DARTS R4.1 Identification and quantification of...

D•A•R•T•S Durable and Reliable Tunnel Structures

Identification and quantification of hazards DARTS R4.1 - May 2004

The DARTS-project was performed by eight companies and institutes from five European countries. The work has been performed by:

Ricardo van den Berg, BAM Hans Derks, RWS Carola Edvardsen, COWI Søren Degn Eskesen, COWI Christoph Gehlen, IBPS Leendert van Geldermalsen, RWS Jan Gijsbers, TNO Adrie de Groot-van Dam, TNO Annette Gudum, COWI Joost Gulikers, RWS Jelle Hoeksma, RWS Niels Peter Høj, COWI Gesa Kapteina, IBPS Niels Buus Kristensen, COWI Laust Ladefoged, CTP Sipke van Manen, RWS Piet Meeuwissen, BAM Jan P.G. Mijnsbergen, CUR Steen Rostam, COWI Peter Schiessl, IBPS Ton Siemes, TNO Yan Taibi, BYTP Fedde Tolman, BAM Hans de Vries, RWS Martin van der Vliet, CUR Ton Vrouwenvelder, TNO Gerrit Wolsink, RWS

Acknowledgement This report is written by Ton Vrouwenvelder, Alfons Krom and Jaap Weerheim, TNO; Jelle Hoeksma, RWS; Niels Peter Høj and L. Schepper, COWI Although the project consortium did its best to ensure that any information given is accurate, no liability or respon-sibility of any kind (including liability for negligence) is accepted in this respect by the project consortium, the au-thors/editors and those who contributed to the reports. Information Specific information on this report: Jan Gijsbers, TNO, PO Box 49, NL – 2600 AA Delft, The Netherlands T: +31 15 2763212, E: [email protected] Project information: Steen Rostam, COWI A/S, Parallelvej 2, DK - 2800 Kongens Lyngby, Denmark T: +45 45 972782, E: [email protected] Jan P.G. Mijnsbergen, CUR, PO Box 420, NL-2800 AK Gouda, the Netherlands T: +31 182 540620, E: [email protected] Information on the project and the partners on the Internet: www.dartsproject.net

D•A•R•T•S Durable and Reliable Tunnel Structures

Identification and quantification of hazards DARTS R4.1 - May 2004 COWI A/S RWS - Ministerie van Verkeer en Waterstaat, DG Rijkswaterstaat Citytunnelkonsortiet i Malmö Koninklijke BAM Groep nv Bouygues Travaux Publics Ingenieurbüro Professor Schiessl TNO - Netherlands Organization for Applied Scientific Research CUR – Centre for Civil Engineering Research and Codes

Project with financial support of the European Commission under the Fifth Framework Programme, Competitive and Sustainable Growth Programme (GROWTH 2000) Project GRD1-25633, Contract G1RD-CT-2000-00467

Identification and quantification of hazards

4 Durable and Reliable Tunnel Structures - D•A•R•T•S

Table of Contents Preface 5

1. Introduction 7

2. General Background model for hazards 9

3. Design stages 11

4. Discussion of hazards 14 4.1 Explosion 14 4.2 Flooding and leakage 23 4.3 Collisions 24 4.4 Dropping and dragging objects 27 4.5 Sunken ships 29 4.6 Earth quakes 31 4.7 Toxic and aggressive materials 39

5. Conclusions 42


D•A•R•T•S – Durable and Reliable Tunnel Structures 5

Preface Tunnels and underground structures are indispensable when installing new infrastructure in congested areas as well as when raising the qualities of existing urban living. In mountainous countries and where waters cause separation tunnels are essential parts of the infrastructure to connect regions and countries. Tunnels represent however the type of structure where serious consequences on environ-mental pollution and time delays, and the largest budget overruns have occurred during construction. To ensure customer satisfaction and user acceptance, safety, performance and reliable availability of tunnels are essential issues All key stakeholders in the creation and operation of tunnels have acknowledged the lack of opera-tional tools for an overall life cycle cost optimisation of tunnels to minimise extensive budget overruns and delays. This is the problem of a European dimension where the main elements are solved by the DARTS-project. The objective of the DARTS-project (Durable and Reliable Tunnel Structures) was to develop opera-tional methods and supporting practical tools for the best pro-active decision-making process for choosing, in each individual case, the cost optimal tunnel type and construction procedures regarding environmental conditions, technical qualities, safety precautions and long service life. Traditionally, all tunnel components, structural as well as non-structural such as joints, gaskets, drain-age, membranes etc, are designed according to usual ultimate limit state (ULS) and serviceability limit state (SLS) requirements. However, when it comes to durability design to verify that the intended life can be achieved with an acceptable level of reliability, the situation is entirely different from structural design. It seems to be acceptable without question to use a grossly over-simplistic approach. Codes fail to define the design life in relation to durability, and fail to quantify the durability limit states that must be exceeded for the design life to be ended. Service life design, environmental impact assessment, and hazard and risk analysis each provide a set of operational tools for the stakeholders in the creation and operation of tunnels. Each set of these tools have been updated in DARTS based on the most updated scientific knowledge and experience from practice. Currently these fundamental tools for tunnel design are treated separately leading to a fragmented problem (project) solving approach, with specific consequences for tunnels as previously highlighted. The approach of DARTS was to formulate the above different sets of fundamental tools in tunnelling into a format being mutually compatible. This is needed to create a truly integrated life cy-cle cost optimisation and a rational long-term economic and performance related decision basis for each specific tunnel individually, taking the environmental issues and risk assessment into account. DARTS has been developed for the main current types of tunnel constructions as follows: rock tun-nels, bored tunnels, NATM tunnels, immersed tunnels, and cut and cover tunnels. The dominating innovation of DARTS is the integration of reliability based structural design, geotech-nical issues, service life design, hazard design including risk assessment, environmental aspects, socie-tal needs, sustainability, and economic aspects. This is merged into a set of practical methods and op-erational tools for choosing, designing, constructing, operating and upgrading the overall structural, environmental and socio-economic optimal tunnel type for each individual tunnel project. The project is performed in seven interrelated work packages (WPs). ‘Process Management’ (WP1) covers the multitude of technical and non-technical issues involved in the creation and operation of tunnels as part of a transportation network. ‘Reliability Based Service Life Aspects’ (WP2), ‘Envi-ronmental Aspects’ (WP3), and ‘Hazard Aspects’ (WP4) including risk analysis, each provide as out-put a set of operational tools for the stakeholders. DARTS developed these fundamental tools in tun-nelling into a format being mutually compatible. This is needed to create a truly ‘Integrated Design



and Redesign’ (WP5) including life cycle cost optimisation. The output of WP 5 has been validated already during the project period through calibration with ongoing and planned tunnel projects through a ‘Benchmarking’ (WP6). Exploitation and Dissemination (WP7) is ensured by enabling partners to optimally exploit the results and ‘products’ of the project in their day-to-day practice.

Project process flow

The DARTS-project is performed with financial support of the European Commission under the Fifth Framework Programme, Competitive and Sustainable Growth Programme (GROWTH 2000). General information on the DARTS-project, including links to the individual project partners, is avail-able through the web site of the project on the Internet: www.dartsproject.net. The Comprehensive decision tool (report R5.1) describes the DARTS Integrated (Re)Design Method-ology. Many reports form the basis of this report. Interested readers are referred to Chapter 8 of report R5.1 where all these reports are listed and to the accompanying report R5.2 with design examples and the CD-Rom which contains all reports.



