But it's impossible to dig all of that out (reasons: too big, too disruptive, no funds, happens everywhere, etc). So what usually happens is: take pictures, save some artifact, cover with concrete and go on with the works.
And the building, or underground parking lot I think, still got built.
For people who have NO idea what was under the new Bloomberg HQ in London, here's a link:
Finding yet another clay pot in Rome, one of the largest centers of population with one of the longest and most complete historical records of anywhere in the ancient world, is probably not going to be of much historical value.
In an age where you can buy 10 Roman coins on Amazon for $25 and have them delivered to your house in two days with Prime shipping, the value of common ancient Roman artifacts is made abundantly clear.
There was an article about this when they were building the tunnel in Istanbul. They did find some significant stuff, but the vast majority of it was stuff they already had tons of. They literally filled warehouses with the stuff, and dumped most of it back in because it was not rare or significant.
Clubbing and nightlife area now, nice place...
An alternative to squeezing a few billion more people onto our already massively overpopulated planet? Madness! (and certainly unpopular among the HN crowd)