Lightweight backfill materials in integral bridge construction

Analysis of integral bridge structures shows that lateral earth pressures on the end abutments have a dominant influence on the sizing of bridge components. Thermal cyclic movements induced by deck expansion cause densification of backfill material, leading to the build-up of high pressures behind the abutments. Measures to reduce these pressures by the use of alternative backfill materials can be highly beneficial to the structure as a whole, with extra expenditure on the backfill material being offset, and in some cases exceeded, by savings in material quantities in the rest of the structure and build time. This paper examines three contrasting backfill options that were considered during the design of Cottington Road overbridge in Kent: 6N granular backfill; lightweight expanded clay; and expanded polystyrene blocks. Although more expensive as a material than the other options investigated, the expanded polystyrene block option was selected as the most economical solution for this particular structure owing to large material savings in the foundations, abutment walls and wing walls as a direct result of reduced lateral pressures. Although it did not form part of the decision-making process, this paper also shows a significant saving in embodied and transport-related carbon dioxide emissions through preferring expanded polystyrene blocks to expanded clay.