# Study on Compaction Characteristics and Compaction Process of an Unsaturated Silt Based on PFC3D
## 1 Introduction  
An unsaturated silt is a kind of cohesive soil with a strong hydrophilicity and water holding capacity, and poor water permeability. It is distributed within layers on the surface. Its soil has special properties, large porosity, and viscosity, and generally contains expansive minerals, which makes it difficult to dry and not easy to crush [1]. At present, domestic and foreign scholars have carried out in-depth and systematic research on the physical and mechanical properties and compaction characteristics of unsaturated soils mainly through laboratory tests and numerical simulations [2; 3; 4; 5]. For the mechanical properties of unsaturated soil, [PERSON] et al. [6] simulated the deformation mechanism and mechanical properties of coarse-grained soil at the mesoscopic level. Uniaxial compression tests were carried out on samples of different sizes to determine the distribution of particle strength and elastic modulus of the sample. The strength and elastic modulus of the sample particles decreases with the increase in particle size. Based on the idea of the discrete element method and the principle of pore elasticity, [PERSON] et al. [7; 8] analyzed the stress of soil samples under different particle combinations and calculated the pore fluid pressure of unsaturated soil. [PERSON] et al. [9] simulated the water-bearing characteristics of unsaturated soils by adding adhesion force at the contact of particles, and compared the discrete element simulation results with the laboratory test results to verify the rationality and accuracy of the application of the adhesion force. [PERSON] [10] used the discrete element numerical analysis software, MatDEM (matrix discrete element) and PFC (particle flow code), to study and analyze the unsaturated soil by the control variable method. The relationship between the macroscopic parameters of the unsaturated soil and the microscopic parameters of the discrete element model were calibrated, and the accuracy of the lsy liquid bridge contact model to simulate the unsaturated soil was verified. [PERSON] et al. [11] used Hill contact to consider the contact between soil particles, and established a discrete element model of unsaturated soil static triaxial test. The water content state of unsaturated soil was characterized by suction, and the influence of water content change on the stress-strain curve of the sample was obtained. In terms of rolling compaction characteristics, based on the discrete element method, [PERSON] et al. [12] improved the rolling compaction simulation from four aspects: micro-particle generation, boundary condition setting, real-time monitoring, and macro-meso parameter correspondence. The rolling compaction model was established more realistically and efficiently, and the effects of rolling combination mode, particle gradation, and particle appearance shape on compaction degree were discussed. [PERSON] et al. [13] tested the soil in the Huaning section, improved the compaction degree of the silt raodbed, and reduced the cost of the raodbed construction by changing the construction technology of the silt raodbed. [PERSON] et al. [14] solved the problem of rolling compaction of water-shortage or unsaturated silt road by using the variable amplitude rolling technology of static pressure-strong vibration-weak vibration-static pressure. The international research on unsaturated soil mainly focuses on the characterization of water content, especially the influence of microscopic parameters on water content, while there is less research on rolling compaction technology. Therefore, the main purpose of this article is to (1) analyze the contact effect between unsaturated silt particles through theoretical derivation.