8The mechanical behaviour of the soil-structure interface plays a major role in the shear 9 characteristics and bearing capacity of foundations. In thermo-active structures, due to 10 non-isothermal conditions, the interface behaviour becomes more complex. The objective 11 of this study is to investigate the effects of temperature variations on the mechanical 12 behaviour of soils and soil-structure interface. Constant normal load (CNL) and constant 13 normal stiffness (CNS) tests were performed on soil and soil-structure interface in a direct 14 shear device at temperatures of 5, 22 and 60 o C. Fontainebleau sand and kaolin clay were 15 used as proxies for sandy and clayey soils. The sandy soil was prepared in a dense state, 16 and the clayey soil was prepared in a normally consolidated state. The results showed 17 that the applied thermal variations have a negligible effect on the shear strength of the 18 sand and sand-structure interface under CNL and CNS conditions, and the soil and soil-19 structure interface behaviour could be considered thermally independent. In clay samples 20 the temperature increase, increased the cohesion and consequently the shear strength, 21 due to thermal contraction during heating. The temperature rise had less impact on the 22 shear strength in the case of the clay-structure interface than in the clay samples. The 23 adhesion of the clay-structure interface, is less than the cohesion of the clay samples. 24 Résumé 27 Le comportement mécanique de l'interface sol-structure est d'une grande importance en 28 1 raison du rôle de l'interface dans la résistance due au frottement et la capacité por-29 tante des structures. Dans les structures thermo-actives du fait de la variation de la 30 température, le comportement de l'interface devient plus complexe. L'objectif de ce tra-31 vail est d'étudier l'effet des variations de température sur le comportement mécanique 32 de l'interface sol-structure. Des essais avec des conditions de charge normale constante 33 (CNL) et de rigidité normale constante (CNS) ontété réalisées dans une boîte de cisaille-34 ment directà différentes températures, 5 o , 22 o et 60 o C sur deséprouvettes sol-sol et 35 sol-structure. Le sable de Fontainebleau et le kaolin ontété utilisés comme materiaux de 36 référence pour les sols sableux et argileux. Les résultats ont montré que les variations ther-37 miques appliquées ont un effet négligeable sur la résistance au cisaillement des interfaces 38 sable-sable et sable-structure dans les conditions CNL et CNS et que le comportement 39 du sable peutêtre considéré commeétant indépendent de la température. Dans l'argile 40é tudiée, l'augmentation de la température augmente la résistance au cisaillement en rai-41 son de la contraction thermique pendant le chauffage, ce qui augmente la cohésion du 42 sol. L'augmentation de température a eu moins d'impact sur la résistance au cisaillement 43 dans le cas de l'interface argile-structure que dans leséchantillons argile. L'adhésion de 44 l'interface argile-structure est infér...

The mechanical behaviour of the soil-structure interface plays a major role in the shear characteristics and bearing capacity of foundations. In thermo-active structures, due to non-isothermal conditions, the interface behaviour becomes more complex. The objective of this study is to investigate the effects of temperature variations on the mechanical behaviour of soils and soil-structure interface. Constant normal load (CNL) and constant normal stiffness (CNS) tests were performed on soil and soil-structure interface in a direct shear device at temperatures of 5, 22 and 60 °C. Kaolin clay was used as proxy for clayey soils. The results showed that, in clay samples the temperature increase, increased the cohesion and consequently the shear strength, due to thermal contraction during heating. The temperature rise had less impact on the shear strength in the case of the clay-structure interface than in the clay samples. The adhesion of the clay-structure interface, is less than the cohesion of the clay samples.

