Experimental and numerical investigation of coupled thermo-hydro-mechanical behaviour of clay with focus to cyclic processes

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In many geotechnical problems either saturated or unsaturated soils are subjected to a coupled thermo-hydro-mechanical (THM) loading. Furthermore, in several important applications the mechanical loads, temperature and moisture or suction vary in a cyclic manner. A highly topical example is the geothermal use of geotechnical structures, which is increasing in the context of renewable energy development. Energy piles as the most prominent example are subjected to a combined mechanical and thermal loading, with the latter one varying cyclically due to the seasonal changes of the ambient temperature. The possible degradation of the pile capacity or settlement increase due to the cyclic thermal loading of the surrounding saturated or unsaturated soil, in particular in case of clay, has not been sufficiently investigated yet. As another example, the global climate change leads to larger seasonal and daily variations of ambient temperature, which will also affect geotechnical structures. Infrastructure like railway tracks induce a cyclic mechanical loading to the subsoil, while the soil undergoes seasonal changes of temperature and moisture. The development of large cracks at a dam of a railway track near Lünen, Germany, composed of fine-grained soil, leading to closure of the track and an elaborate restoration, is supposed to be caused by the more extreme seasonal changes of temperature and moisture during recent years, in combination with the continued mechanical cyclic loading from the passing trains. A similar deterioration of other dams of railway tracks can be expected for the future. Despite its practical relevance for the design of various geotechnical structures the complex behaviour of fine-grained soils under coupled THM loading has not been sufficiently investigated and is thus not fully understood yet. This applies in particular to processes involving a cyclic variation of the mechanical load, temperature and moisture. The available constitutive models are mainly restricted to monotonic THM processes. Therefore, a thorough experimental investigation of a medium-plastic clay under coupled THM loading is planned in the proposed research project, including oedometer and triaxial tests performed at either Ruhr-University Bochum (RUB) or Charles University, Prague (CUP). The focus is laid on the behaviour of both saturated and unsaturated clay under cyclic mechanical and thermal loading. Furthermore, column-type experiments with temperature cycles and a measurement of the development of several quantities (e.g. temperature, suction, stress) along the height are conducted at RUB. Based on the experimental results constitutive models for clay developed by the applicants of both RUB and CUP will be enhanced with respect to the description of complex THM loading paths. The enhanced models are validated and compared based on the element tests and column-type experiments, before being applied in numerical simulations.