Effect of thermal cycles on volumetric and shear behavior of sand–concrete interface

Abstract

Energy piles are dual-purpose foundation elements that transfer the building loads to the supporting soil and act as heat exchangers for heating and cooling. Energy piles are subjected to heating and cooling cycles over days and seasons, resulting in cyclic expansion and contraction. The cyclic expansion and contraction bring about the modification of vertical stress and the increase of vertical displacements on the piles, in other words, the thermo-mechanical behavior of the pile, the soil, and the soil–pile interface is changed by the cyclic thermal cycles. Investigation to know the effect of temperature cycles on soil–pile interface characteristics has high academic and significant engineering values. This paper intends to investigate the thermo-mechanical response of soil–pile interface under different types of cyclic thermal loadings, to better understand the behavior of energy piles and their surrounding soils after the buildings are constructed and the geothermal activation. Direct shear tests are performed by using a new interface direct shear test machine composed of a temperature controlling device to assess the influence of different cyclic thermal loadings on the shear behavior of Fontainebleau sand–concrete interface. Direct shear tests are carried out after three different thermal loadings, i.e., a constant temperature of 13 °C, and two different kinds of cyclic cooling–heating​ (8–18 °C) loadings with 10 cycles (namely CT-1 and CT-2). The horizontal stress and horizontal displacement are evaluated after the 10 thermal cycles in CT-1 and CT-2 programs, and the shear results are then compared. The results reveal that the cyclic thermal cycles increase the peak shear strength of CT-2, however they do not significantly affect the residual shear strength of both CT-1 and CT-2. The peak/residual interface friction angle of the reference test is 29.5∘/28.4°, after the 10 cyclic thermal cycles, it becomes to 28.9∘/27.9° for CT-1, and 31.7∘/28.5° for CT-2.