1 Introduction In present day tunnel design practice the issue of hazards is treated in many different ways. For some projects a fairly advanced approach on the basis of risk analysis is being made, while for other projects hardly justified deemed to satisfy rules are being used. In order to stimulate an explicit and systematic hazard analysis, an open communication on data and experiences in this field is needed. Such type of prenormative research is believed to enable better cost optimised decisions when facing safety related design decisions. The aim of DARTS is to improve the foundation for optimal decisions for tunnel structures. Optimal-ity should be understood from a society perspective, meaning that the decision should be optimal for the society as a whole. Essential in DARTS is that completely different kinds of design aspects should be treated in the same way, according to the same principles and preferably integrated. DARTS Project Task 4.1 aims at the qualification and quantification of hazards (models, statistics) relevant to tunnel design, in terms of ♦ initiating events and subsequent hazard scenarios ♦ probabilities ♦ consequences Consequences may be fatalities, injuries, costs of repair, costs of collapse, traffic disturbances and im-pact to the environment. Next risk reducing measures should be identified and their costs should be compared to their effectiveness in risk reduction. The intention is to provide a basis for improving the present design procedures. Attention is given to both the construction and the exploitation stage. Where relevant a distinction will be made between the various types of tunnel use (road, rail) and con-struction methods (bored, immersed, cut and cover, NATM) for concrete tunnels, also taking non-structural aspects into account. In general, the modelling of a hazard includes the following main aspects: -Modelling aspects / scenarios A complete model describes the initiating events of the hazards as well as the actual actions and the consequences. Such a sequence is often referred to as a scenario. In the case of a fire, for instance, the initiating event may be a lorry defect leading to a release of fuel and some spark to start the actual fire. The subsequent burning process will then lead to the actual actions like smoke and increased tempera-tures. The process may be influenced by sprinkler installations, fire brigade actions, etc. The last step in the modelling concerns the consequences in terms of casualties, injuries, material losses, environ-mental losses and structural damage. The effect of rescue measures plays of course an important role here and should be part of the modelling. -Data Models need data which can be achieved on the basis of laboratory tests and observations in reality. The data serve as input to the models in terms of probabilities of occurrence (or frequencies) for the initiating events, conditional probabilities for follow up events and probability density functions for the various physical parameters. Where scatter is relatively low, deterministic point estimates may be sufficient. An introductory indication of statistics on serious tunnel accidents is presented in Annex 1.



This Annex gives 22 major accidents in tunnels in the past 30 years. The number of casualties varies from 1 to about 1000. When reading the table, one should realise that the total number of tunnel fires is much larger and that in most cases no casualties occur. -Cost aspects In order to find optimal solutions one should be able to express all items into one unit. Although some counter arguments exist, it will be assumed here that monetary values provides a consistent and ra-tional basis for decision making. This means that both consequences and safety measures should be expresses in terms of money. As far as possible, attention will be given to all three aspects for each hazard under consideration. This task will be used later in the DARTS project to define how the various combinations of hazard design options will be analysed and compared in a rational way and what tools are needed for both designers and contractors. Starting point is the Life Cycle Cost analysis approach. Hazards to be considered within this task are: ♦ fire ♦ contracting and construction ♦ explosion ♦ flooding and leakage ♦ collisions ♦ dropping and dragging objects ♦ sunken ships ♦ earthquakes ♦ toxic and aggressive materials Fire may be considered as one of the main hazards to be considered during tunnel design. For this rea-son this issue has received quite a lot of attention within this task, and it has been decided to report the results in a separate report (DARTS Document 138). Another important category of hazards are related to the contract and construction phase. Also for that aspect a separate document has been produced (DARTS Document 89).



2 General Background model for hazards The aim of this task is to develop a tool that can be used to find the relation between the design pa-rameters, the costs of safety measures and the performance. Typical design parameters are: tunnel depth, cross section type, dimensions, cross connections, material specifications, protecting measures, building techniques, traffic measures, etc. The performance can be expressed as the yearly risk for structural damage, traffic disturbances and casualties. For a partial and total Life Cycle Cost optimisa-tion, the risks should also be translated into monetary units. In addition the risk acceptance criteria should be specified. These criteria may put limits to the cost optimisation. In this task, as far as possible in a generic way, quantitative information is presented in terms of occur-rence rates, conditional likelihoods of subsequent scenario's and the effect on society, economy and environment. The task also includes an identification of the main gaps in knowledge and data needed for a design approach for tunnels in present design and re-design methods. Furthermore the effects of safety measures are indicated. The task is also a preparation for the development of integrated design tools (guides, software) in Task 4.2. In order to structure the information in Task 4.1 and the procedures in Tasks 4.2 and 5.3, the scheme as indicated in Figure 2.1 has been adopted. In the centre there is the so called “abnormal” or “un-planned” event. Usually one may identify a number of causes for this abnormal event which may be presented in a fault tree. Given the event, there usually exists a set of circumstances, all leading to different types or different intensities of consequences. This will be represented in an event tree. The consequences can be: casualties, short or long term damage to the structure, economic losses for own-ers and users of the tunnel, damage to the environment and so on. For all branches of the fault tree and the event tree probabilities can be estimated.

event tree consequences

fault tree initiating events

undesiredevent

measures

measures

measures measures

Figure 2.1 Background model for event sequences for a hazard.

In order to reduce the risk involved in accidental type of loading situations one might, as basic strategies, consider probability reducing as well as consequence reducing measures, including contingency plans in the event of an accident. Design with respect to accidental actions may therefore



pursue one or more of the following strategies, which may be mixed in the same design: 1. preventing the action to occur or at least reducing the probability 2. preventing the action to grow or to reach extreme levels (e.g. sprinkler installations) 3. non-structural measures to resist the action (e.g. by barriers against collisions) 4. structural measures to resist the action (strength of the structure) 5. mitigating the consequences (ventilation, evacuation, rescue, quick repair) Figure 2.1 shows the various places in the fault and event trees where measures may be active. Tunnels may be subject to several types of major accidents and hazard scenarios depending on loca-tion and type of structure. Because the probability of major events usually is low, the conditional dam-age consequences may be relatively large without being uneconomic. In the end the cost optimisation should give indications in this respect.



3 Design stages The design of a structure may take several years and can be subdivided into a number of stages. Gen-erally one makes a subdivision into the following four stages: Feasibility Study Conceptual Design Outline Design Detailed Design

In each stage a number of decisions are being made. Ideally all possible measures should be evaluated in detail for all possible design options and tunnel types at the start of the project . In practice this is not possible, or at least far too expensive. For the aspects of hazards we need to identify how to deal with it. Possible options are: consider only standard safety measures in the initial phases and do detailed optimisation only in

the final stages of the design. consider the possible later optimisations in the initial phase as processes with “uncertain” out-

comes for which Bayesian Preposterior analysis [Benjamin and Cornell, 1969] could be used.

A problem is also that stakeholders influencing the outcome of procedures may change. The fire bri-gade, for instance, may at some point in the process interfere with quite strict requirements. Their de-mands may be quite high, as they do not have to pay for the measures themselves. Some thinking on those issues, from the hazards point of view, is needed, at least in task 4.2.

As a general starting point of operation we may formulate the information in Table 3.1 for the various stages of the project. (Note that in every stage uncertainties related to the outcome of the next stages are present. It may be helpful to quantify those uncertainties). Some more detailed information on the various stages:

Feasibility Study

Standard costs of protection measures should be taken into account. Differences between various tun-nel types with respect to the costs of safety measures may be important. For instance, cross connec-tions usually are much more expensive for bored tunnels than for immersed and cut and cover tunnels.

Conceptual design

In the early stages the estimation of risk in the tunnel must be based on more general information. Hereby the resources for the analyses can be kept low and the associated uncertainty of the results will in most cases be acceptable. The estimate of frequency can be based for example on the following:

• Information collected by international associations. For fire in road tunnels: e.g. PIARC (Fire and Smoke Control in Road Tunnels, 1999)



Table 3.1 Information on fire hazards needed in the various design stages

Phase Feasibility Study

Conceptual Design

Outline Design

Detailed Design

Degree of accuracy

qualitative as-sessments.

