Thermoactive geostructures are considered an economically suitable and environment-friendly solution for heating and cooling buildings. Energy tunnels have gained growing interest in recent years because of the large ground volume involved in heat exchange in comparison to building foundations. Heat exchange is obtained by embedding a circuit of pipes into the precast concrete lining, resulting in a reduction of the initial costs of installation with respect to standard low-enthalpy geothermal solutions. In this paper, thermal activation of the twin tunnels of the II line of Warsaw metro, Poland, is considered with the aim of evaluating the geothermal potential of the line. Thermohydraulic finite-element analyses were performed on a number of representative cross-sections, considering site-specific hydrogeological conditions based on the Engineering Geological Database of the Capital City of Warsaw. Results demonstrate that the thermal activation of the two 1·6 km-long tunnels could exchange up to 5·3 and 5·8 GWh in the heating and cooling seasons, respectively. Furthermore, the influence of the local geological conditions on the exchangeable heat rate was investigated. The results form a basis for a preliminary assessment of the shallow geothermal energy utilisation potential for the design of further extensions of the metro line in Warsaw.

The European standards, developed extensively over last 30 years, are driven by the need for continuous evolution and their Authors’ pursuit of better EU-wide quality in civil engineering – combining safety, economy, and sustainable development. The adoption of theory of reliability as the basis for design has played a major role in shaping current geotechnical practice. However, it requires from practitioners a greater understanding of underlying uncertainties. Furthermore, a number of alternative approaches, not generally used in structural design, are also allowed, as some situations in geotechnical engineering require an individual approach. Moreover, the current trends in geoengineering increase the importance of risk assessment and management. The paper presents general philosophy guiding the geotechnical design and pointing to some of the ideas introduced by Eurocode 7 and its requirements, in relation to preexisting practice of geotechnical design in civil engineering.

This paper presents the current conditions of and perspective on development of the use of shallow geothermal energy as a renewable energy source in highly urbanised areas in Poland, using the Warsaw municipality as an example. The type of geothermal energy source considered in the paper is the thermoactive foundation elements of high-rise buildings and underground structures. The rising demand for new office, commercial and housing spaces in Warsaw fosters the development of new engineering projects, mostly involving deep foundations and structural elements well suited for shallow geothermal energy utilisation. In this study, the preliminary assessment has been made, considering the city-scale impact of the thermoactive foundations. New as well as planned construction areas for new engineering projects in Warsaw were evaluated on the basis of spatial management plans and analysis of local geotechnical and geothermal conditions. Furthermore, the legal issues concerning the use of thermoactive foundations are discussed, considering the current state of the national regulations and the typical construction process.

Thermoactive piles ensure a dual function: they support mechanically the superstructure and at the same time they are used as heat exchanger elements. In comparison to standard design practice, this requires considering the associated thermal loadings from the operation of the heat exchange system. To assess the thermal effects on the mechanical behaviour, different design assumptions and modelling approaches can be used. Therefore, within the framework of European Cooperation in Science and Technology Action TU1405 ‘Geothermal Applications for Buildings and Infrastructures’, benchmarking of a single geothermal pile has been performed. Hence, both heating and cooling of the pile were considered while different rigidities (representing the upper structure) on the pile head were assumed. The results show that a broad range of design assumptions could be used, which unfortunately lead to noticeable differences. Although the main focus was on the methods and assumptions employed, the exercise also opens questions about the influence of transient effects, thermal strains in the ground, temperature cycles and so on. Owing to the complexity of the topic, some of them were just mentioned, while others were addressed in greater detail. In general, the findings emphasise the need for further practice-oriented guidance to be presented in design guidelines and standards.

The findings of a joint ISSMGE TC 205/TC 304 discussion group on the evaluation of model uncertainties, i.e. uncertainties related to the chosen geotechnical design method, are presented. The discussion revealed the importance of model uncertainty assessment due to the high sensitivity of many geotechnical design methods for the accuracy of the adopted model. In case of geotechnical designs in which the design quantities are clearly defined, model uncertainties may be considered by a model factor representing the bias of the predicted to the real behavior. The real behavior itself can be derived from well documented prototype and model scale field and laboratory tests. Model factors can be used in rigorous reliability based design but also in LRFD formats adopted by many geotechnical design codes. However, for complex design situations model uncertainty assessment remains difficult. Moreover, model uncertainties are affected by extraneous uncertainties such as spatial soil variability or uncertainties due to test performance which cannot be separated.