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68 earthquake motions [Ref. 17]. The main characteristics of shaking. Data for cut-and-cover and immersed tunnels are these case histories are as follows: not included in the figure. These tunnels served as railway and water links with diam- 4.4.4 Critical Factors in Vulnerability eters ranging from 10 to 20 feet (3 to 6 meters). Assessment of Transportation Tunnels Most of the tunnels were constructed in rock with variable rock mass quality. Because of the nature of underground structures, the vul- The construction methods and lining types of these tunnels nerability of a tunnel must be assessed by considering the varied widely. The permanent ground supports ranged from interactive effects of the blast pressure, the structure, and the no lining to timber, masonry brick, and concrete linings. surrounding ground. The critical factors that could have an impact on structural vulnerability in response to hazards and Based on their study, Dowding and Rozen concluded, pri- threats are summarized below: marily for rock tunnels, the following: Type of tunnel (i.e., construction type): In general, Tunnels are much safer than aboveground structures for a immersed tube tunnels and cut-and-cover tunnels are given intensity of shaking. more vulnerable than bored or mined tunnels because of Tunnels deep in rock are safer than shallow tunnels. the typical shallow soil cover and the nature of the back- No damage was found in both lined and unlined tunnels at fill material surrounding the tunnels. If an immersed surface accelerations up to 0.19 g. tube tunnel is breached, the result could be rapid flood- Minor damage consisting of cracking of brick or concrete ing in the tunnel and potential flooding of significant or falling of loose stones was observed in a few cases for portions of the underground transit system if they are surface accelerations above 0.25 g and below 0.4 g, connected. No collapse was observed due to ground shaking alone up Geological medium (i.e., ground type): Stronger and to a surface acceleration of 0.5 g. more competent rock (accounting for the rock joints and Severe but localized damage, including total collapse, may discontinuity effects) provides better tunnel confine- be expected when a tunnel is subject to an abrupt displace- ment and therefore more resistance to explosions. Even a ment of an intersecting fault. large blast inside a tunnel in good rock will likely induce only limited local damage and could be easily repaired Owen and Scholl documented additional case histories within a reasonably short period. Tunnels constructed in (making a total of 127), including cut-and-cover tunnels and soil tend to be more vulnerable than those in rock. Tun- culverts in soils [Ref. 18]. Owen and Scholl's conclusions form nel structure elements in very soft soil will induce larger their study echoed the findings by Dowding and Rozen dis- bending and shear demands under blast loading condi- cussed above. In addition, Owen and Scholl suggested the fol- tions. Underwater tunnels surrounded by very porous lowing: material (such as immersed tunnels backfilled with gravel or rock fill) are particularly vulnerable to the Damage to cut-and-cover structures appeared to be caused inflow of large volumes of water mixed with surround- mainly by the large increase in the lateral forces from the ing materials. surrounding soil backfill. Soil or rock overburden: Structural damage potential Duration of strong seismic motion appeared to be an increases with decreasing soil or rock cover. Deeper cover important factor contributing to the severity of damage to provides better tunnel protection from both interior and underground structures. Damage initially inflicted by earth exterior explosions. movements, such as faulting and landslides, may be greatly Groundwater conditions: For a tunnel surrounded by increased by continued reversal of stresses on already dam- semi-flowing water (e.g., an immersed tunnel backfilled aged sections. with gravel-sized backfill) or flowing water (e.g., an immersed tunnel backfilled with coarse rockfill material), Using the data presented above as well as additional data the damage potential may be severe because of flooding from the 1995 Kobe, Japan, earthquake (with a moment mag- and associated damage to the operating systems. For a tun- nitude of 6.9), Figure 14 summarizes empirical observations nel on land, better tunnel performance can be expected of seismic effects on the performance of bored tunnels [Ref. when it is surrounded by a dry geological medium (i.e., 19]. The damage state is presented as a function of ground when there is a low groundwater level) than when it is sur- shaking levels (represented by peak ground acceleration) and rounded by a wet geological medium (i.e., when there is a tunnel lining types. The data apply only to damage due to high groundwater level).