Ground-penetrating radar-based damage and porosity assessment of block-stone subgrade in permafrost regions

Climate warming and wetting are accelerating permafrost degradation on the Qinghai-Tibet plateau, leading to widespread settlement and cracking of highway infrastructure. Significant deformation has been observed during service, even in embankments incorporating block-stone cooling layers. This study focuses on a section of the Gongyu expressway situated in a permafrost region, integrating field investigations, geological drilling and ground-penetrating radar (GPR) surveys. The results indicate that the main internal distress of the block-stone embankment includes gravel cushion detachment, infilling-induced densification of the block-stone layer, settlement deformation and loosening of rock blocks. A GPR-based porosity inversion method was further proposed to identify structural anomalies in the apparent porosity of the block-stone layer. The continuous accumulation of internal structural deterioration weakens the effective pore structure and pore connectivity of the block-stone layer, thereby reducing its cooling capacity and diminishing its cooling effect on the underlying permafrost. In addition, water accumulation along both sides of the road acts as a persistent heat source, intensifying thermal field asymmetry and accelerating the development of embankment settlement. These findings provide technical support for the rapid diagnosis of distress in block-stone embankments in permafrost regions of the Qinghai-Tibet plateau.