The Forth Bridge/The North Approach Railway
The North Approach Railway.
This line is nearly two miles in length, and commences with an embankment at the north end of the bridge. The embankment is 34 ft. in depth at the abutment of the masonry arches, and continues for a length of 14 chains, when cutting No. 1 commences. This cutting is through whinstone, and is over 600 yards long, with an average height of 80 ft. The work here was commenced by driving a toplift from both ends at about 40 ft. above formations, and at the same time bottom gullets from the south end and the north end. The material excavated was run down an incline, worked by gravity in the ordinary way, and deposited to form No. 2 bank. The excavation made in the top-lift proved the rock to be of such a nature that an open cutting at the great depth would not have been compatible with safety to the traffic, and it was decided to form a covered way for the portions already excavated and to tunnel the remainder. Accordingly, shafts were sunk in two places, and headings were driven from them in each direction. The tunnelled portions are lined with side walls of roughly dressed whinstone and rubble backing, while the roof is of brick. The covered way has side walls and roof both of whinstone masonry. Starting from the south end of this cutting there are 189 yards of covered way, 229 yards of tunnel, and 154 yards of covered way, at the end of which the cutting terminates and bank No. 2 commences. This is 11 chains in length, and is followed by a cutting through whinstone 11 chains in length and, on the average, 50 ft. in depth. Another embankment follows, gradually deepening and leading to a viaduct crossing the North British Railway and the public high road, and by another bank the outskirts of Inverkeithing are reached.
The viaduct consists of masonry abutments in the form of eliptical arches of 57 ft. span at either end and four spans of steel girders of 100 ft. span each. The girders rest on masonry piers set at an angle of 25 deg. to the centre line, and the whole viaduct is on a curve of 40 chains radius with a gradient of 1 in 70.
At the north abutment of this viaduct the bank is 85 ft. in height, but, situated on rising ground, rapidly falls away.
At 11⁄2 miles from starting another cutting through whinstone commences, this extending for 6 chains, followed by a tunnel 378 yards in length under the town of Inverkeithing. Finally the line terminates in a junction with the North British Railway at Inverkeithing.
The last-mentioned tunnel is also on a curve of 40 chains radius with a gradient of 1 in 70, and was excavated by means of headings driven from each end. The enlargement to full size was principally made from the south end of the tunnel, as the material was required at that end for the filling up of the banks. This tunnel is lined with masonry side walls, and a brick roof.
Apart from the heavy cutting and tunnelling work, the greatest difficulty in this contract was caused by a bog through which bank No. 2 had to be carried. After some quantity of spoil had been tipped from the north end of No. 1 cutting, the weight of this material commenced to force up the ground in front, and this to such an extent as to displace the public high-road running to the south west side of the bank, carrying this road some 60 ft. out of its course and altering the gradients materially. There was at one time a fear that it would displace the North British Railway line between North Queensferry and Inverkeithing as well, but fortunately it stopped short of this. There is in this bank an excess of 69,000 cubic yards over and above the estimated quantity of 115,000 cubic yards, and to obtain the necessary quantity of spoil the east side of No. 2 cutting was enlarged to the necessary extent.
Geologists will have it that this is the site of an extinct volcano, but it will probably be best to leave this question to be settled by geologists.
In the excavation of tunnels and cutting, pneumatic drills were generally used with good results. The explosives used were both dynamite and ordinary blasting powder.
The total rock excavation amounted to 341,500 cubic yards in the solid.
The filling in the banks is as follows:
| Bank No. 1 | 33,400 | cubic yards. | |
| No. 2 | 115,600 | " | (original estimate) |
| " | 69,000 | " | (excess) |
| No. 3 | 198,100 | " | |
| Masonry | 24,850 | " | |
| Ballast | 13,350 | " | |
| Filling | 1,000 | " | |
| Total | 455,300 | cubic yards. |
The total weight of steelwork in the viaduct and road bridges is 460 tons for the former and 190 tons for the latter—total, 650 tons.
The whole north approach line, from the abutment of the bridge to Inverkeithing, is on a gradient of 1 in 70, except at one point, where it is level for about 100 yards.
The sum for this contract was 88,678l., increased through extra work by 18,000l., or a total of 106,6782l.—equal to something over 56,000l. per mile.
A new station has been built at Inverkeithing. At present there is no station between this and the Forth Bridge Station on the south side.
The writer wishes to express his indebtedness to Mr. Louis Neville, the contractors' engineer for the approach railways, who furnished the above particulars.