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Risk Assessment of Operation Period Structural Stability for Long and Large Immersed Tube Tunnel
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 166 (2016) 266 – 278
2nd International Symposium on Submerged Floating Tunnels and Underwater Tunnel Structures
Risk Assessment of Operation Period Structural Stability for Long and Large Immersed Tube Tunnel Mengjun Wua,b,c, *,QiZhang a,b,c,SenyangWua,b,c a
China Merchants Chongqing Communications Technology Research & Design Institute Co., Ltd., Chongqing 400067 b National Engineering and Research Center for Mountains Highways; Chongqing 400067 c National Engineering Laboratory for Highway Tunnel Construction Technology, Chongqing 400067
Abstract Immersed tube tunnel has been widely used in virtue of its characteristics of shallow burial, good waterproof performance, strong stratum adaptability, etc., however, it is also characterized by enclosed space, prolonged submersion of structure in deep water, complicated surrounding environment, etc., particularly, in case of accident of tunnel structure, inestimable losses will be caused, therefore, the control of structural stability risk in operation period is especially important. This article carries out risk source analysis and identification, scenario design and simulation analysis, risk loss judgment standards and weight research for structural stability of long and large immersed tube tunnel in operation period on basis of extra-large immersed tube tunnel of the fourth lane of Shenzhen-Zhongshan Thoroughfare, based on which and in combination with the relevant domestic and foreign risk assessment and expert research results, the structural stability risk levels and risk scores of structural stability in operation period are obtained by quantitative risk calculation method and the relevant suggestions for risk control are proposed. ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Peer-review under responsibility ofthe organizing committee of SUFTUS-2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016 Keywords:Long and Large Immersed Tube Tunnel, Operation Period, Structural Stability, Risk Assessment
1. Introduction As a tunnel construction method, immersed tube has the characteristics of shallow burial, high space utilization ratio, good waterproof performance, strong stratum adaptability, etc. and has been widely used in the construction of underwater tunnels. The world’s first immersed tube tunnel was constructed by America on Detroit River in 1910,
* Corresponding author. Tel.: +86-023-62653128; fax:+86-023-62653128. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016
doi:10.1016/j.proeng.2016.11.549
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
while the first immersed tube tunnel in our country was Hong Kong Hung Hom undersea tunnel constructed in 1972, subsequently, with the fast development of economy and progress of immersed tube construction technology of our country, immersed tube tunnels were applied more in river-crossing and sea-crossing projects, at present, there has been more than 20 immersed tube tunnels that are constructed or under construction in our country. The long and large immersed tube tunnel project is typically characterized by long axis line, enclosed space, dense vehicle traffic, large escape and rescue difficulties, etc., in addition, the structure will be submerged in deep water for a long period, the surrounding environment is complicated, and there are many factors affecting the structural stability and safety, particularly, in case of accident of tunnel structure, inestimable losses will be caused. Risk assessment can reasonably analyze these uncertainty factors and convert the unforeseeable risk factors into quantitative indexes to provide bases for risk control and relevant management. At present, for tunnel risk assessment, many domestic and foreign scholars have carried out a lot of researches on construction period risk assessment[4-11] and relevant guidelines or regulations have been formed since American Einstein.H.H[1-3]introduced the risk analysis theory into tunnel and underground projects and put forward the characteristics of tunnel project risk analysis and the ideas that shall be followed. In general, tunnel construction period risk assessment has been fully popularized and the assessment methods and technologies are relatively mature. Compared to construction period risk assessment, operation period risk assessment was started much later and the technologies are much more backward; Europe carried out operation period risk assessment relatively early but not until the 21st century, subsequently, Britain, France and other countries have enacted a number of assessment methods successively. However, in our country, there are less risk assessments for operation periods of tunnels, especially there is basically no assessment for long and large immersed tube undersea tunnels. This article carries out risk assessment for the stability and safety of tunnel structure in operation period and proposes specific opinions and suggestions which are very important for controlling and reducing the risks as well as ensuring the structural safety and stability on basis of the immersed tube tunnel with extra-large section of the fourth lane of Shenzhen-Zhongshan Thoroughfare. 2. Risk Source Analysis and Risk Identification The geological, hydrogeological and surrounding environment conditions of long and large immersed tube tunnel project are complicated, in addition, the design technologies are immature or defective, the construction quality control is improper, and so on, therefore, there are many potential risk factors that affect the structural stability. According to the operation conditions of domestic and foreign immersed tube tunnels and by reference to the statistical analysis of highway tunnel operation accidents, the risks are screened, determined and classified, and 6 types of key risks of structural stability of long and large immersed tube tunnel in operation period are obtained, including: tube dislocation, section deformation, tube crack, overall settlement and uneven settlement, material deterioration, and other risks. Meanwhile, by risk cause and logic analysis, diagrams of hierarchical relationships of structural stability risks and their inducements can be established (as shown in Figures 1~6).
