Strain softening and navigating karstic risks in the design and construction of large retaining walls

Jess Dalton and Ken Chen

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 presentation will discuss 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 will be presented, along with the instrumentation and monitoring that was adopted to monitor the wall performance.

Finally, the presentation will explore embodied carbon in two bridges across the project – one constructed using top-down methods and the other a gravity structure. Practical insights into emission reduction strategies will be shared, with a focus on how design choices can influence carbon outcomes.

About the speakers

Jess Dalton Principal Engineer & Team Lead, WSP Sydney

Jessica is a geotechnical engineer with nearly 20 years experience in geotechnical investigation and design across Australia, New Zealand and Canada. Jessica applies her knowledge gained from site experience to inform design development and coordinate across interdisciplinary design and construction teams to achieve practical and efficient project outcomes. She has a personal interest in reducing carbon in design and leads the geotechnical decarbonisation effort for WSP Australia.

Jessica was the Geotechnical Lead for the Metronet YRE bridges and station structures.

Ken Chen Senior Associate & Team Lead, WSP Sydney

Ken is a chartered geotechnical engineer with strong analytical skills and has been involved in the site investigation, design and construction of various major projects across Australia. He has a deep understanding of geotechnical theory and practice with the ability to rapidly breakdown complex problems and develop practical and efficient solutions.

Ken leads design teams and provides technical guidance and mentoring to other engineers. He is also experienced in project management, ground improvement design (particularly those which involve soft soils), soil-structure interaction, numerical modelling, supervision of construction activities and planning & supervision of site investigation works.

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