Designing for Unsaturated Clay

Dr G W S Kaggwa, Dr P W Mitchell and Mr R A Herraman

You’re asked to design the walls of an underpass or a cutting in a stiff, unsaturated clay. You think that for short- term analysis you can take j=o degrees and get undrained shear strength from an electric cone penetration test. But how do you obtain values of phi’ and c’ for “long term” or “drained” analysis”? In fact, what does al this mean for an unsaturated clay? You know that the moisture content and hence suction of the clay are likely to change over time and that this will affect its strength. But how do you account for this? Dr William Kaggwa will talk about the theory and testing needed to obtain
effective strength parameters for unsaturated clay, taking into account changes in suction. Mr Herraman will illustrate the use of this theory for the design of a retaining wall and a cutting in a stiff, unsaturated clay.

You’re asked to provide values of pressure likely to be exerted by a stiff clay on a basement wall to be designed. You’re familiar with the values of earth pressure on the wall of the Gouger Street Mail Exchange measured by Richards and Kurzeme and concerned that, if these values are used, the design may be unduly conservative. Dr Mitchell will discuss how to establish design values for the pressure exerted by a stiff clay on a basement wall.

About the speakers

Dr G W S Kaggwa Senior Lecturer in G e o t e c h n i c a l Engineering, Department of Civil and Environmental Engineering, University of Adelaide

Dr P W Mitchell Senior Principal Geotechnical Engineer, URS Corp

Mr R A Herraman Manager, Geotechnical Group, Transport Services Division, Department of Transport, Energy and Infrastructure

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