Leakage characteristics of concrete lining canal with cracks

Concrete lining is one of the primary engineering measures to improve water conveyance efficiency in irrigation canals, thereby supporting 40% of global grain production. However, canal seepage losses account for 15%-50% of diverted water in recent years. Particularly, even with only 1% crack area, seepage can reach 70% of total. Repeated frost heave can also cause progressive cracking in seasonally frozen regions, leading to a vicious cycle of seepage, foundation frost heaves, and further lining damage. However, existing studies focus on crack impacts on concrete structural durability within code-specified small crack widths. It is often required for the seepage hydraulic characteristics of actual canal linings. This study aims to quantify the effects of key factors on cracked lining permeability. A systematic investigation was made to explore the permeability of cracked concrete lining and canal leakage mechanisms using single-crack flow theory and the Poiseuille seepage model. A high-precision prediction model was then established to reveal the hydraulic characteristics of canal leakage. A series of tests were conducted to combine laboratory experiments and 3D fluid-solid coupling simulations. Discrete crack model was adopted for experiments. C30 concrete specimens with prefabricated cracks were evaluated under 63 loading conditions, particularly with the crack widths of 0.1-3 mm, lining thicknesses of 60-80 mm, and water heads of 200-600 mm. A constant-head permeability device was used to measure seepage volume. Grey correlation analysis was implemented to rank influencing factors. A multivariate nonlinear regression was established to predict permeability of concrete lining. In numerical simulation, cracks were equivalent to ideal porous media. The constitutive equation was validated with the maximum seepage volume deviation of 12% using physical tests. The standard D80-U-shaped canal, the most widely used form in China, was selected as the research object. A systematic analysis was conducted to explore the seepage characteristics of cracks at different lining positions. Experimental results show that crack width was the dominant factor influencing permeability, which increased monotonically with crack width in a rapid-then-slow trend. For instance, the permeability increased by 130% under 600 mm water head and 60 mm lining thickness, when crack width increased from 0.5 to 0.8 mm, but only by 20% when further increasing to 1 mm. Water head and lining thickness exhibited stage characteristics: when the crack width was less than 0.5 mm, permeability evolution closely followed the Poiseuille model, with negligible sensitivity to water head and lining thickness. Once the crack width exceeded 0.8 mm, permeability decreased with increasing water head and lining thickness, due to amplified head loss and nonlinear flow effects that narrowed effective water passages. Grey correlation analysis showed that the influencing factor was ranked in the descending order of: crack width, water head, lining thickness, indicating strong correlations. The regression model achieved a high correlation coefficient of 0.998, enabling accurate permeability prediction under various conditions. Numerical simulation indicated that seepage velocity in cracked areas was far higher than that in intact regions, where the minimum velocity in non-cracked areas was only 2% of the maximum velocity in cracked areas. Seepage velocity increased with both crack width and water head. When crack areas accounted for 1% of the total lining areas, the maximum seepage contribution rate reached 65.1%, indicating that the cracks closer to the canal bottom shared higher contribution rates. Inside cracks, water flux followed a parabolic distribution, and the high-flux regions shrank, as total flux increased. A growth zone of negative pore pressure formed at the cracks, due to the suction effect, while the surrounding areas shared positive pore pressure growth. Cracks near the canal bottom exhibited faster and more stable seepage development, as the fluid experienced less gravity resistance. The findings can provide critical theoretical support to assess seepage and anti-seepage performance in concrete lining canals.