Rough estimate Estimate Detailed estimate

Degree of detailing

Initial identifica-tion of hazards, risks and measures

Identification of hazardsand risk Identification of reducing measures Initial quantification Initial acceptance Criteria

Quantification of risk Study of risk reducing measures. Acceptance criteria

Detailed evaluation of cost efficiency of risk reducing measures. Evaluation of acceptance

Input General experi-ence

General experience Rough models General data

Experience Models Statistical information

Experience Models Statistical information Specific studies, Tests etc.

Output Evaluation of importance

Rough estimates of risks and costs as function of major design parameters.

Risk and costs as a function of basic design parameters Acceptability criteria

Basis for design and project documentation

• Information collected directly from tunnel operators (some tunnels similar to the one in question can be chosen)

• Information on open sections modified with the information about the situation in the tunnel • Information on accidents in tunnels as input for an event tree developing the accidents resulting in

fires, explosions, release of toxic materials and so on.

Outline design and detailed design

Frequencies of unwanted events at advanced stages can be estimated by use of for example fault trees and event trees, which make it possible to model the specific features of the tunnel

Further detailing of the frequency estimates can be relevant either because it is feasible to reduce the uncertainty or because the cost efficiency of risk reducing measures will have to be studied.

Fault trees leading up to the top-event may be relevant for evaluation of all the measures which can prevent or mitigate a hazard.



Evaluation of measures to reduce the consequences can be modelled in terms of event trees. Some of the events in the event tree can be modelled by small fault trees.

Figure 3.1 shows the efforts to be done in the various design stages for the various aspects for a bored tunnel. More grey means more effort. As said before, most effort is needed on fire and contracting and construction in the outlined and detailed design stage.

aspect parameter

feas

ibili

ty

conc

eptu

al

outli

ne

deta

iled

fire contracting and construction explosion flooding and leakage dropping/dragging objects sunken ships earthquakes toxic/aggressive materials

hazard aspect

Figure 3.1: Visual presentation of attention for the various hazards in design stages (bored tunnel).



4 Discussion of hazards

4.1 Explosion Description of explosion mechanism A gas explosion is a fast combustion of a cloud of flammable fuel-air premixture. After ignition a flame will propagate through the cloud consuming the fuel. Initially, the flame propagates through the medium in the form of a thin laminar flame front at relatively low velocity (a couple of meters per second). The temperature and the pressure of the burned fraction rise, which causes expansion and thereby a flow field ahead of the flame. The final pressures that may develop depend largely on the possibilities of expansion. Apart form the degree of confinement, the presence of obstacles is determinative for the explosion characteristics. Obstacles lead to an increase of the chemical active flame surface area and increase the intensity of the explosion in this way. If enough obstacles are present in the flow path and/or when a tunnel is long enough. the acceleration of the flame may lead to a transition from a heat transfer-driven propagation process (deflagration) into a pressure-driven propagation process (detonation). In a deflagration the flame propagation is determined by conduction and diffusion of heat and species. The developed overpressure is dependent on the flammable substance, the cloud size, the cloud com-position, the cloud position in the tunnel, ignition location and the presence of vehicles. The overpres-sure in common hydrocarbon-air mixtures may reach values from near zero up to about 1000 kPa. A detonation is a different physical process where shock-compression instead of heat transfer is the reaction triggering mechanism. Pressures at the shock wave front reach usually values of up to 2000 kPa. Models for describing the detonation process are the Chapman-Jouguet (CJ) model (see Nettle-ton, 1987) and the more advanced Zel'dovich- Von Neumann - Dorhing (ZND) model. However com-puter codes are required to solve all the equations involved in realistic scenarios. Some solutions for basic geometries exist, one of them describing the propagation of a detonation wave in a straight tun-nel. Gas explosions in tunnels have been the subject of experimental research programmes. Experiments in tunnels at small scale and at larger scale heve been reported (Wiekema 1978, Zeeuwen and Harmanny 1981, De Maaijer et al. 2000, Van den Berg et al. 2001 and Van den Berg and Weerheijm 2004). Re-ported overpressures in common hydrocarbon-air mixtures range from near zero up to many bars de-pendent on on the flammable substance, the cloud size, the cloud composition, the cloud position in the tunnel, ignition location and the presence of obstacles. In Van den Berg and Weerheijm (2004) tunnel load overpressures and impulses have been tabulated dependent on the cloud length. A tunnel with obstacles like cars is the best environment one can imagine to accelerate flames. Even without obstacles it is just a matter of tunnel length for detonation to occur. In case of the presence of vehicles, just about 10 tunnel diameters are required for transition from deflagration to detonation. It should be mentioned that these observations were made during experiments in which the tunnel was completely filled with a stoichiometric fuel-air mixture. Application of lean mixtures substantially re-duced the explosion overpressures. Application of rich mixtures, on the other hand, did not necessarily reduce the explosion overpressures because rich mixtures may mix up to stoichiometry in the expan-sion flow before combustion.



Another phenomenon is the so called BLEVE, which is the acronym for Boiling Liquid Expanding Vapour Cloud Explosion. A BLEVE is the consequence of the failure of a high pressure vessel con-taining a liquefied gas. If by any chance the vessel gets ruptured, the internal pressure drops to ambi-ent and the liquid starts to quickly evaporate, which leads to a big increase in volume and to a blast wave in the environment of the vessel. The blast depends largely on the rupture characteristics of the vessel. A fully instantaneous and complete rupture may produce substantial blast effects. Modelling (Van den Berg and Weerheijm, 2004) showed that if the release of liquid extends over just a fraction of a second, blast overpressures already substantially reduce. As a BLEVE is often the result of a fire, the flammable gas cloud often ignites directly a fireball is the result. Models and Data Explosions may be the consequence of accidents or incidents with trucks in a similar way as large fires. The event tree is given Figure 4.1. For part of the models and data reference can be made to the Task 4.1 report dealing with fire. The starting point of the tree is an event of a "serious accident or incident". In The Netherlands, for practical reasons, a serious accident is defined as an accident with at least one casualty. Note that this is a different starting point compared to the event tree for fire in the WP 4.1 Report on Fire and Smoke (DARTS report 138) which starts from the fire event in the tunnel. It should further be noted that especially the numbers in the columns "type", that is the relative portion of heavy good transport and dangerous good transport and the subdivision into LF (Liquid, flamma-ble), GF(Gaseous, flammable), LT(Liquid, toxic) and GT(Gaseous, toxic) depends very much on the road. Close to an important industrial area the traffic may be completely different form a typical rural area. Of course one should also keep in mind that these circumstances may change during the design life time of the structure. Based on the tree the probability of an explosion can be calculated as 3 10-14 per vehicle km. For a tunnel with a length of 1 km and a traffic intensity of 107 vehicles per year this leads to an explosion probability of 10-7 per year or order 10-5 in the life time. Typical pressure-time records observed in three locations for a deflagration in a (1:20) mockup of a tunnel tube are presented in Figure 4.2. Table 4.1 offers information on the pek pressures and duration depending on the amount of gas.



type type amount amount ignition MW other 0 <100kg 0,96 0 no 0,8 0 0,5 m3 0,15 indirect 0,1 200-300 pool fire LF 0.7 direct 0,1 200-300 pool fire serious no 0,8 0 1,00E-07 >100kg 0,04 5 m3 0,60 indirect 0,1 200-300 pool fire + explosion per vhkm direct 0,1 200-300 pool fire no 0,8 0 dgv 0.0002 all 0,25 indirect 0,1 200-300 pool fire + explosion direct 0,1 200-300 pool fire <100kg 0,98 0 GF 0,2 no 0 0 jet direct 0,8 300 flare continuous 0.70 indirect 0,2 300 explosion (small hole) no 0 0 jet direct 0,8 300 flare >100kg 0,02 continuous 0.20 indirect 0,2 300 explosion (small hole) no 0 0 cold bleve direct 0,8 300 cold bleve+flash fire instant 0.03 indirect 0,2 300 cold bleve+explosion impact driven instant 0.07 direct 0,2 300 warm bleve+flash fire

temperature driven

<100kg 0,96 0 LT 0.1 0,5 m3 0,15 0 toxic cloud >100kg 0,04 5 m3 0,60 0 toxic cloud all 0,25 0 toxic cloud <100kg 0,98 0 GT 0.01 continuous 0,7 (small hole) 0 jet+toxic cloud continuous 0.2 (large hole) 0 jet+toxic cloud

