EICP固化砂质黏性紫色土的力学性能

    Mechanical properties of EICP solidified sandy viscous purple soil

    • 摘要: 紫色土结构疏松、强度低、抗蚀性和水稳性差,在强降雨、水力冲蚀作用下,干湿交替作用频繁,极易产生水土流失。脲酶诱导碳酸钙沉淀(enzyme induced carbonate precipitation,EICP)技术可有效改良土体孔隙结构,提升土体强度、刚度和抗蚀性,达到固化土体的效果。该研究通过正交试验探究黄豆脲酶浓度(A)、氯化钙浓度(B)与尿素浓度(C)共同作用下的最优产钙率组合(CR组)与最大产钙量组合(CP组),据此开展紫色土的固化试验、干湿循环试验、无侧限抗压试验、扫描电镜试验、X射线能谱分析和低场核磁扫描试验,探究固化效果的变化规律,并分析固化机理。结果表明:1)EICP处理显著提升紫色土无侧限抗压强度与刚度(弹性割线模量),与对照组(CW组)相比,CR组和CP组无侧限抗压强度分别提升了104.47%和60.03%,刚度分别提升了86.36%和36.56%;2)固化试样具有良好的耐久性,在干湿循环作用下仍保持较高的无侧限抗压强度与刚度;3)EICP固化紫色土,生成的碳酸钙晶体尺寸在0.1~2.0 μm,可对土体中的小孔隙进行有效填充,同时将土体中的大孔隙进行胶结、覆膜,而转化为中孔隙,使紫色土孔隙结构更加均匀。研究结果阐明EICP对紫色土具有良好的固化效果,可为紫色土地区的地基、边坡加固和水土流失防护等工程应用提供科学依据和参考。

       

      Abstract: 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 (CR group), as well as the maximum calcium production combination (CP 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 (CR 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 (CP 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 (CW group), the unconfined compressive strength increased by 104.47% in the CR group and by 60.03% in the CP group; while E50 increased by 86.36% in the CR group and by 36.56% in the CP 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 CR group samples and at 507.92 kPa for the CP group samples; E50 reached 24.02 MPa for the CR group and 27.57 MPa for the CP 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.

       

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