Analysis of tunnel collapse disasters during operation and exploration of disaster damage mechanisms

Highway tunnel collapse during the operation period is a serious threat to the stability of tunnel operation. Thus, it is important to investigate the disaster-causing mechanism to prevent the occurrence of disasters. In this paper, based on 155 cases of domestic and foreign highway tunnel collapses during operation, we statistically analyze the causes of the collapses and the year of occurrence, and we explore the disaster-causing mechanism of tunnel collapses through numerical simulation. Tunnel collapses in operation period are affected by geological factors, lining factors, and accidental factors. The primary causes of tunnel collapses are determined to be the post-lining cavity, lining crack expansion, and fire. A single factor cannot lead to tunnel collapse, and the probability of collapse increases under the coupling of multiple factors. Through a numerical simulation method, the evolution mechanism of tunnel collapse caused by the post-lining cavity is analyzed, which lays the foundation for establishing the tunnel collapse prediction model and qualitative risk assessment system of highway tunnel collapse.


Introduction
Tunnel operational safety has been a research hotspot in several countries across the world.Tunnel operational disasters frequently occur due to factors such as the service time of tunnels, complex geological conditions, lining cracking, and occasional loading.Transportation infrastructure in many cities in the United States, European countries, and Japan are facing serious service safety hazards, and these countries have initiated a number of strategic research activities to improve service safety of transportation infrastructure in recent years.With economic development and infrastructure improvement, China's tunnel construction has made 1333 (2024) 012049 IOP Publishing doi:10.1088/1755-1315/1333/1/012049 2 great achievements.The growth of the operating life of China's tunnels and the rapid increase in the number and mileage of tunnels have caused many serious accidents of significant damage or even collapse of tunnels during the operating period.However, the collapse mechanism is still unclear.This has facilitated the need for research on the evolution mechanism of structural instability and collapse disaster-causing mechanism of long tunnels, in addition to the technology of evaluating the service performance for the safety of tunnels during operation period.
Many domestic scholars have studied the disaster mechanism of tunnel collapse.Wang and Zhu [1] statistically analyzed a large number of tunnel collapse cases, and they highlighted that engineering geological conditions, tunnel section form, groundwater, tunnel depth, blasting disturbance, etc. are the main factors affecting tunnel collapse.Zhou and Wu [2] summarized the mechanism of tunnel collapse, established an accident tree for tunnel collapse analysis, and quantitatively analyzed the potential collapse factors in the tunnel.Liu Xuezeng (2010) statistically analyzed a large number of cases of tunnel collapse, and highlighted that engineering geological conditions, underground water, and blasting disturbance were the main influencing factors.Liu Xuezeng (2010) and others evaluated the proportion of tunnel disaster accidents in 2006 and 2007 and concluded that tunnel collapse accounted for 64%, and they thus summarized the specific influencing factors of collapse.Jucai et al. (2013) used the attribute comprehensive evaluation system for the risk evaluation of tunnel collapse in mountainous areas, and they established the attribute identification model of tunnel collapse risk and used it in engineering examples.Wuli et al. ( 2014) combined 11 highway tunnels to perform a statistical analysis of the characteristics of cavities behind the lining, and summarized the main causes and general rules of cavities behind the lining.Zhang [3] statistically analyzed a large number of tunnel collapse cases and adopted the Monte Carlo method to study the distribution characteristics of tunnel surrounding rock pressure and summarize the tunnel collapse law and influencing factors.Based on the fuzzy polymorphic Bayesian network analysis method, Sun Jinglai (2019) obtained the collapse probability by constructing a tunnel collapse Bayesian network and compared it with the risk matrix results.Li et al. [4] summarized the root causes of tunnel collapse accidents and used the risk probability estimation method, and they combined accident tree and Bayesian network to assess the overall tunnel collapse risk.
At present, tunnel collapse research primarily focuses on the construction stage, and there are fewer studies on the risk of road tunnel collapse during the operation period.There is an insufficient understanding of the disaster-causing mechanism of tunnel collapse during the operation period mainly because the assessment of highway tunnel collapse risk is focused on the qualitative level, lack of research at the quantitative level, and high dependency on expert surveys, which is more subjective.With the growth of tunnel service life and the frequent occurrence of tunnel diseases, to further prevent the risk of tunnel collapse, study the disastercausing mechanism of tunnel structural collapse accidents, and establish a qualitative highway tunnel collapse risk assessment system.This paper investigates domestic and international cases of tunnel collapse during operation, statistically analyzes the causes of tunnel collapse and the year of occurrence, summarizes key factors affecting the collapse and the key information characterizing the disaster, discusses the main controlling factors of tunnel collapse disaster during operation, and performs a numerical simulation analysis of the impact of the cavity behind the tunnel lining on the cracking of the lining structure.The research results shed light on the potential risk factors of tunnel collapse disaster during the operation period, and they help reduce the probability of tunnel collapse disaster by controlling them one by one during the design, construction and operation periods.

