紫色土坡耕地埂坎裂隙发育对土壤入渗的影响

为揭示埂坎裂隙发育程度对土壤入渗性能的影响，该研究选取了3种裂隙发育程度（重度发育、中度发育、轻度发育）的埂坎为研究对象，利用双环入渗试验揭示裂隙发育的埂坎土壤入渗规律并采用入渗模型进行模拟。结果表明：1）不同裂隙发育程度下，埂坎土壤入渗率变化趋势均为先迅速下降后逐渐趋于稳定。有裂隙埂坎各时段的入渗率均大于对照组无裂隙埂坎，但其差值均随入渗时间的增加而减小；2）随着埂坎裂隙发育程度的增加，土壤初始入渗率、平均入渗率、稳定入渗率和累积入渗量均增大，其中初始入渗率的增幅最高（98.72%）；3）控制初始含水率条件下，裂隙深度和面积-周长比仍与初始入渗率、平均入渗率及120 min累积入渗量呈显著正相关（P＜0.05），裂隙面密度仅与平均入渗率呈显著正相关（P＜0.05）；4）Kostiakov模型和Mezencev模型对不同裂隙发育程度下埂坎入渗过程拟合效果较好（R2为0.84～0.99），但Kostiakov模型只有在一定时间范围条件下才能有效描述裂隙埂坎入渗过程。研究结果可为紫色土区坡耕地埂坎的建设、维护管理和合理利用及水土保持提供参考。

Water seepage in soil bunds under different crack development degrees on the purple-soil sloping farmlands

Abstract: The sandy clayey purple soil is widely distributed in the middle and upper reaches of the Yangtze River, serving as one of the main soil resources in the mountainous areas of southwest China. Intense physical weathering, loose structure, and low erosion resistance are the main characteristics of sandy clayey purple soil. Therefore, environmental disturbance, such as the water fluctuation near reservoirs, heavy rainfall, and groundwater, often induces soil erosion, landslide, settlement, and soil-water disasters of purple soil. A serious threat has been posed on the village buildings and roads, even the agricultural production. The properties of water immersion disintegration with the sandy clayey purple soil can also be an important reason for water and soil disasters in the southwest mountainous areas. It is necessary to clarify the disintegration characteristics and reinforcement for the water-soil disasters prevention and control. In this study, a disintegration test was performed on the purple soil samples with different initial dry densities, water content, and grain gradation using a self-developed instrument. Meanwhile, the disintegration and evolution of sandy clayey purple soil were also analyzed from the perspective of unsaturated effective stress. Bacillus megaterium was selected to reinforce the soil samples with the Microbial Induced Calcite Precipitation (MICP), as the more suitable as Sporosarcina pasteurii in the sandy clayey purple soil areas. A Scanning Electron Microscope (SEM) was then utilized to characterize the morphologies of the soil sample, thereby determining the MICP improvement on the disintegration characteristics of sandy clayey purple soil. The results show that: 1) Four stages were divided in the whole process of immersion and disintegration of sandy clayey purple soil, including the air-water conversion, equilibrium, disintegration development, and disintegration residual stage. 2) The initial dry density, water content, and grain gradation obviously affected the disintegration characteristics of sandy clayey purple soil. Specifically, the disintegration rate and the average disintegration velocity decreased, with the increase of initial dry density and water content. In addition, the average disintegration velocity of soil increased by the content of fine particles. 3) The evolution of water and air was ranging from the pore water closed, double connected, and pore air closed morphology, with the increasing of the initial saturation. Water was rapidly absorbed into the pores under the matric suction, where the pore pressure was changed significantly. Subsequently, the effective stress of unsaturated soil rapidly reduced to the negative, leading to an interparticle compressive stress (the negative tensile stress). Once the tensile stress reached the value of effective cohesion, the unsaturated strength of purple soil was lost completely. Finally, the soil sample was then destroyed under disintegration. The more severe disintegration was also obtained with the decrease in the initial saturation of a soil sample. The decay process of the unsaturated effective stress depended greatly on the initial saturation after the purple soil was immersed in water. Specifically, the average disintegration velocity attenuated exponentially with the increase of the initial saturation. 4) The disintegration rate and the average disintegration velocity of the MICP treated soil samples decreased by 73 to 97 percentage points and 84%-99%, respectively, compared with the untreated soil. Calcium carbonate crystals formed by solidification and deposition greatly reduced the micro-cracks and large pores in the soil structure. As such, a denser pore structure was achieved to enhance the strength of intergranular cementation for the higher resistance to the disintegration of the soil. Consequently, the MICP technology can serve as an effective measure to prevent the water and soil disasters of the sandy clayey purple soil in southwest mountainous areas.