WH Ward, DSc(Eng), CEng, MICE, FCGI and DL Hills, MSc, CEng, MIMechE. This paper was first published in GE’s July 1987 edition.
Several millions of cubic metres of voids exist underground in the West Midlands as a result of the mining of limestone over the last 200 years. Some mines have collapsed, causing damage at the surface, and others may be in danger of collapsing. The abandoned limestone mines underlie substantial parts of the town centres and other developed land in Dudley, Sandwell and Walsall.
A study to determine the extent and condition of the abandoned mines, and to make recommendations for monitoring and remedial measures, was commissioned by the Department of the Environment and various local authorities in the West Midlands and carried out by Ove Arup and Partners.
Since 1980 grants have been available under the local Government Planning and Land Act (1980) for remedial action to prevent dereliction by reason of actual or apprehended collapse of the surface as a result of mine workings for minerals other than coal and those associated with coal. Publication of the full engineering limestone study report and its summary was accompanied by a government commitment to action to solve potential problems arising from the mines.
Many of the proposed remedial measures involved eliminating the voids; a promising method appeared to be to fill and support the mines, which are mostly pillar and room workings 4 to 10m high and flooded, with waste rock mixed with water to form a stiff paste, referred to as rock paste. This would be produced by a central plant at each site and pumped along surface pipelines to widely spaced boreholes. It would flow down the boreholes, then spread extensively into and fill the mine to the roof under its own weight and under pressure applied in the pipeline. The process is illustrated in Figure1.
The proposal to use rock paste, not as a rigid support but as a means of choking voids to prevent their migration to the surface, seemed to offer a much more cost effective means than conventional infilling techniques by using cheap waste material and fewer boreholes. It also offered the dual environmental benefits of clearing dereliction at the material source and preventing dereliction at the mines.
It was, however, a new idea and it had not been demonstrated in practice that rock paste could be produced and placed using conventional civil engineering equipment or that it would flow adequately within mines. To assess the feasibility of the technique the Building Research Establishment was commissioned by Minerals Division of the Department of the Environment to conduct trials in preparation for the filling of part of a mine at Dudley.
Exploratory small scale mixing and pumping trials using colliery spoil were carried out at the end of 1982. The results were sufficiently encouraging to proceed to further trials, summarised here, to examine the pumping and spreading properties of pastes at a scale comparable to that envisaged for mine filling, that is at pumping rates of up to 80m3/h and through pipelines several hundred metres long.