Investigation of the permafrost beneath the subgrade of Qinghai-Tibet railway using electrical resistivity tomography

Climate warming and disturbances from the Qinghai-Tibet Railway have caused permafrost beneath the railway subgrade to degrade, resulting in structural problems like embankment settlement. Existing ground temperature monitoring systems along the railway are insufficient to monitor the permafrost changes comprehensively, and constructing new systems is time-consuming, labor-intensive, and disruptive to railway operations and permafrost stability. The electrical resistivity tomography (ERT) method offers rapid, non-destructive detection. However, inserting electrodes into the crushed-rock embankment is challenging, creating data gaps beneath the subgrade and reducing inversion reliability. To address this, forward and inverse modeling was employed to assess the impact of missing electrodes. Results indicate that the complete absence of embankment electrodes prevents reliable inversion beneath the subgrade. However, partial electrode insertion on embankment slopes or shoulders allows reliable deeper-layer results despite low data coverage in shallow levels. ERT was applied at 36 locations along the railway where subgrade settlement was observed. Permafrost remained stable or slightly degraded at 21 sites, while complete thawing occurred at 15 sites. Among these, 10 sites exhibited full degradation beneath the embankment, slope toes, and surrounding natural areas. Regional disparities in the cooling efficacy of crushed-rock embankments were observed. The crushed-rock embankments successfully maintained underlying permafrost at two sites where natural permafrost had thawed completely. While they were ineffective in preserving permafrost at five sites. In four of these, natural permafrost degradation coincided with complete thawing beneath the embankments. Notably, even at a location where natural permafrost remained, the embankment still experienced complete degradation. ERT effectively evaluates permafrost changes and the performance of cooling measures, providing a critical tool for Qinghai-Tibet Railway maintenance.