## 3 Results
### GPR-Results  
Stable zone interpretation: Figure 13 highlights the results from the 15L measurement line conducted outside the subsidence zone. The radargram, segmented into distinct layers, reveals key subsurface features. The green line delineates the 2 m-thick soil layer underlying the road structure, while the blue line marks the soil-to-bedrock transition at approximately 4-5 m. A pink line indicates the relatively homogeneous bedrock layer at a depth of 8-9 m, with minor density variations. Observations include a westward slope of the bedrock and road structure, suggesting the potential water seepage direction and the absence of cavities or erosion in the bedrock, confirming structural stability. An example radargram was recorded in the zone out of the subsidence region. Figure 12: Time-series displacement trends for four representative points (**a–d**) along Road 431, showing subsidence rates derived from PSI analysis. The blue line indicates the fitted subsidence trend, while the red line represents the zero-displacement baseline. The Subsidence Zone study: Figure 14 depicts the results of the L1 measurement line along the southern edge of the northern lane. The radargram reveals structural details, including the water transfer pipe (orange) and the central subsidence area (blue). The lower figure uses color-coded interpretations to clarify subsurface layers: asphalt (green), granular fill (blue), additional fill (pink), and clay soil (red). Clear concavities in the asphalt and granular layers reflect significant subsidence within the fill layer. Outside the subsidence zone, stratification remains linear, indicating proper compaction, while the subsidence area shows fragmented layers with anomalies indicative of structural breakdown. Void Analysis and Comparative Insights: Figures 15 and 16 provide further insights into the subsidence zones. Figure 15 focuses on measurement line L5, identifying air voids (karst cavities) concentrated in the center of a 70 m subsidence area, which aligns with water flow paths and fine particle displacement. Figure 16 examines lines L3 and L7, revealing a disintegrated fill layer in the southern lane, extending 2 m deep, compared to a more intact structure in the northern lane, with a depth of 1.7 m. These contrasts emphasize variations in subsurface stability across the lanes. Figure 14: The results of the L1 measurement line along the southern edge of the northern lane. The radargram reveals structural details, including the water transfer pipe (orange) and the central subsidence area (blue). The lower figure uses color-coded interpretations to clarify subsurface layers: asphalt (green), granular fill (blue), additional fill (pink), and clay soil (red). Figure 13: An example of radargram recorded in the zone out of the subsidence region. The green line delineates the 2 m-thick soil layer underlying the road structure, while the blue line marks the soil-to-bedrock transition at approximately 4-5 m. A pink line indicates the relatively homogeneous bedrock layer at a depth of 8-9 m, with minor density variations. Spatial Patterns and Synthesis: Figure 17 offers a spatial cross-section of the subsidence area across all lanes, revealing a \"winged\" depression pattern, with asymmetric elevation suggesting localized water pooling on the eastern side. The asymmetry, with the western wing elevated above the eastern wing, indicates localized water trapping east of the tilt. This configuration amplifies moisture content, increasing particle displacement and structural degradation potential. Figure 18 synthesizes these findings on a map, showing the displacement and structural degradation potential. The figure shows the subsidence area (blue), water channel (orange), and core subsidence zone with air voids (white). These visuals underscore the interplay between water movement and subsurface instability.