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65 The material falling into the tunnel should be confined to The material strength and load-carrying capacity of the the area where the liner is breached. lining may be degraded when exposed to high tempera- Shear zone, broken, or decomposed rocks: Depending on tures resulting from the fire, and whether the shear zone is saturated with groundwater, the Tunnels tend to be thermally restrained in both longitudi- materials may advance into the tunnel under flowing, nal and transverse directions, resulting in increased struc- swelling, and squeezing conditions. tural demand under fire conditions. Plastic, ductile rock: This type of rock, such as shale, behaves similarly to the overconsolidated clay described Fires in tunnels may lead to a high risk of explosive spalling above. It may yield without losing its coherence and thus of the concrete liner, particularly for concrete with high mois- provides self-support capability for a short duration. ture content, such as shotcrete, or for high-performance or Fractured rock held in place by support of dowels or high-strength concrete with low permeability. Explosive shotcrete: The rock mass may yield with small to moder- spalling occurs in the temperature range where chemically ate displacements along fractures. Fresh fractures could be bound water is released from the concrete. Explosive spalling generated, thereby resulting in some loosened rock pieces of high-performance or high-strength concrete is directly falling into the tunnel. related to internal pressures generated during the attempted Fractured rock without reinforcement: Upon blasting release of chemically bound water. loads, this material tends to become severely loosened, Lawson et al. characterized the residual mechanical prop- thereby resulting in a raveling situation. erties of high-performance or high-strength concrete after Stronger, brittle rock: Fractures and local spalling could the concrete is exposed to elevated temperatures [Ref. 11]. occur. Chunks of rock loosened by the explosion could fall Using results from a combination of a heat transfer analysis into the tunnel. and a nonlinear structural analysis conducted for a range of service loads, concrete mixes, and fire types, Caner et al. pro- posed a guideline for assessing fire endurance [Ref. 12]. The Water Inflow and Flooding effects of temperature-induced material degradation and Transportation tunnels are intensively concentrated and ground tunnel liner interaction were considered in these interconnected in urban areas. Therefore, failure of an under- analyses. Caner et al. also recommended techniques for repair water tunnel ranging from collapse or complete inundation of damaged concrete tunnel liners, as summarized below: with water due to local breaching of the liner may lead to flooding in the underground transportation system. Flooding Concrete sections: Concrete sections exposed to tempera- may also introduce large quantities of sand, silt, gravel or tures in excess of 300C (570F) should be investigated. shear zone debris. Significant lengths of tunnel can become They should be removed or replaced if they are found to be filled with debris or mud in short periods of time, causing deficient. The depth of fire-damaged concrete may be tunnel structures to become buried. In addition, loosening of determined by using heat transfer analyses and should be the soil under foundations can undermine structures above verified by condition assessment. Voids and spalls should or adjacent to the tunnel. be patched with patching materials of similar characteris- tics as the concrete mix design used for the original tunnel to maintain its structural integrity. Progressive Failure Reinforcement: If the concrete is removed around the Failure of the tunnel liner and surrounding ground may reinforcement, reinforcement shall also be removed. High- cause instability of adjacent underground utilities and dam- strength alloy bars may lose 40 percent of their initial age to surface structures by piping and differential settle- strength at 500C (930F). The new reinforcement should ments. Flooding of the entire transportation system may also be properly spliced to the existing reinforcement. be considered a progressive failure. Micro-polypropylene fibers: Use of micro-polypropylene fibers in concrete will reduce explosive spalling because the fibers will melt over 130C (270F), making the concrete 4.4.3 Effects of Other Extreme Events more porous, thus accommodating water vapor during a fire. An evaluation of the need for major repair should be Tunnel Lining Behavior During a Fire determined on a case-by-case basis. Furthermore, with the There are three primary adverse effects on concrete or more permeable concrete, the chance of explosive spalling shotcrete tunnel linings that are subjected to fire: may be minimal in the event of another fire. Insulation materials: If the tunnel lining is insulated by The lining may lose its effective section area by spalling, the placement of coatings, and the insulation materials are

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66 damaged, they should be replaced by the same type of Ground failure broadly includes various types of ground material because of the fire performance history of the instabilities such as faulting, landslides, liquefaction, and material. For practicality, spray-on insulation materials tectonic uplift and subsidence. Each of these instabilities may be used to patch the damaged area. can be potentially catastrophic to tunnel structures, although the damage is usually localized. It is often possi- The MTFVTP consisted of 98 full-scale fire tests conducted ble to design a tunnel structure to account for ground in the abandoned Memorial Tunnel. Various tunnel ventila- instability problems, although the cost may be high. For tion systems and configurations were operated to evaluate example, with proper and often expensive ground their respective smoke and temperature management capa- improvement techniques and/or earth-retaining measures, bilities. The fire sizes ranged from 34.1 to 341 MBTU per hour it may be possible to remedy the ground conditions against (10 to 100 MW). For fires below 170.5 MBTU per hour (50 liquefaction and landslides. MW), only cosmetic damage to the tunnel structure was observed (mainly loss of ceramic tiles from the walls and ceil- ing). For the 170.5 MBTU per hour (50 MW) tests, spalling of Vulnerability Screening for Geotechnical Hazards and ceiling concrete was observed. The areas that resulted in Threats. The discussions above show that it is important to exposed reinforcing steel were repaired with reinforced shot- perform a tunnel vulnerability screening