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55 Figure 3. Steel single-shell immersed tube tunnel. confined by one-quarter inch (6.4-millimeter) steel form 18 meters] of overburden). Cut-and-cover tunnel structures plates attached to the shell, protects the shell against corro- may be divided into three types of structures in transporta- sion and acts as a ballast against buoyancy. The space below tion systems: subway line structures, subway stations, and the road slab forms a fresh air supply duct. The segment subsurface highway structures. Figure 6 represents a typical above the ceiling is an exhaust duct. "line" cut-and-cover structure constructed between subway Concrete immersed tube tunnels are generally rectan- stations. In the line structures, the subway tracks are usually gular reinforced concrete sections. The concrete thickness is enclosed in a reinforced concrete double-box structure with determined largely by the weight required to prevent uplift. a supporting center wall or beam with columns. The track Crack controls to achieve impermeability of the concrete and centers are normally located as close together as possible. independent waterproofing membranes are considered to The typical cut-and-cover subway station is a two- or accomplish water tightness. Typical waterproofing membranes three-story reinforced concrete structure in a rectangular used in concrete immersed tunnels are steel membranes made excavation 50 to 65 feet (15 to 20 meters) wide, 500 to 800 feet of one-quarter inch steel plates, multiple-ply membranes of (152 to 244 meters) long, and 50 to 65 feet (15 to 20 meters) fabric and coal-tar layers, and plastic membranes made of syn- deep. Figure 7 represents a cross section of a typical subway thetic neoprene (or vinyl-type rubbers) with epoxy coatings. station. Cut-and-cover structures for older transit facilities Figure 5 represents a typical concrete immersed tube for a four- were constructed using steel frame construction with rein- lane highway tunnel with two 2-lane sections and ventilation forced or unreinforced concrete between the frames. This ducts on both sides. Prestressed concrete has also been used to method is referred to as jack arch construction. construct immersed tube tunnels. Cut-and-cover highway tunnels are often used in urban areas. In addition, they are often constructed at the approaches 4.3.2 Cut-and-Cover Tunnels to subaqueous vehicular tunnels due to the depth required. Fig- ure 8 represents a typical highway cut-and-cover cross section. Shallow-depth tunnels in land are frequently designed as This type of tunnel is often under the groundwater table and structures to be constructed using the cut-and-cover method. typically consists of massive reinforced concrete structures. The cut-and-cover tunnel construction method involves braced, trench-type excavation ("cut") and placement of fill 4.3.3 Bored or Mined Tunnels materials over the finished structure ("cover"). The excava- tion is typically rectangular in cross section and only for rel- When a tunnel is located at significant depth or when over- atively shallow tunnels (typically less than 45 to 60 feet [14 to lying structures exist above the tunnel alignment, bored or

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56 Figure 4. Steel double-shell immersed tube tunnel. Figure 5. Concrete immersed tube tunnel.

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57 Figure 6. Cut-and-cover tunnel, subway line structure. Figure 7. Cut-and-cover tunnel, subway station.

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58 Figure 8. Cut-and-cover tunnel, subsurface highway structure. mined underground tunnel construction is typically the pre- ated with gravity falls of rock wedges from the roof and side- ferred method. Bored tunnels are often excavated using walls. A tunnel in an unweathered, massive rock with few mechanical equipment, such as TBMs, and are usually circu- joints does not usually suffer from serious stability problems lar. Mined tunnels may be excavated using manual or unless stresses in the rock exceed the strength of the rock. As mechanical methods and may be rectangular or horseshoe- the below-surface depth increases or as the number of close- shaped. Bored or mined tunnels are typically divided into two together excavations increases, the rock stress increases to a groups based on the type of surrounding ground: soft ground level at which failure is induced in the rock surrounding the tunnels and rock tunnels. tunnels. This failure may range from minor spalling or slab- For bored or mined tunnels in soft ground (i.e., soft bing in the surface rock to major rock bursts involving failure ground tunnels), the main concerns during excavation are of significant volumes of rock. Various tunneling methods associated with groundwater conditions and stability charac- used in rock and soft ground are summarized in Tables 7 teristics of the soil along the alignment. The control of and 8, respectively. groundwater is of utmost importance in soft ground tunnel- When surrounding ground is massive and rock mass is sta- ing. Typical methods for controlling groundwater are dewa- ble, the tunnel may require no support system or minimal tering, using compressed air, grouting, freezing, and using support systems at portals and weak rock zones. When the pressurized face TBMs. Recent improvements in grouting ground is unstable, the initial support system is installed have made grouting a valuable tool in both groundwater con- before, during, or immediately after excavation to stabilize the trol and soil stabilization for soft ground tunneling. excavation. The final lining system is then placed to provide For bored or mined tunnels in rock (i.e., rock tunnels), sta- permanent support and to provide a durable, maintainable, bility problems in blocky jointed rocks are generally associ- long-term finish. Tables 9 and 10 show the initial support and