Typically, the degree of road damage (Han et al. 2018) reflects the current degree of damage caused to a road, and it can be primarily divided into four categories: severe, medium, minor, and undamaged. The grade of highway protection (Cho et al. 2021), which reflects the current traffic capacity of a highway, can be divided into three levels: prohibitive, restricted, and normal. The degree of highway damage often corresponds to the grade of highway protection; particularly, severe damage often corresponds to traffic prohibition, medium damage corresponds to restricted traffic, and minor or no damage corresponds to normal traffic. However, in certain exceptional cases, the degree of highway damage does not correspond to the grade of highway protection.
As shown in Fig. 5, a total of 507 places were inspected, which included 386 roadbeds, 103 bridges, nine tunnels, and nine culverts. Among these, the roadbeds suffered the most severe and significant disruption, some roadbeds and bridges witnessed minor damage, and most bridges remained undamaged. The main structure of the tunnel in the earthquake area was unaffected, and minor collapse incidents were observed at the entrances and exits of some tunnels. Here, roadbed damage accounted for > 75% of the total road damage, and the sections of the roadbed with damage levels above the medium level accounted for approximately 67% of the total damaged roadbed.
For highway-protection grades, traffic prohibition and restriction were directed toward roadbed sections, and the bridges and tunnels were left open for vehicular traffic. Few bridges were closed owing to collapse and landslide incidents; in particular, approximately 16% of the road sections were closed for vehicular traffic.
The Luding earthquake had varying degrees of impact on highway embankments, bridges, tunnels, and other structures, and the damage characteristics varied accordingly. Roadbed damage included cracking, barrier damage, roadbed collapse, roadbed burial, and washaway. Bridges, tunnels, and other structures witnessed minor cracking, whereas only a few bridges witnessed minor damage, which included beam displacement, support deformation, pier damage, expansion-joint damage, and guardrail damage. Spalling, falling blocks, and collapse incidents occurred at the entrances and exits of tunnels; however, the overall integrity of the tunnel remained acceptable.
4.1 Roadbed damage
(1) Roadbed and pavement cracking
Typically, roadbed cracking produces irregular tension–shear fractures, which causes ruptures, displacement, and sliding. Such cracking often occurs at the outside of roads with steep terrains and filled or walled sections. This is because, under the impact of an earthquake, the soil or retaining wall at the exterior of the highway slides or breaks, resulting in uneven settlement, further deformation, and roadbed damage. Such earthquake damage is common along rivers and steep terrains. Some typical earthquake damage photographs are presented in Fig. 6.
(2) Supporting structure damage
Notably, retaining walls and anchor cables (anchors) were identified as the primary supporting structures along the highway in the earthquake zone. An investigation on the seismic damage caused to the support revealed that the supporting-structure damage primarily occurred on the retaining wall, and the anchor cable remained almost undamaged. This was because the anchor cable was connected to the rock and soil mass with grouting and was subjected to motion and fluctuation together with the slope, resulting in an effective seismic resistance. However, the seismic resistance with the retaining wall alone was weak, and the damage was apparent. The primary forms of retaining-wall disruption included sliding, cracking, collapse, and spalling of the retaining wall.
As illustrated in Fig. 7, any retaining wall can collapse in two ways. In the first scenario, the upper slope cutting wall on the inner side of the highway (Fig. 7a) is subjected to an increasing pressure under the impact of the earthquake, degrading the stability of the slope. Consequently, the stress at the slope foot increases, and the earth thrust force destroys the retaining wall. In the second scenario, the soil on the outside of the highway deforms under the impact of the earthquake, resulting in the sliding of soil at the bottom of the wall and overall instability and collapse of the shoulder wall (Fig. 7b).
(3) Roadbed collapse
Roadbed collapse refers to the overall instability, sliding, and collapse of a roadbed, and it leads to further cracking along the roadbed. While its cause is identical to that of roadbed cracking, roadbed collapse is more severe. Such damage often occurs on the external side of the highway, e.g., free surfaces, half-excavation, half-filling, and roads along the river. Particularly, owing to the disruption of the roadbed and retaining wall, overall instability and collapse are common during earthquakes. Pictures representing the typical damage due to the Luding earthquake are presented in Fig. 8.
(4) Smashed and buried roadbed
Along the highways with numerous high and steep slopes, collapse and landslide incidents frequently occur on the inner side of the highway, consequently burying the roadbed (Fig. 9a).
Landslides occurring at high-altitude locations usually generate considerable potential energy. A higher kinetic energy results when the rock and soil fall to the road surface, posing a significant threat to the roadbed. When the outside of the roadbed is exposed to air, it is likely to cause overall instability, collapse, and even complete disruption of the roadbed (Fig. 9b). Moreover, owing to confined terrain conditions, roadbed repair is often complicated.
4.2 Bridge disruption
Among the 103 bridges surveyed, 89 (86%) were found to be slightly damaged or undamaged, 13 (13%) were moderately damaged, and one (1%) was severely damaged. All other bridges were undamaged, with two exceptions: one bridge in X067, which was buried by a collapsed slope, and one on the Wan Tung Cun Road, which was buried by a mudslide. Generally, the bridges in the earthquake suffered minor damage owing to their expansion joints and rubber buffer, which reinforce the seismic resistance and adjustability of the bridges.
Typically, seismic damage can be divided into direct and indirect damage based on the damage pattern. Direct seismic damage was found to be the major pattern in this earthquake, which resulted in the displacement of the pavement and beam body of the bridge under the action of seismic waves. When the displacement reached a certain threshold, small-scale damage (beam displacement, expansion-joint damage, guardrail damage (Fig. 10a), road cracks (Fig. 10b), support deformation, stop damage (Fig. 10c), etc.) could often be observed.
Indirect earthquake damage is often caused by disasters such as collapse, landslide, and debris flow that bury bridges and cause partial damage. In most cases, indirect damage causes minor disruption of the bridge but has a significant impact on its passage. Only two impassable bridges after the earthquake suffered indirect damage—one was buried by a mudslide (Fig. 4b) and the other by a landslide (Fig. 10d).
4.3 Tunnel disruption
Based on an inspection of nine tunnels and nine undamaged or slightly damaged bridges, we observed that archways of the bridges remained undamaged. Notably, water seepage and road-surface damage had occurred before the earthquake. Only a few tunnels underwent minimal damage, such as smashed pavements and burials due to collapse or landslides. The tunnel body was deeply buried; thus, it was integrated with the surrounding rock and soil mass, allowing it to better endure forces and fluctuations. The seismic resistance of the tunnels mitigated the adverse effects of the natural disaster.