ANGLO AMERICAN’S GROSVENOR MINE, in Australia’s Bowen Basin, is groundbreaking in more ways than just the literal sense. Thanks to changes in the methodology of excavating the access tunnels to the longwall coalface, the Grosvenor mine is being hailed as "industry leading" and "the future of underground mining".
The AUD 1.95bn (USD 1.48bn), green_ eld underground coal project in Moranbah, central Queensland, is one of the few new longwall coal mines to come on stream in Australia. Work began in 2012 and, once operational later this year, the mine will produce around seven million tonnes of metallurgical coal annually, which will be processed into five million tonnes of coking coal for export each year for the next 26 years. The longwall panels will be 300m in width with lengths up to 6,200m and access to the coal seam at the shallowest depth of 160m is achieved via two decline tunnels. The first – the conveyor drift – will transport the coal from the longwall to stockpile areas on the surface, while the second – the transport drift – will allow access for personnel and equipment once the mine is operational.
METHODOLOGY
Grosvenor targets the same Gonyella Middle coal seam as Anglo American’s Moranbah North coal mine, just to the north. The company’s experience of the geology – and a costly coal production -halting roof collapse in the Moranbah North conveyor drift in 2011 – fed into the decision to excavate Grosvenor’s tunnels via TBM methodology, rather than roadheaders, which are the Australian coal mining industry’s usual modus operandi.
Conventional excavation by roadheaders did feature in original plans for the conveyor drift but engineering and design consultancy GHD’s initial detailed design of the conveyor drift (geotechnical investigations; risk assessments; ground support classification and models; design of SCL; design of rock support; portal design, including canopy tubes; and finite element analysis of tunnel support) led to Anglo American’s decision to adopt the TBM methodology.
This marks the first time TBM technology has been used in the Queensland coal industry and, according to Adam Foulstone, Anglo American’s general manager and site senior executive at Grosvenor, it’s the first time an EPBM has been used in a mine in the whole of Australia.
Geology
The geology along both drifts’ alignments consists of varying soil and rock conditions. The soft ground portion consists of sand, sandy clay, clay and conglomerate. The mixed face/ rock portions consist of siltstone, coal, sandstone and basalt.
The conveyor drift is about 900m and declines at a gradient of 1:6, while the transport drift is about 1,100m and is at a gradient of 1:8. Each drift has an internal diameter of 7m and consists of steel fibre reinforced concrete segmental lining.
With the exception of a vertical curve at the top of the tunnel, where the TBM was assembled, there are no curves along either drift. The tunnels are about 120 degrees offset to each other and end underground around 150m apart (the portals above ground are about 2.5km apart).
Both drifts feature man refuges at 200m intervals, allowing pedestrians to take cover when mobile equipment goes past and ventilation is via a 5.5m diameter, 170m-deep shaft and three 1,000kW exhaust fans on the surface. The fans create low pressure in the underground environment, which then turns both tunnels into intake airways.
The geology along both alignments changes from soft clays and soil in the first 300m, to stone and basalt as the tunnels extend down the decline. "We had very mixed ground in a range from about 0.5MPa to about 90MPa, which would have been very hard going for a roadheader," said Foulstone. "We had also had the drift fall in our system line about 7km away [at Moranbah North mine] and these were the main reasons for going with a segmental tunnel.
"Another reason was that the TBM method was much faster, allowing us to access the coal seams 10 times quicker than if we’d used a roadheader. And, excavating through all the coal seams before we hit our predominant seam meant we had to deal with the presence of methane. That would have been very hard to manage with a roadheader."
TBM specification
Contractor Redpath Mining was in agreement that using a TBM as the primary excavation tool was the way forward in providing an extremely safe working environment and "certainty" regarding the development programme, hence Robbins was duly introduced into the process providing TBM and backup equipment.
Robbins project manager Martino Scialpi recalls that the company participated in the multi-stage risk assessment required for Grosvenor even before any manufacturing decisions were taken.
"We made ourselves completely available to interpret new and stringent Queensland legislation relating to the tunnel design and the TBM design with the client, the contractor and the consultants," he said.
"We spent more than 35,000 engineering hours on Grosvenor, which is about three times what we would usually spend on a standard TBM for a civil project of around the same size. We regularly had meetings with 30 people from 30 different companies around the table."
