On the impact of temperature perturbations on the creep of sensitive clay

Abstract: Temperature affects the soil response, but is not fully understood and quantified and thus often ignored. The temperature fluctuations resulting from energy foundations and construction activities, such as excavations and soil stabilization, however, may lead to changes in the hydromechanical response of the clay. Especially, the sensitive clays that are abundant in Scandinavia, may be detrimentally affected. The fragile natural bonds between the clay platelets of sensitive clays are prone to disturbances from environmental changes, including temperature perturbations. The change in hydro-mechanical properties, such as the compressibility and the creep-rate, can affect the long-term stability of the geothermal structures. The aim of the Thesis is to study the impact of temperature perturbations on the creep of sensitive clay. A systematic series of temperature-controlled laboratory tests (5 to 25 ◦C), including oedometer tests with static and cyclic thermal loading steps, has been carried out on natural and remoulded samples of natural sensitive clay from the Utby test-site in Gothenburg, Sweden. An original time-saving test protocol is developed to obtain sufficient data within the duration of the project. The results demonstrate that the influence of the temperature on the creep rate of sensitive clays depends on the amount of natural bonding. Heating increments lead to larger changes in creep rate than cooling decrements. The thermal loading cycles lead to larger permanent strains when compared to the static heating and cooling paths. Thorough analyses of the results using the temperature-invariant rate-dependent model implemented in a multi-physics numerical framework indicates that the temperature dependency can be uniquely linked to the amount of natural bonding in the clay sample. Consequently, the temperature-invariant model is modified by correlating the apparent preconsolidation pressure with temperature and the amount of natural bonding in the clay samples. The temperature-dependent model, Creep-SCLAY1ST, improves the predictions of the mechanical response under static thermal loading considerably, whilst the modelling of the cyclic loading paths proved to be somewhat unsatisfactory. The latter is not a shortcoming of the proposed model modification, rather an intrinsic limitation of the underlying temperature-invariant model.