Effect of Temperature and Radial Displacement Cycles on Soil–Concrete Interface Properties Using Modified Thermal Borehole Shear Test

Thermoactive geostructures (such as energy piles) are used for heating and cooling of buildings, which generate daily temperature changes and cycles in the geostructure and surrounding soil due to the intermittent operation of ground-source heat pumps. For energy piles, daily temperature changes and cycles result in cyclic displacement (expansion and contraction) in both the axial and radial directions of the pile and alter soil properties. In this study, a fully automated modified thermal borehole shear test (modified-TBST) device was utilized to perform tests in normally consolidated clayey soil to investigate the effects of temperature cycles (TC) and radial expansion/contraction displacement cycles (RDC) on the soil-energy pile interaction. In addition to directly measuring the shear stress–vertical displacement curves (t-z curves), the soil temperature at different locations and pore pressure were monitored. The fully automated modified-TBST device uses two concrete plates to simulate the pile surface with temperature and expansion/contraction controls. The tests were conducted with temperature changes ($\mathrm{\Delta }T$) at the soil–concrete interface of $-18$, 0, and $+20°\mathrm{C}$. The radial expansion and contraction displacements ($\mathrm{\Delta }D$) were $+120$ and $-120\mathrm{\mu m}$, respectively. Tests were conducted at different interface horizontal normal stresses and numbers of cycles. This paper focuses on summarizing the results of 16 tests: 6 conducted with temperature changes and cycles, 8 with radial displacement change and cycles only, and 4 with combined temperature and displacement cycles. When the soil–concrete interface was subjected to combined radial displacement and temperature cycles, the interface shear strength experienced significant changes. Therefore, it was concluded that the intermittent operation of heat pumps connected to energy piles installed in normally consolidated clayey soils has a significant effect on shaft resistance.