1902 Encyclopedia > Railway, Railways (Railroad) > Railway Construction: Cuttings and Embankments. Chat Moss.
C. RAILWAY CONSTRUCTION (cont.)
Cuttings and Embankments
Engineers endeavour so to plan the works of a railway that the quantity of earth to be excavated shall be equal to the quantity that goes to form the embankments. The earthwork is the foundation and support of the superstructure, and as such it must be uniformly firm, of liberal width, easy slopes, thorough drainage. Figs. 10 and 11 are type-sections of cuttings and embankments for a double line of way on the national gauge, showing the "formation" surface and the ballast on which the permanent way is supported, with the slopes, the side drains, and the fencing.
Fig. 10. Type section of a cutting.
Fig. 11. Type section of an embankment.
Fig. 12 is a type-section of the permanent way on the national gauge, settled by Mr. John Fowler for the South Wales Railway.
Fig. 12. Type section of permanent way.
Upon the formation level the ballast is deposited, 2 feet in depth at the centre, dressed level, for about 22 feet in width for a double line of way. The sleepers and chairs buried in the ballast, and the rails partially also, these standing 2 or 3 inches above the ballast. The intermediate space between the two lines of way is, as before stated, 6 feet, and, taking the lengths of the sleepers at 9 feet, the total width for two lines of way over the sleepers is 6 feet + 4 feet 8 1/2 inches + 2 1/2 inches x 2 (width of the rails) + 9 feet = 20 feet 1 1/2 inches; and it is seen that, as the ballast is 22 feet wide at the upper surface, it extends to nearly 1 foot beyond the ends of the sleeper at each side and about 3 1/2 feet beyond the outer rails at each side.
The slopes of cuttings vary according to stratification, soil, direction of the vein, moisture. In gravel, sand, or common earth the slopes rise 1 foot for 1 to 1 1/2 or 2 feet of base; in solid rock the slopes are nearly vertical. Cuttings are as deep as from 50 to 100 feet below the surface, and embankments as high above. The London and Birmingham Railway had upwards of 12 million cubic yards of excavation, and 10 1/2 millions of excavation in the original estimates, or above 100,000 cubic yards of earthwork per mile. The heaviest cutting on the line is at Tring, 2 1/2 miles long, averaging 40 feet deep, the greatest depth being 60. In the case of the great Blisworth cutting the strata were unequal in consistency. About halfway up the face of the cutting a stratum of limestone rock, 25 feet in thickness, was found, with loose strata below and above it, and it was necessary to prevent the lower stratum, consisting of wet clay, from being forced out under superincumbent mass by undersetting. A rubble wall, averaging 20 feet in height, was built on each side underneath the rock, strengthened by buttresses at intervals of 20 feet, resting on inverted arches carried across underneath the line. A puddle-drain was formed behind each wall, with a small drain through the wall to let off the water from behind.
Fig. 13 is an elevation of the west end of the cutting where it is about 40 feet deep, showing clearly the method of undersetting, and fig. 14 is a cross section of the side walls at the same place, where the left-hand shows a section of the wall in the water, and the right-hand side shows the section through a buttress, together with the invert and drains. One of the walls is shown in front elevation in fig. 15. The New Cross cutting through the London clay, on the South-Eastern Railway, is 2 miles long, and is for some distance from 80 to nearly 100 feet in depth. This cutting affords an example of the tendency of some soils to slip. The slopes of the cutting were finished at 2 horizontal to 1 vertical; and they remained as thus finished for 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 Winchburgh cutting, on the Edinburgh and Glasgow Railway, is 4 miles long and from 25 to 60 feet deep, through solid rock. It is succeeded by an embankment 1 1/2 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 1/2 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 consumed, 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 months 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 embankment 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.
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