Angus Place operates in a challenging geotechnical environment, resulting in innovative technology and mining methods being developed and used at the operation.
Nestled as it is, not far from Lithgow in the Western Districts of New South Wales, the mine’s personnel have a long and proud history of using innovation to win coal.
The mine is bordered by Baal Bone Colliery (Xstrata) and Invincible Colliery (CET Resources) to the west; Centennial’s Springvale Colliery to the south; and the Wolgan Valley to the north.
Angus Place began production in 1979, after being developed as an extension of the Newcom Mine at Kerosene Vale. Coal is extracted from the Lithgow Seam by a longwall and supporting continuous miner units developing access headings.
Coal is currently extracted for domestic power generation at both Wallerawang and Mount Piper power stations. Approximately 215 personnel are employed by Angus Place, operating across three shifts enabling production 24 hours a day, seven days a week.
In the 2010 financial year Angus Place achieved record saleable production of 3.4 million tonnes per annum of coal.
The colliery has a long history of difficult strata conditions, which have in the past resulted in slow development rates, longwall production discontinuities and associated injuries to personnel.
Technical services manager Peter Corbett, working closely with production manager Ross Dowsett and mine manager Jacques le Roux, has focused on adapting equipment for the harsh requirements of the colliery.
“We’re in a difficult geotechnical environment, with a weak roof, relatively high in situ stresses together with a number of fault zones that traverse the mining area,” Corbett said. “The net effect of these issues is a lot of deformation of the strata – roof, ribs and floor which increases the risk of strata failure in the mine roadways and on the longwall face.
“Only a handful of other underground coal mines in Australia have to deal with similar geotechnical issues.”
Abutment loading from the longwall has been a problem at the mine through most of its history, according to Dowsett, a 25-year veteran at the mine.
The current mining area at Angus Place has a total seam thickness of approximately 7m. The lower 3.2m is the mined section – leaving about 4m of coal in the roof. The roof coal is interbedded with claystone bands which lead to a weak roof highly prone to deformation.
The depth of cover is around 380m. The vertical stress has been measured at 9-12 megapascals and the horizontal stress at 15-23MPa.
The current mining area has significant strike slip fault zones at spacing of approximately 1 kilometre. Although these do not exhibit much vertical displacement, their zone of influence is extensive (tens to hundreds of metres around the mapped structures). One of these zones is known locally as the Wolgan Zone.
The Wolgan Zone is a major fault zone with associated high horizontal stress. It is around 300 metres wide and strikes northeast-southwest across the inbye end of the lease extending from Angus Place through the adjoining Springvale lease over a strike length of greater than 5km. Whereas Springvale has progressed beyond this area, Angus Place still has this feature to contend with for a few more years. Improvement and innovations have allowed Angus to mine this feature successfully.
A new longwall with Joy chocks and a Bucyrus armoured face conveyor was installed for Longwall 950, replacing a 25-year-old longwall system. The crew had the previous longwall from 1984 to 2008, so adapting to the new system was a step change for them.
“I told the guys to get ready for a Ferrari,” Dowsett said. “Unfortunately from day one we encountered pan surfing, due to the centre of gravity of the pans. There were improvements made during that block and the surfing issue was overcome with significant changes made at the longwall changeout.”
Longwall 950 was a challenge for the mine. In addition to introducing a new longwall with some design issues, longwall crews had to learn to operate a system that was significantly different from that used for the past 25 years. Soon after introduction, the crews had to mine through the Wolgan Zone. Water management in the tailgate and on the face was challenging due to its difficult access – this made the pumping system difficult to maintain, and affected its reliability.
“The combination of equipment design issues, operator familiarity with new equipment, very difficult strata conditions and water inflows onto the face gave us some grief for a while,” Dowsett said. “Having said that, the outbye three-quarters of the block went very well, and with the modifications made at the longwall changeout, Longwall 960 is performing beyond expectations.”
The team has learnt lessons from the Longwall 950 experience and has just successfully tackled the Wolgan Zone in Longwall 960.