267
268
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278 Tube joint dislocation Damage of shear connector
Inter-tube joint dislocation
Improp er butt joint
Section conver gence deform ation
Tube size error
Constr uction error
Immersi on and assembl y center deviatio n
Section converg ence deforma tion
Extern al load
Jack extrusi on
Earthqu ake and other disaster s
Longitu dinal deform ation due to uneven settlem ent
Tube joint fracture
Horizont al Founda deformat tion ion due trench to bottom uneven undulat settleme ion nt
Externa l constru ction
Longitu dinal deforma tion due to uneven settleme nt
Horizo ntal deform ation due to uneven settlem ent
Section conver gence deform ation
Founda tion Improp trench er butt bottom joint undulat ion
Tide and underc urrent
Figure 1.Hierarchical Relationship Diagram of Tube Dislocation Material deterioration Thickness reduction
Chemic al corrosio n
Strength reduction
Aggreg ate loss
Impair ment
Chemica l corrosio n
Material damage due to overlarge stress load
Figure 2.Hierarchical Relationship Diagram of Material Deterioration Section deformation Section convergence in horizontal direction
Section convergence in vertical direction
Load factor
Jack extru sion
Overlarge burial layer
Existi ng load
Unev en settle ment
Earth quake and other disast ers
Load factor
Exter nal const ructio n
Jack extru sion
Overlarge burial layer
Existi ng load
Unev en settle ment
Earth quake and other disast ers
Overall settlement and uneven settlement
Uneven settlement
Overall settlement
Exter nal const ructio n
Figure 3.Hierarchical Relationship Diagram of Section Deformation
Foundat ion oil mass consolid ation
Overlarge load of burial layer
Founda tion undulat ion
Uneve n load distrib ution
Uneve n founda tion soil quality
Uneven structur al stiffnes s
Figure 4.Hierarchical Relationship Diagram of Settlements
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Tube crack
Other risks
Overall bending
Prefabr icated microcrack
Longitu dinally
Corner
Circular
Extrusi on
Foundat ion trench bottom undulati on
Foundat Uneven ion settleme trench nt in bottom horizont undulati al on direction
Oblique
Foundat ion trench bottom undulati on
Uneven settleme nt in horizont al directio n
Figure 5.Hierarchical Relationship Diagram of Tube Crack
Shipwre ck collision risk
Ship anchorin g
Extreme ly high water level and wave
In-hole explosio n
Fire condition
Figure 6.Hierarchical Relationship Diagram of Other Risks
3. Typical Scenario Simulation Analysis 3.1. Scenario Design As known from the above risk source analysis and identification, there are many inducements affecting the structural stability of tunnel in operation period, therefore, it is not practical to carry out scenario simulation analysis for all the factors, at the same time, some risk factors such as improper butt joint, immersion and assembly center deviation, foundation trench bottom undulation, jack extrusion, over-large burial layer, prefabricated micro-crack and impairment can be improved or avoided by enhancing the construction quality control and raising the construction process level, in addition, the probability of structural instability caused by such factors is very low, therefore, only the main and typical scenarios are selected for scenario simulation analysis. According to the main accident causes resulting in structural instability, the following typical scenarios are designed: 1. Horizontal checking calculation in normal operation environment; 2. Longitudinal checking calculation in normal operation environment; 3. Shipwreck collision: regardless of the collision load diffusion effects, 58.5kPa evenly distributed load is taken; 4. Ship anchoring: the anchoring positions are at the maximum bending moment points and maximum shearing force points of roof; 5. Extremely high water level and wave: The wave with return period of ten years is Hs=2.49m, the water pressure increased by wave is 2.49m/2=1.245m head; The high water level with return period of ten years is 2.74m, the increased head height is 2.74-0.54=2.2m. 6. Explosion in tunnel: in double-hole tunnel, the number of times of explosion happened in any traffic hole at the same time is once, the equivalent load of explosion in traffic hole is 100kPa˗ 7. External explosion: the checking calculation is based on level VI civil air defense load, the external explosion load is 60kPa, the load acts on the whole peripheral surface of tube in normal direction. 8. Fire: the fire will not change the load conditions of tunnel structure, however, the rise of temperature will cause the reduction of strength and stiffness of structural material. Due to the limited length of this article, only the shipwreck collision and ship anchoring scenarios are expounded. 3.2. Calculation Model and Parameters 1.Model and Parameters Based on the general FEM software ANSYS, the rock is considered as Mohr-Column constitutive criteria, the structure is considered as the elastic-plastic model. The calculation parameters are shown in Table 1. Table1. Numerical Simulation Model Parameters Material
Elasticity Modulus E (N/m2)
Poisson’s Ratio ȝ
Volumetric Weight (KN/m3)
Seawater
-
-
10.06
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Cover layer
25.0×103
0.32
11.00 (floating)
Lining
3.45×107
0.20
25.82
Shear connector
3.45×107
0.20
25.82
2. Load Calculation 1) Vertical soil pressure: the net volumetric weights of siltation layer and lock layer are 21kN/m3 and 15kN/m3respectively. 2) Lateral soil pressure: the calculation formula of lateral soil pressure is ൌ ͳȀ ɔ, in which, ɔis the effective angle of internal friction of backfill gravel and is 40°, k=0.357. 3) Additional load: the length of tube is 22.5m, the value of load that is based on trailer and evenly distributed on floor is 8.4kPa; the volumetric weight of concrete is 23.3kN/m3, the thickness of concrete is 1.2m; the pavement thickness of highway surface is 0.1m, the pavement load is 2.3kPa, the total load is38.75kPa. 4) The hydrostatic pressure of roof (evenly distributed load) and hydrostatic pressure of side wall (trapezium distribution). 3. Boundary Conditions Not the same with sub-surface excavated tunnel, for immersed tube tunnel, the arching effects are not considered, and the loads act directly on the structure. The burial depth of base is 40m, the covering soil is 7.8m, the single side width of base is 3 times the width of immersed tube and is about 132m. The side constraint of model is in x direction, the longitudinal constraint is in y direction, the bottom constraint is in z direction. 3.3. Shipwreck Collision As the tunnel crosses the main channel, according to the Report of Research on Ship Collision Avoidance, when the tunnel top and tunnel protection layer are not less than 1m lower than the seabed and the water depth is 9m or above (corresponding to the draft of the design ship), the tunnel shall be designed to bear the even load of 58.5kPa within the whole width direction and 17.6m longitudinal direction range of tunnel, if siltation layer is already available in the deep trench of tunnel, this load shall be applied on the top of siltation layer.When the tunnel top and tunnel protection layer are less than 1m lower than the seabed and the water depth is 9m or above, the tunnel shall be designed to bear the even load of 95kPa within the whole width direction and 19m longitudinal direction range of tunnel.When the water depth is less than 9m, the load and load action length shall be considered according to the proportion of actual water depth to 9m water depth. In the same time period, only one of the above-mentioned conditions acts on the tunnel structure or seabed.According to the above principles, the structural deformations and stresses under two conditions, i.e. normal operation condition and shipwreck collision condition are calculated respectively, in which, the diffusion effects of shipwreck collision load are not considered and is taken as 58.5kPa evenly distributed load, the calculation results are as shown in the following Figure 7~Figure 12.
