Cracking and vertical preferential flow through landfill clay liners

• Vegetation roots can restrict the crack development, especially in the early stages.
• The saturated hydraulic conductivity of vegetated soil increases 3 orders of magnitude.
• The unsaturated hydraulic conductivity of vegetated soil is 2 orders of magnitude larger.
• The vegetation roots are significant preferential flow pathways.

Cracking is a major concern in the long-term performance of landfill clay liners. Cracking of clay liners is affected by the roots of the vegetation planted to control erosion. While vegetation roots may restrict cracking they may introduce preferential flow through the liners. The impact of vegetation roots on the hydraulic properties of clay liners remains unclear. This study investigates the effect of vegetation roots on crack formation and preferential flow through clay liners. Three experimental soil columns were used to observe crack formation by exposure to the weather and to measure the flow through them caused by artificial rainfall. The clay in each column was compacted to 90% relative compaction. The first column was not vegetated and no crack developed in it. The second column was vegetated and underwent drying–wetting cycles outdoor. The third column did not have vegetation but underwent drying–wetting cycles outdoor. Cracks developed in the second and third columns due to the exposure to the weather. Image analysis in the experimental soil columns revealed that vegetation roots restricted crack formation, especially in the early stages of drying–wetting cycles. Infiltration tests performed on the experimental soil columns showed that the unsaturated hydraulic conductivity of the vegetated soil in the second column was respectively about two orders and one order of magnitude larger than those of the soils in the first and third columns. The experimental results imply that the effect of vegetation roots on the preferential flow through landfill clay liners should be considered in practical designs.