Transient thermal response of a thermally activated pile during a long-term maintained load test at Lambeth College, London

Abstract

The energy pile test at Lambeth College, London was first reported just over 10 years ago; however, the original appraisal of the test only looked at the thermal response at the end of extended cooling and heating stages, and did not examine the transient response within these stages or in the daily cyclic thermal loading stage that followed. This paper revisits the earlier results, allowing for concrete creep under sustained load and using a revised strain-dependent pile modulus. The strong influence of the latter and the possible importance of stiffness changes during load reversal is highlighted. Detailed evaluation of the long-term transient results suggests that within a given thermal loading interval the thermal effects are similar, but there is some further discrepancy in the results, possibly due to an underestimation of the concrete creep. This re-evaluation has mitigated pile head displacement and increased thermal stress in the cooling phase, with the reverse occurring in the heating phase, and appears to be in better agreement with back-analysis reported by others. The transient response within the thermal loading stages illustrates that the development of the thermal stress–displacement response is related to the time for heat to diffuse through the pile body – around 3 days in this case.