Figure 7. Structure Bending Moment Diagrams(Normal Operation Condition)
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
Figure 8. Structure Bending Moment Diagrams(Shipwreck Collision Condition)
Figure 9.Structure Axial Force Diagrams(Normal Operation Condition)
Figure 10.Structure Axial Force Diagrams(Shipwreck Collision Condition)
Figure 11.Structure Shearing Force Diagrams(Normal Operation Condition)
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Figure 12.Structure Shearing Force Diagrams(Shipwreck Collision Condition)
As known from the comparative analysis, with the shipwreck collision load considered, the bending moment, axial force and shearing force of tunnel lining structure are increased obviously, their maximum value and normal operation condition value ratios are 1.13, 1.2 and 1.06 respectively.Therefore, the shipwreck collision load can greatly increase the force borne by the structure and has relatively serious effects on the stability of structure, if shipwreck collision is not considered during the design, overall instability of structure may be caused. 3.4. Ship Anchoring According to project feasibility study report, when the ship is anchored to the tunnel structure, impact load around 1900KN will be produced within the range of 3m diameter above the structure roof during the anchoring, after the anchor is stabilized, equivalent static load around 3800KN will be formed in the same range. According to different anchoring positions, the effects on structural stability are also different. Below, internal force calculation analysis is carried out for two extreme cases of anchoring positions, i.e. at the maximum bending moment point and at the maximum shearing force point of roof.
Figure 13.Model Diagrams under Different Conditions of Ship Anchor(Maximum Bending Moment Point)
Figure 14.Model Diagrams under Different Conditions of Ship Anchor(Maximum Shearing Force Point)
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
Figure 15.Structure Bending Moment Diagrams(Maximum Bending Moment Point)
Figure 16.Structure Bending Moment Diagrams(Maximum Shearing Force Point)
Figure 17.Structure Axial Force Diagrams(Maximum Bending Moment Point)
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Figure 18.Structure Axial Force Diagrams(Maximum Shearing Force Point)
Figure 19.Structure Shearing Force Diagrams(Maximum Bending Moment Point)
Figure 20.Structure Shearing Force Diagrams((Maximum Shearing Force Point)
As seen from the comparison of the above calculation results and normal operation condition calculation results, the local internal forces of the roof of immersed tube are increased by the anchoring, and the overall force borne by
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
the structure is also increased, but to a smaller extent. Therefore, the main risk brought to the structure by ship anchoring is local structural damage, however, the overall damage risk is less. 4. Risk Assessment 4.1. Risk Occurrence Probability The accident probability level standards in design and construction risk assessment of highway tunnel in construction period are used for reference, meanwhile, the risk occurrence probability level number is expanded from 1~4 levels to 1~5 levels in order to further refine the risk occurrence probability level judgment standards for structural stability of immersed tube tunnel, and 0.03% judgment limit is added in the quantitative judgment standards, see Table 2 for the detailed judgment standards. Table2.Risk Occurrence Probability Level Judgment Standards level
Quantitative Judgment Standard (%)
Qualitative Judgment Standard
1
<0.03%
Extremely small
2
0.03-0.3%
Relatively small
3
0.3-3%
Likely
4
3-30%
More likely
5
>30%
Almost certain
According to the risk source surveys and the structural risk analyses corresponding to different mileages of tunnel, in combination with the expert survey results, the risk occurrence probability levels of structural stability of tunnel are evaluated comprehensively from the aspects of design, construction, operation, etc., see Table 3 for the results. Table3.Risk Occurrence Probability Table Risk Event
Section deformation
Risk Source Earthquake and other disasters External construction Over-large burial layer
Probability Level
2
Existing load Prefabricated micro-crack Tube crack
Overall bending
2
Horizontal deformation due to uneven settlement Uneven structural stiffness Settlement and uneven settlement
Uneven foundation soil quality Foundation oil mass consolidation
2
Uneven load distribution Material deterioration
Aggregate loss Chemical corrosion
2
Shipwreck collision Ship anchoring Others
In-hole explosion Fire Extremely high water level and wave
2
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4.2. Risk Loss Assessment Risk loss assessment is to estimate the severities of various losses brought by risks, and the losses caused by tunnel risks can be divided into direct loss (including repair cost, casualty, property loss, environmental loss, etc. after the accident) and indirect loss (including traffic delay, operator revenue reduction, reputation loss, etc.) and sometimes also include third-party losses, etc. The risk loss levels are divided into levels 1, 2, 3, 4 and 5 and mainly judged from three aspects, i.e. casualty, economic loss and environmental loss, at the same time, the weights of different loss levels are determined in consideration of the risk loss acceptance degree differences and in combination with relevant domestic and foreign risk assessment standards and expert survey results, see Table 4 for the detailed judgment standards and weight values. Table 4.Risk Loss Judgment Standards and Weights Level
Casualty Judgment Standard L1
Economic Loss Judgment Standard L2
Environmental Impact Judgment Standard
Weight
L3
W
1
Less than 5 seriously injured persons
Less than RMB 5 million Yuan of economic loss
Very small involved area, without mass effect, less than 50 persons needing emergency relocation
0.8
2
Less than 3 deaths (including missing), or more than 5 and less than 10 seriously injured persons
More than RMB 5 million Yuan and less than RMB 10 million Yuan of economic loss
Relatively small involved area, general mass effect, more than 50 and less than 100 persons needing emergency relocation
0.9
3
More than 3 and less than 10 deaths (including missing), or more than 10 and less than 50 seriously injured persons
More than RMB 10 million Yuan and less than RMB 50 million Yuan of economic loss
Large involved area; normal economic and social activities in the area are affected; more than 100 and less than 500 1 persons needing emergency relocation
4
More than 10 and less than 30 deaths (including missing), or more than 50 and less than 100 seriously injured persons
More than RMB 50 million Very large involved area, partially loss of ecological Yuan and less than RMB 100 functions in the area, more than 500 and less than 1000 million Yuan of economic persons needing emergency relocation loss
5
