Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/251

Rh 233 about two years, when, after continued bad weather, the slopes commenced slipping to such an extent that the line was rendered impassable for some weeks, and parts of the slopes were reduced to an inclination of 4 to 1. The Wiuchburgh cutting, on the Edin- burgh and Glasgow Railway, is 4 miles long and from 25 to 60 feet deep, through solid rock. It is succeeded by an embankment 1^ miles long and 60 feet high, followed in immediate succession by a stone viaduct half a mile long and 80 feet high. The Olive Mount cutting of the Liverpool and Manchester Railway is 2 miles long and at some places 100 feet deep. Perhaps the most interesting case of embankment and cutting in combination is that of the crossing of Chat Moss, on the Liverpool and Manchester Railway. The moss was 4 miles across, and it varied in depth from 10 to 30 feet. Its general character was such that cattle could not stand on it, and a piece of iron would sink in it. The subsoil was composed principally of clay and sand, and the railway had to be carried over the moss on the level, requiring cutting and embanking for upwards of 4 miles. In forming 277,000 cubic yards of embankment 670,000 yards of raw peat were con- sumed, the difference being occasioned by the squeezing out of the water. Large quantities of embanking were sunk in the moss, and, when the engineer, Stephenson, after a month's vigorous operations, had made up his estimates, the apparent work done was sometimes less than at the beginning of the month. The railway ultimately was made to float on the bog. Where em- bankment was required drains about 5 yards apart were cut, and when the moss between them was dry it was used to form the embankment. Where the way was formed on the level drains were cut on each side of the intended line, and were intersected here and there by cross drains, by which the upper part of the moss became dry and firm. On this surface hurdles were placed, 4 feet broad and 9 long, covered with heath, upon which the ballast was laid. Tunnels. The relative costs of rock - cuttings and cuttings in clay do not greatly differ ; for, not only does the vertical rock- cutting require less excavation than the wide yawning earth- cutting of the same depth, with extended slopes, but, when it is executed, the rock-cutting is not liable to the expensive slips which sometimes overtake the other. For depths exceeding 60 feet it is usually cheaper to tunnel. The tunnel (see fig. 16) under Callander ridge near Falkirk station, on the Edinburgh and Glasgow Railway, is a fair representation of I"io. 10. Tunnel under Callander ridge, on the Edinburgh and Glasgow Railway. tunnels as usually constructed. It is lined with brick 18 inches thick, founded on stone footings of greater breadth, in order to throw the, load securely upon the subsoil, as shown in the trans- verse section. The sides and roof of the tunnel are curved from footing to footing, so as effectually to resist the inevitable external pressure of the earth, to a span of 26 feet in width and a height of 22. The sectional view shows also the centering or timber framing employed in the building of the tunnel, which was braced diagonally and transversely to resist the unavoidable inequalities of pressure without alteration of form whilst the arch was in course of construction. Externally the entrances are built of stone, and the flank walls are 3 feet in thickness, with counterforts at inter- vals. This tunnel is not straight, but is formed on a curve of 1 mile radius, and is 830 yards, or nearly half a mile in length. The Kilsby tunnel, on the London and Birmingham Railway, was rendered necessary by the opposition raised to the line passing through Northampton. It is driven 160 feet below the surface and is 2398 yards in length, 30 feet in width, and 30 feet high, constructed with two wide air-shafts 60 feet in diameter, not only to give air and ventilation but to admit light enough to enable the engine-driver in passing through it with a train to see the rails from end to end. .The construction of the tunnel was let for the sum of 99,000, but, owing chiefly to the existence of unseen quicksands, the tunnel is stated to have actually cost nearly 300,000, or 125 per lineal yard. The Box tunnel, on the Great Western Railway, between Bath and Chippenham, was another difficult and expensive work. It is about 70 feet below the surface, and is 3123 yards in length, or rather more than 1 1 miles ; the width is 30 and the height 25 feet. Where bricked, the sides are constructed of seven and the arch of six rings of brick, and there is an invert of four rings. There are eleven air-shafts to this tunnel, generally 25 feet in diameter. The tunnel under the Mound at Edinburgh (see fig. 17), on the Edinburgh and Glasgow Railway, supplies an excellent illustration of tunnels formed with inverts, that is to say, inverted arches built : under the rails. The figure is a transverse section, showing the truly circular arch of the tunnel, 28 feet in diameter and 20 high above the rails, built of brick 3 feet thick, stiffened with counterforts ex- ternally, and with ribs of masonry internally, founded on a solid bed of mason -work, with an inverted i'io. 17. Tunnel under the Moond, arch to distribute the weight. The at Edinburgh. Mound was a mass of loose earth and rubbish on a boggy soil, hence the necessity for the invert arch, on which the tunnel may be conceived to float. The Shakespeare tunnel, or, more correctly, double tunnel, driven through the Shakespeare Cliff near Dover, on the South-Eastern Railway, is in fact two narrow tunnels, carrying each one line of rails (see fig. 18), 12 feet wide and 30 in extreme height, through FIG. 18. The Shakespeare tunnel, on the South-Eastern Railway. the chalk, separated by a solid pier or wall of chalk 10 feet thick. The chalk is of variable quality, and the greater part of the tunnel is lined with brick, strengthened by counterforts at 12 feet intervals, which carry the weight of doubtful beds of chalk. The tunnel is 1430 yards, or upwards of three-quarters of a mile in length, rising westward with an inclination of 1 in 264. The tunnel being within a short distance of the face of the cliff, the material excavated was discharged through galleries about 400 feet long, driven in from the face of the cliff, into the sea, the first operation being to run a bench or roadway along the face of the cliff. There are seven vertical shafts from the surface, averaging 180 feet deep. There were in 1857 about 70 miles of railway tunnelling in Great Britain, or 1 mile of tunnel for 130 miles of railway. There are now (1885) probably at least 100 miles of tunnelling. The cost of tunnelling has averaged 102 per mile. The longest tunnel is the Woodhead, at the summit of the Manchester, Sheffield, and Lincolnshire Railway, being 3 miles and 60 feet long. The tun- nelling on the Metropolitan Railway is noticed below, p. 239. Bridges and Viaducts. There are very few level crossings on Bridges. English railways that is, the crossing of one railway with another, or with a common road, at the same level the chances of accidents having demanded, in general, the construction of bridges over or under the railway. The general appearance of an ordinary stone or brick bridge is represented by fig. 19, showing in elevation a bridge over or under the railway. The minimum height of a bridge over the railway is ruled by the elevation necessary to clear the top of the chimney of the locomotive. An excellent method of carrying roads over railways, where the height is limited and the span is moderate, consists in erecting flat -arched cast-iron beams over the railway, and throwing brick arches of small span between the beams upon their lower flanges, to carry the roadway. Thus the vertical depth from the soffit or crown of the main arch to the roadway above may but very little exceed the depth of the beam, which is apparent in the sectional view. This method of construction is, moreover, well adapted for skew-bridges. Cast-iron XX. 30