Research on typical collapse cases of tunnels during operation period
Due to incomplete statistics, this research study could obtain 155 cases of typical collapse cases of tunnels in the operation period at home and abroad and limited to space.An overview of 10 typical cases of tunnel collapse in the operation period is listed in table 1.It is explained here that the stress redistribution of surrounding rock in the tunnel caused by large-area blocks or spalling of the tunnel lining has a significant impact on the overall stability of the tunnel structure and seriously threatens the safety of people and vehicles.If it cannot be handled in time, it may lead to further damage.Therefore, such accidents are also classified as research on the concept of tunnel collapse in this study.Following a fire and explosion in the tunnel in 1999, a large number of voids were discovered behind the lining, and the interior of the tunnel was severely structurally damaged, with parts of the walls and ceilings collapsing.4 1999 Torn Tunnel, Austria The fire that lasted 17 h and collapsed the tunnel by 600 m.

2001 St. Gotthard Tunnel, Switzerland
The truck collision triggered a fire that lasted 48 h and spread 2 km; the tunnel lining was severely damaged, and the roof slab collapsed in large chunks.

2002 Catskill Highway Tunnel
The fire led to the dislodgement and breakage of about 100 meters of tunnel wall tiles and concrete surfaces, resulting in serious economic losses.The collapse of approximately 130 m of concrete in the tunnel vault was mainly due to residual stresses in the anchoring bolts from the March 2011 earthquake and routine safety inspections that were not thorough.
10 2020 Ya'an Yucheng District, Qingbishan Tunnel Continuous rainfall led to a localized increase in the pressure on the surrounding rock, and about 10 m 2 of concrete in the tunnel vault lining fell off and collapsed.

Causes of tunnel collapse during operation
A total of 155 cases of highway tunnel collapses during operation were investigated, including 101 cases abroad and 54 cases in China.The causes of tunnel collapse were categorized into three major groups: geological causes, lining causes, and accidental causes, considering the tunnel collapse cases caused by hidden dangers in the design, construction, and operation periods, we drew the statistical charts of the causes of domestic and foreign tunnel collapse cases during the operation period (figures 1 and 2).It can be seen that geological and accidental causes of tunnel collapse accounted for 34% and 31%, respectively; lining causes accounted for 19%, and hidden dangers during design, construction, and operation accounted for 16%.Further refinement of various causes, geological causes including cavities behind the lining, stability of surrounding rocks, poor geological bodies, groundwater, bias pressure, high geostress, etc.; lining causes include lining crack expansion, unreasonable lining design parameters, poor construction quality, lining aging, etc.; accidental causes include fire, combustion and explosion, earthquakes, heavy rains, mudslides, etc., and draw a statistical chart of the causes of domestic and international tunnel collapse cases in the operation period.(Figs. 3 and 4).It can be seen that among the causes of the tunnel collapse, the cavity behind the lining accounts for 17%, lining crack expansion accounts for 14%, fire, combustion and explosion accounts for 15,% and poor geological bodies account for 11%.With regard to the cases of tunnel collapse caused by geology, lining, and accidental causes, the occurrence of tunnel collapse can be effectively avoided only by exploring the collapse mechanism.The geological, lining, and accidental causes of tunnel collapses during the operation period across the world were analyzed in figures 5, 6, and 7.It was initially determined that the main causes of tunnel collapses were hollowing out after lining, lining crack expansion, and the impact of fire and combustion explosion.Geological causes and lining causes often affect each other, for example, the cavity behind the lining may lead to lining cracking, the exposed reinforcing bars are corroded, under the action of groundwater and other lining cracks continue to expand, and ultimately the concrete in the pressure zone crushed and fall off.Because fire and other incidental factors cannot be easily avoided, this paper mainly discusses the impact of the cavity behind the lining and the expansion of lining cracks on the collapse of the tunnel.
As seen from the case studies, the causative factors of tunnel collapse during the operation period are complex; a single factor does not easily lead to tunnel collapse, and the coupling of multiple factors leads to an increased probability of tunnel collapse.The analysis shows that foreign tunnels were constructed earlier, and the cases of tunnel collapse during the operation period have occurred before 2000, accounting for 20%.In recent years, with the growth in tunnel service life, the safety problems associated with the operational tunnels have increased, and they include cavity behind the lining becomes larger, lining cracking, water seepage and leakage, and lining aging is serious, etc., and the cases of domestic and foreign tunnels tunnel collapse have increased, and the cases of tunnel collapse during the operation period accounted for 41% from 2015 to the present.Thus, the study of highway tunnel collapse during the operation period is highly significant.

