Reliability-Based Design for Geotechnical Engineering Practice

Ramtin Tajeddin

Engineering design involves application of scientific and mathematical principles to provide functional solutions to technical problems, while considering the limitations imposed by regulations, safety, practicality, time, and cost. To achieve a reliable design, Allowable Stress Design (ASD) traditionally has been used by Australian geotechnical practitioners and introduction of Load and Resistance Factor Design (LRFD) in the Australian Standards has not resulted in a smooth transition from the conventional method and remains a topic of debate in the industry. From personal experience and observations as an Australian geotechnical practitioner, the key Australian Standards, such as AS 2159, AS 5100.3 and AS 4678 are not able to answer some key engineering questions, and are prone to misuse or misinterpretation when adopting design parameters or interpreting analysis results. Furthermore, over-reliance of the Australian industry on expert opinions not only has not reduced such misuse and abuse of ASD and LSD, but also arguably has introduced another layer of uncertainty through additional psychological biases and logical fallacies of such involvements. Reliability Based Design (RBD) is a next generation of geotechnical design that is not only physically and mathematically self-consistent, but also is compatible with use of RBD for structures which makes it ideal as an inter-disciplinary common language. Therefore, Reliability Based Design is actively being introduced into major design codes around the world as the main concept for development of the design codes, standards, and guidelines.

In this presentation, the key concepts of RBD will be introduced, and examples will be provided to illustrate its application. Furthermore, the shortcomings of ASD and LRFD methods will be discussed, and an argument will be made as to why reliance on expert opinions may not enhance the reliability of existing design methods. Additionally, examples of design and management decision-making scenarios that cannot be adequately addressed by ASD and LRFD methods will be presented, demonstrating how RBD can empower engineers to provide more reliable solutions.

About the speaker

Ramtin Tajeddin, Technical Director, GHD

Ramtin Tajeddin Technical Director, GHD

Ramtin Tajeddin has 27 years’ experience in geotechnical investigation, design, and construction support of civil infrastructures (roads, rails, bridges, ports and buildings), mining infrastructures, coastal and hydraulic structures, water and tailing dam engineering. His expertise includes design and risk assessment for deep excavations, foundation and pile design for onshore and nearshore structures, analysis and design of soil and water retaining systems, ground improvement in soft soils, and numerical modelling and analysis for soil-structure interaction. His recent works include geotechnical design lead for reference design of Melbourne’s North East Link and technical advisor to Westgate Tunnel Authority, Major Road Projects Victoria and Department of State Growth (Tasmania). Ramtin has particular interest in risk and reliability based design of soil-structures, and advocates for official inclusion of this design method in Australian Standards and states’ technical specifications and requirements.

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