In the fourth stage of the long-term cooperation with the site, the focus was to engineer a geomembrane containment system compliant with the geometry of the new cell added to the existing facility. Welding procedures and QA/QC workflows established in previous phases served as the reference for this stage; however, the slope angles and transition zones of the new cell required redesigning the panel layout from scratch. The textured HDPE liner was applied in a hybrid manner, utilizing single-sided texturing in sections where interface shear resistance was critical and a smooth surface preference in the base area.

Throughout the stage, non-destructive air channel tests for every weld seam and destructive peel tests at selected locations were implemented. The backfilling of anchor trenches and the protection of panel-to-panel transitions were executed under an accelerated schedule as the rainy season approached. Logistically, intra-West African transit times necessitated the early locking of roll planning; every batch arriving on site was matched with the panel cutting list to keep waste ratios to a minimum.