According to the Sichuan Provincial Ganzi Prefecture Kangding "8·03" Geological Disaster Emergency Rescue Headquarters, in the early morning of August 3, at the section between Tunnel No. 1 and Tunnel No. 2 at Ridi on the Kangding–Luding segment of the Ya'an–Kangding Expressway, a bridge between tunnels collapsed after a sudden mountain torrent and debris flow. As of 23:00 on the 3rd, it has been verified that 4 vehicles carrying a total of 11 people fell in the Kangding-to-Ya'an direction of the Ya'an–Kangding Expressway, of whom 1 was rescued and 10 remain missing.

Figure 1 Accident site

N01. Accident Overview

The bridge involved in the accident is located at K120+200 on the Luding–Kangding segment of the G4218 Ya'an–Yecheng Expressway, between Tunnel No. 1 and Tunnel No. 2 at Ridi, spanning the Ridi Gully. The geographical location and bridge structure are shown in Figures 2 and 3.

Figure 2 Schematic diagram of the accident location

Figure 3 Structural schematic diagram

According to relevant information, the mountain body upstream of Ridi Gully is steep, with the highest point at an elevation of nearly 6,000 meters and the lowest point at the gully mouth at less than 1,700 meters, a difference in elevation of more than 4,000 meters. This area is a region where debris flow disasters occur frequently; historically, four large-scale debris flows have occurred. During the 1952 debris flow, the "head" of the flow reached as high as 7 meters when it emerged from the gully, sweeping away a "Tianxian Bridge" about 10 meters long with a clear height of 6 meters, demonstrating extremely strong destructive power.

N02. Accident Site Analysis

(1) Accident debris

According to publicly available online information, the condition of the bridge debris after the accident is shown in Figure 4. It can be observed that the main girder of the left carriageway collapsed, and part of the main girder is visible at the site after the accident; the piers at higher terrain appear basically intact. The main girder of the right carriageway is no longer visible at the site, and the piers are damaged. It can be seen from the figure that the reserved debris flow channel at this road section was not severely damaged and can still drain normally. The travel path of the debris flow did not coincide with the reserved path.

Figure 4 Schematic diagram of the debris and structural schematic diagram after bridge collapse

Combined with local reports, at the same time as the bridge collapsed, Ridi Village at the mouth of Ridi Gully was struck by the debris flow. As of August 5, this had caused 4 deaths and left 23 missing, with more than 939 people relocated and resettled. The site conditions are shown in Figure 5.

Figure 5 Schematic diagram of the debris after the Ridi Gully debris flow impact

Based on the above information, this bridge accident was a bridge collapse caused by debris flow impact. It is inferred that the main girder and piers of the right carriageway may have directly suffered debris flow impact and been damaged, further impacting the piers of the left carriageway near the drainage channel side, causing the piers to be destroyed and leading to the collapse of the left carriageway. The detailed causes of the debris flow and whether the drainage channel was functioning normally require further disclosure of information.

(2) Secondary accidents

According to descriptions by survivors after the accident, there was considerable smoke and dust inside the tunnel after the accident occurred, and vehicles were traveling relatively slowly. When approaching the tunnel portal, they found that the vehicle ahead had suddenly disappeared; they braked urgently and stopped the following vehicles, preventing subsequent vehicles from continuing to fall. Because the bridge accident was discovered in time, a serious secondary accident was avoided and casualties were reduced.

N03. Disaster Prevention for Bridges in Areas Prone to Geological Disasters

(1) Bridge site selection should avoid high-risk debris flow areas as much as possible

According to the "Code for Investigation of Debris Flow Disaster Prevention and Control Engineering DZ/T 0220-2006" and the "Code for Geological Survey of Highway Engineering JTG C20-2011", detailed engineering geological surveys should be conducted for areas along the route when selecting bridge sites. After obtaining basic information such as the composition type of solid debris flow materials, occurrence frequency, possible scale, fluid properties, and mode of action through surveys, the areas should be classified according to hazard into safe zones, affected zones, dangerous zones, and extremely dangerous zones. When selecting bridge sites, high-risk debris flow areas should be avoided as much as possible. If, due to route constraints or other reasons, a bridge must be located in a debris flow risk area, corresponding mechanical design verification must be carried out and the requirements for debris flow prevention facilities must be raised, such as designing drainage channels with higher conveyance capacity.

(2) Selection of pier cross-section type should focus on resistance to rockfall impact

In debris flow disaster areas, the selection of bridge piers should take into account the piers' resistance to fluid impact and rockfall impact.

Regarding resistance of piers to fluid impact, according to the formula for the overall impact force of debris flow given in Appendix C of the "Code for Design of Debris Flow Prevention and Control Engineering (Trial) T/CAGHP 021-2018":

Under the same pier width and debris flow fluid impact, due to different shape coefficients, square pier columns will receive greater direct impact force. Therefore, circular piers are relatively more advantageous when fluid impact force is the dominant consideration.

Regarding resistance of piers to rockfall impact, the shape of the pier column cross-section is unrelated to the magnitude of the impact force. Simple calculations show that, under the same cross-sectional dimensions (with the circular diameter equal to the square side length) and the same amount of reinforcement, a square cross-section is less likely to reach the critical failure point of the M-N curve when subjected to large impacts such as rockfall, and square piers also allow more flexible reinforcement design and arrangement than circular piers, with relatively more space, giving them certain advantages against rockfall impact.

Therefore, when selecting the cross-section type of pier columns, the geological disaster conditions must be fully considered, the characteristics of the pier cross-section fully utilized, and a comprehensive selection made.

