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 article presents the methodology of conducting serial laboratory measurements of thermal conductivity of recompacted samples of cohesive and non-cohesive soils. The presented research procedure has been developed for the purpose of supplementing the Engineering–Geology Database and its part–Physical and Mechanical Properties of Soils and Rocks (abbr. BDGI-WFM) with a new component regarding thermal properties of soils. The data contained in BDGI-WFM are the basis for the development of maps and plans for the assessment of geothermal potential and support for the sustainable development of low enthalpy geothermal energy. Effective thermal conductivity of soils was studied at various levels of water saturation and various degrees of compaction. Cohesive soils were tested in initial moisture content and after drying to a constant mass. Non-cohesive soils were tested in initial moisture, fully saturated with water and after drying to a constant mass. Effective thermal conductivity of non-cohesive soils was determined on samples mechanically compacted to the literature values of bulk density. Basic physical parameters were determined for each of the samples. In total, 120 measurements of thermal conductivity were carried out, for the purposes of developing the guidelines which allowed statistical analysis of the results. The results were cross-checked with different measuring equipment and with the literature data.

Thermal conductivity is one of the key parameters for estimating low-temperature geothermal potential. In addition to field techniques, it can be determined based on physical parameters of the sediment measured in the laboratory. Following the methodology for cohesive and non-cohesive sample preparation, laboratory measurements were carried out on 30 samples of sediments. Density, porosity and water content of samples were measured and used in thermal conductivity estimation models (TCEM). The bulk thermal conductivity ( $${\lambda }_{b}$$λ b ) calculated with six TCEMs was compared with the measured $${\lambda }_{b}$$λ b to evaluate the predictive capacity of the analytical methods used. The results show that the empirical TCEMs are suitable to predict the $${\lambda }_{b}$$λ b of the analysed sediment types, with the standard deviation of the residuals (RMSE) ranging from 0.11 to 0.35 Wm⁻¹ K⁻¹. To improve the fit, this study provides a new modified parameterisation of two empirical TCEMs (Kersten and Côté&Konrad model) and, therefore, suggests the most suitable TCEMs for specific sample conditions. The RMSE ranges from 0.11 to 0.29 Wm⁻¹ K⁻¹. Mixing TCEM showed an RMSE of up to 2.00 Wm⁻¹ K⁻¹, meaning they are not suitable for predicting sediment $${\lambda }_{b}$$λ b . The study provides an insight into the analytical determination of thermal conductivity based on the physical properties of sediments. The results can help to estimate the low-temperature geothermal potential more quickly and easily and promote the sustainable use of this renewable energy source, which has applications in environmental and engineering science.

Thermal conductivity is a key input parameter in geotechnical and shallow geothermal engineering, directly influencing the design, efficiency, and long-term performance of ground heat exchangers, energy piles, and ground-source heat pump systems. Reliable parameterisation of this property in sandy soils remains challenging due to nonlinear interactions between water content, bulk density, and soil structure. This study develops a machine-learning-based workflow for robust parameterisation of thermal conductivity in quartz-rich sands using a large, internally consistent laboratory dataset comprising 1716 samples, including 1455 moist measurements used for modelling, obtained from nationwide site investigations. Air-dry specimens were identified as laboratory-induced drying states and excluded to restrict the analysis to hydro-mechanical conditions representative of typical shallow subsurface environments. Several regression algorithms representing different modelling strategies were evaluated within a unified and reproducible framework and benchmarked against selected classical empirical formulations. Model performance was assessed using standard accuracy metrics together with diagnostics describing the functional stability of predicted thermal-conductivity surfaces. The results reveal a systematic trade-off between predictive accuracy and functional consistency, indicating that models optimised for accuracy may produce functionally unstable and less suitable parameterisations for engineering applications. Accuracy-optimised models frequently produce locally irregular parameter fields, whereas more strongly regularised models yield smoother and physically more coherent response surfaces. The proposed workflow supports reliable thermal-property parameterisation for geotechnical design and shallow geothermal modelling.

The transition toward Smart City development relies on efficient integration of digital and geoscientific data to improve the sustainability of urban infrastructure. A key component of this process is the optimal utilization of subsurface thermal resources for clean energy systems, as well as the planning of underground infrastructure. This study presents a novel adaptation of the national Engineering-Geological Database of Poland (BDGI) for mapping the very shallow geothermal potential of urban areas. Using more than 35,000 boreholes from the Warsaw region, the database was processed to generate spatial models of thermal conductivity (λ) and thermal resistivity (ρ) at depths from 2 to 30 m below ground level. The shallow thermal conductivity maps so prepared are particularly relevant for the design of energy geostructures (e.g., geothermal piles, thermoactive foundations, energy tunnels) and horizontal ground heat exchangers. The data shown as thermal resistivity maps can also support the design and operation of urban transmission infrastructures such as district heating, electricity, and water networks. The case study area is characterized by loose sediments and soils (compressive strength Rc Ł600 kPa, according to ISO 14689), while solid rocks were not considered as they do not occur at such shallow depths. The resulting thermal maps provide new insights into the spatial variability of the urban subsurface and constitute a practical geospatial tool for energy and infrastructure planning in sustainable city development.