Experimental study on hydrothermal effects of preferential flows in silty clay specimens from the Da Xing’anling Mountains using a color tracer under freeze-thaw cycles

Preferential flow (PF) is the rapid, irregular movement of water through soil channels. In permafrost regions, it is triggered by rainfall, snowmelt, and other hydrometeological factors, and affected by environmental and soil factors. With climate change, permafrost is degrading, especially in boreal areas such as the Da Xing’anling Mountains and along the China-Russia Crude Oil Pipelines (CRCOPs). This study focuses on silty clay from the right of way of CRCOPs. Using indoor color tracers and comparative testing, six PF types were simulated in the samples and compared to a control.

The results show that soil columns with microfracture PF and randomly distributed macroporous PF experienced extended cooling (by 65% and 87%) and warming (by 57% and 39%) periods. Their average minimum temperatures were 0.4 to 2.5°C lower than those of the control, and took 1.9 to 2.4 times longer to reach stable temperatures. Microfracture and funnel PF samples had 18% to 25% higher minimum water content compared to the control. The coloration rate was 15% to 56% higher, and the preferential flow index in PF soil columns was over 48% higher during the first freeze-thaw cycle. Overall, PF type and related factors are crucial for the thermal and moisture characteristics of silty clay. These findings provide valuable insights for pipeline operation and permafrost engineering, contributing to enhanced foundation stability in a changing climate.

Key words: preferential flow; freeze-thaw cycles; hydrothermal effects; color tracing; silty clay; northern Da Xing’anling Mountains