The shaft capacity of foundations highly depends on the monotonic and cyclic loads applied to the soil-structure interface. In energy geostructures which exploit the heat of soil using earthcontact elements, the interface is subjected to cyclic thermo-mechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clayclay and clay-structure interface at different temperatures (22 and 60 o C). An effective vertical stress of 300 kPa was applied to the samples and the cyclic and average shear stress ratios (τ cy /S Ds u and τ a /S Ds u , respectively) were varied between 0.35 and 0.57. The tested soil was a kaolin clay (PI=24) prepared in a normally consolidated state. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. In cyclic clay-structure tests, decreasing the cyclic stress ratio, increased the number of cycles to failure; however decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60 o C for the different cyclic stress ratios tested.

In energy geostructures, the soil-structure interface is subjected to thermo-mechanical loads.In this study, a non-isothermal soil-structure interface model based on critical state theory is developed from a granular soil-structure interface constitutive model under isothermal conditions. The model is capable of capturing the effect of temperature on sand/clay-structure interfaces under constant normal load and constant normal stiffness conditions. First, the developed model was verified for sand-structure interface in isothermal conditions. Then, it was calibrated for clay-structure interface under non-isothermal conditions. On one hand, a well-defined peak shear stress for the clay-structure interface and, on the other hand, the effect of temperature on the void ratio of the clay-structure interface were captured and reproduced by the model. The importance of interface thickness determination and some differences between the interface thicknesses of clay-structure and sand-structure interfaces are discussed in detail.The additional parameters have physical meanings and can be determined from laboratory tests. The modeling predictions are in good agreement with experimental results, and the main trends are properly reproduced.Keywords energy geostructures • non-isothermal model • constant normal stiffness (CNS) • soil-structure interface • temperature • critical state theory.

In energy geostructures, which exploit the heat in soil using earth contact elements, the interface is subjected to cyclic thermo-mechanical loads. Monotonic and cyclic constant-volume equivalent-undrained (CVEU) direct shear tests were performed on clay-clay and clay-structure interface at different temperatures (22 and 60 °C). Different cyclic and average stress ratios (CSR and ASR) were applied to the kaolin clay-structure interface under 300 kPa of normal stress. The results showed that, the number of cycles to failure for the clay-structure interface test was lower than that for the clay-clay case in the same range of cyclic and average shear stress ratios. In cyclic clay-structure tests, decreasing the cyclic stress ratio, increased the number of cycles to failure; however, decreasing the average shear stress ratio decreased the number of cycles to failure. Increasing the temperature, decreased the rate of strain accumulation and the number of cycles to failure increased by 2-3 times. The rate of degradation (degradation parameter, t) decreased by 16% with heating from 22 to 60 °C for the different cyclic stress ratios tested.

Unconfined compressive strength (Su) is one of the soil engineering parameters used in geotechnical designs. Due to the temperature changes caused by some human activities, it is important to study the changes in Su at different temperatures. For this purpose, kaolin, illite and montmorillonite clays with a liquid limit (LL) of 47, 80 and 119 respectively, were tested in a temperature-controlled cell in temperature range of 20 to 60 ℃. The results showed that the pore water pressure is a function of temperature and by heating, pore water pressure in the samples increased. In all three types of clay, the Su decreased linearly with increasing temperature. The reduction of Su in kaolin is more than illite and in illite is more than montmorillonite. The reason for this reduction, might be due the difference in the mineralogy of the clays. The results of unconfined compressive tests at different temperatures were simulated using hypoplastic model.

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In energy geostructures the long-term shaft resistance is affected by cyclic thermo-mechanical loads associated to structural loads and cyclic thermal solicitations. This study aims to investigate the effect of monotonic and cyclic thermal loads on normally consolidated and overconsolidated kaolin clay-rough steel interface. The peak shear stress of interface increased with temperature while the critical state shear strength remained unchanged. For cyclic temperature tests, with applying 10 temperature cycles (5-60°C) to normally consolidated kaolin clay interface, significant thermal contraction was observed but the shear strength increased as much as one single heating test. Normally consolidated interface contracted upon heating while overconsolidated samples dilated. These results highlighted the impact of thermal cycles on clayey interface which was stress history dependent.