The use of PFA: Cement pastes in the stabilization of abandoned mineworkings

A project to treat abandoned limestone workings in the West Midlands had led to the development of low-cost PFA/cement bulk infilling materials (or pastes) using cement contents of as little as 2%. The use of mixtures of PFA, cement and water is well established and the technology involved in producing a relatively high cement and water content mix to produce low-viscosity grouts with rapid strength gain is well understood. However, the large volumes required to infill limestone mines sometimes as much as 20 m high and several hectares in area would make traditional grouts prohibitively expensive. This paper describes studies carried out to identify suitable PFA sources and the case histories of two treatment projects at Castlefields Mine in Dudley and Cow Pasture Mine in Sandwell. The design criteria, plant used, quality control testing carried out and the injection and verification processes are described. The relationship between cement content, moisture content, flowability and strength is examined. The paper also discusses a third case history where the low viscosity/rapid strength gain characteristics of a higher cement content mix paste were used to infill part of the 14 m high Stores Cavern in Dudley using no permanent containment works.