In message , Richard J.
writes

I should have realised that! But to me it had seemed unlikely that
they would go to the trouble and risk of digging shafts in the river
bed rather than sink them on the adjacent dry land. Did they decide to
do the former (a) because they couldn't dig the tunnels unless they had
shafts less than x yds apart (where x is shorter than the width of the
river), or (b) because they couldn't gain access to suitable shaft
sites on land?

(c) It is much easier and cheaper to shift bulk excavated material away
from the site by river than by loading it all onto small horse-drawn
carts and driving it away along the then even narrower streets of
London.

I suspect that sites in the river were also very much easier to find and
cheaper than anything on land.

--
Paul Terry