men laying new rail linesREMARKABLE TUNNELS AND THEIR CONSTRUCTION.CHAPTER I.
men laying new rail linesREMARKABLE TUNNELS AND THEIR CONSTRUCTION.
REMARKABLE TUNNELS AND THEIR CONSTRUCTION.
REMARKABLE TUNNELS AND THEIR CONSTRUCTION.
HOW BRUNEL MADE A BORING-SHIELD.
“I watchedthe worm at work and took my idea from that tiny creature!”
“A worm! Was it an ordinary worm?”
“Oh no, it was the naval wood-worm—Teredo Navalis; it can bore its way through the hardest timber. I was in a dockyard and I saw the movements of this animal as it cut its way through the wood, and the idea struck me that I could produce some machine of the kind for successful tunnelling.”
“Well, it has been brilliantly successful.”
“I looked at the animal closely, and found that it was covered with a couple of valvular shells in front; these shells seem to act as a shield, and after many attempts I elaborated the boring-shield which was used in hollowing out the Thames Tunnel.”
This statement, which we can imagine to have been made by Sir Marc Isambard Brunel to a friend, is no doubt in substance quite true. A writer in the“Edinburgh Encyclopædia” says, that Sir M. I. Brunel informed him, “that the idea upon which his new plan of tunnelling is founded, was suggested to him by the operations of theTeredo, a testaceous worm, covered with a cylindrical shell, which eats its way through the hardest wood.”
Two or three attempts had already been made to drive a tunnel under the Thames, but they had ended in failure. In 1823, Brunel came forward with another proposal, and he ultimately succeeded.
This illustrious engineer must not be confounded with his son—who was also a celebrated engineer—Isambard Kingdom Brunel. There were two Brunels, father and son, even as there were two Stephensons, George and Robert.
Sir Marc Isambard Brunel, the father, whose most notable enterprise was the Thames Tunnel, was a French farmer’s son, and after various experiences in France and America settled in England in 1799, and married the daughter of William Kingdom of Plymouth. He had already succeeded as an engineer so well as to be appointed chief engineer of New York, and a scheme for manufacturing block-pulleys by machinery for vessels was accepted by the British Government, who paid him £17,000 for the invention. He was also engaged in the construction of Woolwich Arsenal and Chatham Dockyard, etc., and in 1823 he came forward with another proposal for the Thames Tunnel.
In that same year, his son, Isambard Kingdom Brunel, entered his father’s office, and assisted in the construction of the tunnel. The son subsequently became engineer to the Great Western Railway, and designed theGreat Westernsteamship.
But though Brunel’s proposal for the tunnel was made public in 1823, the work was not actually commenced until March, 1825. It was to cross under the river from Wapping to Rotherhithe, and present two archways. And if you had been down by the Rotherhithe bank of the Thames about the latter date, youwould have been surprised to see that instead of hollowing out a shaft, proceedings began by raising a round tower.
A space was traced out, some 50 feet across, and bricklayers began to build a circular hollow tower about 3 feet thick and 42 feet high.
This tower was strengthened by iron bars, etc., and then the excavation commenced within. The soil was dug out and raised by an engine at the top, which also pumped out water. And as the hollow proceeded, the great shaft or tube of masonry sank gradually into it. Bricklayers added to its summit until it reached a total height of 65 feet, which in due course was sunk into the ground.
Thus, then, the engineer had, to commence with, a strong and reliable brickwork shaft, 3 feet thick, by which men and materials could ascend and descend in safety. A smaller shaft was also sunk deeper for drainage.
And now the actual boring of the tunnel commenced. It was to be 38 feet wide and 22½ feet in height. On New Year’s Day, 1826, the boring-shield was placed below in the shaft. The shield was composed of 36 cells, 3 cells in height and 12 in breadth, with a workman to each.
The huge “shield” was placed before the earth to be excavated, and a front board being removed, the soil behind it was dug out to a specified extent, and the board was propped against the fresh surface thus made. When the boards had all been placed thus, the cells were pushed forward into the hollow then made. This was accomplished by means of screws at the top and bottom of the shield, and which were set against the completed brickwork behind.
For, while the labourers were working in front, the bricklayers behind built up the sides and roof, and formed the floor of the tunnel, the soil at the roof being supported by the shield until the masons had completed their task.
For nine feet, the tunnel proceeded through clay, but then came an unwelcome change. Wet, loose sand prevailed, and the work progressed with peril for thirty-two days, when firmer ground was reached. Six months passed and substantial headway was made, the tunnel being completed to the extent of 260 feet.
Then, on the 14th of September, the startling intelligence came that the engineer feared the river would burst in at the next tide. He had found a cavity over the shield. Sure enough, at high tide, when the river was brimming full, the workmen heard the ominous rattle of earth falling on their shield, while gushes of water followed.
So excellent were the precautions, however, that no disastrous effects followed, and Father Thames himself rolled earth or clay into the hole and stopped it up. It was a warning, and emphasised the fear that haunted the men’s minds all through the hazardous undertaking—the fear that the river would break through and drown the tunnel.
In October, another small irruption took place, and was successfully combated. Then, in the following January (1827), some clay fell, but still no overwhelming catastrophe occurred. The ground grew so moist, however, that it was examined on the other side. That is, the river bed was inspected by the agency of a diving-bell, and some ominous depressions were found. These were promptly filled by bags of clay.
It may be asked, Why had Brunel not gone deeper? Why had he not placed a greater thickness of earth or clay between his work and the waters of the Thames?
The answer is this—He had been informed by geologists that quicksand prevailed lower down, and the shaft that he sank for drainage below the level of the proposed tunnel, indicated that this view might be correct. In fact, when he got down 80 feet, the soil gave way, and water and sand rushed upwards. He was therefore apparently between the Thames and the quicksand. The Tower Subway, constructed in 1869,and driven through the solid London clay, is, however, 60 feet deep where it commences at Tower Hill.
Work went steadily forward at Brunel’s tunnel until the 18th of May. Mr. Beamish, the assistant engineer, was in the cutting on that day, and as the tide rose he observed the water increase about the shield; clay showed itself and gravel appeared. He had the clay closed up, and went to encourage the pumpers. Suddenly, before he could get into the cells, a great rush of sludge and water drove the men out of the cells, extinguished the lights, floated the cement casks and boxes, and poured forward and ever forward, filling the tunnel with the roaring of the flood.
The Thames had broken in with a vengeance this time, and drowned the tunnel.