Fill

I am building a new house that will need retaining walls in several places.

The builders have left piles of broken bricks, roof tiles and concrete, is this usable as fill behind the walls?

Ross

It's the best stuff for good drainage, but if the pieces are big then you can get a lot of settlement. Best to put plastic down, and around it to prevent fines settling down and filling all the voids.
There's special blanket that you can use that allows water to pass through but filters the fines. Everything will often silt up eventually though.