Very large involved area, seriously loss of peripheral More than 30 deaths (including More than RMB 100 million ecological functions in the area, more than 1000 persons missing), or more than 100 seriously needing emergency relocation; normal economic and social Yuan of economic loss injured persons activities are affected seriously
1.1
1.2
According to scenario simulation results and reference data investigations and researches, as well as in combination with expert surveys, the loss level of each risk source determined according to the judgment standards in Table 4 is as shown in Table 5. Table5. Risk Loss Assessment Table Risk Event
Section deformation
Tube crack
Settlement and
Risk Source
Loss Level Casualty L1
Economic Loss L2
Environmental Impact L3
Earthquake and other disasters
2
2
2
External construction
1
2
2
Over-large burial layer
2
2
2
Existing load
2
2
2
Prefabricated micro-crack
2
2
2
Overall bending
2
2
2
Horizontal deformation due to uneven settlement
2
2
2
Uneven structural stiffness
2
2
2
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Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278 uneven settlement
Material deterioration
Others
Uneven foundation soil quality
2
2
Foundation oil mass consolidation
2
2
2 2
Uneven load distribution
2
2
2
Aggregate loss
3
3
3
Chemical corrosion
2
2
2
Shipwreck collision
3
3
2
Ship anchoring
3
3
2
In-hole explosion
4
4
3
Fire
2
3
3
Extremely high water level and wave
2
2
2
4.3. Risk Assessment According to the each risk loss level of each risk source and in combination with the expert survey results, the casualty, economic loss and environmental impact loss levels can be determined respectively. Meanwhile, for accurate comparative analysis of the size of each risk, this research puts forward the method for quantitative calculation of risk scores, the calculation formula is shown as formula (1). Risk score [2.4-90]= risk occurrence probability level [1-5]×risk loss judgment level (σ୬୧ୀଵ ୧ ή ୧ ) [2.4-18] (1) Where Li is the judgment level of each risk loss, Wi is the weight of each risk level, n range from 1 to 3. By reference, the determination standards of risk levels are as shown in Table 6. According to the risk occurrence probability levels and risk loss levels analyzed and determined above, the structural stability risk assessment results can be obtained and are as shown in Table 7, in which, the difference between “with” and “without” transportation of dangerous cargo is mainly “with” or “without” considering the effects of in-hole explosion risk source. Table6. Risk Level Table Risk Loss
Risk Occurrence Probability
1
2
3
4
5
1
ĉ
ĉ
Ċ
Ċ
ċ
2
ĉ
Ċ
Ċ
ċ
ċ
3
Ċ
Ċ
ċ
ċ
Č
4
Ċ
ċ
ċ
Č
Č
5
ċ
ċ
Č
Č
Č
Table7.Risk Assessment Results Risk Loss Level Risk Source
Structural stability
Type
Risk Occurrence Probability Level
Casualty
Economic
Environmental
Loss
Impact
Risk
Risk
Level
Score
Without transportation of dangerous cargo
2
2
1
2
II
8.8
With transportation of dangerous cargo
3
3
2
2
III
19.8
5. Conclusions This article researches the stability and safety of long and large immersed tube tunnel structure in operation period from the aspects of risk identification, typical scenario design and simulation, risk occurrence probability and loss assessment, etc. on basis of the immersed tube tunnel with extra-large section of the fourth lane of Shenzhen-
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Zhongshan Channel, and obtains the operation period risk levels and the following main results, which are significant for controlling and reducing the operation risks and ensuring the structural safety and stability. 1)Based on the identification of risk sources,risk cause and logic analysis is carried out of key risks,the hierarchical relationship diagrams of risk sources and their inducements are established 2) Nine typical scenarios are designed according to the main accident causes resulting in structural instability and numerical simulation analysis is carried out. As obtained from the analysis: shipwreck collision and extremely high water level and wave can lead to overall structural instability, the main risk of anchoring is local structural damage, explosion can lead to damage to bending shear of structure, fire will lead to reduction of bearing capacity of structure and cause structural instability. 3) In order to further refine the judgment standards, the risk occurrence probability level values are expanded from levels 1~4 to levels 1~5, and the quantitative calculation method for risks is established.According to various analyses and expert survey results, the occurrence probability level of each risk event and the loss level of each risk are put forward, and the operation period structural stability risk levels are obtained. 4) In the case of transportation of dangerous cargo, the risk level of structural stability reaches Level III which is high risk, control measures for risk reduction must be implemented and contingency plan must be formulated. Therefore, it is suggested that the traffic of vehicles for dangerous cargo shall be restricted by measures such as time-limited traffic, speed-limited traffic, maintenance of minimum vehicle distance and traffic escorted by guiding vehicles in operation period to reduce the risks in operation period. References [1]Einstein HH, IndermiueC, SinfieldJ,etal. DecisionDids for tunneling. Journal of the Transportation Research Board1996; 6-9 [2]Einstein HH. Risk and risk analysis in rock engineering. Tunnel and Underground Space Technology 1996; 141-155 [3]Einstein HH, Viek SG. Geologic model for a tunnel cost model. Proceedings of Rapid Excavation and Tunneling Conference 1974; (2): 1701-1720 [4]Huang HW. State of the art of the research on risk management in construction of tunnel and underground works. Chinese Journal of Underground Space and Engineering 2006; 2 (1): 13-20. [5]Chen GX, Huang HW, You JX. Study on life cycle risk management of metro. Chinese Journal of Underground Space and Engineering2006; 2(1): 47–51. [6]Huang HW, Zeng M, Chen L, et al. Development of risk management software(TRM1.0) based on risk database for shield tunneling. Chinese Journal of Underground Space and Engineering 2006; 2(1): 36-41. [7]Chen L, Huang HW. The practice of risk management in Shangzhong road tunnel engineering. Chinese Journal of Underground Space and Engineering 2006; 2(1): 65-69. [8]Chen L Huang HW. Risk analysis of rock tunnel engineering. Chinese Journal of Rock Mechanics and Engineering 2005 24(1): 110-115. [9]Qian QH, Rong XL. Statusissuesand problems of Chinese underground engineering safety risk management and relevant suggestions. Chinese Journal of Rock Mechanics and Engineering2008 27(4): 649-655. [10]Li YK, Zhang DL, Fang Q. Risk assessment and analysis for full process of construction of undersea tunnel. Modern Tunnelling Technology2015; 52(3): 47-54. [11]Xu CB, Tian HN, Zhou N. Risk source identification and risk assessment analysis for AoFeng mountain tunnel entrance section. Journal of Highway and Transportation Research and Development 2012; 29(10): 96-101.
