The role of concrete creep under sustained loading, during thermo-mechanical testing of energy piles

The energy pile test at Lambeth College London was first reported some 10 years ago, and since the results were published, some potential anomalies in the results have been identified. This article revisits the results of this test using a strain dependent pile modulus, accounting for residual strain from unload/reload loops and allowing for concrete creep under sustained load. The result of this re-evaluation has been to substantially modify the interpreted pile response, especially near the pile head, which leads to a mitigation of the maximum pile head displacements in the cooling phase, and the maximum thermo/mechanical stresses in the heating phase. Two other case studies have been examined to illustrate that concrete creep may be a significant factor in the responses of each, and it is concluded that existing studies should be re-examined and future testing should account for concrete creep. It is highlighted however that while the quantitative interpretation of the test has changed, the underlying mechanisms of behaviour remain consistent with those described in the literature.