Modelling creep and swelling after unloading under constant load and relaxation with Bayesian updating

It has been established that preloading of clay is an effective means to reduce creep of the soil under working load conditions. After an initial rebound during unloading, a long-term viscous response occurs involving both creep and swelling. A new model is proposed that allows modelling of the creep and swelling that occurs sequentially under constant load or relaxation. The model is verified with 58 published unloading tests under constant load and further evaluated with new one-dimensional tests with unloading stages under constant load and relaxation conditions. The model applies four viscoplastic parameters that are fit to the data by use of a Markov chain Monte Carlo algorithm to evaluate the posterior probability distribution of the parameters. The reinterpreted test results show that viscous swelling plays a prominent role in the unloading response and affects the creep strain rate significantly. The experimental data do not support the modelling of swelling as an isotache process but rather as a transient process triggered by unloading. Creep, on the other hand, can be modelled as an isotache response; yet after unloading the isotaches are distorted, leading to larger creep strain rates than suggested by non-linear fixed isotache models. The newly proposed model allows description of the one-dimensional unloading response under constant load and relaxation that vary as a function of the reference overconsolidation ratio relative to a reference isotache.