“Negotiating the Wolgan Zone with no major delays or reactive strata management work required is a major victory,” Le Roux said.
A change from a reactive to a more proactive stance to managing strata problems is now being pursued by the Angus Place mine management team.
“I believe in planning in detail well ahead and in taking a conservative, proactive approach. It pays to spend the money and effort early and take remedial action proactively in a controlled environment, instead of in a recovery environment after encountering difficulty. In the longer term this is the safest and most cost-effective method of dealing with challenging conditions,” Le Roux said.
Corbett also had good reasons to get on top of the mine’s problems.
“I got sick of getting calls on Saturday night shift,” he said. “Against the laws of probability, strata issues only occur on public holidays, shutdowns and weekends.
“I had a vested interest in developing more systematic ways of management.”
One of the cornerstones of the proactive management philosophy was more emphasis on monitoring roof movement throughout the mine.
Telltales are now installed at 25m centres throughout the current mining area to ensure that anomalous movement trends are detected early. With the increase in instruments (over 800 active telltales) came an increase in required readings and the ability to process large quantities of data.
“In order to maintain consistency of data and adherence to the reading schedule, we have a full-time technician to gather, enter and process telltale and installed support data,” Corbett said.
The Strata Failure Management System Database was created which allowed the storage and processing of large amounts of data very quickly, with detailed TARP exception reports being generated in a couple of minutes instead of several hours later under the previous spreadsheet system.
It was also made accessible to all computer terminals at the mine (including underground crib rooms) and training was conducted to allow access at all times by personnel.
All relevant strata management information is stored in the database, using the telltales as reference points. It is used as the document control location for telltale data, installed support, geology reports, secondary support plans and strata management documents.
“A project for the next 12 months is to trial the latest real time monitoring telltale system. If we can integrate the real time monitoring of the majority of telltales with our database, we’ll have an extremely robust monitoring system,” Corbett said.
“We have recently introduced LVA software to monitor the longwall face for potential strata issues. In the future, we will use data acquired to develop TARPs to give us prior warning of impending longwall face strata issues.
“Strata monitoring in real time will represent a step change in data acquisition compared to that gathered by people making inspections – even with a rigorous monitoring schedule.
“The importance of the data analysis cannot be over-emphasised. We have found that trends exhibited from the development phase of a roadway are mirrored throughout the remainder of its life cycle. The trending of total displacement, displacement rate, longwall acceleration position and side abutment surge has enabled not only early response to anomalous trends, but also detailed strata support design. We have been able to make and justify decisions to reduce support density in many areas to reduce costs and also to increase in specific areas to prevent strata delays and maintain production continuity.”
Another strategy adopted by the mine is the installation of roadway life cycle support as quickly as possible after development.
“We know what the life cycle of each roadway at the mine will be, for example a single abutment maingate or double abutment tailgate,” Corbett said. “We know from previous data gathered how these roadway types behave throughout their life cycle.
We can extrapolate trends to plan for geotechnical zones.
“With this information we can pre-plan with a high level of probability the support which will ultimately be required for each part of each roadway in the mine. Thus it makes sense to install the planned support as early as possible – to allow it to be done in a planned and efficient manner. It’s the old ‘do it once, do it well’ philosophy.
“What we also know is that the longer the roadways stand, the greater the deterioration we see, and the greater the chances of uncontrolled deterioration requiring remedial support or leading to roof falls. This creates unnecessary hazards. It is a false economy to think that delaying installation of support defers and thus reduces costs.
“Remedial support is expensive, inefficient, often hazardous to install and it requires a much higher density of support to repair a roadway which has badly deteriorated than one which is maintained in good condition. Not to mention the fact that it often requires plans to be changed and undermines the planning process.”
Angus Place has a strong relationship with Strata Engineering, which is involved in mapping, monitoring, design, review, feasibility studies and other projects.
