Clay backfill (puddle clay)

Clay backfill, traditionally called puddle clay or puddled clay, is a low-permeability natural material historically used to seal drainage structures, line canals and form waterproof backfill around manholes and watercourses. The clay is worked with water to a dense, pliable consistency and placed in compacted layers. Original Victorian canal infrastructure across London, including parts of the Regent's Canal and Grand Union Canal, was sealed with puddled clay. In drainage construction, clay backfill is still occasionally specified to prevent groundwater migration along granular pipe surrounds, particularly where excavation passes through running sand. Modern alternatives include bentonite barriers and engineered clay liner products. The material is identifiable by its dense, plastic feel and grey-brown colour. Where encountered during excavation of older drains, the presence of clay backfill indicates a deliberately sealed connection.