Probably the most notable submarine tunnel through rock is that under the river Severn in England, which is known as the Severn Tunnel. The Great Western Railway system west of Bris tol was formerly separated south of Gloucester by the Severn and its estuary, from the lines between Gloucester and the South Wales ports, as well as from the western lines between Hereford and North Wales, Liverpool, Manchester. and the north. This want of connection was only partly remedied by a steam ferry across the estuary of the Severn. A tunnel under the Severn was, ac cordingly, proposed in 1871 to provide for through traffic, and was authorized in 1872. Work was begun in 1873. The site selected for the tunnel was about two miles below the mouth of the Wye, where the width of the estuary at high tide is about miles. The strata trav ersed by the tunnel consist of conglomerate, lime stone, carboniferous beds, sandstone, marl, gravel, and sari; the least thickness of soil between the top of the tunnel and the deepest part of the channel is feet. The total length of the tun nel is 4 miles, 624 yards. The tunnel was exca vated from four shafts. work being commenced in 1873 and completed in 1886. Great difficulty was experienced with water. which flooded the workings on several occasions. On one of these occasions a diver was employed to enter the tun nel and close a bulkhead door, which he did by carrying a knapsack filled with compressed oxy gen to sustain life during his submarine journey.
Another subaqueous tunnel through rock is the Hersey Tunnel. about one mile long, connecting Liverpool and Birkenhead. In this work a venti lating tunnel of smaller size than the main tun nel was excavated under the main tunnel by a rotary boring machine which cut out the passage way to full size at one operation. Except for the Mersey Tunnel, which is a passenger tunnel, all the tunnels previously described are for rail way purposes. They are also all rock tunnels.
Among the many important railway tunnels which have been built through soft ground only two will be mentioned. The Baltimore Belt Tunnel, 8350 feet long, in Baltimore, ma_ was excavated through water-bearing sand, loam, clay, and gravel by the German method of soft-ground tunneling. It provides for a double track railway line. The Saint Clair Tunn(1 carries the double tracks of the Grand Trunk Railway under the Saint Clair River between Michigan and Canada. In 1SS6 the Saint Clair Tunnel Company was formed to build this tunnel. Borings showed the materials to be penetrated to be a soft blue clay throughout at the site chosen, which was just below Sarnia and Port Huron. The excavation consisted of an open cutting on the American side 2500 feet long; a tunnel 6000 feet long under the river, and an open cutting on the Canadian side 3100 feet long. Work was be gun by means of two shore shafts in 1888, but these were abandoned, and in 1889 work was com menced on the cuttings. These were completed so that shields were started on the tunnel proper in the same year. The shields used were feet outside diameter, and the cylindrical shell eon sisted of 1-inch steel plates. The cylinder was stitiened by five diaphragms dividing it into twelve eells. In front the shield had a cutting
edge, and at the back it was prolonged 4 feet to cover the of the tunnel. As the shield was shoved forward a paste of cement and water was forced out through holes left in the lining to fill the spaee left between the lining and the clay. The excavation was done in front of the shields, which were then forced forward by hydraulic jacks, and the erection of the permanent east iron lining followed up. the ring being erected in side the tail of the shield. There were 24 hy draulic jacks at the back of each shield, each capable of exerting a power of 125 tons, but the greatest total power used did not exceed 1800 tons. The lining is 21 feet in outside diameter. Each ring of lining is 18 inches long, measured on the line of the tunnel, and is made up of 13 seg ments and a key piece. Each segment weighs 1050 pounds. and the total weight of east iron in the lining is 27.000 tons. The Baltimore Belt Line tunnel is a fairly representative example of railway tunneling through soft ground on land, and the Saint Clair Tunnel is an equally good ex ample of railway tunneling under water by means of the shield method.
Next to their use for railways, tunnels are more frequently built to conduct water than for any other purpose, perhaps. A good example of rock tunneling for this purpose is afforded by the Niaaara Tunnel, built to earry the water away from the wheel pit of the Niagara Power Com pany at Niagara Falls, N. Y. This tunnel is 7600 feet long. 10 feet wide. and 21 feet high. and runs through solid limestone rock. The Chicago water-works tunnels, through which the city of Chicago draws its water supply from Lake M iebi gam constitute the most elaborate system of wa ter-supply tunnels anywhere in the world. They comprise 21.97 miles of subaqueous tunnel and 15.92 miles of land tunnel. The subaqueous tunnels are as follows: At Milwaukee, Wis., there is a water-supply tunnel 3200 feet long and feet in diameter lined with brick masonry throughout, completed in 1895. At Cleveland, 0., a water-supply tunnel 26,000 feet long and 9 feet in diameter has recently been constructed, and two other older tunnels are also in use, one being 5 feet in diameter for 6662 feet and and 6 feet in diameter for 2580 feet, and the other being 7 feet in diameter and 9200 feet long. One of the most notable water supply tunnels in the world is that which brings water to New' York City, and which is known as the Croton Aqueduct. (See Aciumucr.) This tunnel is 33 miles long and about 14 feet high and 14 feet wide; it was built through rock. The Aqueduct, 77 miles long, for bringing water to the city of Liverpool, England, has three tunnels. The first and longest is the Hirnant Tunnel, 2 miles 3 furlongs in length through solid rock. The second is the Cynynion Tunnel at mile 16. It is seven furlongs long; hut the aqueduct only comes out of the tun nel to pass in a siphon across a valley, entering the Llanforda Tunnel, which is one mile long. These tunnels are circular in section and 7 feet in diameter. The third tunnel is that under the River Mersey, which is 805 feet long and lined with iron rings 10 feet in diameter on the out side.