Joint Geometry Controls Preferential Flow and Contaminant Transport in HDPE Geomembrane Composite Cutoff Walls

High-density polyethylene (HDPE) geomembrane composite cutoff walls are used to limit groundwater flow and contaminant migration, but panel joints can form localized transport pathways. The unresolved question is how predefined interlocking configurations differ in joint-controlled flow and transport. This component-scale study combined individual 20 g geotechnical centrifuge tests with local-scale, engineering-width, and three-dimensional centrifuge-apparatus numerical models to compare crescent-shaped, square, and triangular joint configurations. Two-dimensional local-scale models provided configuration-specific equivalent hydraulic conductivity and diffusion coefficients, which were transferred without calibration to a three-dimensional half-domain model of the centrifuge apparatus. Separate 6.0 m-wide two-dimensional calculations quantified the contribution of the joints and prescribed cracks to overall equivalent hydraulic conductivity. The crescent-shaped joint showed the smallest measured hydraulic-head reduction and the lowest local-scale equivalent hydraulic conductivity, whereas the triangular joint showed the largest values. The centrifuge Na+ changes were within the stated instrumental error and were retained only as descriptive records. Extended 10 d constant-head analyses using the 0.40 m three-dimensional half-domain indicated that the maximum external-domain Na+ concentration increased with hydraulic-head difference and that prescribed cracks accelerated the concentration response; differences among the 0.5–1.5 mm crack cases were small and non-monotonic at 10 d. The simulated concentration fields illustrate joint-controlled contaminant transfer into a common receiving-fluid domain at the component scale. The comparison provides a practical basis for joint detailing, defect control, and construction quality assurance in composite cutoff walls.