“It adds value to have input from external reference sources with a broad overview of the industry,” Corbett said. “It also lowers risk to have a peer review process for critical geotechnical design work and feeds back into the industry database. We also work with a large number of equipment OEMs and suppliers to develop new products. It pays to stay abreast of new developments and to work with those who are leading the way.”
The trigger for a number of the process improvement initiatives at the mine was the Wolgan Zone. “The Wolgan Zone has in the past caused huge problems for roadway development and shortened the length of a previous longwall panel by some 600 metres due to not being able to mine through it in the given time,” Corbett explained.
Typical roof conditions in the Wolgan Zone had guttering approximately 1m high and 1.5m wide and extended for about 300 lineal metres (with about 700m of drivage).
Development rates slowed to 25% of normal prior to process improvement initiatives.
“The zone intersects all new longwalls in the current mining area and represents a business risk with the potential sterilisation of over seven million tonnes of coal reserves if we could not successfully mine through it. We had the choice of taking it on or shorten the mine life by several years,” he said.
“We realised that the technology to overcome these conditions did not exist and we set out to create it.”
The Angus Place team responded to this challenge by working with original equipment manufacturers to develop several innovative technical solutions to allow development in extremely difficult strata conditions.
One outcome was the development of a Continuous Miner Mounted Spiling Rig in conjunction with Rambor which was used in conjunction with Hilti’s OneStep bolt (the first operational use in Australia).
“The timing of introduction to Australia of Hilti’s OneStep bolt in 2006 was perfect to allow it to be teamed with the spiling rig,” Corbett said.
Spiling is a process used to prevent cavities propagating. Bolts are drilled horizontally into the face at roof level at close centres to improve the spanning ability of the exposed roof. Previously coupled IBO bolts were used for spiling and were grouted using a cementitious grout. These were installed by specialist contractors using an elevated work platform. This involved removing the continuous miner and mobilising specialist contractors and equipment – a slow and disruptive process.
When the major cavities form at the face, the spiling rig is mounted on the continuous miner cutter head and the continuous miner is moved to the face. The spiling bolts are drilled into the face so hole closure is not an issue and resin contained within the bolt is injected to anchor them to the strata. Resin encapsulation has been measured at 80-100% – even in difficult strata conditions.
“The operating position of the rig is under the supported roof and the spiling bolts are installed and resin injected remotely, reducing the hazard to operators,” Corbett said.
“The wholly self-contained nature of the Hilti OneStep bolts is a key part of the successful installation using this method.
“The new spiling process reduced our development face spiling cycle time from seven shifts per round back to one shift per round. It saved us two weeks in the spiling process alone in the subsequent panel following introduction”
There have been no accidents or incidents during the new spiling process and cavity propagation has been effectively stopped using this process. Importantly, it also gave the operators the tools to regain ownership of the entire process – not reliant on others to recover the situation when the going got tough. It has been used successfully in 940 Panel in 2006 and 950 Panel in 2008 and 960 Panel in 2009.
The drill rigs on the continuous miner were also modified to allow them to be tilted forward to allow bolts and cables to be installed within 1m of the face.
“This was also a critical part of the strategy to prevent major cavities developing in the area between the last line of installed bolts and the face. This distance had previously been two metres and had allowed major cavities and even roof falls up to four metres in height to develop at the development face,” Corbett said.
In conjunction with Megabolt, a barrel and wedge tensionable, bird-caged, post-groutable cable bolt was developed for installation from the continuous miner.
“Development of this cable bolt also allowed us to support the roadways for the life cycle of the roadway, at the development face,” Corbett said.
“The previously available options were plain strand cables and their load transfer was poor – we proved time and again that they were inadequate for long-term roadway stability. We ended up replacing them all with bird-caged secondary cables and massively increased our costs.
“Although pre-tensioning cable bolts adds value at the start of their life cycle, in our environment it is also essential that high load transfer is achieved. Only fully grouted bird-caged cables have proved themselves throughout the roadway life cycle. I wish I had a dollar for every post-grouted plain strand cable we’ve end loaded and failed at Angus Place. I’ve seen places where the 12-millimetre plates are cracking in half and raining down from the roof.
