Its detailed in the ITG's 'Traction and Travel' books, I know the 3rd edition has lots of info, published around 1997
There were a few proposals made on how to dispose of them, including the above mentioned placing inside a tunnel, offshore disposal at sea, 'controlled' dismantling locally and then burial of the removed Asbestos, and transporting the stock to England for incineration. The quarry was the cheapest and easiest option. Recall seeing pics when the quarry was drained a few years ago, the remains of the various coaches and railcar bodies were still visible