Cyclic loading effects on soil-energy pile interaction

Abstract

To reduce greenhouse gas emission from space heating and cooling, energy piles have been used in buildings as an alternative renewable energy source for approximate by two decades. However, the effects of cyclic thermal loading, due to the intermittent operation of the heat pump, on the soil-energy pile interaction have not been fully investigated. Energy piles are subjected to cyclic temperature changes that affect the properties of soil-structure (or pile) interface. In addition, the effects of temperature variations produce cyclic expansion and contraction of the pile. To evaluate the effects of radial expansion/contraction cycles, a fully controlled thermal-modified borehole shear test (Thermal-mBST) device was developed at Lehigh University to measure the thermo-mechanical behavior of the soil-energy pile interface. A pair of concrete plates, representing the pile surface, were used. The plates have embedded aluminum small diameter pipes that are connected to a heat pump to control the temperature of the Thermal-mBST. Two linear potentiometers were fixed between the two concrete plates to control/measure the horizontal displacement between the two plates. The testing system is capable of simulating temperature change and cycles, expansion/contraction (displacement) change and cycles, as well as the combination of temperature and expansion/contraction cycles. In this paper, the Thermal-mBST device was utilized to conduct tests with temperature changes (ΔT) of 0 and +20°C at Soil-Concrete Interface (SCI) under initial normal pressure of 41.4 kPa utilizing an aluminum tank filled with silty clay. The tests were performed under heating cycles, expansion cycles, and combined heating and expansion cycles. After applying the cycles, the shearing stage was performed to measure the stress-displacement curves of the soil-pile interface (t-z curves).