| Loess is a kind of porous and weakly cemented Quaternary loose deposits.It is prone to instability and damage under a certain external load,such as water and force,resulting in serious engineering problems and geological disasters.Loess is an aggregate composed of a large number of discrete particles.The macro mechanical mechanism can be explained by the change of microstructure.Exploring the structural change mode of loess at the micro scale provides a direction for revealing the nature of the mechanical behavior of loess.This paper uses a combination of experiment and numerical simulation to study the microstructure and evolution of loess.The loess samples L1,L2,L3,L4,L5,L6,L7,L9 and paleosol samples S1,S4,S5,S8 in Jing yang(Shaanxi Province,China)were selected to carry out the laboratory tests of Grain size distribution,Mineral composition,Scanning electron microscope and Mercury intrusion porosimetry method.The strength of pedogenesis was inferred by grain size distribution and mineral composition.The pore distribution curves were obtained by mercury intrusion method,and the particle,contact and cementation characteristics were qualitatively described by scanning electron microscope.On this basis,the microstructure characteristics of the loesspaleosol sequence are studied,and the evolution mode of the microstructure of the loesspaleosol sequence in the process of deposition,pedogenesis and loading is proposed.The results of the study show that due to the differences in the paleoclimate environment of the Quaternary,the microstructures of loess and paleosol are very different.The weak pedogenesis transforms the disordered "single peak" structure of the initial aeolian sediments into the "bimodal peak" structure of loess,and the strong pedogenesis transforms it into the "uniform" pore structure of paleosol.The unstable pores in loess are mainly overhead pores(20?32 ?m)and loose intergranular pores(8?20 ?m)formed during the deposition process and the secondary pores(>32 ?m)formed during the pedogenesis process.The unstable pores in the paleosol are mainly the fissures formed in the process of strong pedogenesis(>8 ?m).When the surface pedogenesis is completed,with the gradual increase of the overlying load,the dominant pore size of the loess gradually decreases.The microstructure of the paleosol is relatively stable,and only when the load is large enough,the fissures in the paleosol will be closed.These findings will help understand the microstructure and evolution mode of loess and paleosol sequences.On the basis of the existing DDA algorithm,the interaction of clay and water are generalized into a force,which is applied between the neighboring particles to establish a numerical simulation method for discontinuous deformation analysis of the structural loess skeleton.In order to construct the initial microstructure model of loess,firstly,on the basis of determining the shape of loess particles,the loess particle group before deposition was generated by Monte Carlo method.Then the free-falling deposition process of aeolian loess was simulated,and the initial structure model is formed after falling.In order to construct metastable overhead structure model,small load is applied to the initial structure model.In order to construct structural loess microstructure model,the interaction of clay and water is applied to the metastable overhead structure model.The metastable overhead structure model and the structural loess microstructure model were simulated by one-dimensional compression tests under different pressures respectively,and the difference in displacement changes before and after cementing was compared.The numerical simulation results are compared with the laboratory test,and the reliability of the numerical simulation results is demonstrated.Subsequently,the movement characteristic of loess particles under load is analyzed from the microscopic view.This research provides the support of analytical methods for the microscopic understanding of the mechanical behavior of loess. |
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