Model tests on thermo-mechanical behavior of an improved energy pile

Abstract

Energy piles are now considered as an efficient energy solution for heating and cooling buildings. However, once the energy pile has a breakdown especially of the heat exchange elements, it is difficult to be repaired. Thus, a new design of energy piles with embedded steel tubes instead of two longitudinal bars is introduced. Heat exchange pipes were placed into the embedded steel tubes, instead of being attached to reinforcing cages, for the purpose of decreasing the damage to the pipes. Model tests on a conventional energy pile and an improved energy pile with and without a mechanical load were conducted in saturated sand. The thermo-mechanical behaviour of the two piles was comparatively analysed when heating and cooling. In the tests, pile temperature, soil temperature, horizontal earth pressure, strain and pile head displacement were all measured at different stages. Based on these measurements, thermal stress and mobilised side shear stress were studied to comprehensively evaluate the performance of the improved energy pile. The results show that the temperature variation of conventional energy pile was maximum 1.11 times after heating and 1.21 times after cooling that of improved energy pile, respectively. The horizontal earth pressure and strain of conventional energy pile also had larger variations than improved energy pile due to more obvious thermal performance. For the improved energy pile, thermal stress during heating was higher with a mechanical load than without load, but the mobilised side shear stresses were distributed similarly irrespective of mechanical load. The pile heat displacement for the two piles also shows a slight difference after heating and cooling. Moreover, the mechanical load also influenced pile head displacement too much.