<p>The promotion of Energy Geostructures (EGs) is strongly related to the use of renewable and clean energy resources for the heating and cooling of buildings. They couple the structural role of geostructures with the exploitation of Low Enthalpy Geothermal Energy (LEGE). During their operation, EGs are continuously subjected to thermal variations, due to the heat exchange between the soil and heat transfer fluid circulating in the pipes inserted in the structure. This can lead to an impact on the mechanical response of the structure, and the role of the soil-structure interface takes on relevance in this operation. Nevertheless, experimental results deriving from the literature on the Thermo-Mechanical (TM) soil-structure interface behavior suggest that the effect of temperature on the shear resistance is quite limited, in the case of interaction with a building material such as concrete, especially for coarse-grained soils. The case of fine-grained soils is more complex: some studies suggest an enhancement of the interface shear strength, showing an increase of adhesion or a slight increase in friction angle at the interface during heating; while other studies show no significant variations of the interface behavior with thermal cycles. Such differences are likely due to the multitude of experimental configurations, development protocols, and composition of the samples used during tests. With the aim of better understanding this controversial framework on the interface behavior, a modified device for direct shear tests was developed at the Laboratory of Geotechnical Engineering of the University of Perugia: starting from the conventional direct shear apparatus, this has been equipped with a heating cement plate, where a thermal resistance and a temperature probe for continuous temperature control have been integrated. The first tests on silty sand reconstituted samples have shown that the thermal effects at the interface are limited to a decrease in shear strength of less than 3%.</p>
<p>Energy Geo-Structures are being increasingly employed over the last decade. They combine the structural and energetic function, allowing the savings related to the absence of additional drilling, required instead by the common geothermal boreholes. Currently, they are a rather mature and deeply investigated technology, with a number of successful applications worldwide; however, some issues related to the thermo-hydro-mechanical (THM) effects induced in soils during heating/cooling cycles still deserve some more analyses, particularly for what concerns the possible non-linear behavior of soil under thermal loading. In this work, this issue has been investigated by means of fully coupled 3D FE modeling, considering a single, small diameter Energy Pile.&#160;The emphasis of the FE numerical modeling activity is the investigation of the effects induced by the pore pressure variations close to the pile during the thermal loading stage, and the assessment of the potential influence of the soil thermal softening effect on the pile behavior. Two different constitutive models have been adopted for the considered fine&#8211;grained soil, both based on the standard critical state theory: i) the classical Modified Cam Clay (MCC) model; and, ii) a similar critical state model incorporating a thermal hardening/softening mechanism for the critical friction angle, assumed as an internal variable that can be modified with temperature. In the FE model, first the mechanical load is applied at the pile head in almost undrained conditions, followed by a consolidation period during which the excess pore pressure dissipates. Thus, the pile is thermally loaded, with the temperature that is assumed to vary with a harmonic function law over periods of 1, 5 and 10 years, to investigate the short term and long&#8211;term effects.&#160;The results show that: <em>a) </em>for the considered case study, the thermal loading conditions produce very small changes in pore water pressure at the pile-soil interface; and no effects observed on the pile head displacements can be related to thermally-induced pore pressure changes; on the contrary, b) significant additional pile head settlements are observed in presence of thermal softening, due to by plastic shear deformations at the soil-pile interface.</p>
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