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Rankine lecturer considers probabilistic risk

Reliability of geotechnical structures is a better approach to considering risk than conventional factors of safety, according to Norwegian Geotechnical Institute technical director Suzanne Lacasse who delivered the 55th Rankine Lecture at Imperial College last night.

Lacasse said that she believed her Rankine Lecture was the first to directly address risk, although she added that past lecturers have addressed geotechnical uncertainty.

Lacasse’s lecture on Hazard, Risk and Reliability in Geotechnical Practice proved popular with the industry with a packed lecture theatre and overflow room, and another 125 groups viewing the live stream online.

Lacasse used a number of projects as examples of the benefits of a probabilistic analysis and the effects of using different probability models and design codes. The examples showed how consideration of reliability rather than an arbitrary factor of safety could enable pile lengths in offshore applications to be shortened.

Through discussion of the Roşia Montaňa tailings dam in Romania, Lacasse explained how event tree analysis (ETA) can be useful to consider the impact and consequence of an initiating event. “ETA is a systematic approach that offers a diagnostic tool that can aid decision making,” she said.

Having delivered compelling evidence on how quantitative assessment of the risk using probabilistic analysis rather than deterministic approaches is better, she questioned why acceptance of the reliability approach is so slow. Lacasse added that she believes it is because reliability theory concepts are unfamiliar to geotechnical engineers.

Although Lacasse advocated the use of computer software to ease the probability calculations to enable the reliability approach, she cautioned that engineering judgement was still vital.

In thanking Lacasse for her lecture, Arup fellow Brian Simpson said that she had challenged the audience to think rationally and more numerically. “Probability terms are necessary to communicate risk to other professions and reliability creates a common language,” he said.

At the end of the Rankine Dinner, held after the lecture in honour of Lacasse, BGA chairman Chris Menkiti announced that the 56th Rankine Lecturer will be Imperial College professor of Geomechanics Richard Jardine.

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