Analysis of effect of cavities behind tunnel lining on cracking of lining structure
Through the case study, it is initially determined that the primary causes of tunnel collapse during the operation period include the cavity behind the lining, lining crack expansion, and fire, and this section conducts a study on the impact of the cavity behind the lining on the lining structure crack damage.Subject to the constraints of surrounding rock factors and construction technology, a cavity is created behind the lining structure, and the lining structure in this part loses constraints.Further, the lining is subjected to the reaction force of the ground, which may lead to the deformation of this part of the lining structure in the direction of the cavity.

Modeling
This chapter considers a 16.34-m long tunnel in Yunnan as an example.The geological conditions in Yunnan are more complicated, and the phenomenon of hollows behind the lining is more common.As the number of operating years increases, the tunnel is prone to diseases and other problems.The finite element analysis is performed using the software ABAQUS, wherein the lining behind the cavity under the lining structure, cracking disease generation rules and lining structure damage pattern are simulated.

Analysis of model calculation results
Through the research and analysis, it is observed that the cavity mostly exists in the arch top part and arch waist part, and thus, the location of the cavity behind the lining structure is determined as 0 °, 22.5 °, 45 ° (the center of the cavity and the center of the tunnel connecting line with the angle of the plumb line, i.e., 0 ° for the top of the arch) three working conditions for analysis.As seen from the finite element cloud map, obvious cracks appeared in the presence of cavities behind the lining, and the extent of the plastic zone appeared to be highly overlapped with the void area.This did not have a significant impact on the lining around the cavities.3.2.2Lining stress analysis.According to Min et al. [5], it can be observed that the peak value of structural stress and deformation changes caused by the cavity behind the tunnel lining is located in the part where the cavity exists, and the cracks primarily appear in the center of the cavity protruding outward from the part of the lining and the edge of the cavity at the place of concentration after the stress redistribution.Therefore, the inner surface node P at the location of the main crack is selected for stress and deformation analysis.The stress variation relationship of node P under the impact of the cavity behind the lining was analyzed to extract the stresses on the path within the cavity at the location of point P (Fig. 15).The maximum principal stress of the lining outside the cavity boundary reaches the maximum value when there is a cavity behind it, and the lining stress at the middle of the cavity is similar to the value when there is no cavity.Further, there is a clear gradient in the value of change in maximum principal stresses in the lining when the cavity is at different angles, with the largest change in stresses of 1.8 MPa in the presence of a cavity at the top of the arch, followed by 22.5°, and the smallest change when the cavity is at the 45° position.

Lining deformation analysis.
The relationship between the deformation variations of node P under the impact of the cavity behind the lining is analyzed, and the displacement on the path within the cavity at the location of point P is extracted (Fig. 16).When there is a cavity behind the lining, the maximum displacement change of the lining in the center of the cavity reaches the maximum value, and the displacement of the lining at a distance of 3 m outside the cavity is close to the value when there is no cavity.There is a clear gradient in the maximum displacement change value of the lining when the cavity is at different angles, with the largest displacement change of 1.1 cm in the presence of the cavity at the top of the arch, followed by 22.5°, and the smallest change when the cavity is at the 45° position.
According to the results of 3D modeling and case studies, the presence of a cavity in the back of the arch leads to the cracking of the concrete structure and deformation of the lining.However, this may not solely lead to the collapse of a large tunnel in the operation period.In reality, the collapse of a large tunnel is often accompanied by abundant groundwater, and the mechanism of tunnel collapse caused by the cavity behind the lining is determined as follows: the cavity in the back of the arch leads to cracking of the lining, and the exposed reinforcement bars are corroded, which may result in the deterioration of the lining and the redistribution of stress in the surrounding rock, and the risk of tunnel collapse if not dealt with in a timely manner.

