UBC Theses and Dissertations

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UBC Theses and Dissertations

An experimental investigation of thermal effects on the axial resistance to relative ground movement of buried district heating pipes Huber, Michael


District heating (DH) systems are commonly used in urban areas to distribute thermal energy from central heat sources. Buried pipes, with a composite cross-sectional construction, are used to transport a heated medium, usually water. These pipes expand and contract radially and axially due to changing water temperatures, invoking soil-pipe interaction situations during operation, and potentially leading to significant pipeline material strains. Measures to account for these soil-pipe interactions are an important consideration and a significant cost factor when designing and installing robust and cost-effective DH pipe systems. A series of full-scale tests were undertaken to provide experimental data on the axial and lateral soil resistance of DH pipes. An existing soil chamber that is part of the Advanced Soil Pipe Interaction Research™ (ASPIRe™) facility at the The University of British Columbia (UBC) was adapted to test full-size water-filled pipes. As a part of this project, a heating system was developed specifically to apply different heating histories to the water mass before the pipe is pulled. Strain gauges were mounted on the pipe at the soil interface to contribute to understanding the mechanisms involved in soil-pipe interaction. It was shown that changes in the temperature of the water mass have a significant influence on axial pullout resistance of the DH pipe. After heating the water mass by ∆T = 50 °C, large-strain resistance increased by roughly 15 % compared to the control tests. Three full cooling and heating cycles reduced the axial soil resistance of the pipe, potentially due to an arching mechanism in the soil. Considerable strain was measured at the soil-pipe interface both in axial and radial direction during heating of the water mass. Based on the development of strain with the heating history, it was inferred that the expansions at the pipe surface result from a combination of strains from both the steel pipe at the core and the high-density polyethylene (HDPE) cover. Consequently, DH pipes have to be treated as a complete system in combination with the surrounding soil mass in order to accurately model their mechanical behaviour under thermal load.

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