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51 CHAPTER 4 Tunnel Elements and Vulnerabilities 4.1 Introduction pressures developed from the tunnel boundaries when an internal explosion occurs, and (4) a coupled behavior of air When considering the role of the 550 U.S. highway and blast inside the tunnel and wave propagation through the sur- transit tunnels in the overall transportation network, and rounding ground. considering the lessons observed from natural disasters An understanding of the characteristics of various types of and the transportation-related consequences of the Septem- tunnels is essential in performing an accurate vulnerability ber 11th attacks, it is clear that loss of a critical tunnel at one assessment. For example, an immersed tube tunnel is partic- of numerous "choke points" could result in hundreds or ularly vulnerable to rapid flooding in the event of an explo- thousands of casualties; billions of dollars of direct recon- sion. This high risk is due to the following features of the struction costs; even greater socioeconomic costs; and ancil- immersed tube tunnel: (1) the tunnel is under high hydro- lary costs to other institutions in the nation's complex, static water pressure; (2) the tunnel is surrounded by porous interrelated economy. For these reasons, transportation backfill materials; and (3) there is a limited thickness of soil agencies must conduct systematic reviews to understand cover over the tunnel. The vulnerability of an immersed tube their facilities, identify their vulnerabilities, and develop pro- tunnel should therefore be assessed not only from a structural tection strategies. damage standpoint, but also from a flooding potential stand- This chapter describes important elements of various point. Similarly, cut-and-cover tunnels are typically built in tunnel construction methods used for transportation tun- shallow depth with limited backfill above the structures; this nels. Discussions on the failure mechanisms associated with build reduces the amount of confinement in the vertical hazards and threats are included. In addition to the con- direction when the tunnel is subjected to either internal or struction method and general tunnel vulnerability assess- external explosions. In addition, cut-and-cover tunnels are ment, a comprehensive description of the various tunnel usually built in soil sites and therefore tend to be less blast liners and structural systems is provided. The critical factors resistant than tunnels surrounded by rock. Conversely, bored that could affect structural behavior in the event of safety- or mined tunnels in rock with deep cover are more resistant related hazards or security-related threats are then intro- than those in soils with shallow cover. Therefore, the type of duced and related to the various features of the different tunnel has a major impact on the tunnel's vulnerability to tunnel systems. extreme events. The results of this chapter lead to the Chapter 5 guidelines for use by tunnel and facility owners and operators to iden- 4.2 Types of Transportation tify (1) critical locations in tunnel structures from the opera- Tunnels tion and safety standpoints and (2) countermeasures appropriate to those critical locations. Tunnel types can be categorized to a certain extent by their Because of the unique features of transportation tunnels, usage, or mode of transportation. The functional types of the structural response of a transportation tunnel to a hazard tunnels included in this report are as follows: or threat differs somewhat from that of a surface structure. The most notable of these features are (1) a high ratio of lon- Road, gitudinal length to cross-sectional dimension, (2) a complete Transit, and confinement by the surrounding soils and rocks, (3) reflected Rail (both passenger and freight).