>100kg 0,02 instant 0.03 impact driven 0 cold bleve+toxic cloud

instant 0.07 temperature driven div

warm bleve+toxic cloud



Figure 4.1, Event tree for fire and explosion (starting with a serious accident)

Figure 4.2: Load-time records observed at three locations for deflagration in a (1:20)

mockup of a tunnel tube [Weerheijm et al., 2002]

Position from point of ignition

Cloud length

0 m 100 m 200 m 300 m 400 m 500 m

4 m p [kPa] ∆t [s]

45 0.22

40 0.27

33 0.30

30 0.32

28 0.32

10 00.02

8 m p [kPa] ∆t [s]

100 0.21

83 0.18

55 0.22

55 0.29

45 0.36

25 0.02

20 m p [kPa] ∆t [s]

190 0.36

180 0.28

120 0.39

110 0.45

85 0.53

55 0.03

40 m p [kPa] ∆t [s]

340 0.51

340 0.28

190 0.52

155 0.59

140 0.61

95 0.04

60 m p [kPa] ∆t [s]

460 0.36

550 0.33

240 0.66

220 0.72

190 0.60

140 0.09

Table 4.1: Peak pressures and explosion duration as a function of the gas cloud dimension



Detonation leads to much higher overpressures. Based on highly simplified modelling, the following approximate overpressure-time function may be taken into account, see figure 4.3:

21211

o cxt

cx

)c/c()x/tc(

p)t,x(p ≤≤

−−

−= for11

0.651 (4.2)

212o c

xLcLt

cxp)t,x(p −

+≤≤= for0.35 (4.3)

0=)t,x(p otherwise (4.4)

where: po is the peak pressure (1500 - 2000 kPa) c1 is the propagation velocity of the shock wave (~ 1800 m/s) c2 is the acoustic propagation velocity in hot gasses (~ 800 m/s) x is the distance to the ignition location (x > 0) (see figure 4.1.3) L is the distance from the ignition point to the nearest end of the tunnel (see figure 4.1.3). t is the time Figure 4.3 shows the pressure relative to the detonation shock properties at a location x relative to the location of the shock. The pressure decay after the shock results in a steady pressure of about 0.35 times the peak detonation pressure. At some point in time the reflection to the open end of the tunnel will lead to a further drop to ambient pressures.

t

p/po

1

0,35

0 x/c 1 x/c 2 L/c 1 + (L-x)/c2

* ignition

L x

Figure 4.3: Pressure in location x as a function of time for detonation initiated in the centre of a tube, filled with stoichiometric hydrocarbon-air mixture



Given the explosion time-pressure-diagrams for all points of the inner tunnel surface, the tunnel be-haviour can be analysed. For good analysis this requires a Finite Element Method analysis with op-tions for dynamic and nonlinear effects. Soil-structure interaction effects should be modelled in an adequate way. Material properties, for steel, concrete and soil, may differ from those used in normal static analysis. Weak points in structures may be inner walls and floors. In some cases collapse of those structural elements may initiate other failures. Even if the structure is strong enough to withstand the explosion, there usually will be fatalities, de-pending on the type of tunnel, the tunnel length, the number of people present in within the influence area of the explosion and so on. Models for estimating these numbers of fatalities are available [CPR 16E, 1992]. Lung damage is lethal beyond blast overpressures of 100 kPa. More critical is the drag effect. For example, at an overpressure of 50 kPa and an impulse of 20 kPa.s, a body is accelerated up to a velocity of 16.5 m/s. Impact at a wall at this velocity will lead to about 50% lethality. When peo-ple are still in their cars when the explosion occurs, the windows and wind shields may be hazardous. These are blown into the cars at blast overpressure levels of 30-60 kPa. In more detailed analysis for both structures and human beings, effects of explosions are often pre-sented in p-I-diagrams; where p indicates the peak overpressure and I the momentum (I = ∫ p(t)dt ) of the pressure wave. The figures 4.4 and 4.5 show some examples. In Figure 4.4 (lower diagram) the limits op p are presented for initial cracking, yielding of the rein-forcement and failure. These diagrams of course depend on the schematisation for the moment curva-ture relationship of the concrete (upper diagram). The figure in the centre represents the load dis-placement relation for the roof structure considered. Figure 4.5 shows a p-I diagram for skull base fracture. Similar diagrams exist for other types of seri-ous damage to the human body. These diagrams for a necessary step in the assessment of the number of casualties in the case of an explosion.



Initial cracking of concrete

Yielding of thereinforcement

M

κ

M

κ

M- diagrams corresponding withpossible plastic hinge locationsnear the clamped edge and in themiddle

R

∆

2 supportrotation

Initial cracking of concrete(edge or middle)

Yielding of the reinforcement(edge and middle)

10

100

1000

10000

1 10 100 1000Impulse (kPa*s)

Pre

ssur

e (k

Pa)

Initial cracking of the concreteYielding of the reinforcementFailure at 2 degrees support rotation

Figure 4.4: ∆P-I-diagram for structural response of an immersed structure [Weerheijm et al., 2002]



Figure 4.5: ∆P-I-diagram for skull base fracture [CPR 16E, 1992] Design options / preventive measures Preventive measures to mitigate the losses from explosions can be taken on various levels: - Limiting of transports of dangerous materials. - Shock absorbers to resist detonation pressures. - Vent openings to reduce the overpressure by deflagrative gasexplosions [14]. - Explosion suppression systems - Fire fighting devices - drainage systems and dense asphalt - explosion proof equipment In [2][4] sandwich type absorbers with soft fill in materials are described, leading to reductions of 200 kPa. In (The and de Bruijn, 1992) an experimental research is reported into the influence of venting through the roof of covered recessed highways on flame acceleration The idea was to prevent transition into detonation by sufficient roof venting, either through holes in the roof or by early failure of light weight roof panels. It appeared that this is a' promising concept. Although the experiments were quite illustra-tive, a tunnel with top venting can not exist in case it has to cross a waterway. In that case the bound-ary conditions for strong deflagration and even detonation are easily met and the corresponding explo-sion loads have to be taken into account.



In immersed tunnels in The Netherlands a relatively small explosion load is taken into account. A static equivalent load of 0,1 MPa (=100 kN/m2) in one tunnel tube, perpendicular to the walls, roof and floor represents the explosion load. For immersed tunnels the extra costs are low. Special attention should be given to a combination of moments, tension forces and shear forces in the center walls and in the roofs near the tunnel entrances. Note that for bored tunnels explosion loads have little or influ-ence on the dimensions of the concrete lining.



