The Tunnel Shaft

Before work could start on the tunnel a shaft first had to be sunk deep down to the level the tunnel was going to start out under the river from.

Marc Brunel started by having an iron hoop laid out on the ground 80 feet in diameter. Underneath the hoop there was a cuttin edge. He then had the whole of the Rotherhith shaft built on top of the hoop completely above ground. People obviously thought he was slightly mad to of done such a thing, but then came his master stroke.

The miners climbed up to the top of the shaft some 60 feet above ground. They then went down inside and started digging into the earth that was surrounded by the above ground shaft. By doing this they deliberatley undermined the shaft which then started sinking under its own weight into the ground. That was until it got to a point two feet above where it was intended to stop. in spite of the miners continuing to dig the shaft was stuck fast and would not move. Marc Brunel had made a single mistake in having the shaft built as a cylinder which was the same diameter from bottom to top.

It was a mistake he would not repeat when it came to building the Wapping Shaft in the same manner. When that was built he had the shaft built as a cone with a very gentle taper, the base having a slightly wider diameter than the top. In that way the pressure of the earth actually pushed the shaft down as the miners dug at the bottom.

But that still left Marc with the problem of the Rotherhithe Shaft being stuck. He order several thousand bricks to be place on top of the shaft. When that added weight did not move the shaft any further into the ground he ordered more and more bricks until finally the weight of 50,000 bricks actually moved it the remaining couple of feet.

Then the problem was to stop it sinking any further. The shaft was underpinned and then completed - a gap being left for where the tunnel shield would be built ready to start work on the tunnel proper heading North under the River Thames in the direction of Wapping.