膨胀土边坡开挖时因干-湿循环作用形成裂隙，导致降雨入渗深层土壤，影响边坡稳定性。针对传统工艺存在生态效果差与使用寿命短等不足，进行了6组兼顾生态和防渗功能的非饱和上细下粗二元结构的边坡排水防渗效果的试验研究。采用亮蓝染色示踪方法直接观测了降雨条件下二元结构水分入渗过程，其结果表明：粗粒层粒径越大，级配越差，越易形成优先渗流通道，难以形成连续的水平渗流场，排水效果差。降雨直接入渗至边坡防护层，增加了其入渗边坡土体失稳的风险。通过监测侧向与底部排水体积，分析结果表明：二元结构具有较好的防渗排水效果，其稳定排水效率和综合排水效率分别介于80%～100%和43%～79%之间，随着粗粒层粒径减小、颗粒级配变好，稳定排水效率呈先减小后增加的趋势，而综合排水效率却呈先增加后缓慢减小的趋势。因此，从排水效果和工程材料造价等角度考虑，认为方案4最优。该研究成果可为南水北调膨胀土边坡治理以及其他工程的边坡防渗问题处理提供技术支撑。

Excavation of the expansive soil slope results in fissure development due to drying-wetting cycles, and affects the stability. Although the materials of cement panel and geotextile are generally used to deal with the drainage problem successfully, their service lives are short and less ecological friendly. Therefore, a capillary barrier with dual soil structure consisting of a relatively fine soil layer above a relatively coarse soil layer, is introduced in this paper considering both ecological and drainage-seepage controlling functions. The aim of this article is to investigate the effects of particle size and distribution of the coarse soil layer on the capillary seepage flow. Six groups of tests are conducted to monitor the soil water infiltration process under the condition of rainfall using the brilliant blue dyeing tracer. The results show that preferential flow and discontinuous horizontal seepage field appear in coarse soil with larger particle size and worse particle distribution, and worse capillary barrier effect. The measured vertical and horizontal drainage volumes indicate that the dual structure presents good drainage control. The parameter of steady drainage efficiency and the total drainage efficiency are used to evaluate the effect of capillary barrier. The steady drainage efficiency between 80% and 100% is small firstly and then increases with smaller grain size and better particle distribution of the coarse soil layer. The variation of total drainage efficiency between 43% and 79% is just opposite compared to the former. In summary, the dual structure of the fourth scheme is suggested to apply the South-to-North Water Transfer Project, according to the optimal seepage control efficiency and the lower cost of coarse material. The research results can be directly applied to the seepage control of expansive soil slopes in other projects.