“Tens of thousands of the barrel and wedge Megastrands have now been installed, leading to a 30 per cent cost reduction in cable bolting cost per metre of roadway. Strata control is the second-highest individual cost at the mine.
“Now, instead of putting two phases of cable bolt support in difficult areas, we do it once and do it well,” Corbett said. “Approximately 40 per cent of our development roadways have cable bolt support installed at the face. Without cable bolting at the face, some areas would be unmineable and others would deteriorate quickly outbye the face. Obviously there is a trade off between cable bolt installation at the development face and development rates. We are always reviewing the balance between primary and secondary cable bolt installation to best suit the needs of the mine.”
“Secondary support involves the use of twin strand mini-cage cable bolts. These are a low-cost bird-caged cable bolt with excellent load transfer characteristics. They have been the backbone of our secondary support hardware for many years.”
Corbett believes there is still room for improvement with cable bolt design.
“The next area we’re focused on is bottom- up grouting, to assist with grout permeation and consolidation of strata, higher grout strengths and higher grouting success rates in highly stressed ground,” he said. “It’s already achievable with outbye secondary cables, but not yet with face installed cable bolts.”
In order to assist with installation of cable bolts from the continuous miner, the development (in conjunction with Conway Engineering) of a cable bolt pushing device mounted on each drill rig has progressed to the point of underground trials.
“We are planning to trial the latest version before the end of the year with the intention of installing them on all continuous miners at the mine,” Corbett said.
“We’re also looking at options for mounting hydraulic tensioners on the continuous miners to reduce manual handling and hydraulic injection hazards.”
Reducing strata hazards
Another focus area for improvement was reducing strata hazards around the longwall face.
Support of the longwall recovery face rib had been a perennial problem at Angus Place with no previously satisfactory solution. The longwall face rib is immediately adjacent to the longwall recovery area and must be secured for safe longwall equipment recovery operations.
Angus Place wanted to reduce operator exposure to working on the AFC and improve productivity during face bolting operations.
Conventional resin anchored bolts could not be used due to hole closure between drilling and resin insertion. Sprayed products were also ineffective due to ongoing convergence during longwall face recovery. Split set friction bolts installed with handheld equipment involved operator exposure to highly stressed rib conditions.
The shearer mounted face drill rig was developed in conjunction with Rambor with a carriage/slide arrangement which allows Hilti OneStep bolts to be loaded in a safe position, then moved forward into drilling position and drilled and resin anchored remotely .
Longwall recovery mesh and face rib bolts were installed to a high standard.
Corbett said the project outcomes included no injuries, the most effective face rib support to date, and a lift in productivity (100 bolts in an eight-hour shift).
Phase 2 of the successful longwall recovery face rib support project was to use Hilti OneStep bolts for the roofbolting of the recovery face.
The Angus Place and Rambor team designed a shearer mounted roofbolter for use during longwall recovery operations.
The space available behind the ranging arm was identified as the operating envelope for a drilling rig. The bolter was designed for installation of the roofbolts on the recovery face. Once again, the use of Hilti OneStep bolts removed the need for operators to work on the AFC. The bolter was designed for reloading by operators on the walk side of the AFC and for remote installation.
“We had problems with a critical component failure, and although many bolts were successfully installed we had to revert toanother method. Unfortunately there’s no time during a longwall bolt-up to manufacture components for prototype equipment,” Corbett said.
Phase 3 of the project involves the development of a drill rig which is permanently mounted on the shearer to allow rapid response to strata deterioration.
“The concept behind this project is that there are currently limited tools available forthe installation of supplementary support on the longwall face which do not involve significant mobilisation time and exposure of operators through working on the AFC,” Corbett said.
“Using purpose designed shearer mounted drilling/installation equipment and Hilti OneStep bolts to minimise operator exposure, this project aims to allow more proactive management of longwall face strata which gives operators the tools required to control the situation before roof falls occur.”
This project has just had the design phase completed.