study for ground crete. The repaired areas were not further damaged during the failure potential (i.e., geotechnical or geological hazards and 341 MBTU per hour (100 MW) tests. Test results are available threats) prior to more detailed evaluation. The objective of from Bechtel/Parsons Brinckerhoff [Ref. 13] and on CD at the vulnerability screening process is to identify which sec- www.tunnelfire.com/cd.htm. tions of the tunnel structures may have risk of poor perform- Full-scale fire tests were also conducted in Norway's ance during earthquakes. For sections identified to have low Runehamar Tunnel in association with the UPTUN earthquake risk, no further evaluations are required. Other- (UPgrading methods for fire safety in existing TUNnels) wise, further assessments may be needed. Factors to be con- Research Program [Ref. 14]. Insulated boards with high- sidered during this screening process include, but are not temperature resistance were installed to protect the tunnel limited to, the following: surfaces. A total longitudinal distance of 75 meters was pro- tected. The boards were installed along the first 25 meters Liquefaction potential: Liquefaction potential exists in downstream of the fire site. Ceramic curtains were installed loose granular soils below the groundwater table only. To beyond the boards; 9 meters upstream and 41 meters down- assess site-specific liquefaction potential in areas where liq- stream were covered. The highest gas temperature measured uefaction is possible, procedures based on the standard was 1,365C. Significant spalling of the tunnel material penetration test (SPT) blow count number from soil bor- occurred both upstream and downstream of the passive fire ings and/or based on cone penetration test (CPT) data can protection system. be used. Both methods compare the soil liquefaction resist- ance (through SPT or CPT data) with the earthquake induced dynamic stresses. Detailed information about liq- Earthquake Effects on Tunnels uefaction and the recommended procedures for evaluating Underground structures are generally less vulnerable to liquefaction procedures are documented in the report from earthquakes than surface structures, such as buildings and the 1996 workshop sponsored by the National Center for bridges, because the surrounding ground confines under- Earthquake Engineering Research (NCEER) [Ref. 15]. ground structures. As long as the surrounding ground is sta- Slope stability: In general, a seismically induced landslide ble and experiences only small ground deformations, the through a tunnel can result in large, concentrated shearing tunnel tends to move along with the surrounding ground and displacements and intense damage to the structure. Evalu- maintains its structural integrity. ations should focus on the following areas: (1) at tunnel In a broad sense, earthquake effects on underground tun- portals (in soil as well as in rock), (2) in shallow parts of the nel structures may be grouped into two categories: tunnel alignment adjacent to soil slopes, and (3) in areas where existing slopes have displayed signs of movement Ground shaking refers to the vibration of the ground pro- under static conditions. The commonly used pseudo-static duced by seismic waves propagating through the earth's method of analysis can be used for evaluating the seismic crust. The area experiencing this shaking may cover hun- stability in areas of concern. If a pseudo-static seismic sta- dreds of square miles near the fault rupture. As the ground bility analysis indicates an insufficient safety margin against is deformed by the traveling waves, any tunnel structure in the landslide movements, then a more refined deformation- the ground will also be deformed. based method of analysis should be used to estimate the

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67 movements. The impact of the potential slope movements on the affected structures should then be assessed. Shear/fault zones: If a shear/fault zone crosses the tunnel alignment, the potential relative movement along the weak plane and its effects on the tunnel structure need to be eval- uated. In general, it may not be economically or technically feasible to build a tunnel to resist potential faulting dis- placements, particularly if the magnitude of the fault dis- placement is large (e.g., several feet). However, avoidance of faults may not always be possible, especially for tunnel systems that are spread over large areas. In highly seismic areas such as California, it may be inevitable for the tunnel to cross a fault. The design approach to this situation is to accept the displacement, localize the damage, and provide means to facilitate repairs. Abrupt changes in structural stiffness or ground condi- tions: Stress concentrations often occur in abrupt stiffness change conditions. Special attention should be paid to the following locations: (1) at a tunnel's junctions; (2) where a tunnel section traverses multiple distinct geological Figure 12. Longitudinal deformation of media with sharp contrast in stiffness (such as a shaft ris- tunnels. ing from solid rock formation up through soft soil over- burden to the ground surface); and (3) where a regular tunnel section in soft ground is connected to rigid station end walls or a rigid, massive structure such as a ventilation building or shaft. Tunnel Response to Ground Shaking. The response of a tunnel to seismic shaking motions may be described in terms of three principal types of deformations: (1) axial deformation, (2) curvature deformation, and (3) ovaling (for circular tunnels such as bored tunnels) or racking (for rectangular tunnels such as cut-and-cover tunnels). Axial deformations are induced by components of seismic waves that propagate along the tunnel axis (i.e., longitudinal response of the tunnel). When the component waves pro- duce particle motions parallel to the longitudinal axis of the tunnel, they cause alternating axial compression and tension strains, as illustrated in Figure 12A. Curvature deformations result from component waves that produce particle motions in the direction perpendicular to the tunnel axis. The cur- vature deformation results in bending and shear demands on the tunnel structure, as shown in Figure 12B. The oval- ing or racking deformation (i.e., the transverse response of the tunnel) is caused primarily by seismic waves propagat- ing perpendicular to the tunnel longitudinal axis. Vertically propagating shear waves are generally considered the most critical type of waves for this mode of deformation, as shown in Figure 13 [Ref. 16]. Tunnel Damage Potential Due to Ground Shaking. Figure 13. Transverse ovaling and racking of Dowding and Rozen reported 71 cases of tunnel response to tunnels.