# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 2 Basic Physical Properties of Expansive Soil Samples
### Basic Physical Properties of Sample Materials  
The construction of fill slopes significantly alters the soil's original structure, resulting in a typical remodded soil. Consequently, this paper centers its investigation on the prop  
Figure 1: Sampling Point at Section K23+800 on the Chong’ai Expressway.  
erties of remolded expansive soils. The fundamental physical properties at the sampling site were determined using standard geotechnical testing protocols [26]. The free swelling test outcomes indicate that the expansive soil sample from sampling point 1 exhibits a swelling rate of 98%, whereas the sample from point 2 shows a rate of 75%. Based on the classification criteria for expansive soils, those with a swelling rate exceeding 90% are designated as highly expansive, whereas those below this threshold are categorized as moderately expansive. Consequently, the soil at sampling point 1 is designated as highly expansive, and that at point 2 as moderately expansive. The physical and mechanical properties of the undisturbed expansive soil are detailed in Table 1.  
Smectite, illite, and kaolinite, among other secondary minerals, are formed through chemical weathering. These fine mineral particles are the primary constituents of clay minerals and significantly influence the engineering properties of soil. Utilizing a Smart-lab XRD analyzer, X-ray diffraction analysis was performed on samples of highly expansive soil, as depicted in Figure 2. The findings indicate that quartz, smectite, illite, kaolinite, and trace amounts of calcite are the predominant minerals, with clay minerals comprising a substantial fraction. Smectite exhibits the highest hydrophilicity, capable of absorbing substantial water molecules within its interlayers. Illite, with a crystalline lattice and interlayer structure akin to smectic, is less hydrophilic. Kaolinite's strong interlayer bonding impedes water molecule penetration, resulting in lower hydrophilicity compared to illite. Consequently, the concentration of smectite and illite is notably elevated in highly expansive soil and rock. The clay mineral's structural unit in expansive soil is composed of two SiO\({}_{2}\) layers and one Al\({}_{2}\)O\({}_{3}\) layer. The weak interlayer bonding facilitates rapid water molecule infiltration, leading to internal crystal lattice expansion and subsequent rapid disintegration of the soil, which is a key mechanism in the development and progression of fissures.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Expansiveness** & **Unit Weight (kN/m\({}^{3}\))** & **Liquid Limit \(W_{L}\) (\%)** & **Plasticity Index \(I_{P}\)** & **Rate of Expansion/\%** & **Maximum Dry Density (g/cm\({}^{3}\))** & **Optim Moisture Content \(W_{opt}\) (\%)** \\ \hline Moderately Expansive Soil & 20.2 & 50 & 25 & 75 & 1.8 & 18.1 \\ Highly Expansive Soil & 17.8 & 73 & 34 & 98 & 1.7 & 20.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic physical properties of the tested expansive soil samples.  
Figure 2: X-ray diffraction spectrum of expansive soil in its initial state.