A Laboratory Investigation into the Effect of Coarse-Grained Layer Mixing with Fine Particles on the Water Storage Capacity of a Capillary Barrier Cover

A capillary barrier cover (CBC) is a geotechnical structure which a coarse-grained soil layer covered by a fine-grained soil layer. A CBC can retain downward water infiltration, increase water storage capacity and lateral diversion, and prevent capillary rise. Geotextiles are usually set up as isolation layers between fine-grained and coarse-grained layers to prevent fine particles entering the coarse-grained layer, resulting in a decrease in downward water infiltration and water storage capacity. However, crustal stress, farming, animal, plant activities, and other factors may cause damage to the isolation layer. At present, there is no reliable and accurate method to determine the location and degree of damage to the isolation layer. The existing methods search for the damage location by excavating the whole fine layer, which incurs high maintenance costs. If the damaged position of the CBC isolation layer can be accurately obtained, it can reduce maintenance costs. Therefore, this study investigated the influence of a coarse-grained layer mixed with different particle sizes and proportions of fine particles on water storage capacity through laboratory soil column experiments. The results are as follows: (1) Fine particle mixing into the coarse-grained layer will reduce water storage capacity, and there is a worse admixture ratio that minimizes water storage capacity. (2) The CBC enhances the fine-grained layer volumetric water content (VWC), but the enhancement degree decreases as the distance from the fine–coarse interface increases. (3) A method has been proposed to determine the location and degree of damage to the isolation layer. When the VWC at the fine–coarse interface reaches a stable level during breakthrough, the CBC effect exists, the higher the VWC at the fine–coarse interface, the stronger the CBC; when the VWC at the fine–coarse interface is unstable during breakthrough, the CBC effect disappears, and the median diameter of the fine particles mixed into the coarse-grained layer is finer than or equal to the fine-grained particles’ median diameter.