DW² - Thermal improvements on diaphragm walls

Status

completed (November 2022)

Starting point, content and results

The storage of energy and the reduction of energy consumption of structures represents a large part of the efforts to create sustainable settlements and housing areas, as well as to achieve the climate goals. Current developments in the field of improving thermal properties usually take place in building construction – mainly the external wall and ceiling structures of residential buildings. Geotechnical construction elements such as diaphragm or bored pile walls, are often not considered, although these can also contribute to reducing the energy consumption of a building and also serve to optimize storage solutions.

In the field of residential building construction, diaphragm walls or bored pile walls are used to enclose the excavation pit and, especially in inner-city construction sites, often as an exterior wall in contact with the ground. Thus, these building components - in relation to their use - have subordinate thermal requirements. However, current developments in the field of thermal storage technologies - such as underground thermal energy storages, which can store and provide thermal energy within the ground - also require enclosures. For this purpose, diaphragm walls or bored pile walls are used, which are subjected to thermal requirements in addition to a sealing and enclosing function. The use of conventional structural concrete for such structural elements results in massive heating of these components in the course of storage utilization. In addition to the associated energy losses, which have a negative impact on the balance of the thermal energy storage, also heating of the subsoil takes place.

In addition to a mechanical influence on the properties of the subsoil (stiffness and strength), a possible heating of the groundwater is of particular importance. Current normative and legal regulations specify a very strong limitation of the permitted groundwater heating in order to maintain the drinking water quality and subsequently to prevent bacterial growth within the groundwater zones. However, this also means massive restrictions on the construction of underground thermal storage facilities.

In the course of the DW² project – Thermally improved diaphragm walls - the possibilities of improving and optimizing the thermal properties of diaphragm walls and other geotechnical elements were investigated. In addition to the application and use of new - thermally optimized - concrete types, comprehensive laboratory and field test series were carried out to demonstrate their applicability and the associated potential. These were supplemented by numerical simulations of thermal energy storages in the subsurface and the associated influences on the surrounding soil and groundwater.

The results of the project showed that in principle the application of thermally enhanced structural concretes for geotechnical structures is possible. However, the requirements for production and placement (e.g. contractor method) but also the requirements for use (e.g. impermeability, compressive strength) have to be considered accordingly. With the laboratory and field tests carried out, it was possible to demonstrate the suitability of a newly developed structural concrete for geotechnical components on a model scale. Likewise, a comprehensive laboratory investigation of the developed concrete showed that, as a result of an optimized mixture, a halving of the thermal conductivity and thus a reduction of the energetic losses is possible. In addition, extensive numerical investigations and parameter studies were able to demonstrate the influence of such a concrete in comparison to a classic structural concrete. It could be recognized that a significant reduction of heat losses and heating of the subsoil is possible.

In the course of the project, however, it was also recognized that despite an optimized structural concrete, there is still a significant heating of the subsoil and the groundwater. For this reason, possibilities for preventing or reducing this were also conceptualized during the project. One of the most future-oriented and sustainable solutions is the arrangement of additional extraction wells in the outer area of thermal underground storage. This would not only prevent the groundwater from heating up, but would also result in the possibility of using already heated groundwater (e.g. inner-city heat islands) as a heat source for the underground storage and reduce the losses of the storage. For this, however, further investigations are required, both in the field of simulation and in field and lab tests.

Project management

Graz University of Technology - Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics

Project or cooperation partners

• Graz University of Technology - Institute of Structural Concrete
• Graz University of Technology - Working Group Sustainable Construction
• University of Graz - Institute for Earth Sciences

Graz University of Technology
Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics
Matthias J. Rebhan
Rechbauerstraße 12
A-8010 Graz
Tel.: +43 (316) 873 6738
E-mail: rebhan@tugraz.at
Web: www.soil.tugraz.at