紫花苜蓿对黄土边坡浅层破坏防护时间效应的数值分析

为探究紫花苜蓿对黄土边坡浅层破坏防护的时间效应，该研究考虑降雨入渗条件并以含不同生长期紫花苜蓿的黄土边坡为例，基于室内土工试验获取参数，分别采用含植物根系的无限边坡模型和数值模拟两种方法进行稳定性分析。结果表明：降雨作用使得边坡潜在滑动面位置由坡体内部转移到降雨最大入渗深度处；随着坡面植被的生长，边坡浅层土体的强度逐渐提高，潜在滑动面又逐渐转移到边坡内部。根据数值模拟强度折减法，边坡稳定系数随着植被生长不断增大。对比无限边坡模型与数值模拟方法计算的边坡稳定系数，植物根系生长到1.0 m且不论在天然状态或降雨条件下黄土边坡均处于稳定状态，植物根系在0～0.8 m且在降雨条件下时处于不稳定状态，随着边坡植物生长期的延长，边坡稳定系数总体变化趋势一致，即先降低再逐渐增大。草本植物随着生长期的延长，对降雨条件下黄土边坡坡面侵蚀有明显的抑制作用，从而草本植物根系生长能够提高黄土边坡稳定性。当草本植物生长时间达到150 d时，能够有效防治黄土坡面侵蚀，并提高黄土边坡稳定性，因此草本植物生长的前5个月为关键期。研究成果对于黄土边坡浅层破坏防护以及实现黄土地区可持续发展具有重要的理论意义和应用价值。

Numerical analysis of protection time effect on planting alfalfa in loess slope with shallow failure

The special microstructure of loess has often posed a great risk on the slope in the loess areas. The loess structure can suddenly collapse, due to the complete loss of strength, particularly when the loess is wetted by water or subjected to a strong earthquake. The loess slope can usually be triggered by the slope surface erosion under the action of rainfall. The local failure of the slope body can often occur under heavy rainfall. There is a serious threat to the overall stability of the slope. For instance, hundreds of slope shallow failures have occurred in the Chinese Loess Plateau every year, as one of the regions with the thickest loess deposition in the world. The loess slope shallow failure can cause the loss and destruction of land sustainable productivity in the loess region. Fortunately, the slope protection with vegetation can be expected to alleviate the current slope shallow failure. A reasonable numerical simulation has been used to analyze the mechanism of root reinforcement soil, providing a strong theoretical basis for the application of slope protection with vegetation. This study aims to explore the protection time effect on planting alfalfa on the loess slope with shallow failure. The example was set as the loess slope with the alfalfa in different growth periods (0, 60, 90, 120, and 150 days). The rainfall infiltration was also considered in this case. A series of tests were carried out using the indoor planting of PVC pipe soil samples, including the quick direct shear tests, consolidated undrained (CU), and unconsolidated undrained (UU) triaxial tests. The physical and mechanical parameters were measured for the root-soil composite under different conditions. The slope stability was analyzed using the infinite slope model with the plant roots and the numerical simulation. The results show that three stages were divided for the loess slope under different conditions in the process of shallow failure. Firstly, a shear yield plastic zone parallel to the slope surface was formed at the maximum infiltration depth of precipitation. Secondly, there was a small amount of soil in the middle and upper position of the slope surface, due to the tension action. Thirdly, the shear yield zone gradually moved along the slope surface toward the slope shoulder. The loess slope was in a stable state under natural conditions. The potential sliding surface was at a deep position inside the slope. Therefore, the rainfall infiltration was attributed to the potential sliding surface of the slope transferring from the inside of the slope to the maximum infiltration depth of rainfall. The soil strength gradually increased in the shallow portion of the slope with the growth of vegetation on the slope. The potential sliding surface of the slope gradually shifted to the inside of the slope. The slope stability coefficient increased with the growth of vegetation, according to the strength reduction of numerical simulation. A comparison was made to determine the slope stability coefficient from the infinite slope model and the numerical simulation. The loess slope was in a stable state, when the plant roots grew to 1.0 m, either under natural or rainfall conditions, whereas, in an unstable state, when the plant roots grew to 0-0.8 m under rainfall conditions. The slope stability coefficient went down and then gradually increased with the growth of vegetation. The overall trend was all the same. The herbaceous plants were used to significantly inhibit the loess slope surface erosion under rainfall conditions, particularly with the extension of the growing period. Consequently, the herbaceous roots can be expected to improve the stability of the loess slope. Once the growth time of herbaceous plants reached 150 days, the loess slope surface erosion was effectively prevented for the better stability of the loess slope. Therefore, the first five months of herbaceous plant growth can be the key period to improving the stability of the loess slope in this case. The finding can provide the theoretical reference and application for the shallow failure protection of the loess slope in the sustainable development of the loess areas.