The Thermal Behaviour of Three Different Auger Pressure Grouted Piles Used as Heat Exchangers

Three auger pressure grouted (APG) test piles were constructed at a site in Richmond, Texas. The piles were each equipped with two U-loops of heat transfer pipes so that they could function as pile heat exchangers. The piles were of two different diameters and used two different grouts, a standard APG grout and a thermally enhanced grout. Thermal response tests, where fluid heated at a constant rate is circulated through the pipe loops, were carried out on the three piles, utilising either single or double loops. The resulting test data can be used to determine the surrounding soil thermal conductivity and the pile thermal resistance, both essential design parameters for ground source heat pump systems using pile heat exchangers. This paper uses parameter estimation techniques to fit empirical temperature response curves to the thermal response test data and compares the results with standard line source interpretation techniques. As expected, the thermal response tests with double loops result in smaller thermal resistances than the same pile when the test was run with a single loop. Back analysis of the pile thermal resistance also allows calculation of the grout thermal properties. The thermally enhanced grout is shown to have inferior thermal properties than the standard APG grout. Together these analyses demonstrate the importance of pile size, grout thermal properties and pipe positions in controlling the thermal behaviour of heat exchanger piles

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

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 degrees C and minimum of 8 degrees 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. (C) 2018 Elsevier Ltd. All rights reserved

Full-Scale Field Testing of Energy Piles

Energy piles are deep foundation elements designed to utilize near-surface geothermal energy, while at the same time serve as foundations for buildings. The use of energy piles for geothermal heat exchange has been steadily increasing during the last decade, yet there are still pending questions on their behavior during temperature changes. A series of field tests on three different energy piles was carried out in Richmond, TX, with the aim of quantifying the thermal influences on the performance and capacity of energy piles. The field test program included conventional pile load tests and application of temperature cycles with maximum temperature of 45°C (113°F) and minimum of 8°C (47°F) over a total duration of six weeks. The test piles with and without maintained structural loads were exposed to heating-cooling cycles to investigate the effect of thermo-mechanical and thermal loads on the energy piles, respectively. With the use of fiber optic cables, vibrating wire strain gauges, thermistors and thermal integrity profile wires, the temperature and strain profiles were monitored on the test piles throughout the field tests. This paper presents the findings inferred from the recorded data along with the details of the in-situ tests on energy piles

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