FEATURE ARTICLE Energy Pile Research is Heating Up in Texas: Part II Ground source heating and cooling systems provide a highly efficient renewable energy solution for buildings. Ground source heat pump (GSHP) systems have been used successfully for over 30 years, providing both lower energy usage and carbon emissions than conventional systems. Energy piles are deep foundation elements integrated with adsorber pipes, and perform as heat exchangers connected to GSHP systems. Unlike traditional furnaces that burn fuels for heat and require separate air conditioning or chiller systems, energy piles simply transfer heat from one place to another for heating and cooling of buildings. The use of energy piles has been steadily increasing during the last decade. However, several questions remain about their performance. A series of field tests on energy piles has been recently performed in Richmond, Texas with funding from the National Science Foundation in collaboration with Virginia Tech, Berkel & Company Contractors, and other partners. The field test program included conventional pile loading tests and the application of temperature cycles over a total duration of 6 weeks to simulate heating and cooling operations. The main purposes of this extensive field test program are to better understand the thermo- mechanical behavior of the energy piles and evaluate if heat exchange operations affect pile capacity. The first part of this article which describes the test pile installation and instrumentation was published in the July/August 2013 issue of Deep Foundations. This is the second part of a two part article on the energy pile research in which we discuss the preliminary results of the tests. General Information on the Test Piles and Test Set-up Three test piles of 18 in (45.7 cm) in diameter were installed using Auger Pressure Grouted (APG) pile installation technique at the Berkel regional office in Richmond, Texas. Two of the piles (TP-1 and TP-3) are 50 ft (15.2 m) and the third one (TP-2) is 30 ft (9.1 m) in length. The test piles are instrumented with vibrating wire strain gauges, thermistors, fiber optic cables and thermal integrity profile (TIP) wires. As described in the earlier article, the soil profile at the site consists of about 30 ft (9 m) of stiff clay overlying 30 ft (9 m) dense sand. The groundwater table is about 12 ft (3.6 m) below the ground surface. More detailed information on the installation and instrumentation of the piles can be found in the first part of this article. Layout of the equipment involved in the thermo-mechanical testing is shown in Figure 1. Figure 1. Schematic layout of Berkel field test Thermo-Mechanical Test Procedure The main purpose of the thermo-mechanical field test is to better understand the effect of temperature changes on the performance and capacity of the energy piles. Temperature changes were applied to investigate the energy pile behavior under static load during heat exchange operations. We were also able to evaluate the effect of temperature cycles on pile capacity by comparing the results of pile load tests conducted before and after thermal cycles. In addition, we investigated the effect of end restraint on the behavior of the energy piles, in terms of development of shear stresses along the pile shaft due to the relative movement of the pile with respect to the soil during thermal cycles. Thermally induced axial stresses can develop along the pile due to the expansion/contraction of the pile and end restraint conditions can play a significant role in this behavior. During heating, with the presence of a firm stratum at the toe of the pile, the expansion of the pile will be restrained, which can generate compressive stresses along the pile. Therefore, the length of TP-2 and TP-3 were determined in order to represent two different end restraint conditions. TP-2 being shorter and extending solely in the clay layer reflects the condition with the absence of a significant tip resistance. On the other hand, TP-3 is longer with end bearing into the deeper dense to very dense sand layer. Thermo-mechanical load tests were performed with two heat pumps which enabled the concurrent testing of two energy piles. One of the heat pumps was connected to TP-1, which was thermally loaded over a five week period. At the same time, the other heat pump was connected to TP-2 and TP-3, which were tested separately, over shorter periods. TP-1 was first mechanically loaded to find its ultimate capacity according to ASTM D1143 and was unloaded before the beginning of thermal loading. Afterwards, five episodes of heating-cooling cycles were applied over a duration of five weeks. During thermal loading, the temperature of the heat AUTHORS Melis Sutman, M.Sc., Virginia Tech; Tracy Brettmann, P.E., D.GE, Berkel & Company Contractors; and Guney Olgun, Ph.D., Virginia Tech DEEP FOUNDATIONS • JAN/FEB 2014 • 77