Study of threshold gradient for compacted clays based on effective aperture

Compacted clay liner plays a crucial role in preventing leakage and migration of pollutants. Clay often appears in Non-Darcy phenomenon, one of which is characterized by having the threshold gradient. The threshold gradients exhibited by different clays are usually quite different. The reason for the discrepancy is currently unknown. To solve this problem, the mineral compositions of two clays were selected for test. The tests included threshold gradient, aperture and the pore water content. The test results showed that kaolin-based Clay A had no threshold gradient, whereas illite and Montmorillonite-based Clay B presented the threshold gradient that decreased with the increase of the porosity. For the samples mixed with Clay A and Clay B, the values of the threshold gradient increased with increasing the content of Clay B in the mixed samples. The reason is that the porosity and mineral content can affect the bound water content. If the value of pF is greater than 3.8, the bound water is gelatinous “immobile water”. Based on the water retention curves, the effective porosity can be obtained by subtracting the volume occupied by “immobile water” from the void of the soil particles. Then the modified effective aperture can be obtained by Hangen-Poiseuille law. The modified effective aperture will decrease with the decrease in the porosity and the increase of the content of illite and montmorillonite. This is the key reason why the different clays have different values of the threshold gradients.