# Applicability Study on Modified Argillaceous Slate as Subgrade Filling for High-Speed Railway
## 3 Experimental Evaluation of Cement/Lime-Modified Soil
### Results
#### 3.1.7 Dry-Wet Cycle Characteristics  
The dry-wet cycling caused by the variability of environmental conditions is an important factor that leads to the degradation of the mechanical properties of railway subgrade filling, and consequently leads to subgrade failure in practical engineering. The ability of modified soil to withstand the dry-wet cycle reflects its ability to withstand water damage in natural environment. This is one of the most important indexes of modified soil's durability. According to ASTMD 559-44 and ASHOT 136-45 \"Standard Method of Compacted Cement-Soil Dry-Wet Test\", the weight loss of modified soil caused by the dry-wet cycle is determined. The standard curing specimen is weighed after being immersed in water for five hours at room temperature, and then is dried in an oven at 71 \({}^{\circ}\)C for 42 h. The floating particles on the surface of the specimen are brushed off by a steel-wire brush, weighed again; this is then repeated 12 times. The weight-loss percentage of the modified soil is used to judge its quality and stability. According to this test method, the variation of strength and saturated mass loss of different dry-wet cycles for cement-improved specimens are shown in Figures 7 and 8, while the quicklime-modified WAS quickly collapsed (Figure 9).  
From Figure 7, it can be seen that the USC of cement-improved soil increases with the increase in dry-wet cycles. This phenomenon may be attributed to the greater influence of curing age on USC than that of cement admixture. In addition, with the increase in cycles, the free water in the sample decreases and the bound water increases gradually until they reach a certain dynamic equilibrium, and this equilibrium state changes continuously with the change in external environmental factors (such as temperature) to form a new dynamic equilibrium. In this way, the bound water gradually replaces part of the free water to fill the pore, forming a stable structure with the skeleton structure of the soil, which increases the strength of the sample.  
From Figure 8, it can be seen that after three dry-wet cycles, the quality of the WAS tends to be stable, and the higher the cement content, the better the water stability of the WAS is and the lower the mass loss of the dry-wet cycles is. After five dry-wet cycles, the mass loss ranges of the WAS with 3%, 5% and 7% cement are about 4-7 g, 2-5 g and 1-3 g, respectively, accounting for 3.2%, 2.3% and 0.8% of the total mass.  
When the cement-modified sample is being saturated in water, the water immersion causes small bubbles to emerge on the surface and surround the sample. Initially, the water bubbles up for nearly 10 min. Thereafter, bubbling slows down until the water in the soil tends to balance with the water in the soil, i.e., until the sample is saturated, and the sample remains intact. This therefore means that the water stability of the cement-improved soil is good.  
The sample of quicklime-modified soil after 14 days' curing disintegrated in water is shown on the right side of Figure 9. Owing to the presence of air in the WAS and the invasion of water, small bubbles appeared on the surface and surrounded the modified soil. Initially, bubbles popped out quickly. After 7 min, the WAS was softened, cracks appeared in the soil and the surrounding particles continued to fall off until the WAS collapsed. This demonstrates that the water stability of the quicklime-improved soil is bad.