An outline of the innovative solutions that were implemented during the successful execution of the Kestrel mine extension and Grosvenor coal mine projects
Kestrel mine extension
Redpath Australia utilised a unique mining method for the Australian underground coal industry at the Kestrel mine extension project in the Bowen Basin, Central Queensland.
The project involved completing two separate drifts from the surface for access to the German Creek coal seam located approximately 230 m below the existing surface. The scope of work was the construction of two drifts – the first 1577 m at one in six gradient and the second 1874 m at one in eight gradient.
It was established that to deliver the requirements of the scope, a method allowing concurrent drift excavation and fitout was required. The basic construction principle carried forward was to provide a completed drift cross-section within 30 m of the advancing drift excavation heading. This would ensure that no delays would be experienced in subsequent work activities once the drifts were completed.
The final drift construction methodology concluded to proceed with the major excavation equipment comprising a S200MA roadheader, combined with an integrated ground support system. It was further concluded that to achieve the desired outcome, systems that minimise delays to the face advance needed to be developed. The systems identified as integral to the success of the chosen method included:
- a machine capable of excavation and support to eliminate place changing at
- a continuous material handling system
- a method of extending the ventilation with minimal disruption to the works
- a pavement and services installation method that would work concurrently with the face advance.
Each drift was developed by a Mitsui S200 roadheader with modifications to allow for a continuous tunnelling/mining cycle operation. A shotcrete boom was fitted to the left-hand side of the machine, with the operator’s console located at ground level. Another operator’s console was elevated on the right-hand side of the machine to allow operation of the drilling boom, which was mounted on a slide rail to allow flexibility of positioning depending on the stage of the cycle.
In support of the roadheader activities, an integrated conveyor system mounted on a sliding floor arrangement will be able to advance as the mining face progresses. This sliding floor consists of four 1.5 × 6 m sections that utilise hydraulic rams to ‘walk’ inbye, allowing room for the installation of precast concrete floor panels for final floor completion. The system allowed for boot end moves to be undertaken during the fibrecrete and bolting works of the cycle while no excavation works are carried out on
This method borrows from the tunnelling and construction industries to eliminate place-change delays in the mining cycle by enabling complete ground support installation without retreat of the primary development machine from the face. The aim was to reduce the project duration with the completion of final floor and services to within 30 m of the operating face as it advances.
The last aspect of the equipment system was the ventilation duct extension and installation arrangements, which are fixed to the sliding floor. The system provides for the installation of 6 m long, 1.8 m or 1.4 m diameter spiral-wound steel ducts to extend the vent system as mining progresses. A telescopic vent duct section located on the inbye end of the sliding floor provided for the ventilation extension between the installed static duct and the moving/advancing duct located on the sliding floor. Additionally, the vent duct system extended to within 3 m of the excavated face to maintain the zone boundary between NERZ and ERZ1.
It should be noted that all equipment outbye of the roadheader was compliant to the appropriate sections of the Queensland Coal Mining Safety and Health Act 1999 Recognised Standard – 04, ‘Underground non-flameproof diesel vehicles’.
Conclusion and lessons learnt at the Kestrel mine extension
The chosen construction methodology for the drifts at the Kestrel mine extension was successful in its application and in meeting the requirements of the project. The integration of the back-end works with the mining cycle to minimise downtime to the face of the excavation assisted with increased advance rates from those previously experienced by Redpath within similar scopes of work. While the system may appear complex in its explanation, once implemented, it was repetitive and assisted with creating standardised work practices.
Subject to the ground conditions and the resultant bolt pattern, advance rates achieved averaged in the order of 24-26 m per week through stone and 18-20 m per week through coal seams. The reduction in the advance rate through the coal seams is influenced by an increase in the ground support required (both bolting and fibrecrete) and an increase in the presence of ground water. Alternative strategies were developed for construction through the coal seams, which assisted with advance rates through subsequent seams.
The lessons learned from the implementation and construction process saw the system undergo improvements from the initial design. Some of these changes included:
- improvements to the ventilation ducting at the ERZ1/NERZ interface, which reduced the likelihood of the roadheader tail conveyor causing damage to the ventilation duct
- installation of a walkway between the precast inverts and the sliding floor to improve access
- modifications to the transit mixer chute to enable the 15 MPa blinding to be poured directly under the bridge conveyor
- enhancements to the sliding floor for hydrocarbon, first aid and tool storage.
The bringing together of knowledge from previous experiences on underground metalliferous and civil tunnelling works enabled Redpath to develop and implement an innovative and integrated system for the construction of the drifts at the Kestrel mine extension.
Grosvenor coal mine
It has been stated that the majority of mining in the future will be underground. We are seeing fewer new near-surface discoveries, and existing mines are expanding their deposits into deeper areas. The excavation of shafts or declines is typically on the critical path of the mining project schedule. Saving time on those activities can significantly improve the net present value of the mining project.
For the excavation of declines, tunnel boring machines (TBMs) can be used in many, if not most, cases. In almost all conditions, these provide considerably higher production rates compared to traditionally practiced excavation methods and, more importantly, offer highly safe environments and operating capacities.
In 2012, a project calling for the development of two access drifts at the Anglo American Grosvenor coal mine anticipated construction using traditional methods of excavation typical for drift development, particular to the underground coal industry. The nominated drifts were located in extremely poor-quality geology, soft soils and variable ground including minor coal seams. The project particulars nominated high quantities of rock bolts, shotcreting and concrete inverts as primary ground support for the drifts, which by necessity were substantial, thus creating elevated cycle times and slower advance rates. This caused prolonged construction periods, which in turn resulted in augmented development costs. Considering these factors, an alternative method to deliver the drifts using an earth pressure balance (EPB) TBM was proposed by Redpath Australia.