4.2 Flooding and leakage For internal flooding the central undesired event in Figure 2.1 is "unacceptable water in the tunnel". Such an event may be the result of the following initiating events: ♦ leakage directly through the concrete ♦ leakage through cracks ♦ leakage at the joints ♦ collapse of the roof or walls due to heavy loads (ships, anchors) ♦ inundation of the land at the tunnel entrance In the building stage of the tunnel, there may be a leakage of the temporary end caps for immersed tunnels and , cut&cover tunnels) or through the bore front for bored tunnels. Extreme flooding has oc-curred in the Storebelt tunnel where piping occurred in combination with a temporary open connection between the TBM and the tunnel. Also in Den Haag (Netherlands) a tunnel has been flooded during construction as result of leakage of the grout floor structure. As a rule the consequences in the exploitation stage are much less serious. Leakage through the con-crete, cracks and joints usually will not lead to complete flooding of the tunnel but maybe to inconven-ient when pumping capacity is not sufficient In DARTS Document 43 formulas are presented for leak-age. The main anticipating counter measure is to have a careful design and construction. In addition one needs sufficient pump capacity, which additionally should be prepared for heavy rain situations. Leakage as a result of calamities (for instance earthquake or landslide) usually will require repair. Serious consequences due to flooding are only to be expected in the case of structural weakness or abnormal loadings like anchor drops, sunken ships, earth quake and so on. In fact in those cases flood-ing is not the hazard, but just an additional serious consequence. In case of a big gap, the water will flow with a very high speed into one of the tubes. Therefore the load in one tube will not be spread immediately as an equivalent uniform load on roof, outer and inner walls or floor. Because of the loads (water and soil) from outside against the outer walls, the roof and the floor, damage on these construction parts however is not likely. What also might happen is that the area around tunnel entrances (either the main entrance or connec-tions with the outside world at stations, service corridors or emergency exits) is flooded because of high water, dike breach or what so ever. In such cases the water may enter the tunnel from one of the open ramps or other entrances and fill the tunnel tubes. In Prague this mechanism was responsible for a large damage to he Metro tunnel during the Moldau flood in 2001. In many cases the water pressures are uniformly and all sided, but for several reasons walls, floors and possibly doors can be damaged or even collapse because of one side (dynamic) loads. All possible consequences for the foundation and the overall stability should be checked. Also the effect of settle-ments may be quite substantial. Note further that not only the tunnel may get damaged, but due to the water flow through the tunnel otherwise well protected area may get flooded as well. Possible counter measures are inside watertight doors or the construction of all exits higher than some design flood level. The risk analysis to find out whether these measures are needed and how (cost) ef-fective they are starts with a normal flood risk analysis.



4.3 Collisions Description During a collision the kinetic energy of the colliding object can be transferred into many other forms of kinetic energy and into elastic-plastic deformation or fracture of the structural elements in both the tunnel structure and the colliding object. To find the forces at the interface one should consider object and struc-ture as one integrated system. Approximations, of course, are possible, for instance by assuming that the structure is rigid and immovable and the colliding object can be modelled as a quasi elastic single degree of freedom system. In that case the maximum resulting interaction force perpendicular to the tunnel wall is equal to [4.3.1]: F = vr sin α √(km) (4.3.1) vr = the object velocity at impact k = equivalent spring stiffness of the object m = equivalent mass of colliding object α = angel of impact This result can be found by equating the initial kinetic energy (mvr

2/2) and the potential energy at maxi-mum compression (F2/2k). In practice the colliding object will not behave elastic. In most cases the collid-ing object will respond by a mix of elastic deformations, yielding and buckling. The load deformation characteristic may, however, still have the nature of a monotonic increasing function (see Figure 4.3.1). As a result one may still use equation (4.3.1) to obtain useful approximations. See [JCSS, 1995] and [CIB W85, 1993] for further discussion. Note that F, of course, will never be larger than the upperbound in Fp in the load deformation curve.The duration of the load can be found to be: ∆t = 0.5 π √ (m/k) ≈ 1.5 √ (m/k) (4.3.2) It should further be taken into account that the load (4.3.1) is a travelling load: the speed of the vehicle in the longitudinal direction of the tunnel is vr cos α. The formula (4.3.1) gives the maximum force value on the outer surface of the structure. Inside the struc-ture these forces may give rise to dynamic effects. If the load is conceived as a step function the dynamic amplification factor is 2.0. However, as the load is a travelling load, it is present at every location for only a short while, a dynamic load factor equal to 1.0 may be assumed as a conservative estimate. In cases of doubt a FEM analysis may be applied to calculate the stresses and deformations in the structure.

Figure 4.3.1: General load displacement diagram of colliding object



Data road tunnels The event that the intended course if left is modelled as an event in a Poison process. In most countries statistics are available for various road types, mainly for highways. To give some indication: according to [4.4] the probability of leaving a highway is about 10-7 per vehicle per km. Higher and lower values will occur in practice, depending on the local circumstances. The probability of a collision for a given tunnel length L and period of time T then may be estimated as: P = λ n L T n = number of trucks passing per time unit L = tunnel length T = reference period under consideration (one year or life time) Taking n = 3000 trucks per day this leads to a collision once every 10 years in a tunnel of 1000 m. The velocity of a vehicle in a road tunnel depends on the type of tunnel, the mass of the vehicle and the traffic intensity at the time. The distribution of the velocity conditional upon the event of leaving the in-tended track, however, is not known. As long as no specific information is available, one might assume the conditional and unconditional velocity distributions to be the same. The direction angle α for road traf-fic varies from 0 to 400 .There are no indications that v0 and α are dependent for straight sections of the road. Table 4.3.1 gives further information on Table 4.3.1: Summary of data for a collision hazards in road tunnels

variable designation type mean stand dev

λ accident rate deterministic 10-10 m-1 -

α angle of collision course rayleigh 10o 10o

v vehicle velocity Lognormal 80 km/hr 10 km/hr

m vehicle mass Normal 20 ton 12 ton

k vehicle stiffness deterministic 300 kN/m -

To give some indications: for a velocity v = 80 km/hour, a mass m = 30 ton, an equivalent stiffness k = 300 N/m and an impact angle of 300 the resulting point load is about 1000 kN. The force is on a dis-tance of about 1.0 m above the road surface and the contact area will be in the order of area 0.4 x 1.0 m. The force will be moving with a forward speed of 40-60 km/hour. Data railway tunnels Trains may derail and hit the structure or some protection system. Initiating events for derailment may be:: • excessive nosing intensity • excessive centrifugal forces • broken parts of the wheels or axles



• broken rails • material/equipment on the rail From research in The Netherlands a derail frequency of 10-8 / km for passenger trains and 3 10-7 / km for freight trains was found; about 50 percent was caused by switches. Derailment may lead to a moving concentrated force of 10 MN tgϕ on the safe guard system at a dis-tance of 0.4 m above the rail (area 0.25 x 0.25 m). The angle ϕ may depend on the geometry of the tunnel. A derailed and/or fallen wagon may give rise to forces of 18 kN/m at a height of 4 m above the rail. Frontal collision forces may go up to 10 MN. For further information see [UIC, 1992] and [Grob, 1993]. Design options/measures As long as the tunnel wall is a smooth one (not a row of columns), the collisions do not give many problems. If good barriers are present and the effects of impact are low. Further possible measures are:

• No switches • Guidance structures • Hot box detection • Detection of rail breaches



4.4 Dropping and dragging objects Description Although it should be prohibited to use anchors in a zone around the tunnel, a ship may be in a situa-tion where the anchor is dropped anyhow. Anchors may be dropped in a normal way (the shipper is not aware of some restriction) or the anchor drops as an accidental action (rupture of the chain of fail-ure of the hoist). Consequently it is normal design practise to design the tunnel for such events. The dropping anchor seems to be more harmful than the dragging one, so we will restrict ourselves to the first category. Data To determine globally the probability of an anchor loss with a certain mass above a tunnel, the follow-ing data are needed:

- Number of movements of ships during a certain time above a tunnel; - Chance (risk, possibility) of a falling anchor per movement of a ship; - Chance (risk, possibility) of a falling anchor, that hits the roof of a tunnel; - Division of mass of anchors of representative ships;

The equivalent static load and the penetration of the anchor in the fill-up material on top of the tunnel mayl be calculated according to CEB Bulletin d’Information no. 187 [3]. Starting points are:

- Mass of the anchor [kg.]; - Cross section area of the anchor [Ae]; - Velocity of the anchor on the moment of impact [m/s]; - Depth of the covering [m]; - Modulus of Elasticity of the fill-up layer on the tunnel [kN/m2]; - Maximum dynamic load factor (pure impulse);

