Boliden – Tara Mine In-Rush Event Assessment

The existing operational mine in the Navan/ South West Extension (SWEX) area is the largest Zinc mine in Europe. The mine processed 2.1Mtonnes of ore in 2021.

There is no current mining in the Tara Deep area, only planned exploration activities and enabling works. The deposit is estimated to be a 28 Mtonnes Inferred Mineral Resource1 as of December 2021 and is regulated under Boliden Tara Mines prospecting licence (PL4502) which comprises 48.7 km2 area of dominantly flat agricultural land. PL4502 was issued in 2015 and covers the same area as retired licences PL1489R and PL1661R. A prospecting licence has been issued to Boliden Tara Mines for this area since 1970

An exploration drift tunnel, referred to as the 0860 TRDEX or ‘TRDEX’, was excavated from the existing South West Extension (SWEX) area of mine workings through the UDL in the Tara Deep area from late 2018 (refer to Figure 4.1). Further information on the chronology of the development of the exploration tunnel is in Appendix B. The exploration tunnel starts c. 730 m below ground level (mbgl) within the ‘860 level’ of the SWEX mine workings. The tunnel has a gradient of -14% and is completed at c. 1040 mbgl within the UDL, in the hanging wall of the Tara Deep deposit. The cross-sectional profile of the tunnel is 6.5 m high by 6 m wide. The current length of the tunnel is 2.1 km (as of September 2021) and the total planned finished length of the tunnel is 2.4 km

The drilling of a 0.381m diameter RAR7 pilot hole (for surface air vent) began on July 2nd 2021 and concluded on November 25th 2021 which initiated the inrush event into the TRDEX tunnel.

By the 22nd of November, a 300m section of the tunnel had flooded and the inflow rate had increased to 800m3/per hour. While some monitoring wells showed ‘normal ground water levels’ during this time, others were recording significant drawdown. The mine operators began planning for packer and grouting interventions to try and stop the flow. By the 23rd of November the flow had approximately doubled again to 1500m3/hour and the raise bore drill string was removed to facilitate the plan for inflatable packer plugs to be installed from surface into the hole. On the 24th of November the flow had increased again to 2200m3/hour and the response plan had been broadened from the initial plugging of the borehole to also include installation of bulkheads and deliberate flooding of the lower SWEX mine to cope with the inflows which started overtopping the tunnel floor level on the 25th of November. The 25th of November saw the first attempt at packer inflation which temporarily worked (7 minutes) and abated the flow but then the drill string broke (including the nitrogen pressure line used for inflating the packers) and the seal failed.

From the 25th of November to the 2nd of December 2021 plans and surveys were developed for a second packer-plugging attempt. During this time (and since the initial onset of the inrush) Boliden deployed major-incident level responses to ensure safety, continued operation of emergency equipment and infrastructure (pump-sets and dewatering systems), water management, monitoring, community, public, workforce and regulatory liaison. The inrush incident was highly dynamic and challenging in terms of management as large-scale underground pump stations became overwhelmed by the inflow and were submerged.

On the 1st of December, the broken drill string was still in place at the base of the RAR7 bore despite efforts to locate it using downhole surveying equipment. An inflatable packer was lowered to a position of 900m depth and secured in place on the 2nd of December 2021 and inflated at 00.30am. Once inflated and stabilised, sand and bentonite were placed on top and then later a grout seal placed which was allowed to cure and the inflow finally stopped.

Dewatering of the flood water commenced on December 13th 2021 with consented discharge to the River Boyne and was still continuing at the time of drafting this (May 2022)

Prior to the inrush the Navan mine monitoring programme comprised :
• Daily inspection of underground headings.
• Monthly compilation of underground groundwater inflows (including probe drilling).
• Monthly water table measurement of surface drill holes.
• Monitoring of groundwater pressures in underground holes.
• Bi-annual hydro-geochemical sampling; and
• Professional consultancy from Boliden Tara’s consultants.

Following the in-rush event, 17 boreholes within the Tara Deep and Robinstown and Shambo areas were selected to be monitored on a daily basis.

EPA license data for the site

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