Full-scale in-situ tests on energy piles: Head and base-restraining effects on the structural behaviour of three energy piles

Abstract

Energy piles, due to their dual roles, are exposed to temperature change throughout their life time, which may cause axial displacements, additional axial stresses and changes in the shaft resistance along their lengths. The extent of these effects highly depends on the level of restrictions along the energy piles, from the surrounding soil, as well as from the superstructure. A full-scale in-situ test has been performed on three energy piles, with the purpose of investigating their thermo-mechanical behaviour with respect to the corresponding end-restraining conditions. Thermal loads with maximum temperature of 45 °C and minimum of 8 °C were applied to the test piles for a 6-week period along with conventional mechanical load tests. Two of the test piles were designed to have base resistance from the very dense sand layer while the shorter pile was tipped into a stiff clay layer with the purpose of representing different end-restraining conditions. Moreover, the thermal loads were applied to the longer piles with and without the presence of mechanical load at the head. In this paper, the full-scale in-situ test setup is presented, along with the test results of the three test piles giving emphasis to the restraining effects of the mechanical load at the head and the base resistance from underlying soil layers. It is concluded from the results that the distribution of thermally induced axial stresses and the mobilization of shaft resistance during heating and cooling episodes are highly dependent on the location of the dominant restriction. Moreover, the end-restraining effects fade away at depths farther from the dominant restriction, leading to the degree of freedom of test piles with diverse end-restraining conditions to converge.