Strain Softening and Navigating Karstic Risks in the Design and Construction of Large Retaining Walls

The METRONET Yanchep Rail Extension (YRE) is one section of Perth’s most ambitious public transport program of works. The passenger rail infrastructure runs 15 km north from the existing Butler Station to the Yanchep stowage yard and includes three new station precincts and over a dozen bridge structures. The rail alignment passes through regions known for karstic limestone conditions and is primarily formed within a large cut setting requiring bulk excavation in the order of up to 10 m to 15 m below the existing ground level.

Due to project and site constraints, contiguous piled walls without propping, anchoring or tiebacks have been constructed to support the ground within these large cuts. Fully cantilevering piled retaining walls of this height and within these geological settings are unconventional. Importantly, they require consideration of post-peak strength loss associated with the predicted strains and lateral wall movements, coupled with effective management of karstic risk.

This paper discusses the development, application, advantages and limitations of an innovative “strain-softening” numerical analysis approach developed for the wall design. An efficient and pragmatic approach used to mitigate karstic risks during design and construction is presented, along with the instrumentation and monitoring that was adopted to monitor the wall performance.