Conclusion
This paper focuses on the frequent occurrence of road tunnel collapse disasters with the increase in service life at home and abroad, on the basis of case study and numerical simulation to explore the disaster mechanism of tunnel collapse.The main conclusions of the study are as follows: (1) The causes of road tunnel collapse during operation in several countries across the world are categorized into three major groups: geological causes, lining structure causes, and accidental causes.The main causes of disasters are post-lining cavities, lining crack expansion, and fire effects.Further, a single factor is not sufficient for tunnel collapse, multi-factor coupling under the collapse risk increases.
(2) In recent years, with the increase in operation years, the number of cases of tunnel collapse has increased both at home and abroad, and thus, it is necessary to study the tunnel collapse during the operation period of highway tunnels.
(3) We use Abaqus software to analyze the disaster-causing evolution mechanism of tunnel collapse damage caused by post-lining cavities.
In the future works, the development trend of tunnel collapse disasters of different causes will be predicted and analyzed, and the critical conditions and judgment indicators for the occurrence of tunnel structural collapse disasters will be established.
The tunnel's vault concrete, weighing about 12 tons, suddenly fell, resulting in a serious accident that killed one person and injured another.82008 Kunming Shipaling TunnelContinuous heavy rainfall and flash floods led to severe scouring and erosion of the soil and rock bodies inside the Shipai Ling Tunnel, and the supporting structure lost its stability, which eventually triggered the collapse of the entire tunnel.92012Sasago Tunnel on the right side of the Chuo Expressway in northeastern Japan.

Figure 1 .
Figure 1.Bar chart of causes of tunnel collapse cases during operation in China and abroad.

Figure. 2
Figure. 2 Pie chart of causes of tunnel collapse cases during operation in China and abroad.

Figure 3 .
Figure 3. Refined bar chart of causes of tunnel collapse cases during operation in China and abroad.

Figure 4 .
Figure 4. Refined pie chart showing the causes of tunnel collapse during operation in China and abroad.

Figure 5 .
Figure 5. Geological causes of tunnel collapse during operation in China and abroad.

Figure 6 .
Figure 6.Lining causes of tunnel collapse during operation in China and abroad.

Figure 7 .
Figure 7. Incidental causes of tunnel collapse during operation in China and abroad.
The research obtained 101 foreign and 54 domestic cases of tunnel collapse during the operation period and categorized them according to the year of collapse.We can obtain 31 cases before 2000, all of which are foreign cases; eight cases from 2000 to 2005, of which six are foreign cases and two are domestic cases; 17 cases from 2005 to 2010, all of which are foreign cases; and 36 cases from 2010 to 2015, of which 25 are foreign cases and 11 are domestic cases; 63 cases from 2015 to present, of which 25 cases are foreign and 38 cases are domestic.Statistical charts are drawn to categorize domestic and foreign cases of tunnel collapse during the operation period by year (Figs. 8 and 9).

Figure 8 .
Figure 8. Bar chart of domestic and international cases of tunnel collapse during operation by year.

Figure 9 .
Figure 9. Pie chart of domestic and foreign cases of tunnel collapse during operation by year.

Figure 10 .
Figure 10.Finite element modeling of tunnel lining arch back cavities.

1 .
Finite element cloud map analysis.

Figure 13 .
Figure 13.Schematic at point P.Figure 14. Stress deformation extraction path at point P.

Figure 14 .
Figure 13.Schematic at point P.Figure 14. Stress deformation extraction path at point P.

Figure 15 .
Figure 15.Stress variation at node P.Figure 16.Variation of displacement of node P.

Figure 16 .
Figure 15.Stress variation at node P.Figure 16.Variation of displacement of node P.

Table 1 .
Cases of tunnel collapse during operation.