Negative creep of soils

After unloading, the deformation of soils cannot be stable immediately, but continues to expand over time even under constant pressure. In this paper, the expansive deformation over time when effective stress is kept constant is defined as the negative creep, while the compressive creep is described as the positive creep. The division between positive creep and negative creep is named the stable normal compression line (SNCL), on which the stress–strain behaviour of the soil is time-independent. Based on the concept of the SNCL and test results, a new formula for creep is proposed. This formula is simple in form and has less parameters, and both negative creep and positive creep can be well predicted. By incorporating this formula into the current yield function of the unified hardening model, a new time-dependent current yield function is built. Combining the yield function, a flow rule, and transformed stress method, a new three-dimensional time-dependent constitutive model considering both positive and negative creep for clays is derived and presented. The new model is then validated by test results, including multistage loading oedometer tests, triaxial undrained creep tests, and triaxial undrained compression tests at the constant strain rates.