Anchor losses have been observed to be roughly once per 20 year per ship [Wens et all, 1222]. Most of them occur during normal anchoring operations. Only 10 percent occurred during sailing. In a tun-nel under a canal or river with busy ship transport, anchor dropping may be expected to occur once per 100-1000 years. Information on anchor shapes and masses can be found in the Anchor Manuals [Anchor Manual, 1990]. Standard type of anchors are stockless anchor, stocked anchor, high holding power anchor and stream anchor. For many purposes, however, the exact shape is not so important and a schematisation as a bar will be sufficient. Anchor masses may vary from 100 kg to several tons and the length may vary form 0.5 m to several meters. The mass of an anchor may be estimated from the ship dimensions: m = 3 [ ∆2/3 +2 B h +0.1 A ] where ∆ [ton] is the water tonnage, B [m] the ship width, h [m] the distance from water level to the level having a width equal to B/4 and A [m2] the total side area. Ship classes and corresponding di-



mensions are usually known for a given location. The velocity at touch down with the bottom is usually (for larger water depths) in the order of magni-tude of 7 – 9 m/s. Given an anchor drop on a tunnel roof, a FEM analysis might be used to check the structural conse-quences. The model should include the top tunnel slab and the ground or grind layers on top. Material properties should include nonlinear and dynamic effects. Especially for the ground, one should keep in mind that penetration may be as deep as 3 m. Design options Dropping anchors are relevant only for immersed types of structures under waterways. Where a nor-mal concrete roof, protected by a standard thickness of ground (1,00 m) is not enough, asphalt or low strength concrete layers (70 mm) could be used as an additional protection material.



4.5 Sinking or sunken ships Description Sunken ships are a threat only to immersed tunnels under a waterway. If the waterway is used by ships, every ship has some small probability to sink on the roof of the tunnel. The loading is relevant only for those tunnels having a small cover, like immersed tunnels and cut and cover tunnels. In tun-nels with a large cover (bored tunnels for instance) the load is negligible. Where relevant, the probability of the distributed load p [kN/m2] exceeding at any point of the exposed cross sections some value x in one year is given by:

P(p>x) = 1- exp{ - 2 λ L n T ∫ f(G) dG ) } (4.5.1) The integration is from BLx up to infinity. Further:

λ = probability of a ship to sink per m n = number of passing ships per year B = width of the ship [m] L = length of the ship [m] T = period under consideration G = weight [ton]

f(G) = distribution of ship weight The load on the tunnel further depend on dynamic effects and soil cover on the tunnel roof Data If no further information is available, the yearly probability of a ship hitting a tunnel may be taken as about 10-5; the design load intensity p is in the following order of magnitude: Sinking ship

Category: Load (including dynamic effects)

Area: (L x b)

Inland navigation ships 75 kN/m2 3 m x b Sea-going ships 200 kN/m2 3m x b

Sunken ship

Category: Load (including dynamic effects)

Area (Lxb)

Inland navigation ships 50 kN/m2 L x b Sea-going ships 150 kN/m2 L x b

The loads in these tables are based on ships with heavy cargo like steel (rods) or iron ore. The combi-nation loads should be placed in a representative situation, depending on the construction part under consideration.



Design options Make an adequate cover to reduce the loads and the corresponding failure probability. No anchoring in the neighbourhood of tunnels should be permitted.



4.6 Earth quakes Description Earthquakes usually result from the movement and interaction of lithosphere plates. During teh proc-ess elastic strain energy builds up, When these stresses reaches their limit values the rock material fails and the accumulated strain energy is released. The release is done in the form of seismic waves and heat [Kramer, 1996]. The effect of earthquakes on underground structures may be grouped into the following two classes [St. John & Zahrah, 1987]: Faulting Shaking

From the structural point of view faulting includes all other cases in which major soils displacements occur e.g. from liquefaction, landslide or other soil instabilities which are indirect consequences of shaking. From a practical point of view, it is not feasible to design structures to restraint major soil displacements, which in the case of faulting could be in the order of several centimetres or even me-ters. It is thus more practical to avoid the sensitive areas, or if that is impossible to localise the poten-tial damages in order to optimise the repairs. As far as shaking conditions are concerned, in the design of a tunnel, an evaluation of the vibration waves needs to be performed. The seismic waves are divided into 2 types of waves namely body waves and surface waves. The body waves are again divided into primary waves and secondary waves and the surface waves are divided into Rayleigh waves and Love waves [B. A. Bolt, 1993]. The equation of motions for body waves and surface waves in an elastic medium can be derived mathematically [Achenbach, 1975].

Models and data

The estimate of the movements of the bedrock at the tunnel site can be made by considering: Historical records of seismic activity Other recent seismic hazard assessments for nearby structures Geology at the site Active faults likely to produce a seismic event affecting the tunnel site and vicinity

The following models may be of a help:

Magnitude model

For seismic design one should have information on the earthquake statistics at the building site. To start with one needs statistics on the earth quake magnitude M for all seismic active areas and faults in the environment of the construction site. Usually the number of earthquakes per year exceeding a given magnitude may be expressed as:



ln ( ) ( )oN m A B m m= − − for ( )omm > (4.6.1) Here )(mN is the average number of earthquakes per year with a dimensionless magnitude M larger than m , which occurs in the given seismic active region This relation is referred to as the Gutenberg - Richter relation. Earthquakes of which omM < are not taken into account. They are considered not to be relevant for the structures ( 0m for example may be equal to 3.0). The number of earth quakes per year with omM > is equal to:

No = exp( )A (4.6.2)

For a given earthquake it can be stated that the probability exceeding m is given by:

{ } [ ]exp ( )oP M m B m m> = − − (4.6.3)

The seismic hazard at a tunnel site can be quantified by a project-specific seismic hazard assessment. Attenuation model

The relation between an earthquake with magnitude M occurring at a distance R and the peak accel-eration at the building site (see Figure 4.6.1) can in general be written as:

ˆ ( , )ga f M R ε= ∗ (4.6.4)

with: M = the magnitude of the earthquake R = the distance to hypocenter ε = a model uncertainty factor with a mean equal to 1.0

gâ = the peak ground acceleration at the construction site

In here R is a random variable too as the hypocentre of the earth quake may happen anywhere along a

fault or within some area.

M (magnitude)

source area j

R

ˆga

site

Fig.4.6.2 Attenuation Relation: ˆ ( , )ga f M R ε= ∗ .



Multiple source model

Given a number of seismic zones and faults in the neighbourhood of the structure, the total number of earthquakes occurring at the construction site with a peak acceleration ˆga larger than a can be ex-pressed as follows:

{ } { }aâPNaâN gjjj

g >∑=> (4.6.5)

{ }aâN g > = the number of earthquakes at the site with ââg > per year

gâ = the peak ground acceleration

jN = the number of earthquakes per year in source area j

{ }aâP gj > = the probability to have an earthquake with ââg > at the site due to area j



Neglecting the model uncertainty and if each source area j is small enough that the distance R may be considered as a constant, we may write for each source area:

{ } { }mMPaâP jgj >=> (4.6.6)

where m follows from ˆ ( , )ga f M R ε= ∗ . Based on the result (4.4.6) we may calculate the probability that that the peak ground acceleration gâ for a given site and for an individual earth quake exceeds a value a according to:

)0(/)()( >>=> ggg âNaâNaâP (4.6.7)

The return period TR corresponding to the earthquake intensity a may be derived from:

{ }aâNT

gR >=

1 (4.6.8)

Vice versa one may use this equation to find a design value for the ground peak acceleration gdâ that corresponds to a given value for the return period TR (see section 2). Remember that the error term ε has been neglected. If it is taken into account the return period for a certain level will go down.

