Performance-Based Design in Geotechnics / Emeritus Professor Malcolm Bolton, Cambridge University

Engineering design should consist of a sequence of decisions
that lead to the creation of an artefact, a geo-structure in this
case, that satisfies the client’s performance requirements.
This lecture will argue that any assessment of geotechnical
performance must involve ground displacements. The
traditional approach of specifying safety factors on soil
strength (whether in the new LSD Eurocodes or the LRFD
codes that are emerging in North America) fails to recognise
that both structural unserviceability and structural failure are
likely to precede soil failure, and that soil stiffness does not
correlate well with soil strength. If engineers want to offer
economic designs that successfully control displacements and avoid structural damage, they
must measure soil deformability and account for it somehow in routine displacement
calculations. But how is that to be achieved in practice? A new design approach will be
proposed, the Mobilizable Strength Design (MSD) method.
Central to the MSD approach will be an assessment of the possible deformability and strength
of the soil that lies within the deformation mechanism of the proposed geo-structure. The lecture
will focus on clays. Regarding deformability, the shear strain required to mobilize half the peak
undrained strength should be assessed, together with the slope of stress-strain on log-log axes.
This enables displacements under working loads to be estimated almost as easily as factor of
safety, and before any later checks by Finite Element Analysis (FEA), ensuring that design
assumptions can be validated during construction by monitoring settlements. Examples will be
given of the application of MSD to shallow foundations, piles and excavations in clay.

Malcolm Bolton is now Emeritus Professor of Soil Mechanics at Cambridge University, where he was Director of the Schofield Centre for Geotechnical and Construction Modelling between 1995 and 2013. He is a Fellow of the Royal Academy of Engineering and holds various prizes of the UK Institutions of Civil and Structural Engineering, the British Geotechnical Association and the Canadian Geotechnical Society. He was founding chairman of the ISSMGE Technical Committee on Geo-Mechanics from Micro to Macro (GM3). He has collaborated on piles with the Giken company of Japan for 18 years, and is the founding chairman of the International Press-In Association. And he served for 4 years on the Slope Stability Technical Review Board for the Hong Kong Government. He helped to draft BS8002 Earth Retaining Structures, and remains committed to improving Eurocode 7 which has replaced it. He has over 230 publications on topics ranging from fundamental soil mechanics to a wide variety of geotechnical engineering applications.

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