Optimisation of temporary support design for the Northern portal cut & cover tunnel

The West Gate Tunnel Project is a city-shaping project that will deliver a vital alternative to the West Gate Bridge, provide quicker and safer journeys, and remove thousands of trucks off residential streets. Delivery of the WGTP project is currently underway by a joint venture of CPB Contractors and John Holland Group. Northern Portal cut and cover tunnel is one of the major structures on the project which requires excavations to a depth of 22 m to allow the launch of the twin 15.6m diameter Tunnel Boring Machines (TBM). Design of the retention system comprised 900 – 1500 mm diameter secant pile wall supported with multiple levels of temporary steel struts. A detailed Soil-Structure Interaction (SSI) analysis together with a review of proposed construction methodology indicated that a two-level propping arrangement as opposed to propping at three levels, which is what would normally be expected for a structure of this scale, would be adequate. The opportunity to remove one level of the proposed steel struts with the potential for a significant reduction in materials and time was considered critical to the completion of this critical path structure. Removal of one level of props would result in a reduction of steel tonnage in excess of 1,000 tons, in addition to improving constructability, productivity and safety. This paper discusses technical aspects of the analysis which enabled the development of the above optimised solution. In addition, the results of the instrumentation and monitoring and performance of the constructed portal structure will be discussed.