Heave of coal shale fill

Washing plant discard, predominantly laminated and micaceous carbonaceous shale, was used as fill beneath a thick concrete floor slab. This paper discusses the cause of heave of the fill and the severe cracking of the floor slab—up to 70 mm—about a year after construction.Observation of the fill and the nature of the fill indicate that the heave is caused by absorption of water (but not clay mineral swelling) and predominantly by alteration of the sulphides in the fill to sulphates.These observations are confirmed by measurement of the chemistry of fresh washing plant discard, weathered fill, and fresh discard subjected to accelerated weathering. Oedometer tests confirm the swelling of the rocks as a result of water absorption. As these processes were still occurring, further heave was anticipated; accordingly, the fill was removed.