Repeated ETRTs in a Complex Stratified Geological Setting: High-Resolution Thermal Conductivity Identification by Multiple Linear Regression
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 148, Issue 4
Abstract
For the design of ground-source heat-pump systems, the local subsoil is an invariant factor. To improve the evaluation of the local heat exchange capability, significant efforts recently have been devoted to identifying the ground thermal conductivity vertical profile. In recent years, an innovative method using hybrid optic fiber cables inserted into the ground has been developed. The technique relies on copper wires that thermally stimulate the ground. Optical fibers measure the temperature variation over time all along the cable at a high spatial and temporal resolution. In this work, the hybrid cable was grouted into a 125-m well located in the Po Plain in Northern Italy. The provided core defined the geological environment as a continuous succession of unconsolidated alluvial deposits of very limited thickness, grouped in 15 different granulometric units. Three enhanced thermal response test (ETRT) data sets were acquired in different seasons; for 5 days of heating followed by 5 days of recovery, the soil temperature was recorded continuously along the well, with a spatial resolution of 1 m. A new approach using a multiple linear regression is proposed to analyze the data sets to distinguish the thermal conductivity of each individual granulometric unit. The obtained thermal conductivity values were compared and discussed considering the standard thermal response test outputs and the thermal conductivity data obtained from direct measurements performed on the cores. The analytical method’s reliability stands due to the high repeatability of the obtained results, despite the increased complexity of the treated geological setting.
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Data Availability Statement
All the data acquired by means of fiber optic measurements and the code that support the findings of this study are available from the corresponding author upon reasonable request.
Acknowledgments
The coring of the well was funded by the project GEO4CIVHIC: Most Easy, Efficient and Low Cost Geothermal Systems for Retrofitting Civil and Historical Buildings, which received funding from the European Union’s Horizon 2020 research and innovation program under Grant agreement No. 792355. The authors thank Mattia Donà for performing the geotechnical classification of the deposits in the Laboratory of Department ICEA at the University of Padova, and Gianpaolo Girardi for his efforts in performing the DTS measurements.
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Journal of Geotechnical and Geoenvironmental Engineering
Volume 148 • Issue 4 • April 2022
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© 2022 American Society of Civil Engineers.
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Received: Nov 23, 2020
Accepted: Sep 28, 2021
Published online: Feb 8, 2022
Published in print: Apr 1, 2022
Discussion open until: Jul 8, 2022
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