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?