I did it, you know. Shifted 7 cubic tonnes of plum slate chippings. Each shovelful, each barrow load, each knackering run between non-returnable bag to dump site, nearly killed me. I staggered in afterwards, and standing at the kitchen sink as the kettle boiled I actually lost the ability to think. Ever had that happen? Then let that be a lesson to you.
Never take on a garden with slate chippings. No matter how much you love your client.