The door being opened, the men found themselves face to face with the earth to be excavated. They cut away as well as they could, perhaps about 2½ feet deep, throwing the earth into trucks in the compressed air chamber; these trucks would be afterwards hauled away through the air-lock by electricity, and the huge iron cylinder would be pushed forward by means of hydraulic power. Twenty-eight hydraulic “jacks” were employed, and they forced forward the 250 ton cylinder with its cutting edge, when the men would resume working through the door as before.
Behind them, the hole of the tunnel thus cut out was being lined. First, it was built round with iron plates a couple of inches thick. This plating was fixed in segments, and formed a huge pipe a little smaller than the actual hollow in the earth. Through holes in the immense piping, liquid cement was forced, thus plugging up the space entirely between the earth and the iron, and forming an outer ring of cement.
Within, the tunnel was completed by a facing of glazed tiles, placed on a thickness of 14 inches of concrete. A road-way was laid 16 feet wide, flanked by footpaths of 3 feet, 2 inches, on either side. The subway is lighted by electricity, and staircases on the banks lead down to it for foot passengers. The stairways give entrance to the tunnel not far from the river, and much nearer than the commencement of the carriage-way approaches.
At the northern side, the slope down commences near the East India Dock entrance, and turns out of the East India Dock Road. The slope is fairly gradual—about one in thirty-four—and it passes under the Blackwall line of the Great Eastern Railway, and near to Poplar Station. The part of the tunnel near to this point—that is the part between the river and the open slope—was executed by what is called “cut and cover” work—that is, a huge trench was dug, then arched in and covered over.
“Cut and cover” work also took place on the south side; and there, at the foot of an immense excavation ninety feet down, and with its sides held up by huge timbers, might have been seen a river of water which had drained in and was being pumped up quickly by powerful machinery.
Not far distant, the shaft was being sunk for the staircase. In principle, the sinking of the shaft was conducted much as Brunel’s shaft at the Thames Tunnel, only it was built up of iron instead of brick. Imagine a big gasometer with a scaffold near the top, where men are busy building the walls higher and higher by adding on plate after plate of iron. On reaching the scaffold you find that there are two great cylinders of iron, one standing inside the other, and concrete is being filled in between them. Men also are down below digging out the earth which is being swung up in iron buckets; and as the soil is gradually removed, the immense double iron and concrete cylinder slowly sinks by its own weight.
In this manner, the great shaft was sunk nearly ninety feet, and within it the staircase has been built, giving entrance for foot passengers, not far from the river. Thus, on either side are sloping entrances to the tunnel, and also, nearer the water, stairways of descent down great shafts.
Engineers have also found their way beneath other great English rivers—the Severn and the Mersey. Much water had to be dealt with in the cutting of the Severn Tunnel. This important work, four and one-third miles long, was driven in some places forty-five feet under sandstone, and at the Salmon Pool—a hollow in the river bed—the tunnel was thirty feet under soil called trias marl. Much greater space, therefore, exists here between the tunnel and river than at Blackwall. But the river burst through. The work was begun in 1873, and completed in 1886.
Six years after its commencement the tunnel was drowned, so to speak, for a long time by a large spring of water which burst out from limestone, and arrangements had to be made to provide for this flood. It is now conducted by a subsidiary tunnel or channel to a huge shaft, where it is raised by pumps of sufficient strength. Then there was the perilous Salmon Pool to be dealt with. The river burst through here, and the rent had to be stopped with clay. The tunnel is twenty-six feet wide by twenty feet high, and is cut through Pennant stone, shale, and marl. It is lined with Staffordshire vitrified bricks throughout—seventy-five million bricks it is estimated being used. The works are ventilated by a huge fan, and pumping continually proceeds, something like twenty-six million gallons of water, it is said, being raised in the twenty-four hours. The tunnel, of which the engineers were Messrs. Hawkshaw, Son, Hayter & Richardson, and Mr. T. A. Walker, Contractor, is for the use of the Great Western Railway, and saves that Company’s Welsh and Irish trains to Milford a long way round by Gloucester.
