Research On Macro And Micro Characteristics Of Loess Based On Microstructure Functional Unit And Implementation Of Discrete Element Method

Loess is a complex multiphase nonlinear system.It is generally believed that there is a close relationship between the deformation and failure of loess and its microstructure.At present,the understanding of the microstructure characteristics of loess is still insufficient,and some current test equipment and technical means can not solve the problems such as real-time monitoring of the internal stress fabric of soil and giving the analytical solution to describe the nonlinear properties of materials mathematically.In recent years,macro and micro soil mechanics,which takes discrete element as an important medium,has been paid more and more attention and respected by scholars.Some scholars try to build the research model of loess mechanical problems by using discrete element,but the modeling is slightly insufficient,and they do not consider establishing the discrete element model from the microscopic characteristics of loess.Therefore,this paper takes Malan loess as the research object.After in-depth study on the material composition and properties of loess through indoor test and micro test analysis,a clear conceptual model of loess microstructure functional unit is established.On this basis,a series of discrete element numerical simulations are carried out to study the influence of loess particle shape and particle size distribution on the macro and micro mechanical properties of loess in the process of compression and deformation,This paper focuses on the evolution of particles and cements in the process of aggregate hydrolysis failure and loess,tunnel erosion.The main research contents and achievements are summarized as follows:(1)A total of 10 groups of loess samples were collected from Yulin,Dingbian,Lanzhou,Yan’an,Luliang,Linfen and Xi’an,of which six groups were distributed in Sandy Loess Zone,Loess Zone and Clayed Loess Zone of the Loess Plateau,and the other four groups were from the loess section of Yan’an slope.The basic physical properties of undisturbed loess are tested mainly from two dimensions(spatial distribution of loess and new and old age).The specific test items include natural moisture content,natural density,liquid plastic limit,particle size distribution,mineral and chemical composition analysis,etc.(2)Using electron microscope scanning(SEM)and computer tomography(CT)as the main microscopic analysis means,the morphological characteristics of particles at different scales are quantitatively analyzed.From the new loess to the old loess and from the northwest to the southeast of the Loess Plateau,the proportion of particles with nearly circular or spherical shape increases.Most of these particles are clay particles or fine powder particles,while the shape of medium coarse powder and sand particles is irregular,The change trend in the above two dimensions is just opposite to that of viscous and fine powder particles.Although the shape of these particles is irregular,they are usually preferentially affected by the "filling and rounding" effect of viscose particles to become a higher-level microstructure unit-single particle aggregate.If the content of clay particles in the soil sample is high,under the effect of "thickening increasing effect",the clay-particle aggregates and single-particle aggregates will further compound into slightly larger multi-particle aggregates or composite aggregates.(3)In order to reveal the shape effect of loess particles,the real shape of particles was obtained from SEM images and mixed into the biaxial compression test model of loess according to the content percentage of particles with different shapes in item(2).Measuring circles with different functions are placed in the model to monitor the changes of various parameters.Finally,the change of internal friction angle,coordination number and porosity were selected to quantitatively evaluate the influence of particle shape on loess biaxial compression performance.Results show that the friction effect of occlusion is mainly embodied in the interaction of particles is irregular,flat if this type of particle concentration percentage decrease,the internal friction Angle of the specimens is reduced,and the micro,with the decrease of macropore also in number,but the average coordination number of the sample is increased,the distribution is uniform in the whole specimen.(4)The inhomogeneity of particle size has a profound influence on the macroscopic and microscopic characteristics of loess loose medium system.The model in this paper has a different approach,which is different from the "binary" and "ternary" models of most scholars.Triaxial compression tests were carried out considering both continuous gradation and particle shape effect,and the basic data for modeling were derived from the results of studies(1)and(2).The results show that the dry density of the samples is low when the flat particles are in the majority.However,when the proportion of nearly spherical fine particles is high,they usually fill in the pore space,and the coordination number of the system increases,which makes the soil more dense,which can be explained by the theory of compact packing.Filling effect and occupying effect are beneficial to increase the dry density of the system,but there are few such medium materials in the ideal nature.Loosening effect,wall effect and wedging effect are the most common,which will lead to the decrease of dry density of the tested sample.Meanwhile,the propagation of contact force is blocked,and the average normal contact force decreases,which is manifested as the decrease of shear strength at macro level.(5)In the analysis of item(2),the stability of aggregates can be roughly ranked as follows:the colloidal clay aggregates are better than the single-particle aggregates and the multi-particle aggregates,while the composite aggregates have a relatively small number and the worst stability.The hydrodynamic properties of loess are closely related to the degree of self-stability of aggregates.Therefore,a CFD-DEM coupling model for hydrolytic failure of single and multiple aggregates was established in this paper,with water pressure as the main external inducing factor.At low water pressure,almost no damage occurs to the aggregates,but at high water pressure,the aggregates are hydrolyzed rapidly.On the other hand,the water stability of the aggregates with small porosity and high density is slightly better.(6)Tunnel erosion of loess is a unique geomorphic landscape in the Loess Plateau,which is also one of the inducements of many adverse geological disasters such as collapse and landslide,and also poses a direct threat to infrastructure such as structures,traffic engineering,power transmission lines and drainage engineering.The quantitative evolution process of tunnel erosion is very slow in nature,and the germination of tunnel erosion begins with the microscopic changes of loess.Nevertheless,the previous researches mainly focus on field investigation,and the understanding of the microscopic mechanism of tunnel erosion is very limited.Or the physical model study can be carried out indoors.This method may use high hydraulic gradient to accelerate the occurrence of tunnel erosion,resulting in the difference between the developed fabric in the model test loess and the naturally occurring fabric.In this study,a tunnel erosion model of loess was established by combining computational fluid dynamics(CFD)and discrete element method(DEM).Virtual cement method was used in numerical tests to convert the dissolution of cement into the reduction of bond strength.The microscopic mechanism of tunnel erosion in three regions was studied from the aspects of the erosion rate of particles and the evolution process of meso-contact mechanics between particles.The results show that the higher the content of fine particles,the stronger the cementation force,the stronger the erosion resistance,the lower the erosion rate of particles,and the dissolution rate of cements is much earlier than the erosion rate of particles,and the variation range of anisotropy parameters of contact force between particles is the smallest.However,under high water pressure,the time difference between the dissolution rate of cement and the erosion rate of particles in the three specimens is very short,and quickly reaches the stage of slow erosion.The anisotropy parameters of the contact force between particles decrease rapidly and then tend to be stable.