All the preceding methods of tunneling described pertain to tun nels not driven below open water, although the material which they penetrate may be fully saturated with water. The most difficult tun neling is that which must be carried on at con siderable depths below the surface of free water above the work. In such cases the water finds its way through either porous material or through crevices or fissures in the overlying rock so that both the completed tunnel and the work in progress are subjected to a water pres sure represented by the static head of the water above them. Occasionally in rock tunneling under such conditions or in stiff clay, water may flow in upon the work in quantities not too great to make its removal by pumping feasible. Whenever, however, water enters too freely for such procedures, compressed air must be used to keep the water out, the pressure of air being determined by the depth of the work below the water surface. When tunneling is prosecuted under these conditions a °shield') is ordinarily used, as will be described further on. An in stance illustrating the method to be followed in sub-aqueous rock tunneling will first be given.
The Severn tunnel, a double track tunnel, a little less than miles long, built under the river Severn in the south western portion of England between 1873 and 1886. Although this tunnel was built largely through rock, for a short distance the material penetrated was gravel. The bed of the river at the tunnel site is composed of strata of con glomerate limestone, carboniferous beds, marl, gravel and sand. The least thickness of the natural cover over the tunnel is 30 feet of marl on the English side of the river. At the deepest part of the river channel the thickness of the sandstone over the tunnel is about 45 feet. The entire tunnel is lined with vitrified brick work to 3 feet thick, laid in Portland cement, the invert having a thickness of to 3 feet. Much water was encountered in the prosecution of this work which produced grave difficulties. Indeed, at one time the entire work was flooded for a period of 13 months. The water gave much trouble at other times, breaking through in large volumes, but in no other instance was the work suspended on account of the water for a period of more than a few weeks. Pumps were employed to raise the water through a side heading connecting with a shaft 29 feet in diameter. The greatest amount of water raised concurrently by all the pumping sta tions was about 27,000,000 gallons in 24 hours, although the total pumping capacity provided was equivalent to about 66,000,000 gallons in 24 hours. The ventilation of this tunnel, which is a matter of great importance on account of its length, is accomplished by a fan 40 feet in diameter in one of the shafts, making 43 revolu tions per minute and drawing out 447,000 cubic feet of air per minute to an 18-foot shaft near to the work.
Tunneling by the Use of a The method which has been employed more than any other for sub-aqueous tunneling in soft FIG. 6.
materials is that which it is believed was first devised and applied by the great French English engineer, Sir M. I. Brunel. Brunel was born in France in 1769, spent six years in the United States, then went to England and became one of the greatest civil engineers of Great Britain, where he died in 1849. He secured his first patent covering the use of a shield for sub-aqueous tunneling in 1818. He first employed a shield in the effort to build a tunnel under the Thames River at London in 1823. His first shield was found to be too weak in actual service and had to be replaced by another and somewhat larger one designed by and built under the direction of Brunel him self. This shield was rectangular in transverse section, 37 feet 6 inches wide by 22 feet 3 inches high, and by the use of it two parallel tunnels of horseshoe shape were built, each being 13 feet 9 inches wide and 16 feet 4 inches high, the two being separated from each other by a wall 4 feet thick. This dividing wall was not continuous, hut was pierced by arch openings, each about 4-feet span about every 20 feet. The total structure built of brick work was 38 feet wide over all and 22 feet high. The tunnel
was finished in 1843, making the total time of construction, including all stoppages and delays, 20 years. Another English engineer, Peter W. Barlow, patented in England in 1865 a method of sub-aqueous tunneling by the use of a circular shield with a cylinder cast-iron lining for the completed tunnel. After 1869 he was associated with the English engineer, James Henry Great head (1844-96), in the construction of the tun nel under the Tower of London, 1,350 feet long laborers stand in order to make the desired excavation through the openings themselves. If the material being penetrated is very soft and porous an inrush of water may take place even when compressed air is used, and the diaphragm must be strong enough to resist the resulting pressure. It may be, and usu ally is, heavily braced with plates and angles, both vertical and horizontaL The illustra tions show how complicated its construc and 7 feet in diameter, which penetrated com pact clay and was completed within a period of 11 months. This was a remarkable record in tunnel building, and from that time until the present numbers of tunnels in soft material under water have been constructed on what is commonly known as the Greathead system, which simply means the use of a cylindrical or circular shield, developed from Brunel's original plan and subsequently perfected by Greathead. The use of the shield has made it possible to construct tunnels under rivers at depths be low the surface of the water as great as the effect of compressed air on laborers will per mit, it being a matter of comparative indif ference how soft the material may be, ex cept that the softer or more easily flowing the material the more carefully must the work be executed. The shield is composed of a cylindrical shell usually constructed at the present time of steel plates and angles or other shapes with a heavy braced diaphragm placed at right angles to the axis of the shell. This diaphragm is of heavy steel plate and shape construction. It may have a num ber of openings in it closed by doors or other suitable devices. These openings in the dia phragm permit laborers to excavate the mate rial immediately in front of the shield. If the tunnel is a large one the openings may have platforms in front of them, on which tion may be. The cylindrical shell in which the diaphragm is located may extend from one to 10 or 12 feet in front of the diaphragm and from 6 or 8 to 26 or 27 feet behind it. The tunnel lining, usually of cast iron from one inch to two inches in thickness and possi bly lined with masonry, is constructed within the rear or tail of the cylindrical shell, so that the latter always overlaps by two or three to four or five feet the finished lining of the tunnel, thus preventing any material or water falling into the completed work. Obviously as the workmen excavate the material in front of the shield, pass it through the dia phragm and take it out in the rear, the shield must be moved forward so as to bring its front end again up to the face of the exca vation. As these shields are very heavy masses of steel, weighing sometimes 40 to 80 tons or more, and as the friction of the sur rounding material on the sides of the shell must he overcome, a heavy force is needed to make the movement. This force is usually supplied by hydraulic jacks so devised and placed around the circumference of the dia phragm as to push against the completed iron lining of the tunnel. These jacks have cylin ders six inches or more in diameter and are actuated with water or other liquid at a pres sure of 1,000 to 3,000 or 4,000 pounds per square inch. A shield about 21.5 feet in diame ter was used in the construction of the Sarnia tunnel under the Saint Clair River above De troit, Mich. This shield was moved by 24 hy draulic jacks, as shown in the illustration, each having a capacity of 125 tons and so placed as to press directly upon the cast-iron tunnel lin ing behind it. By such means the shield may be pushed ahead as fast as the excavation made and the tunnel lining completed behind it.