Investigation of the freeze-thaw states of foundation soils in permafrost areas along the China-Russia Crude Oil Pipeline (CRCOP) route using ground-penetrating radar (GPR)

•Evaluation of thermal states of pipeline foundation soils at 8 sites along the CRCOP using GPR.•GPR records interpreted with the help of stratigraphic and freeze-thaw information from boreholes and hand-dug trenches.•GPR results compared with monitored results, model predictions, and centrifuge model test results.•Insulation and thermosyphon effects of foundation soils did not achieve original expectations, better design, or retrofits recommended.•Causal analysis for larger than anticipated, proactive measures, advantages, and limitations of using GPR for measuring thaw depths of pipeline foundation soils were provided and advised.

The China-Russia Crude Oil Pipeline (CRCOP) was built to transport Siberian crude oil across 1030 km of frozen ground. The Chinese portion of the CRCOP crosses 441 km of discontinuous permafrost zones along the eastern flanks of the Da Xing'anling (Hinggan) Mountains. Significant thaw settlement of some of the pipeline foundation soils has occurred since the pipeline began operating in January 2011. The differential thaw settlement around the pipeline at the transitions between the thaw-settling and thaw-stable zones may lead to displacement or even buckling of the pipeline. To evaluate the engineering safety and long-term stability of pipeline foundations, ground-penetrating radar (GPR) was used to detect the freeze-thaw states of pipeline foundation soils in the permafrost zones. From April 2012 to November 2013, a total of 2780 m of GPR profiles and 62 cross-sections (perpendicular to the CRCOP route) were obtained at eight GPR study sites on various terrains and vegetative coverage, with different thermal insulation configurations around the pipe, and with and without thermosyphons installed. The GPR profiles were interpreted and cross-checked with data from drilling, hand-dug pit or trench excavations, and ground temperature measurements. The results showed that the foundation soils around the pipeline were thawing, with maximum thaw depths under the pipeline of about 1.1 m to 4.5 m from the pipe bottom and increased maximum thaw depths of 0.5 m to 3.7 m during the period of this study. However, GPR profiles from the areas with dense vegetative coverage, and with pipe insulation of 80-mm-thick extruded polystyrene (XPS), and also with vertical thermosyphons installed, showed that permafrost foundation soils were thermally better protected, with only minor thaw settlement. The thermal states of pipeline foundation soils and the underlying permafrost varied markedly with different terrains. The drainage patterns and engineering activities, especially the heating from operating oil flows at an average annual oil temperature of 7.7 °C (with a range of 0.7 °C to 20.4 °C) along the pipeline route may have been responsible for the extensive and substantial thaw subsidence in the ice-rich permafrost zones. Additionally, there was positive feedback between the thaw subsidence of ground surface in the vicinity of the pipeline route and the thaw settlement of the pipeline foundation soils. The greater the earth surface subsidence, the more surface water ponding and soil disturbances, and even more progressive thawing. Therefore, some measures are recommended for stabilizing the pipeline foundation soils. The research results also indicate that the GPR method is effective, nondestructive, and convenient for evaluating the spatiotemporal extent and development processes of the freeze-thaw states of pipeline foundation soils and the underlying permafrost.