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59 Table 7. Tunneling methods for rock tunnels. Type Description Sketch Tunnel Boring Full face advance Machine (TBM) Circular sections High advance rate Roadheader Partial face advance Any cross section Usable in rock with less than about 15,000 psi of unconfined compressive strength Most effective if the unconfined compressive strength of rock is less than 5,000 psi Drill and Blast Conventional method Full or partial face advance Any cross section Cycle involves (1) drilling; (2) charging with explosives; (3) blasting and ventilation; (4) loading and hauling (mucking); (5) scaling and cleaning; and (6) installation of a support system lining systems and the typical application of the initial sup- tion in engineering terms that reflect current technology port and lining systems, respectively. and usage [Ref. 7]. 4.3.4 Air-Rights Structure Tunnels 4.4.2 Modes of Tunnel Failure Air-rights structure tunnels are defined by the National Fire Tunnel failure can range from local spalling (i.e., local fail- Protection Association (NFPA) [Ref. 5] as structures that are ure), local breach, partial or complete collapse, or inundation built over a road using the road's air rights, thereby imposing with water (i.e., global failure) to progressive failure. Figure 10 on the accessibility and operation of the road or train during demonstrates how a threat can lead to progressive failure. emergency operations. Air-rights structure transportation Tunnel failure modes can start from an overstress in the tunnels have been constructed to enclose both road and rail lining caused by explosion or fire. This overstress may lead to operations. failure of the lining if the strength of the lining material is less Figure 9 shows a typical air-rights structure tunnel. The than the applied stress. The failure of the lining may be structure is supported by intermittent columns. The structure restricted to be a local failure such as spalling or local breach. above the tunnel can be a building of any type, a transit or rail When the tunnel lining is damaged locally or globally, fail- station, a parking garage or a parking lot. ure of surrounding ground (i.e., collapse) and/or inundation These structures create transportation tunnels and may be with water (i.e., flooding) may follow. These failures are as dangerous as the air-rights structures constructed above considered global failures. the roads or trains because of the relative ease of access. The It is considered a progressive failure when instability of damage potential of an incident in an air-rights tunnel can adjacent underground structures and/or damage to surface also be greater than those for other tunnel types because structures is involved. Flooding of the transportation system occupancy loads include the people located in the structure. may also be considered a progressive failure. Lining Failure from Explosion 4.4 Structural Elements and Vulnerabilities When an explosion occurs in a transportation tunnel, frag- mentation of the liner is expected near the detonation point. 4.4.1 Ground Characteristics Then, the peak blast pressures and gas pressures from the Terzaghi published the Tunnelman's Ground Classifica- explosion may overstress the lining and the initial support tion System, which describes representative soil types and systems. The fragments and overstress may induce failure of their predicted behavior during various tunneling con- the liner and support systems. The extent of failure depends struction methods [Ref. 6]. As shown in Table 11, Heuer on charge weights, charge shapes, detonation points, types modified this classification system to present the informa- and materials of tunnel liner and support systems, thickness

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60 Table 8. Tunneling methods for soft ground tunnels (as modified by Zosen [Ref. 4]). Type Description Sketch Blind Shield A closed face (or blind) shield is used in very soft clays and silts Muck discharge is controlled by adjusting the aperture opening and the advance rate Used in harbor and river crossing in very soft soils; often results in a wave or mound of soil over the machine Not used nowadays Open Face, Good for short, small tunnels in hard, noncollapsing soils above Hand-Dug groundwater tables Shield Usually equipped with face jacks to hold breasting at the face If soil conditions require it, this machine may have a movable hood and/or deck A direct descendent of the Brunel Shield Seldom used nowadays Semi- Similar to open face, but with a back hoe and boom cutter; Mechanized often equipped with "pie plate" breasting and one or more tables May have trouble in soft, loose, or running ground Compressed air may be used for face stability in poor ground Seldom used nowadays Mechanized A fully mechanized machine Excavates with a full face cutter wheel and pick or disc cutters Manufactured with a wide variety of cutting tools Face openings (doors, guillotine, etc.) may be adjusted to control the muck taken in versus the advance of the machine Compressed air may be used for face stability in poor ground Slurry Face Uses pressurized slurry to balance the groundwater and soil Machine pressure at the face Has a bulkhead to maintain the slurry pressure on the face Good for water-bearing silts and sands with fine gravels; may accommodate boulders Best for sandy soils; tends to gum up in clay soils; with coarse soils, face may collapse into the slurry Can be equipped with disk cutters to bore through boulders or rock in mixed face conditions Earth A closed chamber (bulkhead) face used to balance the Pressure groundwater and/or collapsing soil pressure at the face Balance Uses a screw discharger with a cone valve or other means to (EPB) form a soil plug to control muck removal from the face and Machine thereby maintain face pressure to "balance" the earth pressure Best for clayey soils with acceptable conditions Acceptable for silt and clayey and silty sand Often uses foams and/or other additives to condition the soil Can be equipped with disk cutters to bore through boulders or rock in mixed face conditions EPB High- A hybrid machine that injects denser slurry (sometimes called Density Slurry slime) into the cutting chamber Machine Developed for use where soil is complex, lacks fines or water for an EPB machine, or is too coarse for a slurry machine

