Main Roads WA
South Western Highway Henty Brook Bridge Replacement
This project required the replacement of an existing timber structure bridge with a new 2x3300 diameter Corrugated Steel Pipe structure. The bridge had become unserviceable for major oversized loads planned for the construction of the proposed Perdaman Urea plant in Collie. Project Value $1.5m and WML Fee $111,375
Main Roads WA required traffic flow to be maintained during construction because the South Western Highway is a major heavy haulage route in the South West. This requirement was met by deconstructing a portion of the bridge previously installed to widen the bridge for faster traffic speeds whilst keeping the original bridge open for two way traffic under speed restriction. The culvert could then be installed wider than would be required under normal operational conditions to allow a two way traffic bypass to be constructed to accommodate traffic whilst the original bridge is demolished and the road works completed.
Main Roads WA had to obtain Aboriginal heritage approval prior to any Geotechnical investigation being conducted. This approval process was delayed by two months which put WML under pressure to complete the design on time.
The initial idea on this project was to widen and overlay a section of the South Western Highway over the bridge. This design was almost completed when Main Roads informed WML that the cost estimate was too high, that construction could not proceed on this basis and that the design had to be changed to widen on existing only.
When the Aboriginal heritage approval was eventually obtained the Geotechnical investigation indicated soft clay in the subgrade which meant poor bearing capacity for the concrete inlet and outlet structures with highly likely differential settlement.
The design was subsequently changed again to remove the concrete headwalls, lengthen the culverts, steepen up the embankments for better clearance to the close proximity rail embankments and provide rock pitched embankments on the inlet and outlet. The settlement problems were mitigated by Main Roads choosing the path of accepting a certain amount of settlement whilst still constructing some soil reinforcement to kerb the remainder.
This was a highly complex project going through several iterations in order to try and expedite the work to meet construction time schedules, to meet budget requirements, to meet constraints placed by Main Roads and to mitigate geotechnical problems.
This demonstrates WML’s ability to adapt design work to provide solutions as problems are encountered and to accommodate change, whilst still meeting program requirements.
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