Solid Sherwood Sandstone 20 miles wide.The ground at Bramley Moore will be more steady that that at Wellington, if there is any difference at all. The tunnels were bored through the most solid parts of the Mersey Estuary available, for obvious reasons and, BM is closer to it.
Sink the piling, fill with concrete, repeat as many times as required. Instead of a mile long thin rectangle over water like a bridge, think a nearly square rectangle over filled in dock basin
The engineering to do it is not hard - the money to pay for it will be the question.