[petewailes]

It's partly because they're so big. The actual surface area separating what will be in the bucket from what won't be is fairly small, and the bucket itself doesn't agitate what's in it as much as you'd think.

Also, at dig sites where there's likely to be historic remains in the UK, there's work done to ensure the conservation of the archeology likely to be uncovered.

It's called archeological excavation, if you want to do some Googling. Also, have a look at the Museum of London Archaeology. They do a lot of stuff with this.

Source: I work just down the road from three current construction projects going on at major archeological sites and asked them the same question.

[afarrell]

There is now some controversy[1] over how much work should be done in the name of archeological conservation. This work and the project delays it introduces comes at the expense of construction. There is therefore some amount of tradeoff with affordable housing.

[1] http://new.archaeologyuk.org/news/cba-response-to-new-neighb...