# Development Characteristics and Mechanism of Crack in Expansive Soil under Wet-Dry Cycling
## 3 Indoor Wet-Dry Cycle Test of Expansive Soil  
### Sample Preparation and Testing Methods  
**(1) Sample preparation**  
In the domain of geotechnical engineering, the ring shear specimen stands out as the preeminent choice for testing due to its straightforward preparation and the availability of diverse testing and detection apparatus. Consequently, the current study employs ring shear samples for the indoor wet-dry cycle experiments on expansive soils, which are specified to have a diameter of 61.8 mm. The thickness of these samples is determined based on the stipulations of the experimental protocol. Two types of expansive soil samples were used in the test: medium-expansivity soil and strong-expansivity soil. The initial dry density of the strong-expansivity soil sample is 1.2 g/cm\({}^{3}\), with an initial saturation of 48.7%. The medium-expansivity soil sample has an initial dry density of 1.4 g/cm\({}^{3}\) and an initial saturation of 46.4%. The reconstituted expansive soil ring knife samples were prepared as follows:  
1. Start by drying the expansive soil samples obtained from the sampling point in an oven;
2. After drying, crush the soil samples using a soil crusher and pass them through a 1 mm sieve;
3. Transfer the crushed and sieved expansive soil into a plastic bag and calculate the required amount of water based on the initial moisture content specified in the test design;
4. As per the experimental specifications, uniformly distribute the dried expanded soil onto the container tray. Apply Vaseline to the basin of the stainless steel container to prevent it from constraining crack development at the bottom of the test block. Then, gradually and uniformly add water using a small spray bottle until reaching the pre-determined value, ensuring thorough mixing with the soil. Following configuration, seal the sample in a plastic bag for 48 h to guarantee consistent moisture distribution, as depicted in Figure 3;
5. Once the soaking process is complete, take an appropriate amount of soil sample and pour it into the ring knife. Compact it with a weight and prepare the expansive soil ring knife samples with different compaction degrees and thicknesses according to the test design plan. **(2) Experimental design and methodology**  
The objective of this section is to investigate the characteristics of crack development in expansive soil samples under wet-dry cycles, taking into account different initial moisture contents, compaction degrees, and thicknesses. Table 2 presents the physical parameters of the expansive soil samples under various influencing factors. The compaction degree of the expansive soil subgrade is typically maintained between 90% and 96% to ensure  
Figure 3: Process of expansive soil sample preparation. safe and comfortable driving conditions. However, achieving a compaction degree of over 90% under high-moisture-content conditions is challenging. Therefore, for the indoor wet-dry cycle test in this study, compaction degrees of 80% and 90% were chosen. Given that shallow instability predominantly contributes to the instability of expansive soil slopes, especially during periods of rainfall when the soil in these shallow slopes tends to be saturated, expansive soil specimens are saturated using a vacuum saturation apparatus. The specific steps of the wet-dry cycle test are as follows: First, the expansive soil samples sealed in plastic bags are placed in a constant temperature oven for drying. The oven temperature is set to 35 \({}^{\circ}\)C based on the meteorological data of the soil collection site on the Chong'ai Expressway in Guangxi. Regularly measure the samples' weight to determine changes in moisture content. Considering that the shrinkage limit moisture content of the expansive soil specimens employed in this study is 12%, further data recording ceases once the moisture content falls below this threshold. Consequently, soil shrinkage becomes notably challenging at this juncture, with crack formation essentially halting, marking the conclusion of a single dry cycle process.