Study on the permeability evolution law and the micro-mechanism of CCL in a landfill final cover under the dry-wet cycle

This paper aims to investigate the failure mechanism of compacted clay liner (CCL) impervious structures caused by the dry-wet cycles in a landfill final cover. Experimental research is performed on the evolution of permeability characteristics and microstructure of CCL with different initial compactness under repeated dry-wet cycles. The research results show that the effect of dry-wet cycle varies significantly on clays with different compactness. The permeability of the low-compacted clay (90 %) gradually decreases as the times of dry-wet cycles increases, while those of the high-compacted clay (98 %) gradually increases. A non-linear variation relationship between the permeability and dry-wet cycle times with compactness of CCL has been established. The tests and the prediction outcomes of the models both show that, after several dry-wet cycles, the permeability of all compacted clays are greater than 1 × 10⁻⁹ m/s, which can not meet the anti-seepage requirement of the landfill cover systems. The change in pore connectivity caused by the damage of soil structure under the wet-dry cycles is the main factor for the change of the permeability characteristics of CCL. We’ve determined the quantitative indexes (critical pore size) that can present the connectivity characteristic of the pore structure of clay, for which the evolution law under the dry-wet cycles is the same as that of the permeability of the CCL.