For the probability that the earth quake intensity exceeds the value a in an arbitrary period T we may write:

{ } { } { }1 expT g g gP â a T P â a T P â a > = − −ν > ≈ ν > (4.6.9)

where ν = { }0âN g > is the number of earthquakes per time unit. The last step is valid only if

{ }aâPT g >ν << 1. Note that we may derive similar formulas for the failure probability of the structure according to a full probabilistic analysis. In that case:

{ } { }[ ] { }0ZPT0ZPTexp1FailurePT <≈<−−= νν (4.6.10)

where Z is the limit state function. Earthquake motion model The soil movement during an earth quake is a function of time (see figure 4.6.2) that successfully can be modelled as a zero mean Gaussian stochastic process. In order to describe the frequency content, Kanai and Tajimi have derived the following expression for the (one sided) spectral density function for the stationary strong motion part of the earth quake (see Figure 4.6.3):

Saa(ω) = 2gg

222g

2gg

2o

)/(4])/(1[

])/(41[G

ωωζωω

ωωζ

+−

+ (4.6.11)

In fact, they assumed that the earthquake acceleration process at a given site can be taken as the out-come of a simple one-degree-of-freedom oscillator driven by stationary white noise. In this model Go is a scaling factor, the parameters ζ and 0ω are chosen on basis of the dynamic properties the local soil. The variance corresponding to this spectrum can be determined to be:



∫∞

+==0

g2

g

g0aaA

2 )41(4G

d)(S ζζωπ

ωωσ (3.12)

Usually ζ is in the order of 0.60 and oω may vary from 5 to 50 rad/s, depending on local geological conditions.

Fig. 4.6.2 Time domain record of ground acceleration (example))

Saa(ω)

0G

0ω ω

Fig. 4.6.3: Shape of the variance spectrum aaS

The strong motion duration represents the time interval over which the motion intensity is almost con-stant and near its maximum. It is preceded by a relatively short rise time and followed by a relatively long decay period. Figure 3.2 gives a schematic impression. In the decay time the frequency content of the signal may shift to the lower range. Different strong motion duration definitions can be used for different seismic records.

σ(a) strong motion part

Ts ( ≈10 s)

time Figure 4.6.4 Shape of earthquake intensity

The above time intervals depend on the intensity of the ground shaking. For rock conditions and mag-nitudes of order of M = 5 - 8, one may assume a rise time of 1-3 s and a strong motion time of 5-15 s;



the decay time is usually longer then the strong motion time.

According to the theory of random processes, peaks in a narrow band Gaussian random processes have a Rayleigh distribution:

−=

>∧

2a

2

2aexpaaPσ

(4.6.12)

Although the spectrum is not very narrow, we will base ourselves on this result. The number of peaks to be expected in the strong motion part is equal to:

N = Ts / To (4.6.13)

where Ts is the strong motion duration and oo /2T ωπ= .If we further assume that the peaks and troughs are stochastically independent, the expected maximum peak (or trough) may follow from:

{ } 2a

P exp2aa aσ

∧ > = −

2

= 1

1 2N+ (4.6.14)

From this we get for the expected largest peak:

aˆE{ } 2ln(2 1)a Nσ= + (4.6.15)

Using this equation we may calculate the standard deviation for an earthquake with a given peak ground acceleration.

Horizontal and vertical components Horizontal and vertical accelerations can be assumed independent as well as any two orthogonal hori-zontal components which is exactly true for the radial and tangential components for shear waves. The intensity parameter for vertical accelerations is roughly equal to the corresponding one for horizontal motions for small epicentral distances (≈ 20 km) and falls off down to 50 to 30 % for larger epicentral distances on firm grounds. On soft grounds vertical motions can generally be disregarded. For all translational components the same type of spectral density function can be used.

Spatial Correlation

Given an earthquake ground accelerations are spatially correlated at any point in time. The following form of the auto-correlation function is suggested: ρ(r) = exp [- αr2 ] where r is the distance in [m] between 10-6 (for firm ground) and 10-7 (for soft ground). For r < 100 m full correlation may be as-sumed.



Design methods

Under extreme conditions heavy structural damage due to strong earthquakes never can be ruled out completely, but the design should be such that the probability of complete collapse is sufficiently small. Furthermore, the damage caused by a more common level of earthquakes should be limited. It should also be noted that not only the limitation of direct damage to structures is important. The limit states are situations beyond which the behaviour of the structure is no longer satisfactory. In earth-quake design limit states usually are defined as:

• Initial cracking (Serviceability Limit State) • Repairable local damage (Ultimate Limit State, local level) • Overall collapse (Ultimate Limit State, global level)

The limit state with respect to global collapse is not only concerned with life safety, but also with the survivability of the structure under very severe seismic events at the location. The level of probability of this event is considered to be comparable to values in the range of once in 2000 years. The limit state with respect to local (yielding, crushing) may be compared with normal ULS design. The failure probability should normally be in the order as once in the 100-500 years. The failure probability of the SLS criteria may be in the order of once in the 10-50 years. In the past many immersed tunnels have been constructed in seismic active areas. Advanced computer programmes can simulate the actions on the tunnel structure so that they can be taken into account in the design. The critical components are the joints, which have to be designed for big forces and movements. This makes immersed and cut and cover tunnels more vulnerable than bored tunnels. Randomness exists in both the loads on a structure and in its capacity to resist them. Earthquake mo-tions are inherently random. Even with increased knowledge, there will be large randomness in both the magnitude as in the point in time and place where the earthquake will occur. Further there is uncer-tainty in the attenuation characteristics and the local soil conditions. Structural behaviour is affected by random variations in material properties (in particular strength, deformation capacity and damping), the construction quality and the possible state of deterioration. Another source of uncertainty is the fact that both strength and deformation capacity are sensitive to loading history (low-cycle fatigue) and rate-of-loading effects on material strength and deformability.

A full probabilistic seismic analysis is difficult to accomplish and simplifications are necessary. From the theory of reliability it is known that one usually should take the most dominant variables at the de-sign level and take means or characteristic values for the others. Bearing this in mind, and considering that the seismic load and the local soil conditions are the dominant variables, it is usually not neces-sary to treat other quantities as random.

Another point is the soil -structure-interaction. When a seismic wave hits a rigid boundary like the tunnel lining the wave reflects with opposite amplitude, when a seismic wave hits a free boundary the wave reflects with the same amplitude and if a seismic wave impinges an elastic boundary the wave is reflected and the refracted. The overall behaviour of a tunnel however is more sensitive to the distor-tions of the surrounding ground than the inertial effects. Grounds movements may be determined without considering the tunnel, in a “free-field” ground deformations study. After this “free-field” study an analysis of soil-structure interaction is carried out. The response of the tunnel to the free field soil displacements must consider the stiffness of both the tunnel and the soil. While this complex analysis may be solved numerically using computers, some simplified procedures have been pub-



lished.

Finally there is the reduction of a full 3D calcuations into a number of 2D calculations. The most im-portant deformations of the tunnel are (see figure 4.6.5): worming, snaking and racking (or ovaling for bored tunnels). In most cases it is sufficient to consider only the earthquakes propagating in the longi-tidunal direction. We then need a (two dimensional) beam model to analyse the effects of the P- and the SV waves. For immersed tunnels it is essential that the joints are well modelled, as these are the places where leakage may be expected. A 2D-cross-sectional model is needed for the effects of the SH-waves. Special "anchors" may be needed to simulate the stiffness of the tunnel in the longitudinal direction.

Figure 4.6.5 Deformation modes of underground structure.



