Abstract: The barrier is waterproof, and is used to contain contaminated groundwater within an enclosure. Steel elements are pile driven, the elements having rolled-over forms (5,6) which inter-engage. Upon inter-engagement, an enclosed cavity (90) is created which extends from top to bottom of the piled elements. A scraper (19) on the junior element (8) cleans dirt out of the cavity as the junior is driven down alongside the adjacent senior element (7). The cavity may be cleaned out by inserting a hose pipe to the bottom of the cavity (90) and flushing through with water. Then, a sealant is injected into the cavity, using an injection tube. The inter-engagement of the edge forms (5,6) of the elements is such that the cavity formed by the inter-engagement is constrained to its nominal size and shape throughout the whole height of the barrier.