# Influence of Soil Particle Size on the Engineering Properties and Microstructure of a Red Clay
## 1 Introduction  
Red clay is a shared, but problematic soil characterized by a high liquid limit (LL) and plasticity index (PI) and usually presents a high natural moisture content (NMC). It is generally distributed in the hot and humid regions of South China [1, 2, 3]. With the development of the economy, an increasing number of railways and roads pass through this particular soil area. However, if red clay is widely used in roadbeds as filler, it will inevitably lead to road cracks, uneven road surface settlement, and other engineering problems, which will further cause damages to the roads. Therefore, specialized treatments are required for red clay to be exploited in embankments. Soil mass is a complex particle system, and the grain composition and size of particles play an essential role in their internal deformation characteristics [4]. Accordingly, it is necessary to consider the impact of the particle size on the roadbed when processing red clay. Furthermore, soil particles with different scales have different modes of action [5, 6], resulting in various binding modes between particles. From a microscopic perspective, divergent clay minerals, arrangements, and distribution relationships affect the specific surface area (SSA), micropores, single-grain structure, and cemented structure of soil particles, where van der Waals' force is the main force between particles. From a macroscopic perspective, cracks, layers, and large pores directly impact the deformation and mechanical properties of the soil, where the external load is the main force. Under the imposition of external forces, the soil mass begins to break down and deform from the microstructure. Then, the particles contact and stagger and eventually become dislocated and discrete, damaging the macroscopic structure. Regarding the study on the impact of particle size effect on soil mass, [PERSON] [7] et al. showed that the size and shape of soil particles reflect their formation history, and the macroscale behavior of the soil is, in turn, the result of particle-level interactions influenced by particle shape. The increase in particle irregularity leads to a decrease in stiffness but an increase in sensitivity to stress states. The macroscale behavior of soils explained in microscopic terms is reviewed by [PERSON] et al. [8]. The primary distinction between fine and coarse particulate materials was emphasized. [PERSON] [9] investigated the quasi-static shear deformation of granular media and examined the evolution of internal variables associated with micromechanical processes occurring at the particle scale by numerical simulations. The results illustrate the effect of inter-particle friction on macroscopic mechanical and micromechanical behavior. [PERSON] et al. [10] studied the effect of particle size distribution on the shear strength of the accumulated soil by direct shear and triaxial tests, and found that the shear angle increased with the increase of median particle size and gravel content. [PERSON] et al. [11] explored the effect of particle size on the thermal desorption of PCBs from contaminated soil and found that fine soil particles have higher removal and destruction efficiency than coarse ones. [PERSON] et al. [12] built a mathematical model based on the concept of a dominant particle network to predict the minimum porosity of granular soil with arbitrary particle size distribution. [PERSON] and [PERSON] [13] studied the effects of fineness, grain-size composition, and particle shape on porosity, giving a set of empirical correlations, as well as characterized the importance of grain-size distribution and the presence of interstices in laboratory-produced composite soils or mixtures containing pulverized sands. [PERSON] et al. [14] studied the effect of particle size on the void ratio of loosely packed binary particle mixtures and argued that the relative content of fine particles in the mixture significantly changes the void structure of the material and affects the material properties.