TUNNEL ENGINEERING

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20.48 n Section Twenty buildings, special earthquake joints permit relative movement in all directions. The shell plate is protected against corrosion by a coating and a cathodic protection system. All eight rectangular single-shell tunnels are located in Japan. They are very similar in layout to concrete tunnels, the difference being that the outer steel shell works compositely with the concrete, whereas even if an outer steel plate is present as a waterproofing membrane in a similar concrete tunnel, it is not considered in the strength design. Steel Double-Shell Tunnels n Fifteen of these tunnels have been built, of which five are binocular, the most recent being the Ted Williams Tunnel is Boston. Figure 20.26 shows a two-lane tunnel with a circular interior section. The steel shell plate was 31 ft diameter and about 300 ft long. Exterior diaphragms, approximately octagonal, are spaced about 15 ft apart, and longitudinal ribs of the bars and T sections stiffen the shell. Outside form plates were attached to the diaphragms and supported by angle struts extended from the shell stiffeners. The tubes were erected on shipways. All welds were tested for watertightness with a compressedair stream and soap solution. Before the ends were closed with welded watertight bulkheads, the reinforcing steel for the interior concrete lining was placed. The keel concrete in the space between the outside form plates and the bottom of the shell was cast before launching. The concrete lining and roadway slab were cast while the element was TUNNEL ENGINEERING Fig. 20.25 Transbay Tube of the San Francisco Rapid Transit System (steel single shell). floating at a fitting-out pier, by pumping concrete through hatches in the top of the shell into segmental steel forms. Pouring sequences were regulated to control increments of water pressure on the shell plate and longitudinal bending moments. The hatches were closed with welded plates, and the exterior concrete cap was cast and enough tremie concrete placed in the side pockets to reduce freeboard to about 1 ft. Most, if not all, of these tunnels have been immersed using a catamaran lay barge consisting of a barge each side of the tunnel element connected by two transverse beams from which the element is suspended (Fig. 20.28). Additional tremie concrete can then be added to give the required negative buoyancy for immersing, placing, and joining. More tremie concrete may be needed to achieve the final factor of safety against floating. Figure 20.27 shows a combination of two such cylindrical sections for one of the two four-lane tubes of the Ft. McHenry Tunnel under Baltimore Harbor. Steel Sandwich Tunnels n Although postulated elsewhere many years earlier, steel sandwich tunnels have become a reality in Japan. Rectangular in shape, the principle behind this form of construction is that there is a steel skin inside and outside the tunnel, both acting compositely with the concrete between them. The plate is stiffened with flat and L-shaped ribs, and the interior is divided up into cells by diaphragms Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
in two directions. Self-compacting non-shrink concrete is pumped into each cell though one hole while air is released through others. Osaka South Port and Kobe Port tunnels are being constructed by this method, the latter being the only steel tunnel so far to carry three lanes per duct. Foundations n There are two basic systems in use for supporting immersed tunnels on line and grade, a screeded foundation, and a pumped sand foundation. In addition, a few tunnels are founded on piles where soils are particularly soft or special conditions prevail. Such conditions can include earthquake where stone piles may help to dissipate excess pore water pressure and prevent soil liquefaction. (LC.F. Ingerslev, “Immersed Tunnel Foundations,” Comitato Organizzatore del Congresso, TUNNEL ENGINEERING Tunnel Engineering n 20.49 Fig. 20.26 Cylindrical steel double-shell immersed tunnel (Hampton Roads Tunnel). “AITES-ITA 2001, World Tunnel Congress: Progress in Tunnelling after 2000,” Proceedings pp 209–216, Milan, June 2001.) With a screeded foundation (Fig. 20.22), the tunnel is founded on a leveled bed of sand or stone 2 ft to 3 ft thick, placed prior to the immersed tunnel. The leveling has been done by dragging either a heavy grid of steel beams or a steel box filled with the foundation material along the alignment, suspending them from a carriage on rails set parallel to the required grade. The material has also been placed in narrow passes transverse to the alignment using a pipe, the elevation of which was computer controlled. For a pumped sand foundation, the tunnel is founded on a sand or mortar foundation of similar thickness, placed after the tunnel element is temporarily supported in place. The element can be set on two light pile bents that have been Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
Page 1 and 2: 20 Lars Christian F. Ingerslev, Art
Page 3 and 4: Circular tunnels should be fitted t
Page 5 and 6: Fig. 20.3 Clearance diagram for San
Page 7 and 8: 20.5 Preliminary Investigations Sur
Page 9 and 10: The ventilation system also must be
Page 11 and 12: from larger fires. Supply semi tran
Page 13 and 14: adjustments to meet variable demand
Page 15 and 16: generator sufficient for minimum re
Page 17 and 18: interior. The length of each of the
Page 19 and 20: tunneled with tunnel boring machine
Page 21 and 22: Fig. 20.9 Tangent Piles. deep. The
Page 23 and 24: unless the rock is highly abrasive,
Page 25 and 26: more economical because of their st
Page 27 and 28: layers may be sprayed on as needed.
Page 29 and 30: Steel liner plates are available in
Page 31 and 32: should be restricted. Personnel acc
Page 33 and 34: hoe is used to loosen the soil and
Page 35 and 36: A safety screen placed in the upper
Page 37 and 38: conditions. Stone or brick masonry
Page 39 and 40: effective. Bolts are tightened with
Page 41 and 42: Fig. 20.20 Stresses in liner ring m
Page 43 and 44: ground are directly related to the
Page 45 and 46: deep, as long as risks from ship an
Page 47: stressed together longitudinally fo
Page 51 and 52: constructed to the correct grade. T
Page 53 and 54: seismic hazard at a tunnel site can
Page 55: is not an option, then the tunnel m

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