Strengthening Reinforced Earth Walls — Knox Road Duplication Case Study

In 1979, The Reinforced Earth Company (RECO) designed and supplied Reinforced Earth walls (REWs) at Knox Road bridge over main Western rail line at Doonside, NSW. The arrangement comprised a pair of abutments spanning over rail. In 1979, the REWs were constructed with two traffic lanes on the Eastern side (Stage 1), and with provision for a further two traffic lanes to be added on the Western side (Stage 2). Between 1979 and 2011, the Western side of the abutments were left in a relatively unfinished condition in readiness for the Stage 2 abutment to be constructed.

The original design of the REWs was carried out on the assumption that when the Stage 2 duplication was to occur, the same abutment beam seat, relative founding level and bridge loading would apply. However, the Stage 2 duplication design, developed in 2011, required a larger abutment beam seat and significantly higher bridge loads than the original design assumed. This meant that there was insufficient earth reinforcement capacity within the existing REW structure to safely support the new bridge loading.

To increase the existing capacity, additional galvanised metallic soil reinforcing strips (RE strips) were incorporated into the REWs. This increase in capacity was achieved by a combination of installing new reinforcing strip connections to existing panels and fabricating new facing panels. The work involved a staged construction process with the initial stage comprising removal and replacement of under-strength panels and retro-fitting with new reinforcing strip connections. The second stage of construction comprised conventional construction of the REW to the new finished surface level incorporating newly fabricated facing panels. Impact on existing road and rail users was paramount during the design and construction of the bridge. It was important to keep the existing road open as much as possible, and the reduce the amount of works in the Rail Corridor. This Case Study presents an innovative method of strengthening existing structures, while reducing costs and construction impacts on both road and rail users in an urban environment.