The impact of thermal creep on energy pile systems

Laboratory studies on soils during non-isothermal conditions indicate the existence of thermal creep, i.e., irreversible deformation due to cyclic thermal load at constant mechanical stress. This phenomenon was seen to be significant in soft soils. Energy piles in soft soils could experience higher deformations as a result of thermal creep (dragdown) of the surrounding. Therefore, their computation requires an advanced thermoplastic constitutive model that considers this thermal creep. This study dives into the impact of thermal creep in energy pile systems and its impact on the performance of such piles, through field tests conducted on a new type of energy piles, i.e. a displacement cast in-situ energy pile and numerical simulations by using an advanced rate-dependent thermo-hypoplastic constitutive model. The model was calibrated and validated against non-isothermal tests and then used to simulate the behaviour of energy piles in multilayered soft soils. The results indicated that thermal creep (dragdown) of the surrounding soil during monotonic cooling resulted in a reduction of the compressive stress due to the applied mechanical load, this phenomenon was well tracked and captured by the model. Furthermore, this thermal creep due to the rearrangement of grains during cyclic thermal load resulted in a residual thermally induced compressive stress (and strains), increasing the mechanically induced residual compressive stress, which was also well captured by the model.