Enhanced prediction of settlement in structured clays with examples from Osaka Bay

Abstract

Enhanced settlement and pore pressure predictions in structured clays are shown to be possible by applying a framework specifically developed to identify metastable microstructure and to simulate with appropriate elastoviscoplastic models the associated destruction processes, which are responsible for large time-dependent strains (creep). Two well-documented case histories, Maishima island and Kansai International Airport Phase 1 island, are considered to support this conclusion. The ‘field’ behaviour of Osaka clays at these two sites demonstrates that ‘primary compression’ settlements predicted by elastoplastic models and conventional interpretation of oedometer tests may be substantially lower than observed, casting doubt on the uniqueness of the ‘end of primary’ (EOP) void ratio–effective stress relationship, which is found to provide reliable predictions only under special circumstances. The concepts of primary and secondary consolidation, and parameters such as the preconsolidation pressure , the compression coefficient and secondary consolidation coefficient do not adequately describe soil compressibility in structured clays. More advanced models, which simulate the actual physical processes, do not need to resort to such concepts and parameters and thus this difficulty is overcome.

Keywords:

• Clay
• Consolidation
• Creep
• Soil structure
• Viscoplasticity
• Yield

Acknowledgements

The authors wish to thank Professor M. Jamiolkowski and Professor D. Lo Presti, and more generally Politecnico di Torino (Vercelli) and Studio Geotecnico Italiano srl for promoting and supporting the dissertation that laid the foundations of this study. Special thanks are due to the many Japanese workers who published detailed accounts of their laboratory and field results, without which this paper would have been impossible.