ScienceDirect Procedia Engineering 166 (2016) 266 – 278
2nd International Symposium on Submerged Floating Tunnels and Underwater Tunnel Structures
Risk Assessment of Operation Period Structural Stability for Long and Large Immersed Tube Tunnel Mengjun Wua,b,c, *,QiZhang a,b,c,SenyangWua,b,c a
China Merchants Chongqing Communications Technology Research & Design Institute Co., Ltd., Chongqing 400067 b National Engineering and Research Center for Mountains Highways; Chongqing 400067 c National Engineering Laboratory for Highway Tunnel Construction Technology, Chongqing 400067
Abstract Immersed tube tunnel has been widely used in virtue of its characteristics of shallow burial, good waterproof performance, strong stratum adaptability, etc., however, it is also characterized by enclosed space, prolonged submersion of structure in deep water, complicated surrounding environment, etc., particularly, in case of accident of tunnel structure, inestimable losses will be caused, therefore, the control of structural stability risk in operation period is especially important. This article carries out risk source analysis and identification, scenario design and simulation analysis, risk loss judgment standards and weight research for structural stability of long and large immersed tube tunnel in operation period on basis of extra-large immersed tube tunnel of the fourth lane of Shenzhen-Zhongshan Thoroughfare, based on which and in combination with the relevant domestic and foreign risk assessment and expert research results, the structural stability risk levels and risk scores of structural stability in operation period are obtained by quantitative risk calculation method and the relevant suggestions for risk control are proposed. ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Peer-review under responsibility ofthe organizing committee of SUFTUS-2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016 Keywords:Long and Large Immersed Tube Tunnel, Operation Period, Structural Stability, Risk Assessment
1. Introduction As a tunnel construction method, immersed tube has the characteristics of shallow burial, high space utilization ratio, good waterproof performance, strong stratum adaptability, etc. and has been widely used in the construction of underwater tunnels. The world’s first immersed tube tunnel was constructed by America on Detroit River in 1910,
* Corresponding author. Tel.: +86-023-62653128; fax:+86-023-62653128. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SUFTUS-2016
doi:10.1016/j.proeng.2016.11.549
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
while the first immersed tube tunnel in our country was Hong Kong Hung Hom undersea tunnel constructed in 1972, subsequently, with the fast development of economy and progress of immersed tube construction technology of our country, immersed tube tunnels were applied more in river-crossing and sea-crossing projects, at present, there has been more than 20 immersed tube tunnels that are constructed or under construction in our country. The long and large immersed tube tunnel project is typically characterized by long axis line, enclosed space, dense vehicle traffic, large escape and rescue difficulties, etc., in addition, the structure will be submerged in deep water for a long period, the surrounding environment is complicated, and there are many factors affecting the structural stability and safety, particularly, in case of accident of tunnel structure, inestimable losses will be caused. Risk assessment can reasonably analyze these uncertainty factors and convert the unforeseeable risk factors into quantitative indexes to provide bases for risk control and relevant management. At present, for tunnel risk assessment, many domestic and foreign scholars have carried out a lot of researches on construction period risk assessment[4-11] and relevant guidelines or regulations have been formed since American Einstein.H.H[1-3]introduced the risk analysis theory into tunnel and underground projects and put forward the characteristics of tunnel project risk analysis and the ideas that shall be followed. In general, tunnel construction period risk assessment has been fully popularized and the assessment methods and technologies are relatively mature. Compared to construction period risk assessment, operation period risk assessment was started much later and the technologies are much more backward; Europe carried out operation period risk assessment relatively early but not until the 21st century, subsequently, Britain, France and other countries have enacted a number of assessment methods successively. However, in our country, there are less risk assessments for operation periods of tunnels, especially there is basically no assessment for long and large immersed tube undersea tunnels. This article carries out risk assessment for the stability and safety of tunnel structure in operation period and proposes specific opinions and suggestions which are very important for controlling and reducing the risks as well as ensuring the structural safety and stability on basis of the immersed tube tunnel with extra-large section of the fourth lane of Shenzhen-Zhongshan Thoroughfare. 2. Risk Source Analysis and Risk Identification The geological, hydrogeological and surrounding environment conditions of long and large immersed tube tunnel project are complicated, in addition, the design technologies are immature or defective, the construction quality control is improper, and so on, therefore, there are many potential risk factors that affect the structural stability. According to the operation conditions of domestic and foreign immersed tube tunnels and by reference to the statistical analysis of highway tunnel operation accidents, the risks are screened, determined and classified, and 6 types of key risks of structural stability of long and large immersed tube tunnel in operation period are obtained, including: tube dislocation, section deformation, tube crack, overall settlement and uneven settlement, material deterioration, and other risks. Meanwhile, by risk cause and logic analysis, diagrams of hierarchical relationships of structural stability risks and their inducements can be established (as shown in Figures 1~6).
