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69 Figure 14. Empirical correlation of seismic ground shaking induced damage to bored tunnels [Ref. 19]. Properties of structure, liner, and initial support: In gen- 4.4.5 Damage Potential Rating of Tunnels eral, a structural liner with greater thickness, greater rela- tive structure or ground resistance, more confinement Based on the data and discussions presented herein, as well reinforcement (in concrete lining), higher ductility, and as the hazard and threat scenarios discussed in Chapter 2, better framing design (e.g., moment-resisting properties) Table 13 shows a damage potential rating chart for trans- tends to perform better under extreme loading events, portation tunnels. For rating purposes, the following primary especially if high external confining pressures exist. As hazards and threats were considered from the structural eval- mentioned previously, Figure 11 presents a rough estimate uation standpoint: of the required tunnel liner thickness (for reinforced con- Introduction of small IEDs, which are delivered via one to crete) as a function of the explosive charge weight and the charge standoff distance. five aggressors transporting the payload in suitcase-type bags on foot and consolidating at a critical location inside the tunnel. Table 12 presents relative severity ratings of tunnels based Introduction of medium IEDs, which are delivered either on some of the critical factors discussed above. The infor- by vehicle (car) or by multiple persons acting in concert to mation in this table is based on recent tunnel security proj- transport the payload and consolidating at a critical loca- ect experience and expert opinion. This chart has been tion inside the tunnel. prepared in a qualitative manner, and therefore should be Introduction of large IEDs, which are delivered either by used as such. vehicle (truck) or by multiple persons acting in concert to