Developing the UK’s deepest tunnel and longest shafts for what could become the world’s largest multi-nutrient fertiliser mine has called for some innovative thinking and a flexible approach.
When exploration workfirst started in the North York Moors National Park in 2010, the target was potash for fertiliser. But what was found was far better. The mine now under construction will deliver the first – and the world’s largest – polyhalite fertiliser, which has the potential to be far more valuable to agriculture (see box).
Developing the mine to extract the material from 1.5km below the moors with minimal impact requires some complex ground engineering solutions.The geotechnical challenges and programme changes to bring the mine on stream by early 2021 – six months earlier than originally planned – mean that these solutions are still evolving.
Gaining consent for a new mine in the North York Moors National Park has not been simple. Planning permission hinged on a low impact mine with two 60m deep chambers to house the headgear for the production and service shafts and a 37km long tunnel to take the material directly to port facilities at Wilton on Teeside. Sirius Minerals has worked closely with local communities to explain its efforts to minimise the impact and made firm commitments to ensuring a significant proportion of the workforce is drawn from the area during construction and production.
The polyhalite seam has an average thickness of 25m but is up to 70m at its thickest point and it is at its maximum thickness – below Woodsmith - that the main mine workings are being developed.
Cranes - Potash
The development involves three shafts from ground level at Woodsmith: one for production, another for services and the other for launching of the 5m diameter tunnel boring machines (TBM) that will drive the first section of the 37km material transport system (MTS) tunnel at 360m depth from Woodsmith to Teeside. There is an intermediate shaft at Lockwood Beck from which a second tunnel boring machine will be launched towards Woodsmith. The third TBM, working under a contract recently awarded to Strabag, will be launched from a ramp at Wilton and complete the drive to Lockwood Beck.
The production and service shafts at Woodsmith will extend from the surface to 60m below ground level as 36m diameter chambers with a central 9m diameter shaft extending to the polyhalite seam at 1.5km below ground. The MTS shaft from which the TBM will be launched will extend to 360m and connect to the production and service shafts.
The initial plan was for Bauer to construct six diaphragm wall shafts on the project to include the production and services chambers and take the production, service, MTS and Lockwood Beck shafts to 120m, from where they were to be extended to 1.5km by drill and blast techniques.
In February, Sirius announced that it had changed approach for the extension of the shafts andw ould no longer be working with contractor AMC which had been developing a drill and blast system under an early contractor engagement process. Instead, the contract was awarded to DMC, which will use a shaft boring roadheader from Herrenknecht.
“AMC’s approach would have been slower and also had potential issues with health and safety,” says Sirius Minerals corporate communications general manager Maurice Rankin. “The roadheader proposed by DMC has been used in Canada on a similar scheme but never in the UK before.”
DMC is expected to start on site in 2019 and take two years to drive the shafts.
“Using diaphragm walls was the only viable technique for creating the production and service chambers”
When GE visited the site, a change of approach had just been decided for the upper part of the shafts too. Originally Bauer was to construct six shafts – five at Woodsmith with three from ground level and two smaller diameter at depth to take the shafts to 120m and the one at Lockwood Beck – but it is now working on just two at Woodsmith following a change of plan by Sirius.
The depth of the MTS tunnel was selected to keep it within the Redcar Mudstone, which outcrops at Wilton, to ensure homogenous tunnelling conditions. However, this solution did not help the shafts, and the shallower ground conditions have proved more challenging than expected.
“Using diaphragm walls was the only viable technique for creating the production and service chambers,” says Sirius Minerals shaft project manager Ian Reynolds.
“However, for the smaller diameter shafts there have always been other options out there and the change oftechnique for the deep shafts with DMC led us to re-evaluate the upper sections too.”
Bauer started mobilising to site in October last year and work on the first panel started in December. It quickly became apparent that the ground conditions were not as expected.
Bauer has three cutters on the project – two MC96 machinesand an MC128.
The diaphragm detail
Bauer’s two 36m shafts are formed by 2.8m wide, 1.2m thick and 60m deep panels – 48 for the service shaft and 44 for the production shaft.
The design of the structures is based on hoop force with steel cages extending to the full depth. The cages used in the primary panels are slightly smaller than those in the secondary panels to allow for overbiting.
The cages are manufactured offsite in six sections which are assembled vertically before being installed the panels.
The large diameter of the shaft means that sequencing is key with Bou-Sleiman aiming to have a maximum of ten days between the construction of a primary panel and its adjacent secondary panel.
Verticality is a challenge too with a maximum deviation of 200mm over the 60m depth of the chamber.
“We are well within these tolerances,” says Bou-Sleiman. “We are using a combination of techniques – CIS technology surveys the machine ropes, kodens are used within the panel and the machine operator also has verticality displays within the cab.”
The service shaft has been used to develop the technique and Bauer expects to complete the structure as this issue of GE went to press. Work will then start on the production chamber and Bauer expects to complete that by October.
