Hydro-mechanical behavior of unsaturated soil surrounding a heated pipeline considering moisture evaporation and condensation

Pipelines used for the transportation of waxy oil or high viscous oil usually work under high temperature and some of them are buried in unsaturated soils, e.g., in coastal regions or regions where change in groundwater level takes place. The rise and drop in temperature of these pipelines inevitably cause evaporation and condensation of moisture within unsaturated soils, which in turn could possibly lead to significant change in the pore water/gas pressures and subsequently change the soil behavior drastically. This paper describes the establishment of a coupled model of phase transition and mechanical behavior by implementing the Kelvin-Laplace and Clapeyron functions into the finite element method code OpenGeoSys, and the subsequent numerical study on the hydro-mechanical behavior of soils surrounding a heated pipeline. Results show that the thermally induced vapor condensation could yield significant drop in gas pressure and compression of soil, which is deemed beneficial to pipeline stability, and this effect can be enhanced by increasing the pipeline burial depth. There exists a critical initial saturation that can distinguish differing development trends for both gas pressure and ground settlement. Implications regarding the design of high temperature pipelines in unsaturated soils are also discussed.