Thermomechanical behaviour of an energy pile-raft foundation under intermittent cooling operation

Abstract This paper presents two in situ experimental investigations of the influence of intermittent and continuous cooling operation on thermal and mechanical responses of a foundation. The experiment was conducted with 16 h of cooling operation and 8 h of rest (C-16 mode) for 46 days and with 24 h of cooling operation (C-24 mode) for 32 days. A lower number of daily operating hours promoted natural ground thermal recovery and led to higher heat exchange rates with lower thermal loads on the foundation. The constant heat exchange rate in C-16 was approximately 115 W/m, which was 53.3% higher than the constant heat exchange rate at the same time in C-24. The pile temperatures at all depths in C-16 were decreased by 29.0-36.8% compared to those in C-24. Thermally induced stresses of the pile and raft in C-16 were 25.8-35.7% and 85.9% lower than the thermally induced stresses of the pile and raft in C-24. The mechanical responses of the energy pile in C-16 were reversible. During the cyclic cooling and recovery of C-16, stable changes in axial strain and stress with pile temperature were observed over further cycles, without significant variations in pile end displacement and the vertical stress at the pile toe.