Four somewhat larger tubes, measuring 23 ft. on the outside diameter were built under the East river in 1904-09 for the Penn sylvania and Long Island railroads. Alfred Noble was chief en gineer, S. Pearson and Son, contractors, and E. W. Moir was general manager for the contractors. Blowouts were prevented by placing clay blankets (sometimes 25 ft. thick) on the river bed, which could be carried up to 20 ft. depth of water in the slips and back of the pierhead lines, and by filling the pores of the sand and gravel at the face of the shield with dry blue lias lime or applying plastic clay. Under the Hudson river, two tubes of the same size as those under the East river were built for the Pennsylvania trains to New Jersey.
In 1914 a highway tunnel, started in 1907 and very similar to that at Glasgow, was completed in sand beneath the Elbe river at Hamburg, Germany. Access is here also obtained by elevators, in shafts 1,471 ft. between centres. The shafts are 72 ft. inside and 84 ft. outside diameter and each contains four elevators for vehicles and two for foot passengers. The lift is 78 feet. Each of the two tubes between shafts provides for a single roadway 6 ft. wide and two foot-walks 4 ft. wide. The tubes are of cast iron 19.7 ft. external diameter and the clear width inside is 14.8 feet. The lining is concrete faced with decorative tiles.
The new (1920) water-supply intake tunnel at Cleveland, 0., extends out 2.8 m., below the surface of Lake Erie, at a depth of 95 ft. below the surface and 4o to 5o ft. below the lake bottom. It was driven with a shield and compressed air in soft clay and sand. The air pressure for most of the work varied from 15 to 25 lb. per square inch. The tunnel has an internal diameter of 10 ft. and is lined with concrete blocks ir in. thick.
The notable Holland tunnels connect Canal street, New York, with Twelfth street, Jersey City, passing beneath the Hudson river. The tunnels are named in honour of Clifford M. Holland, the first chief engineer, who died on Oct. 27, 1924, two days before the meeting of the tubes beneath the river. Mr. Holland was suc ceeded by Milton H. Freeman and the latter on his death by Ole Singstad. The tunnels consist of two tubes 1.61 m. in length between the portals. Except for the land approaches, they consist of circular cast iron rings, 29 ft. in exterior diameter. They are solely for motor-propelled vehicular traffic and each tube provides for two lines of traffic in one direction only, with a roadway width of 20 feet. The tubes beneath the river were driven with shields under compressed air. (See Plate, fig. 3.) The maximum grade, with traffic, is 4% and against traffic, 3.6%, both on the New York side of the river. The roadways are paved with granite blocks, and the clear headroom is 13 ft. 6 inches. The interior walls are covered with glazed ceramic tile and the tubes are bril liantly lighted by electricity. The entrance and exit portals are separated at each end by two city blocks and large plazas are pro vided to prevent traffic congestion. Artificial ventilation is pro vided. The tunnels have fire fighting equipment, water lines, sand boxes, telephones, traffic signals, wrecking equipment, etc. A foot
walk is provided for policing and inspection, but the tunnels are not open to pedestrians. Power for the equipment is obtained from two independent sources on each side of the river and three independent cables from each source of supply. Each cable has a capacity sufficient to carry the full load. It was estimated that the tunnels would have a traffic capacity of 1,900 vehicles per hour for each tube. About 52,000 vehicles have used the tunnels in a single day, without reaching their capacity. Con struction began on July 1, 1919, and the tunnels were opened to traffic on Nov. 11, 1927. The cost of construction and real estate was about $48,000.000, borne equally by the States of New York and New Jersey. Tolls are collected. (R. KY.) The River Mersey vehicular tunnel between Liverpool and Birk enhead, England, is the largest subaqueous tunnel in the world, having an outside diameter of 46ft. 3in. and inside diameter of 44 ft. The main roadway is formed at about the diameter line of the tunnel, and provides for four lines of mechanically propelled traffic on a width of 36ft. A narrow footwalk for patrol purposes is pro vided on each side. The whole of the upper part of the tunnel is exposed to view, there being no separate duct for the exhaust air, experiments having established this as unnecessary except near each exhaust shaft. Below the main roadway, in the portion of the tunnel under the river, there is space for a future roadway for two lines of traffic. The fresh air supply ducts are also located under the roadway. The bottom of the under-river tunnel at its deepest point is 17oft. below high water. The distance between shafts on each side of the river is 1,735 yards, and the total length of main tunnel and branch tunnels is 2.62 miles. The branch tunnels have an inside diameter of 26ft. 6in., and only provide for two lines of traffic. They have a length of .6 mile. The gradient is 1 in 3o. The capacity of the tunnel is 4,150 cars per hour, ioo feet apart and moving at 20 miles per hour. At this speed, the time required to pass through the tunnel is 61 minutes. 82,00o tons of cast iron were used in lining the tunnel. The tunnel was opened to traffic on July 18, 1934. The total cost of construction, including land, but ex cluding interest charges during construction, was £6,959,000. The Government contributed £2,500,000, and the balance was borne by the Corporations of Liverpool and Birkenhead. The annual loan charges on sums borrowed are being provided by tolls to the extent of £153,000 per annum, and the remainder out of rates levied on Liverpool and Birkenhead. The late Sir Basil Mott was the engi neer responsible to the Tunnel Committee for the construction of the tunnel. 14,800,000 vehicles passed through the tunnel by end of November, 1938, and 11,200,000 was collected in tolls. (D. A.) Tunnels have been proposed for undersea connection between England and France, beneath the English channel ; even between Alaska and Siberia and beneath the entrance to the Mediterranean at Gibraltar; but aside from the great cost, due to great length and depth, questions of national military defence arise.