###### Abstract  
The capillary barrier is a type of soil cover system commonly used in various geotechnical applications, such as limiting infiltration for slopes or landfills or providing cover for solid waste. It serves to prevent the movement of water through the soil layers by utilizing contrasting particle sizes. This paper focuses on investigating the effect of the granular layer on the performance of a three-layer cover with a capillary barrier, integrating the granular layer within clayey sand. The investigation involved one-dimensional infiltration tests utilizing four uniform granular soils with varying grain sizes. These tests were instrumental in calibrating soil water characteristic curves and hydraulic conductivity curves via back analysis. Subsequently, numerical analyses were conducted using a 15 m long model for each of the four distinct cover types. The results indicated that the fine gravel significantly improved the barrier performance beyond one-dimensional tests, owing to its high permeability and the influence of the slope. After the capillary barrier failure, the intermediate layers transitioned into efficient drainage layers, particularly in the gravel layer with the highest lateral drainage capacity. Clayey sand at the bottom delayed percolation, thereby supporting the conversion of the intermediate layer into an effective drainage component. Overall, the multi-layer system showed superior percolation performance compared to the clayey sand cover lacking a granular layer. three-layer cover system; capillary barrier; precipitation; diversion length; lateral drainage; water balance; unsaturated transient seepage analysis Article  
Effect of the Mid-Layer on the Diversion Length and Drainage Performance of a Three-Layer Cover  
Performance of a Three-Layer Cover with Capillary Barrier  
1421  
Researchers widely explore capillary barrier applications, notably in mitigating rainfall-induced slope failures and as covers for solid waste landfalls [1; 2; 33; 34; 35; 16; 19; 20; 21; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122]. These applications, validated through various testing methods, showcase the superiority of capillary barrier covers in terms of durability, cost-effectiveness, and performance over conventional low-permeability soil covers [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. This study investigates the influence of various granular materials within the middle layer on the barrier effect, diversion length, lateral drainage, and system percolation of a three-layer cover featuring a capillary barrier (Figure 1). This three-layer configuration establishes a two-way capillary barrier, obstructing water movement caused by surface rainfall and rising water tables. However, this study does not consider the upward movement of water from the base, leaving this aspect for future exploration. Previous studies extensively explored the impact of the coarse layer on the capillary barrier [23; 24; 25]. However, this study extends the investigation by incorporating a third layer beneath the barrier. Its objective is to examine the barrier effect and the subsequent effects of drainage and percolation within the three-layer cover after the barrier dissipates. Hence, experimental testing was conducted on the three-layer cover using soil column infiltration tests, employing four distinct mid-layer materials. The experimental results were utilized for calibrating soil parameters, such as soil water characteristic curves and hydraulic conductivity curves, through a process of back analysis.