TUNNEL ENGINEERING

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20.52 n Section Twenty mean water level. Live load includes seabed erosion and siltation, and variations in water level, current, storm loads and earthquakes, each with a return period of 5 years or less. Exceptional loads include loss of support (subsidence) below the tunnel or to one side, and storms and extreme water levels with a probability of being exceeded once during the design life. Extreme loads include sunken ships, ship collision, water-filled tunnel, explosion (e.g. vehicular), fire, the design seismic event predicted for the location, and the resulting movement of soils. Conditions to be investigated should include normal, abnormal, extreme, and construction. 20.21 Shafts In tunnel work, shafts are starting points for excavation in rock or firm material or shields. For long tunnels, such as aqueducts, several shafts are used to divide construction into shorter sections that can be worked simultaneously. For vehicular tunnels, especially subaqueous shield tunnels, the shafts are used as bases for ventilation buildings. In construction of shafts, regulations of the Occupational Safety and Health Administration should be observed (Arts. 20.6, 20.8, and 20.12 to 20.16). Timber shafts are mined and braced in the same manner as tunnels in similar material. Usually, poling boards 5 to 6 ft long are driven into the ground and braced at regular intervals by rectangular timber frames. Then, the soil is excavated to the ends of the polings and a new frame installed at this level. A relatively shallow shaft may be started oversize with sheeting 10 to 20 ft long driven vertically on the outside of the frame bracing. Intermediate frames are installed as the excavation proceeds. At the bottom of the tier of sheeting, the sides are stepped in to make room for the next tier of vertical sheeting. In rock shafts, timbering is used to prevent loose rocks from falling off the walls. Its placement usually lags an appreciable distance behind the excavation. Steel liner plates alone, or in combination with horizontal ribs, may be used in soft ground where excavation can be made in increments equal to the width of the liner plates. H beams driven vertically as soldier piles, with wood or steel lagging and horizontal bracing, may be used for rectangular TUNNEL ENGINEERING shafts. Enclosures of vertical steel sheetpiling, for round or rectangular shafts, are suitable for waterbearing ground. Where ground conditions are poor and waterbearing, shafts may be constructed with a caisson (hollow box), with compressed air as needed to exclude water. Gravity pulls the caisson down as excavation proceeds. Since its weight is relatively small, the caisson may have to be temporarily ballasted or jetted for sinking. The depth to which a caisson may be sunk is limited by the high cost of compressed-air work, which results from the short working hours permitted under high pressure. Open-bottom shafts with heavy walls, often circular or subdivided into compartments, may be built on the ground and sunk by excavating the ground underneath. In dry soil, the excavation may be done directly; if water is present, clamshell buckets and high-pressure jets may be used to loosen the soil and remove it. On reaching the proper depth, the bottom of the shaft is closed by tremie concrete. As an alternative method for shaft construction, water-bearing ground may be frozen in a circular ring around the shaft location and the excavation made in the dry. Closed-end pipes are driven vertically into the ground around the periphery, and open-end smaller pipes inserted into them. A refrigerant, usually brine, is circulated at temperatures as low as 230 8F from the interconnected inner pipes into the larger ones and from them returned to the refrigeration plant. Several months may be required to freeze a deep ring solidly. The ventilation shaft of the Scheldt River Tunnel in Antwerp was built in this manner, as were a number of mine shafts in Germany and France. (J. O. Bickel and T. R. Kuesel, “Tunnel Engineering Handbook,” Van Nostrand Reinhold Company, New York.) 20.22 Seismic Analysis and Design Earthquake loads, or more correctly seismic loads, are included among the loads on a structure that are required to be considered by most current design codes. Seismic effects can occur during the construction phase and should therefore also be considered during that period; an appropriate level of risk should be agreed with the owner. The 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.
seismic hazard at a tunnel site can be quantified by a project-specific seismic hazard assessment. Typically, a functional evaluation earthquake (FEE), likely to occur not more than once during the design life, is used first to design the structure for either limited or full performance following a seismic event, as agreed with the owner. Next as appropriate, either the safety evaluation earthquake (SEE) or the maximum credible earthquake (MCE), both concerned not only with life safety but also with the survivability of the structure under the most severe seismic event considered at the location, is checked to ensure compliance with minimum performance. If necessary, the strength of some parts of the structure may have to be enhanced to comply. Structures buried in soil are generally constrained to follow the seismic deformations of the ground in which they are located. The stiffness of the tunnel is generally small relative to the soil, so that in all but soft soils, tunnel deformations will TUNNEL ENGINEERING Fig. 20.28 Lay Barge. Tunnel Engineering n 20.53 approximate to ground deformations, a conservative assumption. For a tunnel in rock, tunnel deformations will match those of the rock, but in softer soils, the tunnel will resist soil pressures. The response of the tunnel to the free-field soil displacements (as if the tunnel were absent) will depend upon both the stiffness of the tunnel and that of the soil. While the complex seismic analyses may be solved numerically using computers, some simplified procedures have been published. Simplified beam-on-elastic-foundation analysis can also be used to account for the soil-structure interaction effects of soil deformations, especially in soft soils. Horizontal shear S-waves, depending upon the angle of approach, cause transverse bending or axial waves and produce the largest strains that are usually governing. Compression Pwaves should also be considered. At sites where there are deep deposits of soil, Rayleigh R-waves may govern the induced strains. Racking (ovaling) 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 and 48: stressed together longitudinally fo
Page 49 and 50: in two directions. Self-compacting
Page 51: constructed to the correct grade. T
Page 55: is not an option, then the tunnel m

TUNNEL ENGINEERING

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