Serial Laboratory Effective Thermal Conductivity Measurements of Cohesive and Non-cohesive Soils for the Purpose of Shallow Geothermal Potential Mapping and Databases—Methodology and Testing Procedure Recommendations

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.

Download Full-text

A preliminary study of empirical evaluation models for determining the thermal conductivity of sediments

10.5194/egusphere-egu21-8777 ◽

2021 ◽

Author(s):

Simona Adrinek ◽

Mitja Janža

Keyword(s):

Thermal Conductivity ◽

Water Content ◽

Empirical Evaluation ◽

Physical Parameters ◽

Cohesive Sediments ◽

Sediment Samples ◽

Geothermal Potential ◽

Evaluation Models ◽

Preliminary Study ◽

Estimation Models

Shallow geothermal energy is a renewable energy source that will play an important role in future energy management plans. Densely populated areas are often developed on alluvial plains, which consist of unconsolidated sediments. These have different thermal properties, so their accurate determination is important for planning subsurface heat utilization for heating and cooling of buildings in urban areas. Bulk thermal conductivity (&#955;b) is one of the most important ground thermal properties for estimating shallow geothermal potential, as it controls the ability of sediments to transfer heat. The &#955;b&#160;can be determined with empirical bulk thermal conductivity estimation models (&#955;b EM), which define &#955;b&#160;as a function of the measured physical parameters of the sediment (water content, bulk density) and the fluid. In this contribution, we present a preliminary study of three empirical evaluation models for determining the thermal conductivity of sediments &#8211; the Kersten (1949), the Johansen (1975) and the Cote & Conrad model (2005). Validation was carried out with laboratory-measured &#955;b using 30 unconsolidated sediment samples classified into 2 different groups (cohesive, non-cohesive) and by water content. The modelled results were evaluated using the coefficient of determination (R2) and root mean square error (RMSE). The modelled &#955;b&#160;for non-cohesive sediments has the highest &#955;b&#160;with the Johansen model. The lowest RMSE was obtained with the Kersten model. For cohesive sediments, the highest &#955;b and lowest RMSE, and consequently the best model, are based on the saturation of the sediments. It varies between the Cote & Conrad and the Kersten model. By dividing the sediment samples based on shear strength and water content, we obtained the better agreement of individual groups with estimation models. This showed the importance of the physical parameters in better predicting the modelled results. In the future, we will need to upgrade results with the use of more estimation models, that could improve the modelled results. With such an approach the estimation models can become a useful tool for a faster determination of the shallow geothermal potential.

Download Full-text