The Robbins ‘Crossover’ TBM used at Grosvenor – Lucia – is an 8m, high-performance rock/mixed face convertible shield TBM.
"It’s capable of conversion between a pressurised EPB mode and a non-pressurized single shield mode," said Scialpi. "Because of the requirement to build two blind tunnels quickly, while maintaining full ground support, the machine was also designed for quick disassembly so it could be relaunched on the second tunnel."
The TBM is fitted with a back-loading cutterhead powered by 12, 330kW explosion proof electric motors, providing a total installed cutterhead power of 3,960kW.
"The cutterhead is designed to operate in different modes depending on the type of ground," said Scialpi. "It can be used in EPB mode with cutting bits, a relatively open mixing chamber and a screw conveyor for muck pick-up. Or it can be operated in rock mode by exchanging the knife bits with disc cutters, the scrapers with bucket lips, adding modular radial loading plates into the mixing chamber, sliding forward the hopper built into the centre bulkhead and extending the screw conveyor forward into the mixing chamber, underneath the hopper."
He added that the screw conveyor is designed to operate in changing ground conditions and in gassy conditions. "The mixing chamber and the screw conveyor form a sealed chamber. Methane gas may be contained within the excavated material flowing through the screw conveyor, which will escape and be removed at the conveyor discharge by the snuffing box (a steel frame bolted to the discharge gate). The suction created at the screw conveyor discharge draws the methane inside the main duct and out of the tunnel."
Training requirement
Part of Redpath’s scope was to operate and maintain the TBM and it engaged in-house TBM expertise and sourced experienced tunnelling operators and miners.
With no pre-existing experience of operating a TBM among Queensland’s mining community, extensive training was vital. In fact, as Scialpi admitted, the Crossover design was so new that even the company’s own field engineers had to undergo internal training.
"A programme of formal training and competencies were developed – as far as we understand, this is a first in the tunnelling and mining industry – to ensure proof of training and compliance with the mine training guidelines were met," said Rob Nichols, CEO of Redpath’s Australian operations. "The engineering and supervision was by experienced tunnelling personnel with a mix of coal mining engineers and other professionals to ensure understanding of both industry requirements were combined and integrated." The project did have a lucky break, however, because while there may have been limited or no TBM experience among the miners, there was a pool of knowledge within the civils industry.
"We were pretty fortunate in that the Brisbane airport link tunnel was demobilised just as we were beginning to ramp up, so we got some experienced operators from that project," said Foulstone.
"The only real challenge we had was that their experience was with a Herrenknecht machine, so we had to retrain them on the Robbins equipment. It took around two to three months to gain that understanding during the ramp up period but then the second drive was nearly twice as fast as the first one." Assembly at the jobsite started in July 2013, the TBM was ready to be walked down into the first launching tunnel in November and boring operations on the conveyor drift started on December 20.
This first drive was completed in 20 weeks, on 13 May 2014, reaching the coal seam at a depth of approximately 160m. The total length was 798.41m, with an average production of about 40m per week and an average advance rate of 1.32m per hour. The first four weeks were relatively slow due to the carrying out of final load tests, fine-tuning of equipment and the learning curve of the operators, while progress in weeks six and seven was hindered by the need for a replacement gearbox for the screw conveyor drive.
After that the pace picked up, reaching about 65m per week, with a best day on 28 March of 13.9m and an impressive 85m in week 20. The last six weeks were characterised by the presence of gas, with routine implementation of the procedures for mitigation and management of the detected gas concentration levels.
Mixed ground operation
As mentioned, although the TBM was designed for dual mode, Redpath took the decision to use it in its mixed ground configuration (disc cutters and scrapers) for the conveyor drift, rather than in either of its ‘extreme’ hard rock or soft ground modes. This, said Scialpi, resulted in a lower performance in the areas of soft ground but avoided the 7-10 day downtime that would have been needed to switch from soft ground to hard rock modes.
"The project did operate in open, hard rock mode for a short period on the first drift, however, due to the lower strength of rock encountered, the type of muck developed by the softer mixed soils and the control of methane gas, the TBM operated mostly in closed EPB mode," said Redpath’s Nichols.
