I refer to your article on piling to counteract ground heave (NCE 11/25 December), in which Steve Mulholland is quoted as saying that blast furnace slag has the potential to expand to several times its size.
At the risk of being pedantic, blast furnace slag is, in fact, extremely stable and is a valuable construction material, for general fill, sub-base, and, when finely ground, as a cement substitute.
It is steel furnace slag which can, in certain circumstances, exhibit marked expansive properties. As Consett had both blast and steel furnaces, the chances of encountering steel furnace slag would be pretty high, so the above comments do not negate the need for this interesting piling process.
Richard Wilson(F), Lichfield, Staffs WS13 6TU