.Obtain their physical property indexes through field surveys and indoor experiments.With the help of electron microscope scanning and digital image analysis system,the relevant microscopic parameters such as particle size,shape factor,direction angle,porosity and so on were extracted,and the differences and changes of microstructure characteristics of the three types of loess were analyzed by variance analysis.Based on the particle flow PFC as a platform,based on the digital image microscopic particle equal area circle diameter automatic expansion filling model method,the three types of loess were numerically simulated.The simulation results are compared with the indoor direct shear test to further reveal the macro mechanical properties and micro deformation mechanism of the loess.In this paper,the microscopic parameters of loess particle morphology,direction,size and pore size are analyzed in detail from the perspective of the horizontal(the same depth of loess samples in different regions)and the vertical(the same region of different depths of loess samples).Qualitative analysis and quantitative calculation of the changes of the above micro-parameters in the horizontal and vertical directions.The results show that the particle size distribution,pore characteristics,shape factor,and direction angle of loess in Qinghai Xining,Lanzhou New District and Qingyang at the same depth are significantly different.Among them,the average particle radius of loess in Xining area of Qinghai is the largest,and the gradation is the worst.Lanzhou New District is in second place.The average particle radius of Qingyang is the smallest and the gradation is the best.The porosity and shape factor of the loess in Qinghai,Lanzhou New District and Qingyang gradually increased in sequence.The loess skeleton particles in Xining area of Qinghai are clearly visible,mostly with large particles and large pores.The loess in the Lanzhou New District and Qingyang area is dominated by semi-circular and rounded particles,and the particle size is smaller than that of Xining in Qinghai,and there are more clay particles and fillers.Compared with the loess samples of different depths in the same area,the porosity of the loess in Xining,Qinghai and the loess in Qingyang decreases with increasing depth,but the shape factor increases with increasing depth.The calibration of the simulation parameters is based on the particle gradation of the micro-parameter particle size and previous research data.Simulation results show that the width-to-diameter ratio,contact stiffness,bond strength and friction coefficient all have significant effects on the peak shear stress.The greater the width-to-diameter ratio,the smaller the impact.The greater the contact stiffness,the higher the initial modulus and peak stress.As the friction coefficient increases,the peak stress gradually increases,and the increase is significant.The simulation results of micro-particle equal area circle diameter automatic expansion filling method are basically consistent with the experimental results,which is feasible and practical.The monitoring of the shearing box shows that the shearing surface particles in the three types of loess shearing are reversed and the bonding is destroyed.A shear band is formed at the upper and lower interfaces of the shear box. |
