The monitoring of open underground mine workings beneath a major road construction

Abstract

Abstract The Al Edinburgh to London trunk road has been diverted in the vicinity of the Blue Circle Cement Works near Dunbar, in order to free an area of land for opencast extraction of limestone. Limestone has been extracted in the past in this area both by opencast and underground mining techniques. A section of the new Al route passes directly over an area of shallow underground workings which were abandoned in 1959. In 1979 Lothian Regional Council appointed consulting engineers Thorburn Associates to examine the condition of the abandoned workings in order to assess their long term stability in the light of the new alignment of the Al, and to determine whether any additional safety measures might be required. Room and pillar extraction techniques were employed in the mine. The limestone pillars and roof appear to be in good condition and a convergence gauge monitoring system was introduced into the mine in 1977 to determine the extent of any movements. The potential for roof collapse required careful consideration, and, therefore, a ground investigation was carried out and additional support measures were considered to reduce the probability of failure. The paper outlines the background history of the mine, the results of the ground investigation and laboratory testing, and the remedial safety measures considered, together with details of the monitoring system employed. Conclusions are drawn concerning the monitoring results and their implications with regard to stability and the need for continued monitoring. In addition, the usefulness of such a monitoring exercise for this type of problem is assessed, and compared with the immediate implementation of structural/stabilization works.