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Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278 Tube joint dislocation Damage of shear connector
Inter-tube joint dislocation
Improp er butt joint
Section conver gence deform ation
Tube size error
Constr uction error
Immersi on and assembl y center deviatio n
Section converg ence deforma tion
Extern al load
Jack extrusi on
Earthqu ake and other disaster s
Longitu dinal deform ation due to uneven settlem ent
Tube joint fracture
Horizont al Founda deformat tion ion due trench to bottom uneven undulat settleme ion nt
Externa l constru ction
Longitu dinal deforma tion due to uneven settleme nt
Horizo ntal deform ation due to uneven settlem ent
Section conver gence deform ation
Founda tion Improp trench er butt bottom joint undulat ion
Tide and underc urrent
Figure 1.Hierarchical Relationship Diagram of Tube Dislocation Material deterioration Thickness reduction
Chemic al corrosio n
Strength reduction
Aggreg ate loss
Impair ment
Chemica l corrosio n
Material damage due to overlarge stress load
Figure 2.Hierarchical Relationship Diagram of Material Deterioration Section deformation Section convergence in horizontal direction
Section convergence in vertical direction
Load factor
Jack extru sion
Overlarge burial layer
Existi ng load
Unev en settle ment
Earth quake and other disast ers
Load factor
Exter nal const ructio n
Jack extru sion
Overlarge burial layer
Existi ng load
Unev en settle ment
Earth quake and other disast ers
Overall settlement and uneven settlement
Uneven settlement
Overall settlement
Exter nal const ructio n
Figure 3.Hierarchical Relationship Diagram of Section Deformation
Foundat ion oil mass consolid ation
Overlarge load of burial layer
Founda tion undulat ion
Uneve n load distrib ution
Uneve n founda tion soil quality
Uneven structur al stiffnes s
Figure 4.Hierarchical Relationship Diagram of Settlements
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Tube crack
Other risks
Overall bending
Prefabr icated microcrack
Longitu dinally
Corner
Circular
Extrusi on
Foundat ion trench bottom undulati on
Foundat Uneven ion settleme trench nt in bottom horizont undulati al on direction
Oblique
Foundat ion trench bottom undulati on
Uneven settleme nt in horizont al directio n
Figure 5.Hierarchical Relationship Diagram of Tube Crack
Shipwre ck collision risk
Ship anchorin g
Extreme ly high water level and wave
In-hole explosio n
Fire condition
Figure 6.Hierarchical Relationship Diagram of Other Risks
3. Typical Scenario Simulation Analysis 3.1. Scenario Design As known from the above risk source analysis and identification, there are many inducements affecting the structural stability of tunnel in operation period, therefore, it is not practical to carry out scenario simulation analysis for all the factors, at the same time, some risk factors such as improper butt joint, immersion and assembly center deviation, foundation trench bottom undulation, jack extrusion, over-large burial layer, prefabricated micro-crack and impairment can be improved or avoided by enhancing the construction quality control and raising the construction process level, in addition, the probability of structural instability caused by such factors is very low, therefore, only the main and typical scenarios are selected for scenario simulation analysis. According to the main accident causes resulting in structural instability, the following typical scenarios are designed: 1. Horizontal checking calculation in normal operation environment; 2. Longitudinal checking calculation in normal operation environment; 3. Shipwreck collision: regardless of the collision load diffusion effects, 58.5kPa evenly distributed load is taken; 4. Ship anchoring: the anchoring positions are at the maximum bending moment points and maximum shearing force points of roof; 5. Extremely high water level and wave: The wave with return period of ten years is Hs=2.49m, the water pressure increased by wave is 2.49m/2=1.245m head; The high water level with return period of ten years is 2.74m, the increased head height is 2.74-0.54=2.2m. 6. Explosion in tunnel: in double-hole tunnel, the number of times of explosion happened in any traffic hole at the same time is once, the equivalent load of explosion in traffic hole is 100kPa˗ 7. External explosion: the checking calculation is based on level VI civil air defense load, the external explosion load is 60kPa, the load acts on the whole peripheral surface of tube in normal direction. 8. Fire: the fire will not change the load conditions of tunnel structure, however, the rise of temperature will cause the reduction of strength and stiffness of structural material. Due to the limited length of this article, only the shipwreck collision and ship anchoring scenarios are expounded. 3.2. Calculation Model and Parameters 1.Model and Parameters Based on the general FEM software ANSYS, the rock is considered as Mohr-Column constitutive criteria, the structure is considered as the elastic-plastic model. The calculation parameters are shown in Table 1. Table1. Numerical Simulation Model Parameters Material
Elasticity Modulus E (N/m2)
Poisson’s Ratio ȝ
Volumetric Weight (KN/m3)
Seawater
-
-
10.06
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Cover layer
25.0×103
0.32
11.00 (floating)
Lining
3.45×107
0.20
25.82
Shear connector
3.45×107
0.20
25.82
2. Load Calculation 1) Vertical soil pressure: the net volumetric weights of siltation layer and lock layer are 21kN/m3 and 15kN/m3respectively. 2) Lateral soil pressure: the calculation formula of lateral soil pressure is ൌ ͳȀ ɔ, in which, ɔis the effective angle of internal friction of backfill gravel and is 40°, k=0.357. 3) Additional load: the length of tube is 22.5m, the value of load that is based on trailer and evenly distributed on floor is 8.4kPa; the volumetric weight of concrete is 23.3kN/m3, the thickness of concrete is 1.2m; the pavement thickness of highway surface is 0.1m, the pavement load is 2.3kPa, the total load is38.75kPa. 4) The hydrostatic pressure of roof (evenly distributed load) and hydrostatic pressure of side wall (trapezium distribution). 3. Boundary Conditions Not the same with sub-surface excavated tunnel, for immersed tube tunnel, the arching effects are not considered, and the loads act directly on the structure. The burial depth of base is 40m, the covering soil is 7.8m, the single side width of base is 3 times the width of immersed tube and is about 132m. The side constraint of model is in x direction, the longitudinal constraint is in y direction, the bottom constraint is in z direction. 3.3. Shipwreck Collision As the tunnel crosses the main channel, according to the Report of Research on Ship Collision Avoidance, when the tunnel top and tunnel protection layer are not less than 1m lower than the seabed and the water depth is 9m or above (corresponding to the draft of the design ship), the tunnel shall be designed to bear the even load of 58.5kPa within the whole width direction and 17.6m longitudinal direction range of tunnel, if siltation layer is already available in the deep trench of tunnel, this load shall be applied on the top of siltation layer.When the tunnel top and tunnel protection layer are less than 1m lower than the seabed and the water depth is 9m or above, the tunnel shall be designed to bear the even load of 95kPa within the whole width direction and 19m longitudinal direction range of tunnel.When the water depth is less than 9m, the load and load action length shall be considered according to the proportion of actual water depth to 9m water depth. In the same time period, only one of the above-mentioned conditions acts on the tunnel structure or seabed.According to the above principles, the structural deformations and stresses under two conditions, i.e. normal operation condition and shipwreck collision condition are calculated respectively, in which, the diffusion effects of shipwreck collision load are not considered and is taken as 58.5kPa evenly distributed load, the calculation results are as shown in the following Figure 7~Figure 12.
