Page:The New International Encyclopædia 1st ed. v. 19.djvu/624

* TUNNEL. 54:.4 TUNNEL. had to be conveyed these distances on wagons and sleds. By the teims of the contract the size of the tunnel was I61/2 feet wide and 22 feet high. The method of driving it was as follows : An 8- foot heading was driven along the top of the tun- nel and as kept about 30 feet in advance of the bench where the tunnel was excavated to its full width. With the exception of about 500 feet in- ward from each portal, where the rock as firm and stable, the entire tunnel was timbered as fast as the work progressed. The timbering made it necessary to enlarge the section called for by the contracts to a width of lOVi feet and a height of 23 feet 10 in. The total length of the tunnel was 9850 feet, with a rising grade from each end to- ward the middle. Po^^•er drills operated by com- pressed air were used. The amount of explosive used was 300,625 pounds. The total cost of the tunnel was .$1,160,000. The Cascade Tunnel carries the Great Northern Railway through the summit of the Cascade Range in Washington. This tunnel is of particu- lar interest because of the representative illus- tration which it affords of modern rock -tunneling methods and machineiy in the United States. The crossing of the summit proper of the Cascade Jlountains since the construction of the Pacific extension of the Great Northern Railway inlSOl- 92 was effected by means of a switchback. This switchback was built merely as a temporary line. Business conditions, however, did not justify the construction of a tunnel l)etween 1892 and 1897, but in the latter year orders were given to com- mence the work and push it as fast as possible to completion. The length of the tunnel is 13,813 feet, of which about 400 feet (or 200 feet at each end) is an extension of the permanent lining, to take llie place of the wooden snow shed, but the length of the tunnel proper, from face to face of portals, is 2.61 miles. The width in the clear, inside of the permanent lining, is 16 feet, and the height from top of rail to bottom of arch is 21 feet 6 inches. For the first 500 feet at the west portal, the tunnel was driven through a slide com- posed of gravel and large boulders. This ground was heavily impregnated with water, and the pressure from above was tremendous. The pres- ence of the large boulders made it difficult to drive poling boards, and a hole a few inches in diameter, if left open, would soon let in sand enough to fill the heading. To carry this heavy ground required three concentric sets of timbers, as well as tie-rods, connecting the wall plates. The ordinary American system of tunnel exca- vation was employed. The overhead heading was taken out to the full size of the rock section, 10 X 20 feet, and the bench taken nut in two lifts. Rock from the heading and top bench was wheeled in barrows out onto a large traveling carriage called a 'jinnbo,' where it was dumped through chutes into the muck cars. Material from the lower bench was shoveled directly into the cars. On a secondary floor, under the main lloor of the 'jumbo,' was a hoist operated by compressed air, which was used to load large pieces of rock, some of them weighing six tons, on the flat cars. This obviated the necessity of blockholing large pieces of rock, to enable them to be handled. This cable and hoist was also )ised to move the 'jumbo' back from the rock face when blasting was to be done. Compressed air. at 100 pounds' pressure, operated drills, hoists, and pumps with the exception of one large pump which was operated by elec- tricity. Four vertical columns in the heading carried six 314-inch Rand 'Slugger' drills. From 24 to 28 holes, 12 feet deep, were drilled in the breast, and fired by electricity, in three rounds. Holes were drilled to lift the top bench, and ver- tical holes for the main or lower bench, enough to break the rock to subgrade. Altogether an average of fourteen 314-inch drills were used in each heading and benches. All hauling out of muck, and hauling in of con- crete, was done with electric motors, of which eight were in service. Six of these were con- structed at the tunnel machine shop. One of these motors hauled loaded trains of from 16 to 20 dump cars, of 1 cubic yard capacity, up the 1.7 per cent, grade, at 10 miles per hour. The foul air was exhausted from the heading through 24- inch galvanized iron pipe, by means of a No. 9 Sturtevant fan, running at 1700 revolutions. The tunnel is permanently lined throughout its entire length with concrete, this lining being no- where less than 23 inches thick, and in places 3% feet thick. Where the section was larger than would admit of solid concrete, it was backfilled with spawls and broken stone, the result being a monolith of concrete from portal to portal. 'The tunnel excavation proper was commenced August 20, 1807, and completed October 13, 1900. The average monthly progress in the tunnel was 350 feet; average daily progi'ess 11.53 feet. The work of concrete lining the tunnel was commenced November 17, 1899, and completed November 17, 1900. The average monthly progress was 1115 feet. Probably the most notable submarine tunnel through rock is that imder the river Severn in England, which is known as the iievern 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 onl.v partly remedied by a steam ferrv across the estuary of the Severn. A tunnel under the Severn was, ac- cordinglv, proposed in 1871 to provide for through traflic. 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 W.ve, where the width of the estuary at high tide is about 2% miles. The strata trav- ersed bv the tiuinel consist of conglomerate, lime- stone, carboniferous beds, sandstone, marl, gravel, and sand : the least thickness of soil between tl>e top of the tunnel and the deepest part of the channel is 44-?4 feet. The total length of the tun- nel is 4 miles, 624 yards. The tunnel was exca- vated from four shafts, work being conuuenced 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 o.xy- gen to sustain life during his submarine journey. Another subaqueous tunnel through rock is the ^ferfse!l Tunnel, about one mile lone, 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