Design solution to a heritage piled rail bridge foundation using numerical modelling technique

This paper presents a case study of replacement works for a more than 100-year old rail bridge over Guess Avenue, Wolli Creek, Sydney. Firstly, the as-built drawings for the existing railway bridge and brick abutments were reviewed and a summary of findings is presented. A geotechnical investigation program was devised to assess the existing timber pile and pile cap conditions below the brick abutments, which is heritage listed, and the subsurface geological profile for the bridge site. The preferred option using filler beam units enabled the clearance of the new bridge above the existing road to be increased from original 4.13 m to 4.6 m, which is in compliant with the current Australian bridge standard AS5100. The ultimate capacity of a single pile was assessed as the lesser of its structural capacity and geotechnical capacity using the obtained investigation results; the geotechnical capacity of the pile governs. A factor of safety of 2.5 was considered appropriate to evaluate the adequacy of the pile capacity under the existing bridge and future bridge loading conditions. The pile loading was initially assessed using program Piglet based on the as-built drawings and the findings of the geotechnical investigation. Due to much higher loads calculated for the edge piles using the Piglet program which exceeds the assessed capacity of a single pile, Plaxis 3D modelling was carried out for both existing and the future bridge loading conditions. The results of Plaxis 3D modelling indicate that the piles are unlikely to be over-loaded beyond the assessed capacity of a single pile. Based on this comprehensive assessment of the pile load and the use of the piled raft concept we recommended to Sydney Trains that the existing pile foundations and brick abutments be used for the future bridge without any underpinning or foundation strengthening. At the time of writing this paper the bridge replacement has been completed, with the monitored movement of abutments being within the prediction.