There has been an increased interest in geotechnical engineering applications that require an understanding of the impact of temperature on the thermo-hydro-mechanical response of soils for their design and sustainable long-term operation. Examples include the implementation of ground source heat exchangers (GSHEs) into deep foundations to form energy piles, high-level nuclear waste repositories, burial of high-voltage cables, and the utilization of waste heat for the improvement of unsaturated backfill soils in earthen embankments or mechanically stabilized earth walls. An issue of particular interest in these applications is the drained volumetric response of unsaturated soil due to changes in temperature. The underlying mechanisms responsible for the thermal volume change of unsaturated soil are still not well understood, which is the main motivation for this study. Volume changes may have an adverse effect on the long-term settlement response of thermally-active geotechnical systems.;A new high pressure thermal isotropic cell was developed in this study to evaluate the thermal volume change mechanisms in unsaturated silt, including the impact of the initial degree of saturation and secondary compression behavior on the thermal contractile volume change of compacted Bonny silt. The thermal isotropic cell includes suction control using the axis translation technique, saturation control/monitoring using a pore water pressure flow pump, cell pressure control using a high pressure flow pump, and a stainless steel cell to permit application of isotropic net mean stresses up to 10 MPa. The cell fluid temperature is regulated by circulating heated water through a heating coil within the cell. Further, a circulating fan in the cell is used to homogenize the temperature within the cell fluid. Three non-contact proximity probes are used to monitor soil deformation, avoiding the need to consider complex thermo-mechanical cell deformations. Four thermal volume change tests were performed on compacted specimens of Bonny silt at different initial degrees of saturation. The testing procedure involved drying via the application of matric suction, drained isotropic loading to achieve normally consolidated conditions, and drained heating.;The results from thermal volume change tests on Bonny silt were used to evaluate the roles of initial degree of saturation and secondary compression (creep) behavior on thermal volume change. The magnitude of thermal volume change was observed to be unaffected by the degree of saturation and is explained based on the influence of changes in degree of saturation on mean effective stress during drained heating. For the Bonny silt, the changes in degree of saturation were not sufficient to cause a significant change in mean effective stress. Accordingly, the volume change that corresponds to this small change in mean effective stress is negligible compared to the actual thermal volume change observed in the experiments.;An alternative explanation proposed in this study is that the thermal volume changes observed in soils are due to an acceleration of the secondary compression process that was underway in the soil due to changes in effective stress prior to heating. The use of the secondary compression index to define a thermally accelerated creep deformation was found to provide a consistent interpretation of the thermal volume changes, for heating and cooling, of both normally consolidated and overconsolidated soils based on results from this study as well as those from the literature. The thermally accelerated creep concept differs with previous mechanistic theories that assume that thermal volume change is associated with dissipation of thermally induced excess pore water pressures, which can be used to explain the contraction of normally consolidated soils, but not the expansion of overconsolidated soils. Further, the thermal accelerated creep theory provides a simpler and more consistent approach to explain unsaturated soil behavior than the empirical thermo-hydro-mechanical constitutive models where thermo-elastic expansion of the soil skeleton is superimposed atop compression associated with dissipation of thermally induced pore water pressure.
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