## 1 Introduction  
As a new additive material, nanomaterials have been widely used in the fields of information, materials, energy, environment, medical equipment, biology and agriculture, electronic equipment, coatings, and other industries [1; 2; 3]. Size, quantum, surface, and interface effects can be produced by nanomaterials. For example, the NS will increase the density of microstructure and reduce the pore permeability [4]. NS improves the stability of soils significantly by the hydration products that fill the pores between soil particles [5]. Meanwhile, the Atterberg limits can be modified by the formed permanent silicide in the surface and interior of the soil-water system [6]. The microparticle reactions in the soil-water system will be accelerated due to nanoparticles with a high specific surface area [7]. The microscopic characteristics and modified mechanisms of the soil-water system with nanomaterials have been widely studied, especially the micro-physicochemical and mechanical properties. Nanomaterials as an additive can fill the interspace of a soil-water system, and ion exchange and the hydration reaction can be promoted by nanomaterials. These result in the change of physicochemical and mechanical properties. For instance, the gel structure can compact the soil-water system, which is formed upon the soil particle surface. The dry density of nanomaterials modified soil increases and hydraulic conductivity reduces significantly because nano-particles fill the pores of the soil-water system and bond the soil particles together [5; 8; 9]. The hydration reaction and ion exchange are promoted by the nanostructured palm beam ash, and stabilizers flocculation effect is increased [10]. Because of the filling and nucleation effects of nanoparticles, the smaller the nanoparticles' diameters are the larger the pores of loess [6; 11]. The stability of loess will be enhanced effectively in the generation of hydrates due to the chemical reactions between NS and soil particles. In civil engineering, many researchers point out that mechanical characteristics can be enhanced by nanomaterials to meet the requirements of design and construction. For example, with the addition of nanomaterials, the expansion ratio of expansive clay, expansion pressure, and coefficient of permeability reduce, while the California bearing ratio (CBR), stiffness, shear strength, plasticity, and unconfined compressive strength increase [6; 12; 13; 14; 15; 16]. In short, the mechanical properties of a soil-water system can be enhanced by nanomaterials, and it varies with soil texture, nanomaterial type, additive content, and so on. In essence, nanomaterials affect the physicochemical and mechanical properties of modified soil by mainly optimizing the micro-pore structure of the soil-water system. SEM, computerized tomography, and MIP are the three common methods to study the microstructure of the soil-water system [17; 18]. For instance, it was found that NS fills the pores and aggregates with nanomaterials and soil particles by SEM [7]. The tensile strength of clay can be improved by nano-clay, and the mechanism is summarized as follows: a viscous gel material is produced when nano-clay particles contact with pore water, and the friction between soil particles increases. Nanoparticles fill the pores of a soil-water system, and the interfacial bond strength is improved as the contact acreage of soil particles increases. Following this, the mechanical properties of clay with viscous gel fibers around soil particles are improved effectively [12; 19; 20; 21]. The pozzolanic reaction is promoted by nanomaterials under wetting conditions, then improving the durability of stabilized soils [22]. As shown in SEM images of modified clay, the micro-pore structure becomes more uniform and denser, large pores at the surface of the particles reduce, and the pores are filled by nanoparticles [23].