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Deep excavations: Going deep

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Development of a new mine in North Yorkshire is pushing the boundaries and setting some new UK records too, as work progresses to sinking of the shafts for the scheme.

The UK’s first new mine for 30 years is expected to start active production in 2021 and will result in the country’s deepest shafts and the world’s longest conveyor. If those engineering challenges were not enough, project promoter Sirius Minerals is developing the mine to produce a new fertiliser product in the form of polyhalite.

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When Bauer completes its shaft work at the site, it will have installed 7km of diaphragm wall panels.

The focus of the work on site near Whitby in North Yorkshire is all about the end date when the first polyahlite material rolls off the end of the world’s longest conveyor at Sirius’ new processing facility in Wilton. In order to bring that date forward, Sirius has looked to bring new techniques to the UK and search for partners with the same innovative approach to help it realise its ambitions.

The development involves three shafts from ground level at Woodsmith: one for production, another for services and the other for launching of the tunnel boring machines (TBM) that will drive the first section of the 37km material transport system (MTS) tunnel at 360m depth from Woodsmith to Teesside. There is an intermediate shaft at Lockwood Beck (see box) from which a second TBM will be launched towards Woodsmith. The third TBM 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 1,595m below ground. The MTS shaft from which the TBM will be launched will extend to 360m and connect to the production and service shafts.

Recent start of MTS shaft boring by Careys using the first vertical shaft machine (VSM) in the UK is a case in point and the equipment is expected to fast track the upper sections of two of the three shafts at the mine’s main Woodsmith site. Deep shaft contractor DMC will take all three shafts at the site to full depth using the UK’s first shaft boring machine (SBR) and drill and blast techniques but before it can move onto site, the upper parts of the shafts must be completed and excavated.

The production and service shafts will eventually extend to 1,594m and 1,565m below ground, respectively, and are formed in two sections – a larger diameter outer, shallower shaft that will be used to place mine equipment out of site below ground; and a smaller diameter central shaft that will extend to full depth. The MTS shaft will extend to 360m below ground and be used by tunnelling contractor Strabag to drive the MTS tunnel from the mine to the MTS portal at Wilton.

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Careys has already started to excavate the 35m diameter service shaft where Bauer has constructed the diaphragm wall to 60m

Bauer is just completing the diaphragm walls for the production shaft and the diaphragm wall for the service shaft has already been completed.

Careys has already started to excavate the 35m diameter service shaft where Bauer has constructed the diaphragm wall to 60m. Excavation to 45m below ground level is scheduled to be completed by the end of the year. The capping beam has been installed and a 9m deep section of the wall has been broken down to allow construction of the winder house.

Once excavation is complete, a base slab will be cast and preparations made to launch a Herrenknecht vertical shaft sinking machine (VSM), which is like a vertical road header lowered using strand jacks, and that will take the 6.75m internal diameter shaft from 43m below ground level to 110m. From there DMC Mining Services will take over and use a Herrenknecht shaft boring roadheader (SBR) to take the shaft to its final depth.

“As well as being the first site in the UK to use a VSM, launching it from 43m below ground is another first,” says Sirius Minerals project civil engineer Mark Pooleman. “The ground conditions at 43m below ground level have added challenges as there is a weak layer in the Eller Beck Formation at that depth.”

Careys is using a clam shell excavator, which is commonly used on cramped city centre basement excavations, but at Woodsmith it is being used to overcome the weather conditions.

The open site with views across to the North Sea is an attractive location but one that is frequently subjected to high winds, which make it challenging to use the conventional excavator, skip and crane set up.

The production shaft features two diaphragm walls: the outer one within which equipment will be located below ground is 32m in diameter and extends to 60m; and the inner one which has an internal diameter of 6.75m diameter and extends to 120m.

The outer diaphragm wall is complete and Bauer is expected to finish work on the inner one by early December.

Sirius Minerals operations director Graham Clarke says that the production rates on the deeper wall have been much faster than in the early stages of the shallower shafts. “The rate of work on the production shaft has been twice that achieved on the service shaft,” he says.

ge dec 2018 cover pic

ge dec 2018 cover pic

The project is the first in the UK to use a vertical shaft machine

Completion of the 120m inner shaft of the service shaft will result in what Pooleman believes is the deepest diaphragm wall in the UK.

The original plan was for the five shafts to be built by diaphragm wall techniques but early issues with cutting tools clogging led to a change of approach.

“Our focus is all on the end date and enabling other parts of the project to get started quicker,” says Clarke. “With the change of contractor to take the shafts to their final depth from AMC, which was going to use conventional drill and blast to take the shafts to depth to DMC and its SBR approach, we wanted to fast track the early work to bring DMC on site earlier too.

“If the diaphragm walling work had progressed at the speed it is now, we may not have changed our approach. However, the innovations that we are using give us greater certainty over timescales.

“We are pushing the boundaries of existing technologies and bringing different techniques together in combination to create the best result possible.”

Pooleman describes the diaphragm wall work as “quite a task that has been fraught with challenges”.

Nonetheless, when Bauer completes its shaft work at the site, it will have installed 7km of diaphragm wall panels.

Work on the MTS shaft was about to start when GE visited site with the VSM poised to drive the shaft to 120m. According to Sirius, this is the first time the technology has been used in the UK.

