Mode-based characterisation of swell deformations in a high-plasticity Paleogene clay

Accurate prediction of soil deformations is important in unloading as well as loading. Historically, however, the loading scenario has been the most common and thus the most extensively studied phenomenon, leaving unloading less well described. Overconsolidated high-plasticity clays are particularly challenging in this regard due to their complex deformation behaviour that has previously shown two conceptually different unloading behaviours. Based on a series of incremental loading and constant rate of strain compression and swelling tests on folded Rosnaes Clay, these unloading behaviours are unified in a framework as different swell modes, and an additional swell mode is identified. These different modes represent a variation in swell-inhibiting structure, seemingly unrelated to the structure in compression. The use of constant rate of strain tests greatly enhanced the detailed description of stiffness development in each mode, which may be characterised by up to three swell phases. The parameters governing the occurrence of the swell modes are identified along with the variables that define the transition between the swell phases and their detailed development.