On the time-dependent stress-strain behaviour of soft soils

It is commonly understood that the stress-strain behaviour of soils, especially soft clays, is time-dependent. Study of time-dependent behaviour is essential for (1) understanding the mechanism and analysis of deformation, pore water pressure responses and failure of soft soils, and (2) the design and construction of geotechnical structures on soft ground. This paper addresses the following fundamental issues associated with the time-dependent stress-strain behaviour of soft clays and the implications of these issues for analysis of the performance of geotechnical structures on soft ground under loading. 1. Under one-dimensional (oedometer) conditions (a) What is the true 'instant' compression line? (b) Is the pre-consolidation pressure time- (or strain rate-) dependent? (c) Is 'primary' compression bigger than 'creep' (or 'secondary') compression? (d) Does creep settlement increase forever with time or is there a creep limit? (e) Should total strain rate be divided into (i) elastic strain rate plus visco-plastic strain rate or (ii) elastic strain rate plus plastic strain rate plus visco-plastic strain rate? (f) How should time-dependent stress-strain behaviour be modelled? What are the relationships between creep, strain rate effects, relaxation, loading rate, etc? 2. Under triaxial stress and general stress conditions (a) From consolidated undrained (CU) triaxial tests, how does undrained shear strength change with strain rate and overconsolidation ratio? (b) Why are strain rate effects in extension larger than those in compression.? (c) How can we model time-dependent stress-strain behaviour in triaxial and more general stress conditions? 3. Implications for the analysis of settlements/deformations and pore water pressures in soft clays (a) What procedures should be used to calculate short-term and long-term settlements of reclaimed land or foundations on soft ground under 1-D straining? (b) How does creep influence deformation and pore water pressure responses in soft soils under 2-D (axisymmetric) conditions? (c) Under what conditions can excess porewater pressures increase under constant external loading?