Time effects on the stress-strain behaviour of natural soft clays

Laboratory tests on a wide variety of lightly overconsoli-dated natural clays show that important engineering properties such as undrained strength and preconsolidation pressure are time-dependent. Relaxation and step-changing procedures suggest that these properties decrease by 10–20% with a tenfold decrease in the speed of testing. The influence of strain rate on undrained shear strength appears to be independent of soil plasticity, test type or stress history during laboratory reconsolidation. Its influence on stress-compressibility curves from oedo-meter tests is independent of soil plasticity and test conditions. Ageing and delayed compression in clays produce a stiffer arrangement of the soil particles, and a quasi-preconsolidation pressure that is in part available for foundation design. The results from a series of conventional tests and stress-probe tests on two different clays show that yield envelopes in lightly overconsoli-dated natural clays are time-dependent, and contract in p&rime, q-space...