Pore-Scale Study On The Micro Spatial Structure And Hydraulic Characteristics Of Coarse Grained Soil

The physical essence of coarse grained soil is the accumulation of particles,which is not the continuous medium like typical solid or fluid.The previous mathematical models of soil based on continuum mechanics are inconsistent with the physical essence of coarse grained soil.The application of these models in engineering practice is limited for the main drawback.With the economic development of our country,some large and complex engineering such as deep foundations,large-scale underground engineering and coastal engineering continue to emerge,bring new challenges to the modern soil mechanics,which requires us to take more essential and accurate understanding of soil.Based on the basic point of view that coarse grained soil is the accumulation of particles,a systematic and in-depth study on the most basic geometric characteristics of soil such as particle size distribution and particle geometry was performed firstly.Then a series of meso-scale simulation algorithms for particle packing,pore segmentation,wetting,drying and seepage was developed.Through a large number of numerical simulations and theoretical analysis,the relationship between the basic geometric characteristics of particles and pore structure and hydraulic characteristics is established.The main contents and achievements of this paper are as follows.(1)The continuous function characterization of soil particle size distribution was studied systematically.In reality,soil is generated by two key processes: crushing by weathering and transporting by geological force.In this paper,based on the maximum entropy theorem and Kolgomorov cascade crushing theory,the general equation of particle fragmentation was established,and the analytical solution was obtained.For the transport process,a random transport model was established according to H.Einstein's sediment random transport theory,and the transport process of non-uniform particles is numerically simulated by cellular automata.Based on the calculation results of the above models of the two key processes,a continuous function with five parameters was given as the unified expression of particle size distribution,which verified by lunar soil database and UNSODA2.0 database.(2)The characterization of irregular particle morphology was studied systematically.By integrating the advantages and disadvantages of previous Euclidean parameters and frequency domain parameters,the particle morphology analysis was divided into two steps: scale segmentation and morphological parameter extraction.The scale segmentation was carried out by circular(2D)and spherical(3D)empirical mode decomposition.Based on the scale segmentation results,the geometric measure theory was used to extract the morphological representation parameters on different aspects and different levels.Using the above method,a complete characterization system of irregular geometry was established,the previous parameters were unified under one framework,and several new parameters were proposed.At the same time,based on empirical mode decomposition,a simple,flexible and stable construction method of real sand particle was proposed.(3)The rules of particle packing with different particle size distribution and different particle morphology were studied.For the influence of particle size distribution on packing density,a semi-empirical prediction formula of packing density was given by combining binary packing theory and geometric packing theory.The derivation is suitable for all the particle size distributions which can be expressed by continuous functions.The semi-empirical formulas were verified by Discrete Element Method.For the influence of particle morphology on packing density,a fast packing algorithm for particles with arbitrary shape was presented by combining the minimum potential energy principle with Simulated Annealing Algorithm,and the effect of particle morphology on packing density was studied by numerical simulation.It was proved that there is a quadratic relationship between sphericity and packing density,and a linear relationship between roundness and packing density.(4)The pore structure characteristics of coarse grained soil were studied.Without distinguishing pore and pore throat,based on three-dimensional continuous open operation,a new pore segmentation algorithm was presented,and the pore size distributions of particle accumulations with different particle geometric characteristics were calculated.The results show that no matter how the geometric characteristics of the particles change,the pore size distribution is always stable as the Weibull distribution,indicating that the pore size distribution can only be characterized by two parameters: the scale parameter and the shape parameter.In this paper,the empirical relationship between these two parameters and particle geometric characteristic parameters is given by multivariate fitting.(5)The soil-water characteristic curve(SWCC)of coarse grained soil was studied by meso-scale numerical simulation.The continuous opening operation considering the direction was used to capture the ink-bottle effect and contact angle lag effect,and this method was used to simulate the main wetting and main drying process of different porous media with different geometry characteristics.The simulation results show that among so much SWCC equations,Assouline's Weibull-type equation has the best fitting result,and its form was simplified in this paper.At the same time,through multivariate fitting,the empirical relationship between the SWCC parameters and the basic particle geometric parameters are given.(6)The permeability of coarse grained soil was studied by meso-scale numerical simulations.The hyperfine grid numerical simulation of seepage in pore-scale was carried out by using the CPU-parallel lattice Boltzmann method.In this paper,taking the porous media with different PSD and particle morphology generated on the previous chapters as the calculation objects,through a large number of meso-numerical simulations,the relationship between permeability and the basic geometric characteristics of particles was established,and a modified formula of KC formula was given,which is as accurate as KC formula,but all the quantities included are the basic geometric parameters of particles which are easy to be measured in engineering practice.