TUNNEL ENGINEERING

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20.6 n Section Twenty the purpose of the tunnel. Construction of a tunnel in the upgrade direction is preferred whenever possible, since this permits water to drain away from the face under construction. Alignment and Grades for Railroad Tunnels n Straight alignments and grades as low as possible, yet providing good drainage, are desirable for train operation. But overall construction costs must be taken into account. Grades in curved tunnels should be compensated for curvature, as is done for open lines. In general, maximum grades in tunnels should not exceed about 75% of the ruling grade of the line. This grade should be extended about 3000 ft below and 1000 ft above the tunnel. Short (under 2500 ft), unventilated tunnels should have a constant grade throughout. Long, ventilated tunnels may require a high point near the center for better drainage during construction if work starts from two headings. Radii of curves and superelevation of tracks are governed by maximum train speeds (Art. 19.9). Alignment and Grades for Rapid-Transit Tunnels n Radii of curvature and limiting grades are governed by operating requirements. The New York City IND Subway has a 350-ft minimum radius, with transition curves for radii below 2300 ft. Maximum grades for this system are 3% between stations and 1.5% for turnouts and crossovers. The San Francisco BART system is designed for train speeds of 80 mi/h. Relation of speed to radius and superelevation of track for horizontal curves is determined by E ¼ 4:65V2 R where E ¼ superelevation, in R ¼ radius, ft V ¼ train speed, mi/h U (20:1) U ¼ unbalanced superelevation, which should not exceed 2 3 ⁄4 in optimum or 4 in as an absolute maximum For 80 mi/h design speed, the radius with an optimum superelevation would be 5000 ft. For a maximum permissible superelevation of 8 1 ⁄4 in, a minimum radius of 3600 ft would be required. The absolute minimum radius for yards and turnouts is TUNNEL ENGINEERING 500 ft. Maximum line grade is 3.0% and 1.0% in stations. To ensure good drainage, grade should preferably be not less than 0.50%. Alignment and Grades for Highway Tunnels n For tunnels under navigable water carrying heavy traffic, upgrades are generally limited to 3.5%; downgrades of 4% are acceptable. For lighter traffic volumes, grades up to 5% have been used for economy’s sake. Between governing navigation clearances, grades are reduced to a minimum adequate for drainage, preferably not less than 0.25% longitudinally and a cross slope of 1.0%. For long rock tunnels with two-way traffic, a maximum grade of 3% is desirable to maintain reasonable truck speeds. Additional climbing lanes for slower traffic may be required when grades exceed 4%. Radii of curvature should match tunnel design speeds. Short radii require superelevation and some widening of roadway to provide for overhang and sight distance. 20.4 Pavements and Equipment for Highway Tunnels Roadway base is a reinforced concrete slab; on this is placed a renewable pavement. Well-designed bitumastic concrete has given good service and has good riding qualities. Average daily traffic capacity of a two-lane twodirectional tunnel is about 20,000 vehicles with a maximum of 1200 to 1500 vehicles per lane per hour. For single-direction traffic in both lanes, capacities are 10 to 15% higher. Red-amber-green traffic lights are installed at about 1000-ft intervals, or at such spacing that the driver always sees at least one light. Telephones are placed in recesses about 500 ft apart for service and emergency calls. Most tunnels, particularly those under water, are equipped with fire mains and hose outlets every 300 ft. Booster pumps in ventilation buildings raise supply pressure to 120 psi for use of foam. Fire extinguishers are mounted in recesses of hose outlets. Fire-alarm stations and phones are at the same locations. Emergency trucks with heavy hoists, fire hose, foam equipment, and emergency tools are kept in readiness at each portal. 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.
20.5 Preliminary Investigations Surveys should be made to establish all topographical features and locate all surface and subsurface structures that may be affected by the tunnel construction. For underwater tunnels, soundings should be made to plot the bed levels. Knowledge of geological conditions is essential for all tunnel construction but is of primary importance for rock tunnels. Explorations by borings and seismic reflection for soft ground and underwater tunnels are readily made to the extent necessary. For rock tunnels, particularly long ones, however, possibilities for borings are often limited. A thorough investigation should be made by a geologist familiar with the area. This study should be based on a careful surface investigation and examination of all available records, including records of other construction in the vicinity, such as previous tunnels, mines, quarries, open cuts, shafts, and borings. The geologist should prepare a comprehensive report for the guidance of designers and contractors. For soft ground and underwater tunnels, borings should be made at regular intervals. They should be spaced 500 to 1000 ft apart, depending on local conditions. Closer spacing should be used in areas of special construction, such as ventilation buildings, portals, and cut-and-cover sections. Spoon samples should be taken for soil classification, and undisturbed samples, where possible, for laboratory testing. Samples not needed in the laboratory, boring logs, and laboratory reports should be preserved for inspection by contractors. Density, shear and compressive strength, and plasticity of soils are of special interest. All borings should be carried below tunnel invert. For pressure face tunnels, borings should be located outside the tunnel cross section. For rock tunnels, as many borings as practicable should be made. Holes may be inclined, to cut as many layers as possible. Holes should be carried below the invert and may be staggered on either side of the center line, but preferably outside the tunnel cross section to prevent annoying water leaks. Where formations striking across the tunnel have steep dips, horizontal borings may give more information; borings 2000 ft in length are not uncommon. All cores should be carefully cataloged and preserved for future inspection. The ratio of core recovery to core length, called the rock quality TUNNEL ENGINEERING Tunnel Engineering n 20.7 designation (RQD), is an indicator of rock problems to be encountered. Groundwater levels should be logged in all borings. Presence of any noxious, explosive, or other gases should be noted. Where lowering of groundwater may be employed during construction of cut-and-cover or bored tunnels on land, the permeability of the ground should be tested by pumping tests in deep wells at selected locations. Rate of pumping and drawdown checked in observation wells at various distances should be recorded; as well as recovery of the water level after stopping the pumps. Geophysical exploration to determine elevations of distinctive layers of soil or rock surfaces, density, and elastic constants of soil may be used for preliminary investigations. The findings should be verified by a complete boring program before final design and construction. 20.6 Tunnel Ventilation Tunnels will be required to be ventilated to dilute or remove contaminants, control temperature, improve visibility and to control smoke and heated gases in the event of a fire in the tunnel. 20.6.1 Ventilation Requirements for Construction Occupational Safety and Health Administration (OSHA) establishes standards, regulations, and procedures necessary to maintain safe, sanitary conditions for all workers on construction sites. Employers are required to initiate and maintain programs that will prevent accidents. Also, employers are advised to avail themselves of safety and health programs provided by OSHA and are required to instruct and train employees to recognize and avoid unsafe, unsanitary conditions, including prevention and spread of fires. OSHA requirements also cover underground construction. Following are some of the requirements applicable to ventilation. Fresh air should be supplied to all underground work areas in sufficient quantities to prevent dangerous or harmful accumulation of dusts, fumes, mists, vapors, or gases. Unless natural ventilation meets this requirement, mechanical ventilation should be supplied. At least, 200 ft 3 of 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: Fig. 20.3 Clearance diagram for San
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 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

TUNNEL ENGINEERING

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