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61 Table 9. Initial support and lining systems. Type Description Sketch Rock Untensioned rock dowels or tensioned rock bolts Reinforcement To help rock mass self-support capacity and to mobilize the inherent strength of the rock mass May provide only temporary support until a final lining is placed To protect against spalling and fallout of rock wedges between reinforcements, a surface skin may be required such as chain link mesh or shotcrete Shotcrete Early construction support in rock with limited stand-up time to prevent loosening of the rock mass and raveling failure Used in soft ground tunnels when a sequential excavation method (SEM) is used. Sometimes used as a permanent lining May be reinforced for additional long-term ductility in poor or squeezing ground Steel Ribs and Considerable appeal in poor rock conditions Lagging Lateral spacer rods (collar braces) are usually placed between ribs For soft ground tunnels, the ground between ribs is stabilized by lagging or by segmental plates Precast Usually associated with soft ground tunneling Concrete Bolted or unbolted segments Segment Lining One- or two-pass lining system Segments are bolted with a gasket for water tightness Cast-in-place Plain or reinforced Concrete Lining Commonly used second stage lining in two-pass lining system Waterproofing membrane layer may be installed between initial support systems and the inner lining Fabricated Steel Required when leakage through a cracked concrete or Cast Iron lining is a concern. Designed for an exterior water Lining pressure and furnished with external stiffeners for high- external-pressure conditions Concrete placement is required to ensure firm contact between steel and ground Table 10. Typical application of initial support and lining systems. Steel Ribs Rock Bolts Rock Bolts Cast-in- and Concrete Ground Rock Bolts with Wire with Place Lattice Segments Mesh Shotcrete Concrete Girder Strong Rock Medium Rock Soft Rock Soil

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62 Figure 9. Typical air-rights structure tunnel. Table 11. Tunnelman's ground classification for soils. Classification Behavior Typical Soil Types Firm Heading may be advanced without initial Loess above water table; hard clay, marl, support, and final lining may be cement sand, and gravel when not constructed before ground starts to move. overstressed. Raveling Slow Chunks or flakes of material begin to drop Residual soils or sand with small amounts Raveling out of the arch or walls some time after the of binder may be fast raveling below the ground has been exposed, due to water table and slow raveling above. Stiff Fast loosening or overstress and "brittle" fissured clays may be slow or fast Raveling fracture (ground separates or breaks along depending on degree of overstress. distinct surfaces, as opposed to squeezing ground). In fast-raveling ground, the process starts within a few minutes; otherwise, the ground is slow raveling. Squeezing Ground squeezes or extrudes plastically Ground with low frictional strength. Rate of into tunnel, without visible fracture or loss squeeze depends on degree of of continuity, and without perceptible overstress. Occurs at shallow to medium increase in water content. Ductile, plastic depth in clay of very soft to medium yield, and flow due to overstress. consistency. Stiff to hard clay under high cover may move in combination of raveling at execution surface and squeezing at depth behind surface. Running Cohesive Granular materials without cohesion are Clean, dry, granular materials. Apparent Running unstable at a slope greater than their angle cohesion in moist sand, or weak of repose (3035). When exposed at cementation in any granular soil, may Running steeper slopes, they run like granulated allow the material to stand for brief periods sugar or dune sand until the slope flattens of raveling before it breaks down and runs. to the angle of repose. Such behavior is cohesive running. Flowing A mixture of solid and water flows into the Below the water table in silt, sand, or tunnel like a viscous fluid. The material gravel without enough clay content to give may enter the tunnel from the invert as significant cohesion and plasticity. May well as from the face, crown, and walls, also occur in highly sensitive clay when and may flow for great distances, such material is disturbed. completely filling the tunnel in some cases.

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63 the U.S. Army recommends that dynamic yield strength 10 Threat percent greater than the static yield strength be used [Ref. 9]. When the blasting induced peak overpressure is greater than the dynamic strength of the lining materials, the lining is con- Overstress in Liner sidered overstressed. Therefore, estimation of the blasting induced peak overpressure provides a critical input in tunnel lining vulnerability assessment. Breach failure potential may be determined by comparing Failure of Liner breach threshold thickness and effective thickness of the tun- nel lining. The liner may be considered breachable when the effective thickness of the liner is less than the breach thresh- Failure of Ground Inflow & Flood old thickness. The effective thickness of the lining includes the final lining thickness and the thickness of the portion of the initial support system that can be considered a permanent Progressive Failure application, such as shotcrete. Breach threshold thickness of normal reinforced concrete with a strength of 4,000 psi (2,812,400 kilograms per square meter) for a spherical deto- nation is shown in Figure 11. Breach threshold thickness is Figure 10. Path to progressive failure. expressed as a function of explosive charge weight and set- back distance (i.e., the distance from the face of the lining to of liner, size and shape of tunnel, and type and amount of sur- the center of the charge) [Ref. 10]. Note that Figure 11 is not rounding ground confinement. applicable for contact charges. This information allows a When tunnel linings are subjected to extreme blast load- rough assessment of the tunnel lining vulnerability to an ings, the stressstrain relationship of reinforced concrete is explosion inside the tunnel. quite different from that under static load. This difference is due to the increased dynamic compressive and tensile Joint Failure strengths and the increased displacement capacity at ultimate stress. For reinforced concrete, dynamic strength magnifica- Joints between immersed tube segments or between the tion factors as high as 4 in compression and as high as 6 in end tube and the connecting structures (e.g., ventilation tension for strain rates in the range of 102 to 103 per second buildings) may be potential weak points in the structural sys- have been reported by Grote et al. [Ref. 8]. For steel members, tem and may be more susceptible to flooding in case of 100 Large Medium Small Breach threshold thickness (inch) 80 60 40 20 0 0 2 4 6 8 10 12 14 16 Range from lining face to charge center of gravity (ft) Figure 11. Breach threshold thickness for reinforced concrete [Ref. 10].