"This allowed the soil conditioning to be optimised and the cutting chamber to handle and remove the muck efficiently through the centrally located double screw. The screw accommodated a plug that assisted with the continuity flow of muck and in the management of the methane gas encountered nearer the coal seams."
Once excavating the conveyor drift was complete, the TBM was backed out in order that it could be used for the second tunnel, the transport drift. This was, said Redpath, a major challenge given the 1:6 decline – and a challenge to be repeated, of course, when the transport drift was completed.
The TBM design incorporated special features to allow the quick demobilisation in a blind heading without the need for a large disassembly chamber and without any hot work. The cutterhead was designed with an inner/outer bolted construction and the TBM was backed up 600mm to provide access in front of the head so that these bolts could be removed. Roof support consisting of rock bolts and shotcrete was applied from within the safety of the cutterhead before personnel entered this area.
The outer cutterhead segments, consisting of two 180-degree sections were then ‘parked’ in the invert and rock bolted to the face – they were then recovered from this position once the core of the TBM had been removed. "The TBM core (the cutterhead core, cutterhead support, main drives, screw conveyor, segment erector and bridge) and back-up were retracted as a selfpropelled single unit up the 1:6 slope on a special ‘walking dolly’ system," said Scialpi.
He added that, while the concept was simple, the walking system was complicated by the limited space beneath the TBM core and the need to distribute the 1,000 tonne weight over seven precast segments in order to prevent damage to the tunnel liner. "The system consisted of three dolly units working in unison to distribute the load," he said. "The back-up gantries were fitted with lift jacks to provide antislide."
TBM move and recomm issioning
The TBM was walked out between July and August, loaded onto two "super-trucks" and taken to the transport drift portal 2.5km away.
"An advantage from the first use of the machine was that it provided lessons that allowed upgrading and modifications to the machine during reassembly phase, thus increasing productivity and introducing further confidence in applying stretch advance rate targets," said Nichols.
The machine was made ready between September and October and relaunched on 11 November 2014. Based on the experience gained on the conveyor drift and the almost identical geology, Anglo American and Redpath reconsidered the most efficient way to approach the second drive.
As a result, the cutterhead was completely redressed with soft ground tools (scrapers and knives), as these were deemed more appropriate for the initial soft ground conditions. Recommissioning the TBM for the second drive was a much smoother process and that, along with the now experienced operators meant that production was much improved from the outset.
After about 540m (between weeks nine and 10) the changing geological conditions necessitated a four-day stoppage while soft ground tools were exchanged for disc cutters. However, despite this delay, plus 100 hours of downtime to manage high concentrations of methane, completion of the 988.4m transport drift was achieved after just 13.5 weeks on 9 February 2015, almost three weeks ahead of schedule.
Average weekly production was 70.6m, the average advance rate was 1.83m per hour and the best day’s production was 25.2m.
Extracting the TBM from the second tunnel was also a much quicker process than the first. "It took 10-11 days and we achieved a walk back speed of more than 100m per day," said Scialpi. "To achieve that speed with 1,000 tonnes of steel on a steep decline is quite remarkable."
Explosion prevention Methane gas
Looking back, the real challenge of the excavation of both tunnels was in predicting where methane would be encountered (when the machine approached and excavated through the coal seams) and at what levels, said Nichols. It estimates that methane was an issue in approximately 15 per cent of the length of each drift.
"The TBM was designed to the Queensland Coal Mine regulations for equipment to operate in an explosive atmosphere," said the spokesperson. "Gas detection devices and associated interlock systems were designed and integrated into the machine and this, essentially, provided the confidence that, should methane gas escape the system and into the underground atmosphere, the TBM and related equipment would trip and shut down any potential ignition sources.
Normal coal mining practices would then be introduced and carried out, including the evacuation of the environment, purging the drift of gas, increasing ventilation and the like. "The fundamental system of managing the methane was through diluting the gas into the muck and providing an inert atmosphere within the cutting chamber. Soil conditioning was also used to arrest the potential for sparks during the cutting phase.
"Conveyor belts and hosing were flame resistant and antistatic (FRAS) and, as such, were compliant with coal mine regulations. The ventilation system for the TBM extracted gas through steel ventilation tubing via a surface fan and into the and August, loaded onto two "super-trucks" and taken to the transport drift portal 2.5km away.