Figure 7. Structure Bending Moment Diagrams(Normal Operation Condition)
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
Figure 8. Structure Bending Moment Diagrams(Shipwreck Collision Condition)
Figure 9.Structure Axial Force Diagrams(Normal Operation Condition)
Figure 10.Structure Axial Force Diagrams(Shipwreck Collision Condition)
Figure 11.Structure Shearing Force Diagrams(Normal Operation Condition)
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Figure 12.Structure Shearing Force Diagrams(Shipwreck Collision Condition)
As known from the comparative analysis, with the shipwreck collision load considered, the bending moment, axial force and shearing force of tunnel lining structure are increased obviously, their maximum value and normal operation condition value ratios are 1.13, 1.2 and 1.06 respectively.Therefore, the shipwreck collision load can greatly increase the force borne by the structure and has relatively serious effects on the stability of structure, if shipwreck collision is not considered during the design, overall instability of structure may be caused. 3.4. Ship Anchoring According to project feasibility study report, when the ship is anchored to the tunnel structure, impact load around 1900KN will be produced within the range of 3m diameter above the structure roof during the anchoring, after the anchor is stabilized, equivalent static load around 3800KN will be formed in the same range. According to different anchoring positions, the effects on structural stability are also different. Below, internal force calculation analysis is carried out for two extreme cases of anchoring positions, i.e. at the maximum bending moment point and at the maximum shearing force point of roof.
Figure 13.Model Diagrams under Different Conditions of Ship Anchor(Maximum Bending Moment Point)
Figure 14.Model Diagrams under Different Conditions of Ship Anchor(Maximum Shearing Force Point)
Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278
Figure 15.Structure Bending Moment Diagrams(Maximum Bending Moment Point)
Figure 16.Structure Bending Moment Diagrams(Maximum Shearing Force Point)
Figure 17.Structure Axial Force Diagrams(Maximum Bending Moment Point)
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Figure 18.Structure Axial Force Diagrams(Maximum Shearing Force Point)
Figure 19.Structure Shearing Force Diagrams(Maximum Bending Moment Point)
Figure 20.Structure Shearing Force Diagrams((Maximum Shearing Force Point)
As seen from the comparison of the above calculation results and normal operation condition calculation results, the local internal forces of the roof of immersed tube are increased by the anchoring, and the overall force borne by
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the structure is also increased, but to a smaller extent. Therefore, the main risk brought to the structure by ship anchoring is local structural damage, however, the overall damage risk is less. 4. Risk Assessment 4.1. Risk Occurrence Probability The accident probability level standards in design and construction risk assessment of highway tunnel in construction period are used for reference, meanwhile, the risk occurrence probability level number is expanded from 1~4 levels to 1~5 levels in order to further refine the risk occurrence probability level judgment standards for structural stability of immersed tube tunnel, and 0.03% judgment limit is added in the quantitative judgment standards, see Table 2 for the detailed judgment standards. Table2.Risk Occurrence Probability Level Judgment Standards level
Quantitative Judgment Standard (%)
Qualitative Judgment Standard
1
<0.03%
Extremely small
2
0.03-0.3%
Relatively small
3
0.3-3%
Likely
4
3-30%
More likely
5
>30%
Almost certain
According to the risk source surveys and the structural risk analyses corresponding to different mileages of tunnel, in combination with the expert survey results, the risk occurrence probability levels of structural stability of tunnel are evaluated comprehensively from the aspects of design, construction, operation, etc., see Table 3 for the results. Table3.Risk Occurrence Probability Table Risk Event
Section deformation
Risk Source Earthquake and other disasters External construction Over-large burial layer
Probability Level
2
Existing load Prefabricated micro-crack Tube crack
Overall bending
2
Horizontal deformation due to uneven settlement Uneven structural stiffness Settlement and uneven settlement
Uneven foundation soil quality Foundation oil mass consolidation
2
Uneven load distribution Material deterioration
Aggregate loss Chemical corrosion
2
Shipwreck collision Ship anchoring Others
In-hole explosion Fire Extremely high water level and wave
2
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4.2. Risk Loss Assessment Risk loss assessment is to estimate the severities of various losses brought by risks, and the losses caused by tunnel risks can be divided into direct loss (including repair cost, casualty, property loss, environmental loss, etc. after the accident) and indirect loss (including traffic delay, operator revenue reduction, reputation loss, etc.) and sometimes also include third-party losses, etc. The risk loss levels are divided into levels 1, 2, 3, 4 and 5 and mainly judged from three aspects, i.e. casualty, economic loss and environmental loss, at the same time, the weights of different loss levels are determined in consideration of the risk loss acceptance degree differences and in combination with relevant domestic and foreign risk assessment standards and expert survey results, see Table 4 for the detailed judgment standards and weight values. Table 4.Risk Loss Judgment Standards and Weights Level
Casualty Judgment Standard L1
Economic Loss Judgment Standard L2
Environmental Impact Judgment Standard
Weight
L3
W
1
Less than 5 seriously injured persons
Less than RMB 5 million Yuan of economic loss
Very small involved area, without mass effect, less than 50 persons needing emergency relocation
0.8
2
Less than 3 deaths (including missing), or more than 5 and less than 10 seriously injured persons
More than RMB 5 million Yuan and less than RMB 10 million Yuan of economic loss
Relatively small involved area, general mass effect, more than 50 and less than 100 persons needing emergency relocation
0.9
3
More than 3 and less than 10 deaths (including missing), or more than 10 and less than 50 seriously injured persons
More than RMB 10 million Yuan and less than RMB 50 million Yuan of economic loss
Large involved area; normal economic and social activities in the area are affected; more than 100 and less than 500 1 persons needing emergency relocation
4
More than 10 and less than 30 deaths (including missing), or more than 50 and less than 100 seriously injured persons
More than RMB 50 million Very large involved area, partially loss of ecological Yuan and less than RMB 100 functions in the area, more than 500 and less than 1000 million Yuan of economic persons needing emergency relocation loss
5
