Thermal Hazard on Clayey Sediments Induced by a Borehole Heat Exchanger

Abstract

The heat exchange of a borehole heat exchanger (BHE) of a closed-loop geothermal system alters the natural thermal status of the subsoil. An incorrect design of the ground BHE array or particular working conditions related to climatic forcing in cold areas can require the reduction of the BHE carrier fluid working temperature below 0°C, in order to increase the ground thermal withdrawal in winter. The subsequent seasonal freezing-thaw cycles induced in the subsoil surrounding the BHE produce irreversible processes in cohesive sediments as changes in soil texture and non-negligible settlements. This behavior is particularly hazardous in environments showing abundance of cohesive layers in the subsoil. The case-study of the Venice area (Italy) is considered, representing a typical geological context of floodplain, coastal and lagoonal areas. The changes in the mechanical behavior caused by thermal stress on representative samples of venetian typical cohesive soils was studied by using an expressly made thermostatically controlled oedometer. The research focalizes the role of the pore water salt contents, in order to point out a way to prevent the subsoil thermal hazard due to a working BHE system in critical contexts.