"An advantage from the first use of the machine was that it provided lessons that allowed upgrading and modifications to the machine during reassembly phase, thus increasing productivity and introducing further confidence in applying stretch advance rate targets," said Nichols.
The machine was made ready between September and October and relaunched on 11 November 2014.
Based on the experience gained on the conveyor drift and the almost identical geology, Anglo American and Redpath reconsidered the most efficient way to approach the second drive.
As a result, the cutterhead was completely redressed with soft ground tools (scrapers and knives), as these were deemed more appropriate for the initial soft ground conditions.
Recommissioning the TBM for the second drive was a much smoother process and that, along with the now experienced operators meant that production was much improved from the outset.
After about 540m (between weeks nine and 10) the changing geological conditions necessitated a four-day stoppage while soft ground tools were exchanged for disc cutters. However, despite this delay, plus 100 hours of downtime to manage high concentrations of methane, completion of the 988.4m transport drift was achieved after just 13.5 weeks on 9 February 2015, almost three weeks ahead of schedule.
Average weekly production was 70.6m, the average advance rate was 1.83m per hour and the best day’s production was 25.2m.
Extracting the TBM from the second tunnel was also a much quicker process than the first. "It took 10-11 days and we achieved a walk back speed of more than 100m per day," said Scialpi. "To achieve that speed with 1,000 tonnes of steel on a steep decline is quite remarkable."
Explosion prevention Methane gas
Looking back, the real challenge of the excavation of both tunnels was in predicting where methane would be encountered (when the machine approached and excavated through the coal seams) and at what levels, said Nichols. It estimates that methane was an issue in approximately 15 per cent of the length of each drift.
"The TBM was designed to the Queensland Coal Mine regulations for equipment to operate in an explosive atmosphere," said the spokesperson. "Gas detection devices and associated interlock systems were designed and integrated into the machine and this, essentially, provided the confidence that, should methane gas escape the system and into the underground atmosphere, the TBM and related equipment would trip and shut down any potential ignition sources. Normal coal mining practices would then be introduced and carried out, including the evacuation of the environment, purging the drift of gas, increasing ventilation and the like.
"The fundamental system of managing the methane was through diluting the gas into the muck and providing an inert atmosphere within the cutting chamber. Soil conditioning was also used to arrest the potential for sparks during the cutting phase.
"Conveyor belts and hosing were flame resistant and antistatic (FRAS) and, as such, were compliant with coal mine regulations. The ventilation system for the TBM extracted gas through steel ventilation tubing via a surface fan and into the Thinking ahead
The TBM is now in storage, ready for Anglo American’s next project – and the potential remains for it to be used in its full dual mode capacity.
"The ground conditions at Grosvenor were more thoroughly investigated than is usual for some civil work," said Scialpi. "For a short tunnel we had 10 times the data we usually have and this is an advantage because you can design the machine to satisfy the geology. Of course, an abundance of information means you add features onto the machine that probably won’t be used but Anglo American wanted us to consider all the aspects and, wherever possible, to ‘over-design’ the machine because it was intended for use not just at Grosvenor but for other projects."
Business perspective
Foulstone is convinced that, notwithstanding the current slump in Australia’s coal mining sector, those projects will come and the TBM will be redeployed.
"There is no way you would ever develop a mine any other way now," he said. "You only need to look at how much safer excavation is for the workers – they are not exposed to unsupported rock or fumes at any stage of the tunnel construction. And the speed. The rate is 10 times faster than a roadheader, which means we can access coal 10 times faster and get a return on the investment a lot quicker.
"The capital expenditure up front is a lot greater but the return at the back end of the project and the quicker access to the ore body makes it worth it," said Foulstone.
"And the final product is superior to what would be achieved using drill and blast or a roadheader. With those methods you are forever going back to do remedial work and the bolts and mesh systems only have a life span of 10-15 years, but the tunnel lining [from the TBM method] means that, from a maintenance perspective, you don’t have to worry about it.
"It’s definitely the way of the future, not only for coal but for any other commodity where ore bodies have to be accessed from the surface through a drift. It’s something we definitely need to look at"
