Recently, the No. 6 Bridge of the Da-Wan section of the Enshi-Guangyuan Expressway in Dazhou, Sichuan, and the Sibu River Railway Bridge in Wanzhou, Chongqing, collapsed respectively. Combining codes and engineering experience, this paper attempts to analyze the possible causes of the accidents, providing reference for engineering operation and maintenance and new construction projects.

I. No. 6 Bridge of the Da-Wan Section of the Enshi-Guangyuan Expressway

1.1 Accident Overview

In the early morning of July 6, 2023, a sudden rockfall occurred on the mountain slope around the No. 6 Bridge on the Wanzhou-to-Dazhou direction of the Da-Wan section of the G5012 Enshi-Guangyuan Expressway. Boulders destroyed the bridge piers, causing part of the bridge deck to collapse. The accident caused 2 vehicles to be damaged and then spontaneously combust, and 3 people were injured, with no fatalities.

The accident bridge is located at K230+679 on the Wanzhou-to-Dazhou direction of the Da-Wan section of the G5012 Enshi-Guangyuan Expressway. It was completed and opened to traffic on December 18, 2012. The detailed bridge location is shown in the figure. The superstructure of the bridge consists of skewed double-width prefabricated simply supported T-beams, and the substructure consists of columnar piers. Pier No. 1 is approximately 35 m high, and the span arrangement is 3×40 m, as shown in Figure 2.

According to on-site photos and video materials, the accident section has the following characteristics: (1) Pier No. 1 has a bent cap at the top of the pier, but no intermediate transverse tie beam is provided on the pier body, which may lead to weak pier stability. (2) Pier No. 1 is located at the bottom of a valley. This arrangement facilitates the application of standard bridge spans and also provides sufficient space for pier construction, facilitating construction design. However, the valley bottom near the steep slope bank is susceptible to dangerous rockfall, and similar site selection should be avoided for bridge piers.

Figure 1 Schematic diagram of the accident bridge location

Figure 2 Elevation view of the accident bridge

1.2 Damage to the Accident Bridge

According to the damage at the accident site, the outer Pier No. 1 on the east side collapsed, and girders No. 1 and No. 2 fell. The on-site wreckage is shown in Figure 3. The damage to the main components of the collapsed bridge section is as follows:

1) Damage to the substructure: The outer pier fractured into two sections, with obvious punch-through shear failure marks observed on the fracture surface. The pier top separated from the bent cap, and the main body of the pier remained basically intact except at the fracture location. For the inner pier, the pier top separated from the bent cap, it toppled toward the west, and part of the concrete on the pier body was crushed.

2) Damage to the superstructure: Girder No. 1 fell onto the rock surface on the south side and fractured into multiple sections along the mountain trend; Girder No. 2 fell to the valley bottom on the west side.

Figure 3 On-site wreckage of the accident bridge

According to the on-site wreckage, the cross-section of the outer pier is flat, exhibiting the characteristics of punch-through shear failure under impact. A similar failure mode can be seen in the 2020 vehicle collision accident on the Haizhang Expressway, where a pier fractured under the impact of a large equipment transport vehicle, as shown in Figure 4. Based on this, it is inferred that this accident was caused by recent heavy rainfall in the area, which loosened and collapsed rocks along the riverbank on the south side. The collapsed rocks impacted the outer Pier No. 1, causing the pier to fracture and pushing over the inner pier, thereby triggering the overall collapse of the bridge, as shown in Figure 5.

Figure 4 Schematic diagram of the punch-through shear failure surface

Figure 5 Schematic diagram of the collapse location of the accident bridge

Based on the above analysis, the reconstructed failure process is shown in Figure 6.

Figure 6 Schematic diagram of the collapse location of the accident bridge

1.3 Punch-through Shear Failure of Piers Caused by Rockfall Impact

1) Punch-through shear failure caused by high-speed impact: From the on-site photos of the accident, the failure cross-section of the outer pier column is flat, exhibiting the characteristics of punch-through shear failure. Under static action, however, punch-through shear failure occurs in members with a very small shear span ratio, while slender pier columns are more prone to flexural failure. As shown in Figure 7, it can be preliminarily judged that the pier column underwent shear failure under the high-speed impact of rockfall.

