Shaft sinking operations at UK potash mine 12 Apr 2018

With the first contract the materials transport system tunnel for the new Woodsmith potash mine in the UK awarded to Strabag, preparations for advancing deep shaft sinking operations at the mine head is underway by appointed contractor DMC Mining Services.

Fig 1. Main elements of the Woodsmith Mine development

DMC Mining Services, headquartered in Canada, and its UK sister company partner are both subsidiaries of KGHM of Poland, a leading company operating globally in the extraction and processing of valuable metals.

The contract awarded to DMC in February 2018 is for design-build construction of the four shafts associated with the project. These include the deep shafts at the Woodsmith minehead comprising the two 6.75m i.d. x 1,500m deep production and service shafts into the 70m thick seam of polyhalite potash and the 360m access shaft into the working chamber of the mineral transport system (MTS) which will connect to the production shaft and convey the raw material for processing at Wilton on Teesside. The shaft will also operate as a launch and working shaft for the third of three TBM drives for the 37km long x 4.7m diameter MTS tunnel. The fourth deep shaft is on the MTS tunnel alignment at Lockwood Beck. This shaft will operate as a permanent ventilation shaft and provide access for launch and advance of the second of the three MTS TBM drives (Fig 1).

The Herrenknecht shaft boring roadheader machine (SBR) will be used to excavate the two deep level production and service shafts and potentially faster than originally programmed

DMC is a leader in deep shaft contraction and is currently sinking two 1,000m potash mine shafts in Canada and using new shaft boring roadheader (SBR) technology developed and introduced by Herrenknecht. DMC is to procure similar machines from Herrenknecht for its UK Woorsmith Mine contract where application of the technology is expected to accelerate excavation of the production and service shafts by six months over drill+blast and other excavation alternatives (Figs 2 and 3).

The value of the design-build contracts with DMC and Strabag is not revealed by Sirius other than to say that contract award and pricing is in line with competitive tendering practices. DMC is also further incentivised to complete shaft sinking ahead of existing target price and schedule guidance based on a risk/reward commercial model with financial incentives for completing the scope of work under budget and ahead of the first polyhalite production base schedule by Sirius of Q4 2021.

With its knowledge of the SBR system DMC has said that shaft sinking rates with the machines are significantly faster than what is possible with conventional methods and is confident there are real opportunities to achieve the first polyhalite production milestone by May 2021, up to six months earlier than currently planned. DMC is further incentivised to work to identify and analyse all opportunities to accelerate the development of the critical path deep shafts and associated infrastructure. Delivery of the project by May 2021 and in line with the accelerated schedule, would result in “significant profit bonuses for DMC”. Under the arrangement, financial penalties would also be incurred should completion be late or rise above the target price. Payments under the contract are expected to be in a combination of currencies and to cover some exposure to the Polish Zloty by the KGHM parent company.

Layout of the main mine head site at Woodsmith

Diaphragm walling for the service shaft by Bauer

The contract award with DMC ends an association Sirius Minerals had with AMC, a joint venture between Thyssen and Redpath, for construction of the deep shafts. Sirius explained in a statement that AMC was the preferred shaft sinking contractor for the project since June 2016 but that discussions to finalise the contract a notice of award in July 2017 had became protracted through the second half of 2017 and as a result, Sirius approached a number of alternative contractors, including DMC, to create options and explore opportunities to improve on the commercial and technical proposal presented by AMC. DMC, it said, “proposed a significantly more efficient and safer shaft sinking solution and the commercial terms and risk sharing contained in the contract are a more appropriate alignment of incentives for project partners”.

AMC has since been serviced notice of termination in relation to its shaft sinking contract. Under its contract, AMC had mobilised to site to undertake geotechnical work and has completed a 1,627m deep pilot hole in the location of the production shaft some 53m into the polyhalite deposit. It is to complete a 90 degree side borehole into the deposit to obtain an additional intersection of the deposit, which is expected to be complete in the second quarter of 2018, and AMC is then to demobilise and depart the site. The notice terms from AMC to DMC includes transition and novation arrangements around the engineering and design work undertaken to date, on-site activities including operation of the concrete plant, and procurement of the main winders that are on order from a related AMC entity.

An improved excavator cutterhead is set to make up D-wall programme slippage

Engagement on the project by DMC is advancing, with orders for the mechanised SBR shaft boring roadheader machines placed with Herrenknecht within a week of signing the contract.

Sirius had already taken steps to prevent any impacts on the project schedule due to the prolonged nature of the discussions with AMC and these include the direct contracting and delivery of site preparation and diaphragm wall construction works and direct engagement with a preferred contractor for the near surface civil construction activities and with Arup as its design engineer.

Diaphragm wall activities on site for the three shafts at the Woodsmith mine head site are being completed for Sirius by Bauer. Bauer was originally engaged by AMC and is now contracted directly to Sirius. Construction of the D-walls for the service shaft started first and 20 of the 48 panels required were in progress or completed by end of March 2018. A number of initiatives are said to have been undertaken to improve D-wall construction and recover lost time, which is reported by Sirius as being a matter of weeks on the planned schedule. These initiatives include changing the cutterhead design of the diaphragm wall construction machines and increasing operating hours in the production cycle. Once D-wall activities are completed, excavation and fit-out of the foreshafts and subsurface basements will commence.

Diaphragm walling and pilot hole drilling at the main mine head site

In speaking of the award of the shaft sinking contract with DMC, Chris Fraser, Managing Director and CEO of Sirius, said: "We are confident that DMC can deliver the North Yorkshire polyhalite project shafts significantly earlier than all previous expectations and we look forward to working with the team." Graham Buttenshaw, Managing Director of DMC, said: "We are confident of leveraging our leading technology and experience to accelerate development of the mine and enable Sirius to bring its fertilizers to market earlier than previously planned."