Figure 6 Schematic diagram of impact resistance of different types of bridge piers

(3) Layout of bridges between tunnels

For tunnel-to-bridge sections, since they are generally located in canyon and gully areas, it is recommended to increase the single-span length of the bridge and reduce the number of piers to lower the probability of being impacted by debris flows.

N04. Recent Bridge Accidents Caused by Debris Flows and Other Factors

(1) Collapse of Bridge No. 6 on the Da-Wan Section of the En-Guang Expressway

In the early morning of July 6, 2023, a sudden rock collapse occurred on the mountain body around Bridge No. 6 on the Wanzhou–Dazhou direction of the Da-Wan Section of the G5012 En-Guang Expressway. A huge rock destroyed the bridge piers, causing part of the bridge deck to collapse. The accident bridge is located at K230+679 on the Wanzhou–Dazhou direction of the Da-Wan Section of the G5012 En-Guang Expressway. Pier No. 1 on the east side collapsed, and girders No. 1 and No. 2 fell. The debris at the site is shown in Figure 7.

Figure 7 Schematic diagram of the collapse accident of Bridge No. 6 on the Da-Wan Section of the En-Guang Expressway

(2) Damage to the Water Highway in Jinkouhe Grand Canyon due to rockfall

On July 21, 2024, continuous rainfall caused frequent rockfalls along the Jinkouhe section of G245, damaging the road to varying degrees. The accident occurred during the daytime, and traffic control was implemented very quickly, resulting in no casualties. The road damage is shown in Figure 8.

Figure 8 Diagram of road damage on the water highway in Jinkouhe Grand Canyon

(3) The Pingmi Bridge at Huilongmen, Changle Town, Miluo, Yueyang, Hunan, was broken by floodwaters

At 5 p.m. on July 1, 2024, the Pingmi Bridge at Huilongmen, Changle Town, Miluo, Yueyang, Hunan, was overtopped by floodwaters. Under the sustained violent impact of the flood, the 90-meter section of the 180-meter-long old Pingmi Bridge was washed away. Fortunately, the accident caused no casualties. The bridge piers were circular piers. The bridge washout is shown in Figure 9.

Figure 9 Collapse of the Pingmi Bridge in Changle Town, Miluo, Yueyang, Hunan

(4) Collapse accident of the Pengshan Minjiang Bridge

The Pengshan Minjiang Bridge is a transportation hub connecting the main urban roads of Pengshan District to the provincial scenic areas of Pengzu Mountain and Huanglongxi as well as surrounding cities. Due to continuous scouring of the Minjiang Bridge foundation by floodwaters, the bridge deck of the Minjiang Bridge shifted. At 13:00 on July 27, 2018, Pier No. 13 first experienced girder falling; after the superstructure of Span 13 fell, Pier No. 14 was subjected to lateral horizontal thrust and vertical compressive stress, causing Pier No. 14 to collapse, which in turn triggered progressive collapse of the bridge, as shown in Figure 10.

Figure 10 Collapse of the Pengshan Minjiang Bridge

(5) Collapse of the Saji River Bridge in Japan

Flood damage is the main cause of bridge collapse and also occurs frequently in Japan and the United States. On local time August 16, 2023, in Tottori Prefecture, Japan, Typhoon Lan struck the area and a bridge over the Saji River collapsed.

Figure 11 Collapse of the Saji River Bridge in Japan

N05. Conclusions and Recommendations

The number of highway bridges in China has exceeded 1 million, and much transportation infrastructure has extended into mountainous areas where geological disasters occur frequently. Relying entirely on raising design standards to resist natural disasters such as debris flows and floods is too costly and difficult to achieve. With such a huge number of bridges in China, probabilistically speaking, natural disasters occurring within a short period are almost inevitable. For example, a design flood frequency of once in 300 years may seem to have a low probability for a single bridge, but considering that China has more than 1 million highway bridges, the probability of natural disasters occurring each year is very high, and severe cases will cause bridge collapse.

In the future, human subjective factors such as structural defects causing bridge collapse should be reduced as much as possible. On the other hand, for bridge collapses caused by natural disasters, information technology should be applied to achieve **time identification and early warning and to block traffic, thereby reducing casualties from secondary disasters. Specifically, the following recommendations are made:

(1) When selecting bridge sites, the scale and discharge of debris flows should be estimated based on engineering geological survey results, and high-risk areas should be avoided as much as possible. When economically feasible, the bridge should directly cross the area to reduce the probability of direct action by natural disasters such as debris flows.

(2) In areas where rockfalls occur frequently or where piers are otherwise likely to suffer direct large impacts, if the area cannot be directly crossed, square pier columns are recommended to improve the structure's resistance to rockfall impact.

(3) Affected by global warming, the intensity of natural disasters such as floods and debris flows is increasing. It is recommended to update design parameters such as design flood levels based on ** statistical data and to conduct safety assessments of bridges in mountainous areas.

(4) To avoid secondary disasters, bridge collapse warning devices based on the Internet of Things should be installed where conditions permit, so that after a bridge collapses, they can issue sound or flashing warning signals in **time, making it easier for drivers to promptly identify abnormalities on the bridge deck.

(5) Inspection of the effectiveness of disaster prevention facilities near important structures should be strengthened to avoid situations in which facilities cannot serve normally, for example by ensuring that drainage channels remain unobstructed and free of siltation at all times.

The above was completed through collaboration by Professor Peng Weibing of Zhejiang University of Technology and graduate students Zhu Zhixiang, Shen Zenan, Zhu Yuting, Yao Lefan, Shao Yuchun, and Fan Jiaxu.