Following Anglo American’s review and acceptance of the proposed TBM excavation method, three parties (Anglo American, the Robbins Company and Redpath Australia) worked together to bring the concept of TBM-driven drifts to operation. The result was two 7 m (finished) diameter, fully lined drifts totalling 1815 m completed over a total of eight cutting months. Subsequently, the mine operator gained early access to develop the pit bottom through the conveyor drift while the TBM was relocated to excavate the next drift. This provided schedule advancement and surety for the longwall to commence production.
The key to this innovative solution was to combine comprehensiveness with reliability, repeatability and simplicity. This was something that the proposed TBM operation set out to deliver while reducing time and delivering cost effectiveness.
Why a tunnel boring machine at Grosvenor?
The success of TBM technology in establishing underground civil infrastructure and providing alternative means of rapid and safe access in poor ground conditions resulted in the consideration of a TBM at the Grosvenor coal mine to establish the conveyor, personnel and material (transport) drift access from the surface. Considering the geotechnical challenges, the TBM excavation method utilised EPB technology, which also required simultaneously addressing ventilation, gas and cooling management elements along with other coal mining-related hazards.
A study of the conditions at the Grosvenor mine led to an 8 m diameter hybrid purpose-built soft ground (EPB type) machine, with special features being designed and manufactured to efficiently bore mixed-face conditions while installing pre-cast concrete segments as final lining. This also allowed the machine to operate safely within the coal mine regulatory environment with the presence of methane gas and build multiple decline tunnels within the same
coal mine development.
The selected hybrid TBM is capable of conversion between a pressurised EPB mode and a non-pressurised, single-shield mode. Because of the requirement to swiftly build two blind drifts while maintaining full ground support, the machine was also designed for quick disassembly so that it could be removed from the first drift and relaunched on a second drift.
The back-up trailers (nine in total) were specifically incorporated to handle the concrete lining segment units, extend the muck transfer conveyor, handle dewatering pumps, install the main ventilation ducts and fix the TBM and permanent service pipes and communications.
Consideration of the supporting equipment was a fundamentally important aspect to ensure that the TBM maximised the advance rate. Tunnel boring production can be limited by the speed of muck removal and the supply of critical construction material to the TBM. The backup trailer system was as important as the machine.
Performance evaluation – both drifts
Figure 1 provides a summary of the production levels achieved over both drift developments. The conveyor drift advance rates, while poor at the beginning of the drift construction, can be accounted for through the slower productivity rate during the learning curve period, the extended wet testing and commissioning of the TBM (a disadvantage of the OFTA process), poor cutter head muck flow (incorporating soil conditioning that did not function correctly) and the catastrophic failure of the screw gearbox. However, the excavation period for the CV drift did meet the program advance rates. The cutter head interventions accounted for approximately 40 per cent of the recorded TBM downtime.
The transport drift was delivered 29 days ahead of the assigned stretch target and 42 days ahead of the baseline schedule.
Figure 2 compares the two methods for excavating drifts. As stated previously, the initial tender request required the two drifts to be excavated using excavators and roadheader machines. Considering the geology and the associated strata support, the advance rates achievable would be lower than in more competent ground. The graph represents the actual period of excavation that the TBM delivered, including the extended periods for the retraction of the TBM from the CV drift and the modifications required.
The productivity rates for the roadheader/excavator combination are plotted on the same graph (defined in the legend box as RH). Neither the roadheader nor the TBM could commence cutting until the access to the drift face through the portal was provided. The graph starts the roadheader excavation against the completion date of the portal (August 2013). The TBM dates and excavation period are adopted from actual production records.
Overlaying the two methods shows that the roadheader productivity is constrained by the poor soft soils for the first 250-400 m of the drifts. Assuming no breakdowns or an increase in the strata support and accounting for the slow TBM performance for the first 400 m of the CV drift, the TBM completed both drifts some 29 weeks ahead of the roadheader in the CV drift. This allowed immediate access to the coal seam to commence development.
Conclusions at Grosvenor
An obvious barrier to early adoption is the capital cost to purchase a TBM, which can be undoubtedly higher than a drill and blast or roadheader operation. However, through technology and innovation advancements, the opportunity to implement alternative excavation methods in the mining industry is becoming a reality. If a robust and well-managed feasibility review of alternative excavation methods is conducted, it is possible to pursue the various opportunities of mining through poor ground conditions and subsequently deliver economical solutions for resource development that may otherwise be unviable. Several factors should be considered before implementing any alternative excavation method:
- Is the method compliant with local legislation and standards?
- Does the method of excavation improve safety?
- What productivity can be achieved?
- Are the necessary skills available
in the locality to implement the excavation method?
- Can a workforce be trained in the required skills to implement the alternative excavation method?
When consideration is given to the matters raised in this article, the economics of the solution must be tested. If the economic hurdles are met,
a decision can then be made as to whether the implementation of an alternative excavation method will deliver an increase in value to undertake the development of the resource.
The bringing together of knowledge from previous experiences on underground metalliferous and civil tunnelling works enabled the development and implementation of alternative excavation systems to construct the Grosvenor drifts.
This article is based on a presentation given at the 2016 New Leaders’ Conference in Brisbane.