Very large involved area, seriously loss of peripheral More than 30 deaths (including More than RMB 100 million ecological functions in the area, more than 1000 persons missing), or more than 100 seriously needing emergency relocation; normal economic and social Yuan of economic loss injured persons activities are affected seriously
1.1
1.2
According to scenario simulation results and reference data investigations and researches, as well as in combination with expert surveys, the loss level of each risk source determined according to the judgment standards in Table 4 is as shown in Table 5. Table5. Risk Loss Assessment Table Risk Event
Section deformation
Tube crack
Settlement and
Risk Source
Loss Level Casualty L1
Economic Loss L2
Environmental Impact L3
Earthquake and other disasters
2
2
2
External construction
1
2
2
Over-large burial layer
2
2
2
Existing load
2
2
2
Prefabricated micro-crack
2
2
2
Overall bending
2
2
2
Horizontal deformation due to uneven settlement
2
2
2
Uneven structural stiffness
2
2
2
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Mengjun Wu et al. / Procedia Engineering 166 (2016) 266 – 278 uneven settlement
Material deterioration
Others
Uneven foundation soil quality
2
2
Foundation oil mass consolidation
2
2
2 2
Uneven load distribution
2
2
2
Aggregate loss
3
3
3
Chemical corrosion
2
2
2
Shipwreck collision
3
3
2
Ship anchoring
3
3
2
In-hole explosion
4
4
3
Fire
2
3
3
Extremely high water level and wave
2
2
2
4.3. Risk Assessment According to the each risk loss level of each risk source and in combination with the expert survey results, the casualty, economic loss and environmental impact loss levels can be determined respectively. Meanwhile, for accurate comparative analysis of the size of each risk, this research puts forward the method for quantitative calculation of risk scores, the calculation formula is shown as formula (1). Risk score [2.4-90]= risk occurrence probability level [1-5]×risk loss judgment level (σ୬୧ୀଵ ୧ ή ୧ ) [2.4-18] (1) Where Li is the judgment level of each risk loss, Wi is the weight of each risk level, n range from 1 to 3. By reference, the determination standards of risk levels are as shown in Table 6. According to the risk occurrence probability levels and risk loss levels analyzed and determined above, the structural stability risk assessment results can be obtained and are as shown in Table 7, in which, the difference between “with” and “without” transportation of dangerous cargo is mainly “with” or “without” considering the effects of in-hole explosion risk source. Table6. Risk Level Table Risk Loss
Risk Occurrence Probability
1
2
3
4
5
1
ĉ
ĉ
Ċ
Ċ
ċ
2
ĉ
Ċ
Ċ
ċ
ċ
3
Ċ
Ċ
ċ
ċ
Č
4
Ċ
ċ
ċ
Č
Č
5
ċ
ċ
Č
Č
Č
Table7.Risk Assessment Results Risk Loss Level Risk Source
Structural stability
Type
Risk Occurrence Probability Level
Casualty
Economic
Environmental
Loss
Impact
Risk
Risk
Level
Score
Without transportation of dangerous cargo
2
2
1
2
II
8.8
With transportation of dangerous cargo
3
3
2
2
III
19.8
5. Conclusions This article researches the stability and safety of long and large immersed tube tunnel structure in operation period from the aspects of risk identification, typical scenario design and simulation, risk occurrence probability and loss assessment, etc. on basis of the immersed tube tunnel with extra-large section of the fourth lane of Shenzhen-
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Zhongshan Channel, and obtains the operation period risk levels and the following main results, which are significant for controlling and reducing the operation risks and ensuring the structural safety and stability. 1)Based on the identification of risk sources,risk cause and logic analysis is carried out of key risks,the hierarchical relationship diagrams of risk sources and their inducements are established 2) Nine typical scenarios are designed according to the main accident causes resulting in structural instability and numerical simulation analysis is carried out. As obtained from the analysis: shipwreck collision and extremely high water level and wave can lead to overall structural instability, the main risk of anchoring is local structural damage, explosion can lead to damage to bending shear of structure, fire will lead to reduction of bearing capacity of structure and cause structural instability. 3) In order to further refine the judgment standards, the risk occurrence probability level values are expanded from levels 1~4 to levels 1~5, and the quantitative calculation method for risks is established.According to various analyses and expert survey results, the occurrence probability level of each risk event and the loss level of each risk are put forward, and the operation period structural stability risk levels are obtained. 4) In the case of transportation of dangerous cargo, the risk level of structural stability reaches Level III which is high risk, control measures for risk reduction must be implemented and contingency plan must be formulated. Therefore, it is suggested that the traffic of vehicles for dangerous cargo shall be restricted by measures such as time-limited traffic, speed-limited traffic, maintenance of minimum vehicle distance and traffic escorted by guiding vehicles in operation period to reduce the risks in operation period. References [1]Einstein HH, IndermiueC, SinfieldJ,etal. DecisionDids for tunneling. Journal of the Transportation Research Board1996; 6-9 [2]Einstein HH. Risk and risk analysis in rock engineering. Tunnel and Underground Space Technology 1996; 141-155 [3]Einstein HH, Viek SG. Geologic model for a tunnel cost model. Proceedings of Rapid Excavation and Tunneling Conference 1974; (2): 1701-1720 [4]Huang HW. State of the art of the research on risk management in construction of tunnel and underground works. Chinese Journal of Underground Space and Engineering 2006; 2 (1): 13-20. [5]Chen GX, Huang HW, You JX. Study on life cycle risk management of metro. Chinese Journal of Underground Space and Engineering2006; 2(1): 47–51. [6]Huang HW, Zeng M, Chen L, et al. Development of risk management software(TRM1.0) based on risk database for shield tunneling. Chinese Journal of Underground Space and Engineering 2006; 2(1): 36-41. [7]Chen L, Huang HW. The practice of risk management in Shangzhong road tunnel engineering. Chinese Journal of Underground Space and Engineering 2006; 2(1): 65-69. [8]Chen L Huang HW. Risk analysis of rock tunnel engineering. Chinese Journal of Rock Mechanics and Engineering 2005 24(1): 110-115. [9]Qian QH, Rong XL. Statusissuesand problems of Chinese underground engineering safety risk management and relevant suggestions. Chinese Journal of Rock Mechanics and Engineering2008 27(4): 649-655. [10]Li YK, Zhang DL, Fang Q. Risk assessment and analysis for full process of construction of undersea tunnel. Modern Tunnelling Technology2015; 52(3): 47-54. [11]Xu CB, Tian HN, Zhou N. Risk source identification and risk assessment analysis for AoFeng mountain tunnel entrance section. Journal of Highway and Transportation Research and Development 2012; 29(10): 96-101.