On the 20 June 1992, a sudden collapse of ground occurred in the rear garden of a house in Gunnislake, Cornwall, leaving a crown hole eight metres plus in diameter and some 10 metres deep. The size of the hole relative to the back garden and its proximity to the rear entrance to the dwelling (no more than 2.5 metres away) was alarming! The collapse was centred on Michael's Shaft, main engine shaft. The collapse occurred despite the shaft having been 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 indicated up to 30 metres of mine waste at Michael’s Shaft.
Michael's Shaft was located within an old stock working which was centred on the convergence of two lodes (Great Green Lode and Middle Lode). 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. Impeded mine drainage resulted in groundwater levels rising significantly into the mine waste above rockhead, triggering subsidence and ground water issues. A maximum rise of 13 metres was recorded over a period of three days following extreme wet weather. Stability analyses led to a contingency plan being drawn up to evacuate several houses.
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. An unnamed shaft on Great Green lode, south of Michael’s Shaft, which was one of the three actively subsiding shafts, was also secured and provided the site for the construction of the upper of two relief wells. The ground water relief wells were connected via tunnelled relief drainage to South West Water’s existing surface water drainage system. 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 and 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!
The webinar is hosted by the ICE South West Plymouth City Club and presented by Dr John Grimes, Director of John Grimes Partnership.
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