Figure 7 Schematic diagram of failure modes of long and short columns

2) Tie beams should be provided for flexible column-type high piers: According to the Seismic Design Details for Highway Bridges, for areas with seismic intensity 8, columnar piers and bent piers with a height greater than 7 m should be provided with transverse tie beams. According to relevant engineering experience, in the design of columnar piers, when the pier height is 7–20 m, one transverse tie beam between pile tops should preferably be provided; when the pier height is 21–30 m, one tie beam between columns should be provided; when the pier height is 31–40 m, two tie beams between columns should be provided. The pier height of the accident bridge is about 35 m, and no transverse tie beam was provided between the two pier columns. Whether tie beams are provided is not directly related to this collapse accident, and this is raised only as a point requiring attention in design.

1.4 Rockfall Prevention Measures

1) Bridge site selection should avoid dangerous rockfall sections as much as possible.

When avoidance is not possible, the impact of dangerous rockfall on tunnels, subgrades, bridges, and other works should be comprehensively compared and selected, and the route scheme with the least impact should be chosen.

2) Rockfall prevention and control should be well implemented in dangerous rockfall sections.

Rockfall prevention and control technical measures can be divided into two major categories: active protection technology systems and passive protection technology systems. I. The prevention concept of active protection technology is to enhance the stability of dangerous rock masses and prevent their collapse, so as not to cause disasters. Commonly used methods include: support, anchoring, filling, inlay patching, drainage, etc.; II. The prevention concept of passive protection systems is to assume that dangerous rock will collapse, but provide protection by preventing rockfall from reaching the threatened object range. Commonly used methods include: setting up rockfall retaining walls, rockfall nets, etc.

3) A case of bridge site selection in a rockfall area is as follows:

Within the bridge site range of a certain extra-large bridge, there are 4 dangerous rock masses in total. Dangerous rock masses No. 1 and No. 2 have little impact on the project, while dangerous rock masses No. 3 and No. 4 are relatively close to the bridge site, posing major safety hazards during construction and operation. As shown in Figure 8.

Figure 8 Schematic diagram of bridge site selection affected by dangerous rock

Dangerous rock mass No. 3 is the main control area of the geological hazard. At the fishpond location, the dangerous rock mass is relatively close to the alignment, posing a safety hazard. To ensure safety, the alignment was slightly adjusted eastward during the construction drawing stage. According to the rockfall trajectory simulation mapping of dangerous rock mass No. 3, the bridge was shifted 8 m eastward to reduce the disaster impact of rockfall on the bridge, as shown in Figure 9.

Figure 9 Schematic diagram of bridge site shift due to dangerous rock

After adjustment, the distance from the most dangerous rock mass is about 15 m or more. The right line alignment was appropriately adjusted, and the circular curve radius changed from 900 m in the original preliminary design to 800 m. Route selection and bridge site determination followed the basic principles of geological route selection, appropriately reducing local horizontal curve alignment indicators, staying away from dangerous rock masses, and ensuring bridge safety.

II. Sibu River Railway Bridge in Wanzhou, Chongqing

2.1 Accident Overview

On the morning of July 4, 2023, some piers of the Sibu River Railway Bridge between Yubeishan Station and Luotian Station on the Wanliang Railway collapsed, causing part of the railway bridge to collapse, as shown in Figure 10. At present, trains passing through this section have been arranged to detour, turn back, or suspend operation. The bridge collapse caused no casualties.

Figure 10 Site before and after the collapse of the Wanzhou Sibu River Railway Bridge

The Sibu River Railway Bridge is a single-track bridge located between Yubeishan Station and Luotian Station on the Wanliang Railway. According to relevant data, the total bridge length is 375.45 m, and the span combination is 3×24+1×32+60+108+60+1×32 m, with 8 spans in total. The bridge bottom width is about 60 m, the main pier height is 105.3 m, and the bridge height is 113 m. The accident section consists of the main bridge part of a three-span continuous beam and the approach bridge part of a four-span simply supported precast T-beam bridge. Detailed bridge location and bridge information are shown in Figure 11.

