Independent Review of Investigation into Collapse of Workings at Drumgoosat, Co. Monaghan

There was a subsidence event at the site of the closed Drumgoosat underground gypsum mine in Co. Monaghan on 24 September 2018, which affected the Magheracloone GAA club and the associated Community Centre. SRK Consulting (SRK) was commissioned by the owner of the mine, St Gobain Mining (Ireland) Ltd (SGMI), to investigate this subsidence event. SRK has previously undertaken work on the stability of underground mine workings in the area.

Following SRK's report on the investigation into this subsidence event, the then Department of Communications, Climate Action and Environment (DCCAE) commissioned Wardell Armstrong International (Wardell), a multidisciplinary engineering, environmental and mining consultancy, to independently review and verify the findings of SRK's work.

Wardell Armstrong's report entitled 'Investigation of the Collapse of Workings at Drumgoosat; An Independent Review of the Works Completed by SRK' was published on 17 December 2018.

In December 2018, a 10 metre diameter crownhole developed above the abandoned mine workings at Drumgoosat, some 35 metres from a local road (LP4900). The road was not affected by the crownhole. However, as a precautionary measure, the LP4900 was closed whilst a detailed investigation of the crownhole was completed by SGMI. As part of this investigation, SRK Consulting (UK) Ltd was again commissioned by SGMI to:

• Establish the likely cause of the crownhole development
• Carry out assessments to determine the stability and potential failure of any mine workings underlying the LP4900
• Make recommendations to provide early warning of, manage or mitigate future subsidence risk along the line of the LP4900

DCCAE retained Wardell to verify the findings of the SRK investigation. The Wardell report concluded that it was safe to re-open the LP4900, subject to the completion of road drainage works and provided that regular monitoring of the road is undertaken in the future to monitor for any potential underground movements. The report verified the SRK Investigation which stated that the risk of further crownholes impacting on the LP4900 road was very low.

The southern part of the Drumgoosat mine workings additionally extend beneath the R179 Carrickmacross to Kingscourt Road. Although this road was unaffected by the previous subsidence events, SGMI further engaged SRK to investigate the stability of the inaccessible mine workings in the vicinity of the R179. As with the previous LP4900 investigation, SRK’s work has involved both geotechnical stability modelling and drilling into the workings at multiple locations to laser scan the mine voids. Laser scanning allows a 3-dimensional image of the underground mine to be constructed and enables a very accurate picture of the stability condition of the underground workings to be determined. Information gleaned from the investigation was also used by SRK to assess the potential for crownhole development along and adjacent to the R179. SRK’s work concluded that there had been virtually no deterioration in mine conditions beneath the R179 over the past 40 years, that the risk of crownholes developing was very low and that the R179 continues to be safe to use.

Wardell has carried out an independent review of the works undertaken by SRK on the stability of the mine voids in the vicinity of the R179. A draft interim report produced by Wardell was initially used by the regulatory authorities to frame a number of queries for follow-up by SGMI/SRK (Wardell’s finalised Interim report is available below). SGMI’s responses to the follow-up questions have been assessed by Wardell and are also presented below as a separate addendum to the Interim Report. This addendum also incorporates an overarching Executive Summary which concludes that the R179 is safe to use. Wardell consider it prudent that the ongoing R179 monitoring programme should remain in place for the time being, as it provides for early warning of any potential underground instability.