Join the Institution of Civil Engineers (South West) for this lecture by Dr John Grimes, from John Grimes Partnership Ltd, about the Gunnislake Mine collapse that took place on 20 June 1992, the subsequent investigation and the lessons learnt.
A sudden collapse of ground occurred in the rear garden of No1 Woodland Way, Gunnislake, leaving a crown hole 8m plus in diameter and some 10m deep. The size of the hole relative to the back garden and its proximity to the rear entrance to the dwelling, no more than 2.5m away was alarming. The collapse was centred on Michaels Shaft, the main engine shaft, despite it being previously capped in the early 1970s.
In the following six months, further mining-related subsidence, collapses and ground water issues were recorded.
Extensive and detailed ground investigations were carried out across the Old Gunnislake Mine sett. The investigation and subsequent groundwater monitoring, carried out between 1992 and 1995, identified rapidly varying groundwater levels caused by various collapses and blockages in the adit drainage system.
Intricate ground engineering techniques were implemented to secure three shafts (namely Michael’s, William’s and Russell’s), as well as the development of a pressure relief well scheme, which successfully reduced groundwater level variation. As part of a risk mitigation scheme, regular monitoring and maintenance of the drainage assets has been successfully implemented.
A number of lessons have been learnt from this project that are pertinent to abandoned mine workings throughout the South West. The collapse and blockages of decaying adit drainage systems must be expected, this can result in significant fluctuation in draining mine/groundwater that can trigger instability. Assessment of stability and indeed treatment of old mine sites must be more extensive than the consideration of capping of old shafts only!