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I have read the Brunel's shield directly exposed ground which was then excavated by the workers. But the construction was done just a few meters under the seabed. How wasn't the tunnel flooded immediately?
I think of small pits that children dig on beaches in the sand near the water. As soon as water level is reached, the hole is flooded very quickly. Shouldn't the same have happened to the tunnel? Was the soil water-tight enough so that the water didn't seep so fast?
At the risk of performing a pure lmgtfy, take a look at this page
An interesting quote from that page:
The operation was started in 1825 and was halted a number of times when the river burst in and flooded the workings. Isambard Brunel was put in charge of operations, and at one time nearly lost his life in a flood. In November 1827 Marc’s son, Isambard Kingdom Brunel, organised a lavish underwater banquet in the tunnel – to help convince people that it was safe.
And as to the basic construction method,
The Thames Tunnel, from Wapping to Rotherhithe was the first tunnel under a river anywhere in the world. It was designed by Marc Brunel (Isambard's father) using a revolutionary tunnel shield which he invented, comprising 36 cells in which a workman was engaged working independently of the others. Propulsion for the device was a screw which drove the device forward in 114mm/4.5in steps(the width of a brick)
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