EIC’s geotechnics team was engaged as part of a value engineering exercise to review the design of the Northern Portal cut and cover tunnel for the West Gate Tunnel project.
The team undertook a detailed soil analysis which showed that a two-level propping arrangement (as opposed to propping at three levels) would provide sufficient support for the structure.
The removal of the third prop resulted in substantial cost savings, a reduction in steel tonnage while improving constructability, productivity, and safety outcomes.
About the project
CIMIC Group company CPB Contractors was selected by Transurban and the State Government of Victoria as the preferred contractor for the West Gate Tunnel project, in a 50:50 joint venture with John Holland.
The West Gate Tunnel project, one of Victoria’s largest ever urban road projects, will deliver a vital alternative to the West Gate Bridge, provide quicker and safer journeys, and remove thousands of trucks from residential streets.
The Northern Portal
The Northern Portal cut and cover tunnel is one of the major packages of the works on the project.
The portal is used in the temporary condition to facilitate the launch of twin TBM machines for the bored tunnels and forms the final tunnel portal upon construction of the permanent structural lining.
The portal structure is over 330m in length and up to 22.2m in depth at the interface of the cut and cover tunnel and the TBM tunnel.
The original design proposed three levels of temporary steel props.
Scope of the geotechnics’ team involvement
The results of a detailed soil structure interaction (SSI) analysis delivered by the team indicated that a two-level strutting arrangement opposed to propping at three levels, would provide adequate support to the 22 m deep excavation at the portal structure.
The level of the lowest strut was found to be critical in optimising the temporary support requirements for the portal structure.
Close monitoring of the performance of the structure during construction provided confidence in the suitability and adequacy of the two-strut design.
Data from the ongoing monitoring indicated a generally better performance of the retention design than was forecast.
This underlines the significance of adequate site investigation and testing to enable detailed assessment and adoption of refined geotechnical parameters.
The adoption of an optimising strutting design at two levels as opposed to three levels which would normally be seen reasonable for excavation depths such as those required at the northern portal, provided significant cost savings to the project in addition to an efficient, smoother and safer construction program.