Mechanical properties of EICP solidified sandy viscous purple soil

Purple soil is a very representative type of agricultural land in Chongqing, which is widely distributed in the Sichuan Basin and the Three Gorges Reservoir area. However, purple soil exhibits a loose structure, low strength, poor corrosion resistance and water stability. It is prone to soil and water loss under the frequent alternating action of dry and wet conditions caused by heavy rainfall and hydraulic erosion. Enzyme Induced Carbonate Precipitation (EICP) technology can effectively enhance the pore structure of the soil, improve its strength, stiffness, and corrosion resistance, thereby achieving soil solidification. In this study, self-extracted soybean urease was used to induce calcium carbonate precipitation for solidifying purple soil. The relationship between temperature and urease concentration was investigated through test tube tests. Furthermore, an orthogonal test was conducted to explore the optimal calcium yield combination (C_R group) as well as the maximum calcium production combination (C_P group) under the combined influence of soybean urease concentration (A), calcium chloride concentration (B), and urea concentration (C). Subsequently, curing tests, wetting-drying cycles tests, and unconfined compressive tests were performed on purple soil samples to investigate the impact of different curing combinations and numbers of wetting-drying cycles on sample quality, appearance, unconfined compressive strength, and stiffness. Scanning electron microscopy (SEM), X-ray energy dispersive analysis (EDS), and low-field nuclear magnetic resonance imaging (NMR) were employed to investigate the evolving nature of the curing process and analyze its mechanism. The results showed that: 1) The yield of calcium and the productivity of calcium carbonate were influenced by the concentrations of urease, calcium chloride, and urea. The optimal calcium yield combination (C_R group) was a urease concentration of 100 g/L, a calcium chloride concentration of 1.0 mol/L, and a urea concentration of 1.5 mol/L. The maximum calcium production combination (C_P group) used a urease concentration of 150 g/L, a calcium chloride concentration of 2.0 mol/L, and a urea concentration of 2.0 mol/L. 2) EICP treatment significantly enhances the unconfined compressive strength and stiffness of purple soil (elastic secant modulus E₅₀). Compared with the Control group (C_W group), the unconfined compressive strength increased by 104.47% in the C_R group and by 60.03% in the C_P group; while E₅₀ increased by 86.36% in C_R group and by 36.56% in C_P group. The cured sample has good durability, after undergoing seven wetting-drying cycles, the unconfined compressive strength remained at 440.65 kPa for C_R group samples and at 507.92 kPa for C_P group samples; E₅₀ reached 24.02 MPa for C_R group and 27.57 MPa for C_P group. 3) Combined with SEM and NMR tests, the pore structure of soil was quantitatively characterized. Microscopic analysis showed that EICP solidifies purple soil with generated calcium carbonate particles sized between 0.1～2 μm. For small pores, calcium carbonate mainly fills and cements them. While for large and medium pores, the generated calcium carbonate provides cementation and film coating, resulting in transformation of large pores into medium-sized ones. Thus, the proportion of small pores and large pores decreases while the proportion of medium pores increases. These effects lead to a more uniform pore structure in purple soil, and effectively improve its integrity and compactness. Nuclear magnetic imaging analysis also intuitively proved that EICP technology has a good curing effect on purple soil. This research can provide scientific basis and recommendations for engineering applications such as foundation construction, slope reinforcement, and soil erosion protection in the area of purple land.