Experimental investigations and analysis of piles as heat exchangers in pyroclastic soils

Sustainability and the greenhouse gases containment are the main purpose of the world policies to combat climate change. These are certainly in contrast with the world's growing demand for energy that is still too heavily based on fossil fuels, which are the main causes of gas emissions. The European Energy policies for more than 20 years have been based on the reduction of the carbon dioxide emissions using renewable energy sources and the reducing the final energy consumption. Shallow Geothermal Energy (SGE) is a rapidly growing technology all over Europe as a support for the Renewable Energy policies and European Directives because of its low greenhouse gas emissions into the atmosphere. It is considered a renewable source on the timescales of technological/societal systems because do not require the geological times of fossil fuel reserves such as coal, oil, and gas. Low enthalpy geothermal energy is used for heating and/or cooling building by exploiting the ground heat by ground heat exchangers connected to a geothermal source heat pump (GSHP). Energy piles represents a rather innovative technology that couples the role of the structural foundation with the role of the heat exchangers for GSHP plants to satisfy the building heating and cooling needs. Compared to the traditional pile foundations, these structures are loaded both by mechanical and thermal loads, where for thermal loads is commonly intended the application of a thermal distortion. During the last years, thermal and thermomechanical behavior of energy piles has been investigated by different approaches. In this PhD thesis the main aim was to investigate on the thermomechanical behavior of energy piles contextualized in Neapolitan context both by a geotechnical and energy point of view. First of all, a general overview about the social and energy European context and about the geothermal energy, an introduction to energy piles, by both a mechanical and an energetical point of view, was reported. The research was carried out following three different approaches: numerical modelling, small-scale tests, and field scale tests. As regard the numerical modelling, two types of analyses were carried out. In the former case by an axisymmetric FEM model, the impact of different surface thermal boundary conditions on the thermomechanical behavior of a single end bearing energy pile embedded in pyroclastic multilayer soil is investigated. The latter case is about the study of the interaction factors for a couple of energy piles where only one is thermally loaded while the other is embedded as a passive element in the deformation field generated by the loaded pile. The results were obtained for different pile spacings and for different subsoil and are presented in the chapter 4. Chapter 5 is dedicated to the small-scale test carried out on an aluminum energy prototype pile embedded in Neapolitan pyroclastic dry sand. Both thermal and thermomechanical tests were carried out considering a cyclical application of the thermal loads both in heating and in cooling mode and also considering the impact of different mechanical loads. The thermal loads provided to pile was obtained from a dynamic energy simulation of a building in the city of Naples. The results showed different axial forces distribution depending on the kind and magnitude of thermal and mechanical load applied on pile. Moreover, it was observed irreversible pile displacements during the application of cyclic thermal loads. Finally, in the chapter 6 a field test was carried out in the province of Naples on a bored concrete energy pile 12 m in length and 0,60 m in diameter embedded in pyroclastic soil and equipped with a spiral heat exchanger configuration. Three heating thermal tests with different time duration were carried out. From the tests was observed that the null point of the pile was located at the same depth for all the tests. Anyway, the magnitude of the axial forces depended on the duration of the test and the magnitude of the inlet heat carrier fluid. The pile heating did not affect the surrounding soil temperatures during the tests and a high flow rate of heat power exchanged between the pile and soil was measured. The measured pile displacements ranged between the 75% and 78% of the theoretical free displacement. Moreover, a long-time monitoring of the pile and surrounding soil was carried out for about 7 months. The data collected allowed to study the site underground temperatures trend over the time and for different depth. It was also possible to find the mean value of the subsoil thermal diffusivity and consequently predict a yearly temperature trend over the time and for different depth for the site