Sinking The Tunnel Shaft
(Extract from the Brunel Museum publication The Brunels’ Tunnel)
Like Vazie before him, Marc’s first requirement was a shaft on the Rotherhithe side of the river to provide access to the place where the tunnel would start. The traditional method was to dig a shaft and line the walls with bricks. This meant holding up the digging while underpinnings were driven into the sides of the shaft in order to keep the lining in place. But Marc’s ingenious idea was to build a brick tower and then simply allow it to sink it into the soft river-bank through the downward force of its own weight, thereby saving both time and money.
William Smith, M.P., now Chairman of the Thames Tunnel Company, performed the opening ceremony, Marc laid the first brick and his son Isambard laid the second.
Afterwards, the important guests sat down to lunch and to listen to earnest and optimistic speeches about the tunnel, to admire the model of it made for the occasion out of icing sugar and to drink a toast to its success. Twelve bottles of Bordeaux were purposely put aside to be drunk at the celebration on the far side of the river on a day, it was then thought, about three years in the future.
In three weeks, the circular shaft—or tower as it appeared at first—had been built: its strong wall consisted of an inner and an outer surface of bricks a yard apart, the cavity between them filled with cement and rubble. It was 42 ft high and 50 ft across, built on top of a 25-ton iron hoop and was strengthened with another hoop at the top, the two of them tied by iron rods running vertically between the shaft’s two brick walls. A superstructure was then set on top of the tower on which a steam-engine was assembled to pump away the water which the shaft encountered as it sank and to bring up buckets of earth from the bottom.
Thus, the excavation began and the enormous construction, weighing nearly 1,000 tons, was carefully sunk into the ground under its own weight at the rate of a few inches a day. Very soon, the downwards progress of the shaft at Rotherhithe became one of the most popular and fashionable sights of London. The Duke of Wellington was among the first to inspect it.
On 3rd June 1825, the shaft had just two feet more to sink but would go no further under its own weight. So Marc ordered another 8,000 bricks to be added to the top rim. When this failed to have an effect, he ordered more… and more … until 50,000 had been added. With still no movement, he ordered the steam pumping engine to be turned off. The cylinder began to fill with water from underground springs, softening the earth and slowly the shaft began its downward journey again. By 6th June 1825 the top of the brick tower was below ground level. The shaft was not yet complete, however: it had to be given a foundation. The diggers continued downward below the bottom of the pre-fabricated brickwork for another 20 ft or so, and bricklayers were employed again to finish the walls, leaving an opening 36 ft wide facing north for the tunnelling shield.
At the very bottom of the shaft a reservoir was dug and covered, which would hold the water drained from the tunnel workings. Above the shaft Marc installed a new, more powerful steam-engine of his own design, with a boiler house beside it, to drive the tunnel pumps and bring up the earth in buckets. Finally, the great shield, built for Marc by Henry Maudslay, was lowered into place 63 ft below the ground and on about 25th November 1825 the boring of the tunnel began.
On advertised dates (see Events page), this space is accessible to the public, who may descend by temporary staircase into the huge underground space for the first time in 145 years. The Museum is fundraising to convert the shaft into a gallery and performance space.