Yes, they have to build a shaft. One way is a diaphragm wall. A ring of smaller shafts are dug and filled with concrete to form a circular wall. Then the cent

Yes, they have to build a shaft. One way is a diaphragm wall. A ring of smaller shafts are dug and filled with concrete to form a circular wall. Then the centre of the ring can be excavated with the retaining wall already in place.