Figure 11 Schematic diagram of spans of the Wanzhou Sibu River Railway Bridge

2.2 Damage to the Accident Bridge

At the time of the accident, obvious landslide traces occurred on part of the mountain in the Yichang direction. It is speculated that the landslide soil pushed over Piers No. 4 to No. 7, causing the approach bridge section to completely collapse. After the collapse, scattered girder bodies and pier wreckage were visible on site, and the railway track was suspended in midair; after Pier No. 4 collapsed, the three-span continuous beam of the main bridge did not collapse, and the side span was suspended, as shown in Figure 12. Further cause analysis requires the collection of clear photos and damage traces at the collapsed piers.

Figure 12 Schematic diagram of bridge collapse

2.2 Damage to the Accident Bridge

At the time of the accident, obvious landslide traces occurred on part of the mountain in the Yichang direction, as shown in Figure 13. It is speculated that the landslide soil pushed over Piers No. 4 to No. 7, causing the approach bridge section to completely collapse. After the collapse, scattered girder bodies and pier wreckage were visible on site, and the railway track was suspended in midair; after Pier No. 4 collapsed, the three-span continuous beam of the main bridge did not collapse, and the side span was suspended, as shown in Figure 14. Further cause analysis requires the collection of clear photos and damage traces at the collapsed piers.

Figure 13 Landslide traces at the collapse site

Figure 14 Schematic diagram of bridge collapse

According to officially published local meteorological data, Wanzhou District of Chongqing had sustained rainfall for many days, and the bridge location was hit by heavy rain. Many townships around the Sibu Bridge received extremely high rainfall, and the maximum rainfall measured at a nearby weather station was 261.2 mm, exceeding the historical value. Continuous rainfall leading to landslides may be the main cause of this accident.

2.3 Landslide Prevention and Control

To prevent the adverse effects of landslides on engineering structures, relevant code provisions and engineering measures for landslide prevention and control were collected and organized.

1) Bridge sites should avoid landslide areas. According to the Code for Geological Survey of Highway Engineering, routes should avoid landslide and landslide group sections that are large in scale, complex in nature, poor in stability, and difficult to treat; when the landslide scale is small and the treatment scheme is technically feasible and economically reasonable, the route should pass through a safe location conducive to landslide stability; when the route passes through a landslide section, the slope toe must not be excavated, and it should not pass above the landslide body in the form of fill; in specific selection, it should be determined after comprehensive analysis and comparison based on actual geological conditions.

2) Landslide prevention and control measures. The occurrence of landslides is often closely related to the action of water, and the action of water is often the main factor causing landslides. Landslide treatment can be approached from the following two major aspects:

a) Eliminate and reduce the harm of water to slopes. By reducing pore water pressure and hydrodynamic pressure, preventing softening and dissolution decomposition of rock and soil, and eliminating or reducing the scouring and wave impact effects of water. Commonly used methods include: ① horizontal drilling drainage; ② vertical hole drainage; ③ shaft pumping; ④ tunnel drainage; ⑤ supporting blind ditches.

b) Improve the mechanical strength of slope rock and soil. Through certain engineering technical measures, improve the mechanical strength of slope rock and soil, increase its anti-sliding force, and reduce the sliding force. Commonly used measures include: ① slope cutting and load reduction, using reduced slope height or gentler slope angle to improve slope stability; ② building retaining walls, protective walls, and other retaining structures for unstable rock masses; ③ reinforced concrete anti-slide piles or reinforced pile supports as anti-sliding support works; ④ prestressed anchor rods or anchor cables, suitable for reinforcing rock slopes with fissures or weak structural planes; ⑤ consolidation grouting or electrochemical reinforcement methods to strengthen the strength of slope rock or soil; ⑥ SNS flexible slope protection technology, etc., as shown in Figure 15.

