Modified-thermal borehole shear test device and testing procedure to investigate the soil-structure interaction of energy piles

The intermittent operation of heat pumps connected to energy piles causes cyclic variation of temperature in the pile and surrounding soil, affecting the soil-pile interaction in ways that have not been fully investigated or directly measured. The temperature variation and cycles produce expansion and contraction of the pile in both the axial and radial directions and affect soil properties. In this study, a fully automated modified–thermal borehole shear test (Modified-TBST) device was developed to measure the thermo-mechanical behavior of the soil-pile interface properties considering the effects of radial expansion/contraction and temperature variation and cycles. Unlike other devices, the developed Modified-TBST device is fully automated and capable of combining the effects of temperature cycles with radial expansion/contraction (displacement) cycles or separating their effects. This paper describes the testing device, including the control method for expansion/contraction displacement cycles, the calibration of the shear head under non-isothermal conditions, and the measured load and displacement controls during the shearing stage. Furthermore, the paper presents procedure recommendations for performing the soil-pile interface tests, simulating energy piles and presenting preliminary results of shear stress-vertical displacement (t-z curves), considering temperature and displacement effects.

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