A Case Study of an Impact Assessment of an Excavation in the Sydney CBD: Adopting Industry Guidance for Numerical Modelling

The use of numerical models for geotechnical analysis and design is now becoming routine. Numerical modelling is a complex process which can have enormous benefits but can also be misused which can lead to dangerous applications. This has been recognised within the industry through C791 published by the Construction Industry Research and Information Association (CIRIA). The guidance within C791 aims to improve the outcomes for modelling projects by providing advice on the development of clear and achievable objectives, and the required roles and responsibilities to achieve them. It also seeks to ensure the roles, responsibilities and experience of the project team are appropriate and that strategies for model calibration and testing of sensitivity to assumptions and robustness are identified and implemented. This paper summarises how the guidance was followed during a project to assess ground movement around an excavation for the proposed Atlassian Building Central, a 39-storey hybrid timber building adjacent to Central Station. The ground movements were to be used to assess risks to third-party assets around the excavation. Clear objectives were identified and individual models were developed to specifically satisfy these objectives. A design team was formed with the appropriate skills and experience which ensured key questions were asked and risks were identified and mitigated in a timely and efficient manner. Modelling complexity evolved sequentially and element calibration was performed to ground the analysis in reality. Critical inputs and variability of these inputs were identified and quantified. Sensitivity to variations in these inputs was explored, thereby identifying potential project risks and opportunities. The resulting design was transparent, efficient and verifiable. Risks and opportunities were easily communicated to the client, and third-party stakeholders were satisfied the risks to their assets could be managed effectively. At a time when numerical modelling and analysis techniques are becoming more prevalent, more complex and more relied upon, it is deemed critical that processes are put in place to reap the benefits while also managing the risks. This paper demonstrates how the processes and advice set out in C791 can be used to benefit all projects where advanced numerical analysis techniques are employed.