On 18/06/2015 23:54, Adam Aglionby wrote:

Saw a motorway uderpass being constructed with a large , 3` mebbe,
auger drilling holes either side of underpass, rebar frames dropped
in holes and concrete poured im , provided intial piers for beams
placed in just as they started digging out the underpass itself,
finished with slab wall and back fill, was neat to watch over a
couple of weeks.

Its a little more complex than that..

You will have seen the temporary metal struts across the top to stop the
whole thing falling in while they were being dug out.
When they get to the bottom of the dig they have to cast beams across to
tie into the uprights and wait for them to set before they can remove
the supporting struts. This makes a reinforced concrete trench.
They usually finish the walls while waiting for it to set.