The location of the site within the North York Moors National Park has impacted on the approach to construction, as well as the final operation of the site. Sirius has set up a Liebherr concrete plant on site to ensure the 55MPa concrete is not delayed en route to the site and Bauer has had to use shorter, larger diameter silos for the bentonite plant and also paint them a specific hue of green to blend in.
According to Bou-Sleiman the plant is probably the largest in the UK with 1,600m3/he desanding capacity and 4,000m3 of bentonite storage.
If the location, sequencing and ground conditions were not enough of a challenge, the weather has also impacted the work. The North Sea is clearly visible from the site, which creates a nice view on a sunny day but “The Beast from the East” and strong winds have stopped work a number of times.
“We have had issues with fog that have made it unsafe to operate the cranes and we also have to stop work if winds get above 14m/s,” says Bou-Sleiman. “The low temperatures have also caused pipes to freeze. We had to stop work for four days when the snow was at its worst.”
Rankin estimates that the whole project has lost two months to weather and technical issues but is confident that the coming summer and new techniques will bring the work back on track.
“We expected the ground conditions to be better for the cutters than they are,” says Reynolds. “Bauer has worked on a number of cutting strategies and it has taken time to hone the techniques for this site.”
The Woodsmith site is underlain by a 5.8m thick layer of sandstone formed by the Moor Grit Member, followed by a 6.8m thickness of mixed sandstone, siltstone and mudstone of the Scarborough Formation.
Interbedded sandstone and mudstones of the Cloughton Formation then extend to 46m below ground level and is underlain by around 10m of mudstone from the Ellerbeck formation. Bauer’s diaphragm wall chambers end within the interbedded sandstones and mudstones of the Slatwick Formation, which extends to 93m below ground level.
A 2m layer of the Dogger Formation, which is formed from sandstone with some ironstone and siderite, is then encountered overlaying the Whitby Mudstone Formation, which extends to 167m depth.
The strength of the ground through which Bauer is cutting the diaphragm wall panels is classed as weak to medium strong and it is this variation which has been technically challenging.
“The material is hard when dry but when there is ground water or it comes into contact with bentonite means it acts more like a clay,” says Bauer diaphragm wall manager Cyril Bou-Sleiman.
Reynolds adds: “The clay-like material blocked the cutters.”
Bauer brought in alternative cutter heads and teeth and used a variety of combinations until a viable alternative was found.
“At the start progress was limited to 1m at a time but now we are cutting the full depth to 60m,” says Reynolds. “Bauer’s international back up has been invaluable in resolving the problem.”
Bou Sleiman explains: “We used lots of combinations to find the right solution. We used three types of teeth with nine variations such as size, position and strength. We even develop a bespoke design for the project.
“Our production rates are now four times what they were now we have the right combination for the ground conditions.”
Nonetheless, Sirius has opted for an alternative method of construction to extend the service and production shafts to 120m, construct the MTS shaft at Woodsmith and the TBM shaft at Lockwood Beck to 100m.
Under the redesign, Careys will extend the four shafts using a vertical shaft machine (VSM) before DMC takes them to full depth.
Reynolds says that the VSM solution offers programme advantages that create synergies with the programme savings delivered by DMC.
“When we were working with AMC, the shafts were forecast to be completed by the end of 2021 but DMC’s technique has brought that forward by six months so we wanted to match those savings to bring the start of operation forward,” says Reynolds.
“The DMC technique could not be used at 60m though as the system does not have the wall support to cope with ground water. At 120m the shaft is sealed within the Whitby Mudstone.”
Carey is expected to mobilise to site by mid-Summer and will integrate with Bauer’s operation until it completes its shafts in October this year.
In addition to the work to refine the shaft and tunnelling techniques and build the processing plant and wharf facilities at Wilton, Sirius also has the pressing need to raise a further US$3bn this year, on top of the US$1bn already raised, to fund the remainder of the work.
First production is expected in 2021 and will gradually ramp up over three years as the infrastructure is installed to reach optimum production of 10M.t per year in 2024 but Sirius believes that it has ability to double that to 20M.t per year by 2026.
Developing the product
Exploration for the project started in 2010 and was led by a former financier Chris Fraser who looked at various potash resources around the world in the UK, Mexico and Australia in search of one for development. It quickly became apparent that the Yorkshire resource was the best and York Potash was listed on AIM to raise the funds to development.
It took five years to quantify the resource and through that a polyhalite seam, formed in an evaporating marine environment around 260M years ago, was discovered below the potash.
Potash has long been used as a fertiliser but polyhalite is a new concept but one that could significantly help agriculture meet the demand to grow more food from less land. Crops essentially need six different macro nutrients added during growth and polyhalite has four – potassium, sulphur, magnesium and calcium - of those essential ingredients so it means fewer products are needed.
Sirius could have chosen to process the material into a more conventional product but Rankin describes this as a “high impact process” and the planned polyhalite product just require crushing.
Polyhalite as fertiliser is a new concept for the farming market so developing the mine without a ready market was a risk but some of the barriers have already been broken down and Sirius Minerals already has deals for sales of 4.4M.t per year in place.