Figure 15 Schematic diagram of landslide prevention and control measures

III. Bridge Collapse Accidents Caused by Landslides and Rockfalls

Bridge collapse accidents caused by geological disasters such as landslides and rockfalls have occurred repeatedly:

(1) Xikang Expressway bridge collapse (bridge collapse caused by landslide)

At 9:24 on September 16, 2014, a large-scale landslide occurred at K1056+100M in the Ankang-to-Xi'an direction of the Baotou-Maoming Expressway (north entrance of Angou Tunnel), causing about 30 m of the Luoqiangwan Bridge (Xi'an direction) in the Ankang-to-Xi'an direction to collapse, and the north entrance of Angou Tunnel was completely buried; rockfall occurred on the road surface in the Xi'an-to-Ankang direction, and the deck of the Luoqiangwan Bridge (Ankang direction) was displaced.

Figure 16 Wreckage site of the Xikang Expressway

(2) Hanwang bridge collapse (bridge collapse caused by landslide)

From 14:00 on July 8 to 7:00 on July 9, 2013, Mianzhu City experienced extremely heavy rain. The maximum rainfall in Yuanbao Village, Qingping Township reached 435.4 mm. The peak flow of the main river, Shiting River, reached 2,710 cubic meters per second, and the flow of the Mianyuan River was 1,560 cubic meters per second. A debris flow of 200,000 cubic meters occurred in Yongjia Gully, Qingping Township, and landslides occurred in Yanjing Village and Yuanbao Village, forming slope debris flows. The disaster caused the collapse of the Hanwang Niubizi Bridge and the Xinglong Mianyuan River Bridge.

Figure 17 Wreckage site of Hanwang

(3) Collapse of the main span of the Hongnishi Arch Bridge in Pengshui County, Chongqing (bridge collapse caused by landslide)

On May 26, 2010, the main span of the Hongnishi Arch Bridge, about 90 m long in total and located about 5 km from Pengshui County town toward Wulong on National Highway 319 in Pengshui County, Chongqing, collapsed, and an engineering vehicle traveling on the bridge fell off the bridge.

Figure 18 Wreckage site of Pengshui

(4) Ya'an-Xichang Expressway (bridge collapse caused by rockfall)

On September 20, 2020, at around 12:00, a high-level collapse occurred at the Yaoheba Bridge at K2084 in the Chengdu-to-Xichang direction of the Ya'an-Xichang section of the G5 Beijing-Kunming Expressway. Rockfall caused the Yaoheba Bridge (a 2-span prestressed reinforced concrete T-beam bridge) to break and fall, interrupting two-way traffic on the Ya'an-Xichang Expressway. It was verified that there were no casualties on site.

Figure 19 Wreckage site of the Ya'an-Xichang Expressway

(5) Chediguan Bridge (bridge collapse caused by rockfall)

At around 5:00 on July 25, 2009, due to the impact of continuous heavy rain, the pier of the Chediguan Bridge on the Wenchuan section of National Highway 213 was broken by a huge rock, causing the bridge deck to be destroyed. Vehicles and people traveling on the bridge fell into the Minjiang River, National Highway 213 was interrupted, and 3 people were killed, 12 injured, and 7 vehicles damaged.

Figure 20 Wreckage site of the Chediguan Bridge

IV. Conclusions and Recommendations

1. When selecting bridge sites in mountainous areas, geological disaster areas with risks such as landslides and dangerous rockfalls should be avoided as much as possible; if unavoidable, effective engineering means should be adopted to improve the stability of the geological disaster area;

2. For such events, prevention and monitoring should be the main focus, with key attention to dangerous areas such as steep slopes, cliff edges, dangerous rock, and areas prone to debris flows, landslides, and rockfalls. Monitoring and early warning of geological conditions around mountain bridges should be strengthened, the stability status of surrounding geological hazard hidden points should be dynamically grasped, and a geological disaster response mechanism for mountain bridges should be established and improved to prevent accidents, promptly avoid secondary disasters, and eliminate safety hazards.