Caissons Sinking Tubes

The vehicular tunnel beneath the Detroit river, from Detroit, Mich., to Windsor, Canada, was started in 1928. It consists of one tube, 0.95 m. long, between portals, with an inside diameter of 28 ft. 4 inches. The portion beneath the river, 2,500 ft. long, is to be built by the subaqueous trench method as in the case of the Michigan Central tunnel at Detroit, and the second Harlem River tunnel, at New York. The land portions, i,000 ft. and Soo ft. in length are shield-driven. The estimated cost is $10, 000,000. Because of the international character of this tunnel, it is necessary not only to provide for collection of tolls but for customs and immigration inspection, which requires unusual plaza facilities at both ends of the tunnel. Parsons, Klapp, Brinckerhoff and Douglas are engineers and Porter Brothers, contractors.

The largest diameter tunnel built by the subaqueous trench method is the Oakland-Alameda estuary tube, in California. It is a single tube, 0.67 m. in length between portals, of which 2,436 ft. is made up of 12 sections of tubes, each 203 ft. long, sunk in a dredged trench, with a depth of water over the tubes of 42 feet. The tube sections present the novelty of being made of reinforced concrete, 37 ft. external diameter, with a shell thickness of 3o in. and they are enveloped with a membrane of three-ply water proofing. They were cast in forms in a dry dock at San Francisco, then floated to position and sunk. The roadway is 23 ft. wide for vehicles, two lines of street railway tracks and also foot-walks pro tected by railings. The cost is estimated at $4,5oo,000 or about one-half the estimated cost if done by the shield method. C. E. Posey is chief engineer.

Tunnelling by Sinking Caissons.

The first New York rapid transit tunnel under Harlem river, built in 1904-05, is an example of tunnel caissons built under water and in part of the work a portion of the permanent tunnel was itself made to serve as the roof of the caisson. The tunnel has two tubes, each about 15 ft. in interior diameter, the portion beneath the river being 400 ft. long, with a surrounding shell of cast iron itself surrounded by concrete. The outside width of concrete is about 33 feet. Its top is 28 ft. below high water and about 3 ft. below the bed of the river. The method of construction was devised by McMullen and MacBean, the contractors, who dredged a trench in the river to within 7 or 8 ft. of the required depth. They then enclosed a space of the width of the tunnel, from shore to mid-stream, with 12 in. sheet piling, which was evenly cut off some 2 ft. above the determined outside top of the tunnel. On top of this piling was sunk and tightly fitted a flat temporary roof of timber, 3 ft. thick in sections, which was covered with about 5 ft. of dredged mud. An air lock was provided in the roof and the water was expelled from this subaqueous chamber by compressed air, after which the remaining earth was easily taken out, and the cast iron and concrete tunnel walls were then built in the chamber. For the remaining part of the river the foregoing process was varied by cutting off the sheet piling at mid-height of the tunnel and making the upper half of the tunnel, which was built above and lowered in sections through the water, serve as the roof of the chamber in which the lower half of the tunnel was built in compressed air.

Subaqueous tunnels are usually started from shafts near the margins of the streams and, when in soft ground, the shafts are sunk by the use of caissons under compressed air. The caissons remain as part of the permanent construction and from them the shields are started. It may therefore be said that nearly all tun nels of this class are built in part by caissons. (See Plate, figs. I and 2.) The tunnels of the Metropolitain railway of Paris (F. Bienve nue, engineer-in-chief) under the two arms of the Seine, between Place Chatelet and Place Saint Michel, were made by means of compressed-air caissons sunk beneath the river bed, L. Chagnaud being the contractor. They were built of plates of sheet steel and masonry, with temporary steel diaphragms in the ends, filled with concrete, making a cross wall with a level top about even with the outside top of the tunnel and about 2 ft. below the bottom of the Seine. The caissons were sunk on the line of the tunnel so that adjacent ends and the walls just described, were nearly 5 ft. apart with (at that stage) a core of earth between them. Side walls joining the end walls and thus enclosing the earth core on four sides (fig. 3) were next made by the aid of temporary small caissons sunk through about 26 ft. of earth under the river. The tops of the side walls were made even with the end walls. A steel rectangular coffer-dam (figs. 4 and 5) was sunk to rest with rub ber or clay joint on these surrounding walls. The coffer-dam had shafts reaching above the surface of the water, so that the earth core was easily taken out in free air, after removing the water. The adjacent chambers under the caissons were then connected together. Three caissons, of a total length of 396 ft. were used under the large arm and two, of an aggregate length of 132 ft. under the smaller arm of the Seine. Construction was started in 1905 and operation was begun in Jan. 191o; the cost of the tunnel was 2,134 francs per linear foot.

At San Diego, Calif., a tunnel 1,200 ft. long was constructed in 1928 by the caisson method. The tunnel has a cross section of I I by 12 ft., and forms the intake for cooling water for a power plant.

Tunnels Built in Coffer-Dams.

The first subaqueous high way tunnel in the United States was that at Washington street, beneath the Chicago river, in Chicago. It is not properly a tunnel, having been built in a coffer-dam. It was constructed in 1866-69 and has two roadways each II ft. wide and 13 ft. high, and a foot way io ft. wide and 1 o ft. high. It has twice been rebuilt to pro vide a deeper waterway, the original depth being only 14 feet. After the great fire of 1871, it formed the only means of com munication between the west side and the business district pend ing the reconstruction of the bridges. In 1869-71 a similar road way tunnel was constructed at La Salle street and in 1889-94 one at Van Buren street, both beneath the Chicago river. These also required rebuilding at later dates to provide deeper waterways. All these tunnels are now used for surface cars only.