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52 4.2.1 Typical Road Tunnels ducts shown in the figures. Both of the figures depict one walk- way on the right side of the road, although some multilane tun- Road tunnels that are longer than 1,000 feet (304 meters) nels may have walkways on both sides. Tunnel utilities such as typically have forced air ventilation systems. Prior to 1995, power and communication conduits and fire standpipes can when the Federal Highway Administration (FHWA) run along the benchwall, as is shown in both sketches, or in the approved the use of jet fans in tunnels based on results of the opposite sidewall, as shown in the bored tube tunnel. Memorial Tunnel Fire Ventilation Test Program (MTFVTP), the majority of tunnels were ventilated with ducted systems. A tunnel that is served by a full transverse ventilation system 4.2.2 Typical Transit and Rail Tunnels has a supply air duct and an exhaust air duct, and a tunnel Typical transit and rail tunnels are shown in Figure 2. that is served by a semi-transverse ventilation system can have Shorter tunnels can be ventilated naturally by the train's pis- either a supply duct or an exhaust duct. ton action. Longer tunnels without forced ventilation typi- In cut-and-cover tunnels, the air ducts typically run side by cally have intermittent ventilation shafts that relieve the side to save on excavation costs, as shown in Figure 1A. In bored tunnel pressure through sidewalk gratings, as shown in Fig- or mined tunnels, the ducts typically fill the available space ure 2D. Longer tunnels with forced air ventilation can be above and below the road, as shown in Figure 1B. Tunnels served by midtunnel and/or end-of-station-platform fan served by longitudinal ventilation systems typically have shafts. Individual tracks in cut-and-cover transit and rail tun- ceiling-mounted jet fans in the road space in lieu of the upper nels can be separated by columns, porous dividing walls, or solid dividing walls. Similar to road tunnels, utilities are routed along the tunnel benchwalls. 4.3 Tunnel Construction Methods In the U.S. transportation system, tunnels have been con- structed by a variety of methods, as shown in Table 6. In gen- eral, the types of tunnels are identified by the principal types of tunnel construction and include (1) immersed tube tunnels, (2) cut-and-cover tunnels, (3) bored or mined tun- nels, and (4) air-rights structure tunnels. Determination of the appropriate method of construction typically depends on the depth, cross section, and soil/rock/groundwater condi- tions along the alignment. Other constraints include geo- graphical and environmental factors, presence of existing (A) Typical cut-and-cover road tunnel. structures and utilities, and constructability issues. The dif- ferent materials (i.e., structural and geological), tunnel con- figurations, and construction procedures used for these tunnels impact their resistance to hazards and threats. It is therefore important to identify the various types of tunnels and the factors that could have a major impact on their vul- nerability to hazards and threats. 4.3.1 Immersed Tube Tunnels Immersed tube tunnels are employed to traverse a body of water. Tunnel sections, usually 300 to 450 feet (91 to 137 meters), are placed into a pre-excavated trench. The tunnel construction method involves (1) construction of tunnel sections in an offsite casting or fabrication facility that are finished with bulkheads and transported to the tunnel site; (2) placement of the sections in a pre-excavated trench, joint- ing and connecting together and ballasting/anchoring; and (B) Typical bored tube road tunnel. (3) removal of temporary bulkheads and backfilling the exca- Figure 1. Typical road tunnels. vation. The top of the tunnel should be at least 5 feet (1.5

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53 (A) Typical bored tube rail tunnel. (B) Typical mined horseshoe rail tunnel. (C) Typical cut-and-cover rail tunnel. (D) Typical cut-and-cover transit tunnel. Figure 2. Typical transit road tunnels.

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54 Table 6. Types of transportation tunnels. Type Description Sketch Immersed Employed to traverse a water body Tube Tunnel Preconstructed sections are placed in a pre- excavated trench and connected Typical materials include steel and concrete immersed tunnel sections After placement, tunnel is covered with soil Cut-and- In urban areas Cover Tunnel Excavated from the surface, then constructed in place and backfill placed to bury structure For subway line structures, subway stations, and subsurface highway structures Typically concrete cast-in-place or precast sections Steel framing and concrete fill Bored or In urban or remote locations in land, on Mined Tunnel mountains, or through water bodies Bored using a variety of techniques Supported by initial and final support systems Soft ground or rock tunneling Structure may have various liner systems, including rock reinforcement, shotcrete, steel ribs and lattice girder, precast concrete segment, cast-in-place concrete, and fabricated steel lining Air-Rights In urban areas Structure Created when a structure is built over a roadway Tunnel or trainway using the roadway's or trainway's air rights The limits that an air-rights structure imposes on the emergency accessibility and function of the roadway or trainway that is located beneath the structure should be assessed meters) below the original bottom to allow for an adequate of an inch (9.5 millimeters) thick and stiffened by interior protective backfill. transverse steel ribs spaced 6 feet (1.8 meters) on center and Two distinct types of immersed tube tunnel construction two longitudinal vertical interior trusses encased in the rein- have emerged over the years: (1) steel shell immersed tunnels; forced concrete walls of the gallery. The interior lining of rein- and (2) concrete immersed tunnels. Steel shell immersed tun- forced concrete has a minimum thickness of 2 feet 3 inches nels are categorized according to the construction method: (68.5 centimeters). The exterior shell is protected against cor- single-shell or double-shell construction. The first trans- rosion by a cathodic protection system. Ballast pockets 2 feet portation tunnel constructed by immersed tube methods in 6 inches (78.2 centimeters) deep on top of the tube are filled the United States was completed in 1910 for the Michigan with gravel to provide adequate weight to overcome buoyancy Central Railroad Tunnel under the Detroit River. The 1993 during sinking of the tube. report by the International Tunnelling Association (ITA) pro- The basic elements of the double-steel-shell tube is a steel vides a technical inventory of 91 immersed tube tunnels com- shell that forms a watertight membrane and, in combination pleted since 1910 [Ref. 3]. with a reinforced concrete interior lining, provides the neces- For the steel single-shell construction, an outer steel shell sary structural strength for the completed tunnel. Figure 4 serves as a permanent watertight membrane and an exterior represents a typical double-steel-shell tube, which shows the form for the final concrete lining. The steel shell also takes cross section of a two-lane tunnel on an Interstate highway. flexure forces along the exterior face of the tube before and In this example, the circular steel shell has a diameter of 36 after the placement of the concrete lining. The steel shell tube feet 2 inches (11 meters) and is made of five-sixteenths inch behaves as a composite steel-concrete structure after the inte- (8 millimeters) welded steel plate. It is stiffened by external rior concrete is completed. diaphragms spaced 14 feet 10 inches (4.5 meters) apart and Figure 3 shows a typical single-shell tube for two rapid tran- external longitudinal stiffening ribs. The interior is lined with sit tracks, separated by a service gallery and an emergency ven- a minimum thickness of reinforced concrete. An exterior con- tilation exhaust air duct. For this example, the shell plate is 3/8 crete envelope of 2-foot (61-centimeter) minimum thickness,