Investigation on the behaviour of a thermo-active diaphragm wall by thermo-mechanical analyses

- The layout of the exchanger pipe loop governs the energy performance.
- A realistic definition of the thermal boundary conditions and thermal inputs at the pipe inlet is crucial.
- The thermally induced variations of the wall axial forces and bending moments are not negligible.
- Different cross sections of the wall behave differently and mutually interact.
- A three-dimensional analysis is required, instead of the more conventional plane strain analysis.

The thermo-active diaphragm walls are traditional retaining structures that embed heat exchangers for the exploitation of the near surface geothermal energy, used in the thermal conditioning of buildings and infrastructures. The coupled energetic and structural function of these so called energy walls requires some investigation in order to optimize the embedded circuit and assess the possible occurrence of significant consequences, in terms of temperature variations within the soils mass and thermal effects on the stress/strain state of the structure. In this contribution, the behaviour of an energy wall is assessed by finite element thermal analyses, that allow to investigate the energy performance and the short and long term influence on the soil temperatures, and by finite element thermo-mechanical analyses, to highlight the wall geotechnical and structural response. A one year cycle of heating/cooling operating mode of the geothermal system has been considered and the effects have been discussed in terms of soil-structure interaction and structural internal actions. The results show that the thermally induced mechanical effects are not negligible, especially as variations of the internal axial forces and bending moments. Although they seem to be not detrimental to the geotechnical and structural safety, they require a careful evaluation in order to predict possible situations of unexpected overstress conditions.