Mechanical properties of EICP solidified sandy viscous purple soil

Purple soil, one very representative type of agricultural land, has been widely distributed in the Sichuan Basin and the Three Gorges Reservoir area, such as Chongqing. Among them, purple soil often exhibits a loose structure, low strength, poor corrosion resistance, and water stability. Therefore, 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 be expected to effectively enhance the pore structure, strength, stiffness, and corrosion resistance of the soil, thereby achieving soil solidification. In this study, self-extracted soybean urease was used to induce calcium carbonate precipitation, in order to solidify the purple soil. A series of tube tests were carried out to investigate the relationship between temperature and urease concentration. 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 the 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. The evolving nature of the curing process was characterized to analyze its mechanism by using scanning electron microscopy (SEM), X-ray energy dispersive analysis (EDS), and low-field nuclear magnetic resonance imaging (NMR). The results showed that: 1) The yield of calcium and the productivity of calcium carbonate are dependent on the concentrations of urease, calcium chloride, and urea. The optimal combination of calcium yield (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 combination of calcium production (C_P group) was 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 enhanced 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 the C_R group and by 36.56% in the C_P group. The cured sample shared excellent durability after seven wetting-drying cycles. Among them, the unconfined compressive strength remained at 440.65 kPa for the C_R group samples and at 507.92 kPa for the C_P group samples; E₅₀ reached 24.02 MPa for the C_R group and 27.57 MPa for the C_P group. 3) SEM and NMR tests were carried out to quantitatively characterize the pore structure of soil. Microscopic analysis showed that EICP was used to solidify the purple soil with generated calcium carbonate particles sized between 0.1-2.0 μm. In small pores, calcium carbonate was used to fill and cement them. While in large and medium pores, the generated calcium carbonate was provided cementation and film coating, resulting in the transformation of large pores into medium-sized ones. Thus, the proportion of small and large pores decreased greatly, while the proportion of medium pores increased. As such, a more uniform pore structure was obtained in the purple soil, in order to effectively improve its integrity and compactness. Nuclear magnetic imaging analysis also intuitively proved that the EICP technology shared a better curing effect on the purple soil. This finding can provide the scientific basis and recommendations for engineering applications, such as foundation construction, slope reinforcement, and soil erosion protection in the area of purple land.