I should like, first, to thank the British Geotechnical Society for inviting me to deliver the Fourteenth Rankine Lecture, and then to thank Professor Nash not only for his kind introductory remarks but also for his generous support and for the relative freedom I have enjoyed during my time at King's College. It is also a pleasure to have this opportunity to acknowledge the help and encouragement I received from Professor Skempton while I was at Imperial College —and, indeed, this was most generously given even when I was occupied elsewhere. To these names I should like to add that of Dr Cooling who allowed me while at the Building Research Station to learn some mathematics from Dr McNamee; and who also required me to engage in field work between January and March.