Tunnelling Under Rivers and Harbours

TUNNELLING UNDER RIVERS AND HARBOURS actual cross-section varied. It is stated that 30,000 labourers were occupied II years in its construction. With modern appliances and a small percentage of the men, such a tunnel could be driven from the two ends without intermediate shafts in far less time.

No practical advance was made on the tunnelling methods of the Romans until gunpowder came into use. Old engravings of mining operations early in the 17th century show that excavation was still accomplished by pickaxes or hammer and chisel, and that wood fires were lighted at the ends of the headings to split and soften the rock in advance. (See fig. I.) Crude methods of venti lation by shaking cloths in the headings and by placing inclined Through Tunnelling Methods.—In 1818 Marc Isambard Brunel took out a patent for a tunnelling process, which included a shield, and which mentioned cast iron as a surrounding wall. His shield foreshadowed the modern shield, which is substituted for the ordinary timber work of the tunnel, holds up the sur rounding earth during excavation, affords space within its shelter for building the permanent lining, overlaps this lining in telescope fashion and is moved forward by pushing against the front ends. The advantages of cast iron lining are that it has great strength in small space as soon as the segments are bolted together, and its joints can be caulked water-tight.

First Use of Shield.

In 1825, Brunel began, and completed in 1843, after several suspensions of operations, the Thames tun nel between Rotherhithe and Wapping. It was constructed for a highway, but was never used for that purpose. It was sold in 1866 to and has since been used by the East London railway, which operates its trains through it. This early tunnel, built of brick, in the form of two arches with frequent openings between them, has a length of 1,200 ft., and required an excavation opening 27 ft. in width, which is still one of the widest ever built under such conditions. Brunel employed a peculiar form of shield, made of timber, in several independent sections. Part of the ground pene

trated was almost liquid mud, and the cost of the tunnel was about £433 per linear foot. In 183o Lord Cochrane (afterwards the loth earl of Dundonald) patented the use of compressed air for shaft sinking and tunnelling in water-bearing strata. Water under any pressure can be kept out of a subaqueous chamber or tunnel by introducing sufficient air of a greater pressure, and men can breathe and work therein—for a time—up to a pressure exceeding four atmospheres. To confine the compressed air it is Lecessary to provide a substantial bulkhead across the workings. To pass men and materials through the bulkhead there is a mechanical device called a lock, which is a large steel tube, with doors at each end, both of which open inward toward the working chamber, and both of which can never be opened at one time because of the difference in air pressure between that in the working chamber and that back of the bulkhead. Valves are provided to admit compressed air to the lock from the working chamber, and also to exhaust it from the lock to back of the bulk head, in order to manipulate the doors.

The Severn tunnel 4.33 m. in length, for a double line of railway, was begun in 1873 and finished in 1886. Hawkshaw, Son and Hayter were the engineers, and T. A. Walker the con tractor. At the lowest part the depth of water was 59 ft. at low water and 104 ft. at high water, and the thickness of sand stone over the brickwork was 45 feet. Under a depression in the bed of the river on the English side there is a cover of only 3o ft. of marl. Much water was met with throughout. In 1879 the works were flooded for months by a land spring on the Welsh side of the river, and on another occasion from a hole in the river bed at the Salmon pool. This hole was filled with clay and the work completed beneath. The total amount of water raised at all the pumping stations was about 27,000,00o gal. in 24 hours.