The injection of polyurethane (PUR) into longwall faces to consolidate broken strata is a routine operation at Angus Place but until now Angus Place has used MBS drill rigs to drill injection holes. These require the drilling offsider to stand on the unprotected face side of the AFC to change drill steels.
“We investigated options for longwall face drilling and came up with a modified version of the Alminco ‘Spyder’,” Corbett said.
Following the initial trial, the rig mounting position was shifted to allow drill steels to be changed from behind the AFC spill tray. In addition, the rig was made more modular by enabling the drill motor to be removed to reduce weight for carrying onto the longwall face. The drill collaring arrangement, the rotation joint and the AFC mounting bracket were also modified to better suit the task of extension drilling of PUR holes.
“Working on the AFC is a practice that we’re trying to eliminate,” Corbett said.
Tailgate strata conditions at Angus Place are also aggressive.
“By the time we hit the tailgate roadway with a second abutment load, the roadway often closes from a five-metre by three-metre roadway to a three-metre by two-metre roadway adjacent to the face. The floor heaves, the roof and ribs converge and it’s a pretty dynamic place,” Corbett said.
Options to improve tailgate conditions include the use of an extremely wide chain pillar (recommendations varying between 55m and 90m) or to install extensive secondary support.
“We’re currently monitoring pillar loading with stress cells in three adjacent gateroads to monitor the effect of increasing our pillar size from 40 metres to 55 metres width,” Corbett said.
Roof control is currently achieved using 1200mm 9-point Link’n’Lock timber chocks and 8m cable bolts, but rib control was not adequate. The primary support of 1500mm resin encapsulated bolts is not adequate to prevent up to 2m of rib spall.
Due to longwall abutment loading, major rib spall occurs leading to an effective increase in roadway span. The roof can gutter adjacent to the longwall tailgate chock due to the increased span. These gutters have sometimes developed into major roof falls adjacent to the longwall tailgate.
Angus Place currently uses 2.5m Hilti OneStep bolts in the tailgate pillar side ribs as a secondary rib support. “As a secondary support they are much cheaper and faster to install than the cable bolts we trialled as an alternative for this application,” Corbett said.
Gateroad secondary support involves the installation of twin strand mini-cage cable bolts. Working at heights was recognised as a major hazard in this process with people working off ladders for a number of tasks.
“We set out to eliminate the need to use ladders for any secondary support process,” Corbett explained.
“We went through the process and identified the elements where heights access was required, and designed equipment and processes which could be conducted from the floor. Now we have purpose designed manual cable pushers, we seal cable bolts with Silent Seal, we extend grouting tubes to floor level.
“Plates, barrels and wedges are installed using a purpose designed tool with an extension arm. The final piece in this strategy involves mounting lightweight Rock Australia tensioners on an extension arm with a safety lanyard, which will be trialled in the near future.”
Another project which will imminently be trialled underground is the Conveyor Mounted Bolting Platform (again developed with Rambor). It is purpose designed to allow access to the roof on the off walk side of the belt, where there has only been room to work with handheld roofbolters.
Following trials as an access platform, it will be fitted with a drill rig which will eliminate the requirement to use handheld bolters.
“We’re trying to eliminate the use of handheld bolters for secondary support work,” Corbett said.
“We’re already 95 per cent there and this is another element in the strategy”
Another project involves the development of a bulk grout handling and pumping system.
“Currently we handle around 30,000 20-kilogramme bags of grout each year for cable bolting,” Corbett said.
“We have developed systems where we have vehicle access to all grout mixing and pumping locations yet we still manually handle all the grout we use.
“It makes no sense. We plan to use a system of interchangeable grout pods which will be pre-loaded on the surface and are machine handled all the way to the grout mixer and pump.”
Angus Place has a proud history of innovation, which is enabled by a workforce that is very accepting of change, according to Corbett.
“Many of the ideas that have been or will be implemented have come from the workforce,” he said.
“The mine has a stable workforce that has very strong ownership of its workplace.
Because of this, the workforce seeks to improve the way things are done through change.”