4.7 Toxic and aggressive materials Description

It is a rare event that toxic or aggressive materials cause unwanted consequences in tunnels. However, these materials are a potential hazard, which is important to explore. Toxic or aggressive materials in a tunnel may originate from various sources:

• As an immediate consequence of an accident involving hazardous substances • As a subsequent event to another unwanted event, e.g. as the result of fire • As a consequence of a non-accidental release of a hazardous substance transported • Originating from combustion engines in the vehicles • The tunnel material itself • The surrounding environment Considering the nature of the release (short or long-term), chemical materials in a tunnel may have an impact on:

• The users of the tunnel or an emergency crew. Consequences of sudden releases in high concen-trations may be immediate fatalities or injuries requiring hospitalisation

• Maintenance personnel. Consequences of a long-term exposure may be industrial injuries. • The environment: releases may pollute terrestrial habitats, freshwater or marine habitats, or under-

ground water • The tunnel itself. Consequences may be damage to tunnel structure, lining, or installations causing

disruption of traffic or requiring repairs. Hazardous/aggressive goods Release of unwanted materials in the tunnel may occur as a consequence of a car or railway accident involving hazardous/aggressive goods or they may occur due to simple spills from such containers. If the container is damaged/opened toxic/aggressive material in high quantities may be released within a short time interval. The released compounds may have various hazardous characteristics amongst which the most impor-tant are: flammable/explosive (considered elsewhere), toxic, corrosive, radioactive or abbrasive. The extent of the unwanted consequences depends on the hazards of the particular substance involved and the conditions for the release. Consequences of a sudden release of a hazardous substance may result in harm to humans, damage to the environment and damage to the tunnel and its installations. For these accidental events in particular the following preventive or consequence limiting safety measures are of importance: • Prohibition of certain types of transport (extremely toxic or aggressive chemicals, radioactive sub-

stances) • Systems allowing rapid awareness of an emergency (video cameras, gas alarms) • Ventilation system removing toxic gases and supplying fresh air. • Evacuation planning allowing fast escape to safe areas • Planned emergency response • Materials selected for drain systems and installations resistant to the corrosive characteristics of

substances known to occur frequently in the tunnel.



Exhaust gases Exhaust gases from petrol and diesel engines in combination with oxygen deficiency may cause poi-soning/suffocation of persons if they are caught inside the tunnel and no fresh air is supplied. How-ever, unless there is a deficiency in the ventilation system or the ventilation system is broken down due to an accident, suffocation is unlikely. The time until the exhaust gases have developed into a critical situation with respect to suffucation (i.e. lack of oxygen) will give good chances for evacua-tion. The poisoning effect of the exhaust gases is limited with modern engines, catalytic converters, etc. Exhaust gases may also have an impact on the ageing process of the materials used for tunnel lining and installations. This issue is a part of DARTS Workpackage 2. Toxic substances from construction materials Within a shorter or longer time after construction of the tunnel or after major repair works the tunnel materials might give off toxic fumes or toxic substances dissolved in water. The concentrations will be low and this is not likely to cause harm to humans. However, authorities may require the tunnel to be closed until conditions are improved. In case new and untested construction materials are used hitherto unknown toxic characteristics may give rise to unexpected high costs for remediation. Toxic substances from the surrounding environment Toxic fumes may be released inside the tunnel due to the occurrence of toxic substances in the sur-rounding geological/man-made environment. As above this is not likely to cause harm to humans, but it may have an impact on tunnel availability and costs (See Workpacckge 3) Models and data Hazardous goods Numerous projects on transportation of hazardous substances have been carried out. In connection with design of a number of major infrastructure links, studies of traffic load and types of hazardous substances transported have been carried out. Further a number of general studies have been under-taken. Statistical material describing accidents during transportation of hazardous goods on road and rail have been collected and analysed for European and North American countries. Frequencies for both accidental (typical: collisions) and non-accidental spills (typical: valve left open) are available. A review [OECD/PIARC, 1998] of previous analyses [Meyers ,1981], [CETU, 1997], [SAVE, 1997], [Amundsen et all, 1997], and new data provides estimates of accident frequencies in road tunnels. Furhter material may be found in [Heilig, 2002]. Studies of accidents with hazardous goods apply calculation models for assessment of consequences of releases of the most common chemicals. Often the wide range of chemicals transported is repre-sented by a few "worst cases". Typical compounds are: chlorine, ammonia, cyanides, sulphuric acid and other acidic compounds.. Calculation models comprise estimation of release and evaporation rates, estimation of concentration of toxic substances in the air and assessment of consequences on humans. Several simple computer models are commercially available at a low cost. More elaborate assessment of e.g. the performance of a ventilation system may be carried out by use of CFD-methods.



Design options For some cases it may be relevant to check pump- and ventilation capacities in relation to quick re-moval of toxic material.



5 Conclusions Summary This report describes the modelling of initiating events as well as further processes and final conse-quences for a number of hazards in road tunnels. Data is presented that has been found in the literature on the basis of observations in the laboratory as well as in the field. The data serve as input to the models in terms of : • probabilities of occurrence (or frequencies) for the initiating events • conditional probabilities for follow up events • probability density functions for the various physical parameters. In order to find optimal solutions the effectiveness and costs of anticipating and mitigating measures needs to be estimated. Comments During the project it is concluded that a complete data set for all possible tunnel types and circum-stances could not be produced. For that reasons a choice was made to lay emphasis on the (uncer-tainty) analysis rather then on the completeness. Another question was whether the work in this task should concentrate on structural safety or on hu-man safety. DARTS, of course, is primarily concerned with the structure and not with human safety. However, optimisation is concerned with both. As a result, safety of persons has to be taken into ac-count, but no special research in this direction has been (or will be) developed. In some cases it is possible to find generic probabilities and costs for the standard safety situation and the safety measures, in some other cases the probabilities depend on so many specific parameters like tunnel type and traffic flow characteristics that a generic description is not possible and specific de-scriptions involve too much work. However, the procedure itself and the demonstration to example cases are considered as being of major importance. Gaps of knowlegde In the process of producing this document the following main gaps of knowledge were observed: • probabilistic models for initiating events • effects and costs of quite a number of measures



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Earthquakes

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Annex 1 Major tunnel fire accidents in the past 30 years [Ammundsen, Haack, OECD, Ineris] Date Tunnel accident Type of

tunnel Casualties

14-02-71 Vranduk, Yugoslavia. On the track Zepce – Zenica. Derailing of the train

Railway 34

16-06-72 Soissons, France. Collision of two car trains.

Railway 108

6-11-72 Fukui tunnel, Japan. Fire in the restaurant car of the Kitaguna Express

Railway 29

28-02-75 London, UK Failure of the driver of metro at Moorgate station.

Metro 43

11-08-78 Velser, the Netherlands Collision

Road 5

11-07-79 Nihonzaka, Japan Collision and consequential 7 days fire.

Road 7

17-04-80 Kajiwara, Japan Gearbox

Road 2

15-07-80 Sakai, Japan Collision

Road 5

07-04-82 Caldecott, USA Collision

Road 7

3-11-82 Kabul, Afghanistan. Convoy of army trucks collides with tank-car that explodes.

Road 700-1000

1983 Peccorila Galleria, Italy Collision

Road 9

09-09-86 L'Arme, France Collision

Road 3

1987 Gumefens, Switzerland Collision

Road 2

18-11-87 London, UK A burning match causes fire at King Cross metro station.

Metro 31

1993 Serra Ripoli, Italy Collision

Road 4

10-09-95 Pfänder, Austria Collision

Road 3

28-10-95 Bakoe, Azerbeidzjan. Fire caused by a short cut.

Railway 337

10-02-96 Haikkaido, Japan. Collapse of the roof of the tunnel

Road 20

18-03-96 Palermo, Italy. Tank-car explodes in car tunnel after a collision.

Road 5

24-03-99 Mont Blanc, France – Italy. Lorry loaded with flour and margarine catches fire.

Road 39

29-05-99 Tauern, Austria. Lorry loaded with paint collides with car and explodes.

Road 12

11-11-00 Kaprun, Austria. Fire in train in the Kitzsteinhorn tunnel.

Funicular railway

187

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