1. Senior Geotechnical Engineer, Geotechnical Consulting Group, 52A Cromwell Rd., London SW7 5BE, U.K. (corresponding author). E-mail: f.c.schroeder@gcg.co.uk
2. Professor of Geomechanics, Head of Geotechnics, Dept. of Civil and Environmental Engineering, Skempton Building, Imperial College London, London SW7 2BU, U.K.
3. Director, Geotechnical Consulting Group, 52A Cromwell Rd., London SW7 5BE, U.K.
4. GCG Professor of Geotechnical Engineering, Dept. of Civil and Environmental Engineering, Skempton Building, Imperial College London, London SW7 2BU, U.K.