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64 breach. There are various types of joints used in immersed redundancy) and (2) local failure or collapse of one or more tube tunnels: of the cross passageway tunnels may not affect the stability of the main tunnels or prevent their continuous use, except Tremie joints: These joints have been used in a number when flooding results. of steel shell tubes in the past, but have rarely been used recently. The tremie joints in one particular underwater Portal Failure tunnel are steel formed in soil trenches and rock encased in rock trenches. For these tremie concrete joints, the From a stability standpoint, the tunnel portal area is gen- steel reinforcement and the steel plate were welded and erally one of the critical locations due to the inherent slope continued through the joints after internal dewatering. stability problem. Landslide, rock fall, or even collapse at and Thus, in this case, they are as strong as the main body of near tunnel portals may be triggered by certain extreme the tunnel. The tremie concrete is anticipated to provide events, such as earthquakes and blast waves, thereby blocking additional resistance to loading resulting from blast the passageway and potentially affecting structures or facili- waves. ties at the top of the slope. Tunnel portals are therefore con- Flexible joints: The initial seal of the flexible joint is pro- sidered to be particularly vulnerable during such extreme vided by the compression of rubber or neoprene gaskets events. However, at the portal, the blast is less confined and attached to the face of one tube and bearing against a the energy will dissipate. To stabilize the portal area, soil smooth surface on the adjoining tube. Many tunnels in the anchors or rock reinforcement systems are often used. Other United States have used temporary gaskets that may form remedial measures, such as flattening the earth slopes or using a seal, but the load is carried on solid stop bars. The two various ground improvement treatments, may also be effec- most recently built tunnels in the United States have used tive. Nevertheless, the damage potential of a portal failure is Gina-type joints that have soft noses and bodies capable of generally considered to be less than that of a tunnel lining fail- carrying the compressive load. Particularly in seismic areas, ure because the repair for a portal failure can be done in the the flexible joints are designed to carry expected shear and open space. In addition, flooding is normally not an issue tension loads and may sometimes be referred to as seismic when a portal is damaged or collapses, so the repair time and joints. In such cases, a joint cannot open or have offset dis- associated costs are relatively low compared with the other placements under seismic loading conditions, which could parts of the tunnel. lead to life-threatening ingress of water. This type of joint presents potential weakness for ingress of water and flood- Ground (Soil and Rock) Failure ing under blast wave conditions resulting from detonation of an explosive. Blasting may also cause the geological media surrounding Rigid joints: Rigid joints may be designed to have the same the tunnel to yield or fail, particularly when the tunnel liner section properties as the rest of the tunnel, effectively mak- is breached or in unlined tunnels (such as those constructed ing the tunnel continuous without joints. The resistance of in sound rock). The post-yield behavior of the surrounding the joints is therefore the same as the tunnel lining. geological media depends on the types of the materials encountered and their characteristics under high-energy transient loads. Following is a brief description of post-yield behavior of various types of soils and rocks: Cross Passageway Failure The general lining response of cross passageway tunnels Sand and gravel: These materials may quickly collapse into subject to blast loading is approximately the same as the tunnel. When sand and gravel are saturated with water, described above. Special attention should be given to the fol- semi-flowing to flowing conditions may occur. Flooding of lowing considerations: (1) high stress concentration may the tunnel could also happen if the surrounding material is occur at the junctions with main tunnels and (2) given the very porous (such as gravel or rock fill) under a high same amount of explosive charge, the resulting blast peak groundwater level. This is particularly true for immersed pressure in a cross passageway tunnel may be greater than that tube tunnels. in the main tunnel due to its smaller cross-sectional geome- Soft cohesive soils: Because of its low strength, soft cohe- try. Therefore, cross passages are more vulnerable to damage. sive soils, such as clay and silt, could demonstrate slow In general, however, from an operational standpoint, cross flowing behavior (i.e., creeping), eventually collapsing into passageway tunnels are not considered to be more critical the tunnel. than the main running tunnels because (1) there is generally Stiff and highly overconsolidated cohesive clay: Local more than one cross passageway tunnel (i.e., greater degree of failure of this type of material into the tunnel is likely.