A drive is being completely relaid locally in a snakes and ladders manner.

There's a concrete base already there.

The builders laid down grids of rebar on top of one bit of it, shuttered
that bit, then poured 4" of concrete.

The next day they started digging the newly laid bit up again, using a
SDS drill. They managed about 20% of the new concrete by close of play
for the day. Today, when I passed in the morning, they were still at it,
only now they have a decent breaker, so progress is faster. I haven't
checked progress since then, but I guess they finished digging it up by
lunchtime.

What on earth is going on?

I mentioned to DW that the site slopes down towards the front door of
the house, and the top of the concrete looked like it was level with the
floor of the house. But, surely they'd have spotted that before pouring
concrete. I know they are planning some sort of resin-bonded finish on
top of the concrete.

I'll keep you posted.