Failures, Factors of Safety and Soil Strength

Professor John Atkinson, Professor of Soil Mechanics, City University, London

In routine geotechnical design of foundations, slopes and retaining walls a factor of safety is applied to ultimate collapse loads. But very different factors of safety are used for different structures. Also soil has several different strengths; peak, constant volume (or critical state) and residual. Moreover, it is not always clear what constitutes failure of geotechnical structures, or who decides when failure has occurred.

The talk will consider the bases of conventional geotechnical design, the relationship between soil strength and stiffness, and choices of soil strength and factor of safety for routine design. This will lead to some simple methods for determining characteristic values for soil strength parameters for routine design of foundations and slopes.

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