Pooleman says that Sirius has worked with Careys to draw on Herrenknecht’s worldwide expertise – and knowledge gained by Clarke from his years work at nearby Boulby potash mine – for this project.

The VSM has been lowered into the launch shaft structure and is attached firmly to the shaft with its three machine arms. A rotating cutting drum equipped with chisel tools is attached to a telescopic boom. This roadheader excavates and breaks the soil at the base of the shaft using a milling head mounted on a telescopic shaft that and can swivel up and down or rotate. This allows the entire cross-section of the shaft plus an overcut to be excavated gradually. The excavated material is removed hydraulically through a submersible pump and transported to the separation plant on the surface.

Herrenknect has said that advance rates of up to 5m per shift are possible with the VSM, but Clarke is expecting to achieve around 1.75m per shift based on experience gained during the diaphragm wall construction.

Careys project manager Chris Grannan says: “Originally the VSM was only excavating the MTS shaft to 91m but that has now been extended to 120m due to ground conditions, so the machine has been upgraded to cope with the additional depth. The seals have been upgraded and the pumps have a higher head for the slurry return and have been supplemented with additional pumps.

“It is effectively the same technology as the SBR.It will be lowered using nine strand jacks, each with 30 strands, and these can be used to pull the VSM back out and place it on a cradle for inspection, or repair if necessary.”

Once at 120m, which is expected by the end of the year, DMC will take over and construct a Galloway frame for the drill and blast work that will extend the MTS shaft to its full 360m depth. The shaft will be used by Strabag to launch one of three tunnel boring machines that will drive the 37km MTS tunnel.

Work to launch the first TBM is underway at Wilton Docks where Sirius will construct a processing plant for the polyhalite material. The TBM is expected on site early next year and Strabag’s ground engineering division Züblin is currently undertaking piling work for the launch portal.

The project has achieved many key milestones already this year but next year will bring additional ones with the start of work by the SBRs, the TBM launch at Wilton and DMC starting work on a shaft at Lockwood Beck, 32km from Woodsmith.

Although the work is progressing well, Clarke says that the project team is constantly looking to revise the techniques to add further efficiencies to the programme.


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One of the TBMs will be launched from the Lockwood Beck site

Grouting Guarantees

Sirius’s third tunnel boring machine will be launched from an intermediate shaft at Lockwood Beck, 32km from the Woodsmith site, and excavation work for the shaft is set to start early next year.

Work on the Lockwood Beck shaft site started in October with construction of a secant piled wall by Bachy Soletanche. The firm is currently working on constructing a grout curtain to extend below the piles.

The secant piled shaft is formed by 44, 1.2m diameter piles constructed to 20m depth.

“The hard-firm wall has a 40% overcut to ensure the verticality of the wall,” says Sirius Minerals project engineer (shafts) Steve Hickey. “The wall contains reinforcement in the male piles only but these feature reservation tubes for the inner array of grouting.”

According to Hickey, the verticality of the shaft was achieved but the overcutting needed for the high interlock created some challenges with casings becoming jammed as a result of overheating during the cutting process.

Before piling work finished, work started on trials for the grouting that will form a 160m deep grout curtain using single stage rock grouting techniques to create a cut off below the Whitby Mudstone aquiclude.

“The trial was an opportunity to learn more about the ground conditions ahead of the main shaft construction and to refine the design of the grouting,” says Hickey.

The main question was whether the grouting could be undertaken as an ascending or descending exercise with ascending being the preference as it is a faster solution.

“Taking the grout curtain to 160m is very deep – it is probably one of the biggest grouting jobs in the UK in the last 10 years,” says Hickey.

Secant piling and grouting were not the first options considered for the site – ground freezing and drill and blast was the initial concept.

“Use of that technique at the nearby Boulby potash mine was challenging and they lost a year on the project when they ran into issues in the Sherwood Sandstone,” says Hickey. “Piling and grouting reduced the risks.”

Three trial bores were undertaken for the grouting trial.

“We wanted to see if the ground was supportive enough to allow open holes,” he says. “From the trials we split the ground into three main parts – the top 30m is unstable, then it becomes stable from 30m to 50m, and although there is voiding at 50m to 52m, it is fairly strong and then from 55m to 160m the ground is fractured but competent.”

The result was that ascending grouting was possible, which Hickey says is four to five times faster.

Understanding of the ground has also helped guide which grout will be used where. “A C3S grout will be used in the upper layers as it is more thixotropic,” says Hickey. “A12 to A6 grouts will be used below that as these microfine grouts can cope better with fissures and we will treat the voided area using a thick cement bentonite mix.”

The first 20m of ground is not being treated and the grouting is designed to create a 5m overlap from 20m to 50m to cover the base of the secant pile wall.

Bachy will use the Grouting Intensity Number method for the grouting to track flow and injection volume against pressure to restrict the zone of influence from the 22 grouting points in the inner ring and 22 points in the outer array.

Work is now underway on the main grouting work and is on track to be completed by Christmas. “We have comfort in the programme thanks to the trial,” says Hickey. “The descending approach would have extended the work to February.”

Once the grouting is completed, work will start of the pre-sinking work to take the shaft to 50m below ground level from there DMC will construct a Galloway frame to undertake drill and blast to take the shaft to its full 260m depth.

Hickey expects that work on the shaft construction will be completed by late 2019.


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