CHAPTER IV.
UNDER WATER AGAIN.
“Howto cross the Thames at Blackwall, far east of the Tower Bridge?” That was a problem which the citizens of London had to face in the latter part of the nineteenth century.
An immense population dwelt on either side, and some means of easy communication became a pressing necessity. Should it be effected by means of a bridge, fixed or floating, or by means of a tunnel?
Finally a tunnel was decided upon, with sloping approaches on either side. Its entire length was to be 6200 feet including the approaches; but herein lay the danger and the difficulty—it was to be driven only seven feet below the bed of the river, and through loose soil and gravel.
How then was this perilous task to be accomplished? If the great river burst through Brunel’s fifteen feet, would it not be much more likely to rush through this seven feet of loose soil?
But the engineers in charge had an appliance in hand, which was unknown to Brunel—viz., a compressed air chamber, a piece of apparatus which has facilitated several great engineering achievements, besides the Blackwall Tunnel.
When the excavation of the tunnel was commenced, a stout apartment was formed at the end of the cutting, into which air was pumped until it exerted a pressure of some thirty-five pounds to a square inch, in addition to its usual weight.
This is generally reckoned at an average of 14·7 pounds to a square inch. We are so used to this pressure that we do not feel it; but let us enter a room where the air has been much more compressed, as in this air-chamber, and serious consequences would be likely to ensue, especially at first.
The human body, however, has a wonderful power of adaptability, and after a time some men get used to the change and can work in the compressed air without injury. But at first it may cause bleeding from the nose and ears, sometimes indeed affecting the hearing more or less seriously, and also causing great pain.
The reason for using this compressed air chamber was to keep out Father Thames. The great pressure of the air resisted the great pressure of the water, and held up the seven feet of soil between.
Powerful engines were maintained at work to provide for the pressure of the air, and the chamber in which the compressed air was kept was entered and left by the workmen through an “air-lock”—that is, a small ante-chamber having two doors, one leading to the compressed air and the other to the ordinary atmosphere, and neither being opened at the same time.
The men, then, worked in this compressed air chamber, which prevented irruptions of the river. But the method of excavation was also another safeguard, both against irruptions of water and of earth.
In essence, it was much the same as that pursued in boring the tunnel for the South London Electric Railway; that, however, was through thick clay and about 10½ feet in diameter, and this was 27 feet across, and through loose and stony stuff. The shield, instead of containing as in Brunel’s time a number of cells, consisted of an immense iron cylinder, weighing some 250 tons; closed in front, but having a door in the closed part; the rim of the cylinder round this part having a sharp edge for cutting into the soil.
THE ENTRANCE TO THE AIR-LOCK.(Men waiting to enter the Compressed Air-Chamber through the Door.)
THE ENTRANCE TO THE AIR-LOCK.
(Men waiting to enter the Compressed Air-Chamber through the Door.)
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 hydraulicpower. 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 ThamesTunnel, 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 linedwith 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.
THE BORING MACHINE USED IN THE PRELIMINARY CONSTRUCTION OF THE ENGLISH CHANNEL TUNNEL.
THE BORING MACHINE USED IN THE PRELIMINARY CONSTRUCTION OF THE ENGLISH CHANNEL TUNNEL.
In cutting the Mersey Tunnel, which was completed in 1886, machinery was used for some of the work. The machine bored partly to a diameter of seven feet four inches, but hand labour had to be largely depended upon. The plan pursued was to sink a shaft on either side of the river and drive a heading, sloping upward through the sandstone to the centre; this heading acting as a drain for any water which might appear. The thickness between the arch of the tunnel and the river bed is thirty feet at its least, and the tunnel, which occupied about six years in construction, and of which the engineers were Messrs. Brunlees & Fox, is provided with pumps raising some thirteen million gallons of water daily. As in the case of the Severn Tunnel, ventilation is provided for by huge fans.
A boring machine was also used in the preliminary efforts for the construction of a tunnel under theEnglish Channel. Holes, seven feet across and to the length of 2000 yards, have been bored by a compressed air machine, working with two arms furnished with teeth of steel. The construction of the tunnel is held to be quite feasible from an engineering point of view, and it is believed that it would pass through strata impervious to water, such as chalk marl and grey chalk.
Still, the huge tunnel at Blackwall, which was carried out by Mr. Binnie, Chief Engineer of the London County Council, with Mr. Greathead and Sir Benjamin Baker as Consulting Engineers, is probably one of the most daring and stupendous enterprises of the kind ever undertaken. To hollow out a subway hundreds of feet long under the Thames, only seven feet from the bed of the great river, and through loose gravelly soil, was a great triumph. It was achieved not by uncalculating bravery, but by a wise combination of cool courage, superb skill, and admirable foresight.
To design effectively, to provide for contingencies, to be daunted by no difficulties—these qualities help to produce the Triumphs of Engineers, as well as do great inventive skill, the power of adapting principles to varying circumstances, and high-spirited enterprise in planning and conducting noble and useful works. These works may well rank among the great achievements of man’s effort and the wonders of the world.
THE END.
LORIMER AND GILLIES, PRINTERS, EDINBURGH.