Effect of End-Restraint Conditions on Energy Pile Behavior

Energy piles are deep foundation elements designed to utilize the relatively constant temperature of the ground for efficient heating and cooling of the buildings while at the same time serve as foundations. The temperature changes during the operation of energy piles result in axial displacements, a part of which is restrained by the surrounding soil or the building on top. The restrained part of the axial displacements induces compressive stresses during temperature increase and tensile stresses during temperature decrease along energy piles. Moreover, the unrestrained part of the displacement results in changes in the mobilized shaft resistance, which need to be taken into consideration during design of energy piles. With the aim of quantifying these effects, a series of full-scale field tests on three energy piles with different end-restraint conditions was carried out in Richmond, TX. The field test program included conventional pile load tests and application of temperature. Temperature changes were applied to the test piles with and without maintained mechanical loads to investigate the effects of structural loads on energy piles. Moreover, the lengths of the test piles were determined to represent different endrestraint conditions at the toe. In this paper, a comparison of the thermally induced axial stresses and mobilized shaft resistance of two identical, end-bearing test piles with and without maintained mechanical loads are presented along with the details from the full-scale field test.