The influence of initial shaft resistance mobilisation in the response of seasonally, thermally-activated pile foundations in granular media

The thermal-activation of pile foundations for use within shallow geothermal energy systems has received much attention and the results from various studies have allowed the development of a broad understanding of the thermo-mechanical behaviour of thermally-activated (TA) pile foundations. One area that requires further attention is the clarification of the foundation response under seasonal cyclic thermal loading. This study systematically assesses the impact of balanced cyclic thermal loading in relation to the initial mechanical loading, for isolated floating piles and pile groups in a frictional soil medium. The pile thermal response was evaluated for differing levels of mobilisation of the shaft resistance with respect to the initial mechanical loading and for piles at varying spacing based on a unit cell approach. Isolated piles with high shaft mobilisation were found to develop significant irrecoverable settlement during thermal cycling however the maximum thermal stress was similar irrespective of the initial shaft mobilisation. These results are remarkably similar to those reported in small scale tests of TA piles in sand, confirming that the behaviour reported in these is likely to be due to the result of low shaft restraint, rather than thermally-induced loss of resistance on the pile–soil interface. On the other hand, piles in close proximity were found to behave quasi thermo-elastically at the same initial mechanical load, which is attributed to an alteration in mobilised pile resistance due to pile–soil–pile interference under the initial mechanical loading and, to a lesser extent, thermal activation of the ground between the piles both of which reduce the relative movement between the pile and the soil. This pile group thermal effect has major beneficial implications for the geotechnical and structural design of TA pile foundations.