Over two decades, the use of Shotcrete in underground mining has grown rapidly in Australia due to its effectiveness in tunnel stabilisation. As the use of Shotcrete has increased, the industry has learned and improved.
This article reviews the history of Shotcrete use in underground mines in Australia. It explores the early drivers, the challenges, the benefits, the breakthroughs and the future of Shotcrete in Australian mining.
Warren Mahoney is regarded by many as one of Australia’s leading authorities on Shotcrete. He has worked with Shotcrete in mining since 1995 and is currently Asia Pacific Technical Manager for Ground Support for BASF Australia. Warren is one of the founding committee members of the Australian Shotcrete Society (AuSS), founded in 1998. He was one of the steering committee members for the development of the “Recommended Practise of Shotcreting in Australia”.
Paul Meneghel invented the first submersible pump specifically designed for underground mines where Shotcrete is used for tunnel stabilisation. Paul is the Managing Director of PumpEng Australia.
How Shotcrete Use Started in Underground Mining in Australia
Longevity of Ground Support
Initially, Shotcrete was principally used to give some longevity to ground support. In many cases this incorporated a lot of rehabilitation work. Shotcrete was used to seal the surface and provide an excellent option for long-term ground support.
Before Shotcrete was used for underground tunnel stabilisation, there were many incidents – including injuries and fatalities – caused by ‘scats’. ‘Scats’ is a term that refers to loose rocks that weigh 20kgs or less. Where conventional ground support systems were in use, scats could easily fall from a tunnel roof. Shotcrete firms up a tunnel surface like bricks and mortar, reducing the risk of that falling scats will cause injury.
From a technical point of view, the introduction of Shotcrete got away from a lot of the focus on point loading from the installation of bolts and cables. The industry still uses bolts and cables as the principal method of supporting the ground. But concrete, when sprayed, distributes the load that is acting on that rock. The number of bolts that are required to pin that ground together under conventional methods often is less. This represents saving on materials and stabilisation work time.
Early Lessons Learnt
Accelerators and Strength
The most important early lesson that the industry learned was that accelerating Shotcrete strength too rapidly had a negative effect on the long-term strength (long term being a measure at 28 days). From an engineering perspective, whenever a concrete structure is designed, whether it be a multi-storey building or underground tunnel, 28 days is when there is 98% of strength within that concrete. By accelerating too quickly, there was a reduction in the ultimate strength of the concrete.
Accelerators and Safety
In the early days, the industry used highly caustic accelerators. These caused a lot of safety issues such as skin burns. The industry learned quickly to do away with that style of chemical accelerator and much safer and easier to use accelerators have been adopted.
Mining Industry Response
Reluctance To Change
Initially, a lot of experienced people in the industry did not want to move away from putting wire mesh up in roof support or as part of roof support. They felt that if the roof moved, the wire mesh moved with the ground. They could see this movement taking place, and it was telling them something. As Shotcrete covers the surface of the rock, they felt they wouldn’t be able to see the movement occurring. So there was resistance to changing from mesh and bolt to Shotcrete and bolting.
The Arguments for Change
Shotcrete advocates proposed that Shotcrete would still show the cracks when aggressive movement occurred. Eventually, this argument won the day. More people in the industry accepted that Shotcrete distributed load and stopped a lot of the ground from moving. It distributed the load like a suspended concrete slab. Cracks could still be seen if there was a lot of movement. If this was to occur, another layer of Shotcrete could be added where there was cracking, or bolts could be installed in that area to support a major movement behind it. Advocates also had a very strong argument that Shotcrete, as a lining, stopped a lot of air getting into your rock and that in turn stopped a lot of weathering. Mesh and bolt stabilisation didn’t have that ability.
Key Benefits of Shotcrete Stabilisation
Shotcrete is more expensive up front than bolt and mesh. Despite the higher up front cost, Shotcrete delivers more benefits over the longer term, so the net result is that Shotcrete is in a better overall value proposition.
Shotcrete Speeds Up Tunnel Stabilisation
In many cases, the use of Shotcrete has reduced the time taken to achieve stabilisation. That is certainly the case now with the use of robotic arms that can spray an area that has recently been blasted. The Shotcrete seals up scatty ground very quickly. Once a certain amount of strength is achieved by the Shotcrete application, the crew can start their bolting regime.
Other Shotcrete Applications
Assists Slope Stabilisation
Shotcrete is also used on civil sites for slope stabilisation.
Shotcrete is useful for barricades where there is a need to seal off certain areas of a mine. A barricade will be erected and then it will be sprayed with wet Shotcrete to give it some permanency.
Where There Is Seismic Activity
Many mines will use a combination of both stabilisation methods. It is particularly helpful where mines are operating in an area where there is seismic activity.
Challenges To Overcome
Problems With Sprayed Concrete On Site
In the early days of Shotcrete, the mines had problems trying to get fresh concrete batched above ground and then taken underground. The mix had a very short shelf life as concrete does. Once it was batched, it started to go off and harden. At that time Shotcrete crews found that they didn’t have enough open time from the point of batching to the point of getting it underground and spraying. There was concern about the high wastage factor. Concrete was being dumped because it wasn’t usable and workable any longer. Another concern arose when crews would add water in an attempt to bring the slump back up. This had a negative effect on the overall strength of the Shotcrete.
Fugitive Shotcrete Fibre in Waste Water
In some situations, fugitive polymer reinforcing fibres can enter the wastewater cycle in an underground mine and clog or damage machinery. Impacts are most frequent in submersible wastewater removal pumps and downstream mill equipment. Potential strategies to mitigate the risk of damage include keeping Shotcrete fibre out of wastewater during the spraying process, filtering wastewater or ensuring key components use toughened parts such as metal that are capable of processing the fibre.
Improvements in Mix Design Proportioning
There is now a better understanding of Shotcrete mix design proportioning. The industry has become a lot smarter and better educated in how to design an adequate Shotcrete mix. There’s more science involved in it as compared to early days when it was maybe a bit more hit and miss.
Advances in Cement Hydration
Many problems with concrete setting have been overcome by technical advancements. BASF patented the technology of putting concrete to sleep through the use of a stabilisation chemical. This meant concrete setting time could be kept open and depending on its dose rate not go off for up to three days. An accelerator is added at the nozzle that nullifies the effect of stabilisation and then triggers the cement to set at a very fast rate. This is now part of the normal procedure for spraying the Shotcrete. The development of this technique has been a significant advancement in the industry.
The change from hand spraying to robotic spraying was most probably one of the biggest turning points in the evolution of Shotcrete use. Hand spraying was limited in output, and it had safety issues associated with it because operators were spraying in unsupported ground. The use of robotics means that operators are now operating joysticks rather than having to lift large heavy hoses and stand underground to physically spray.
Bigger Companies Switch To Shotcrete
On-site Concrete Plant Operations
Mesh and bolt stabilisation still has a predominant market share in Australia. For a lot of mines, it is still more cost effective to use mesh and bolt rather than Shotcrete. This is because of the need for an onsite concrete plant operation. If a mine site has no concrete plant or one that’s not nearby then this may be a barrier. A mine operator would need to invest money to set up a concrete plant and to engage the personnel to run it. Smaller mines stay with mesh and bolt stabilisation for this reason.
The Future of Shotcrete in Australia
Adoption Rate in Australia
Shotcrete’s proved its worth to the industry. Around 800,000 cubic metres of Shotcrete was used for ground support in Australia in 2017 alone. Twenty years ago that might have been around 10,000 cubic metres. There are now sixty mine sites in Australia using Shotcrete and this is likely to increase significantly.
In future, more sites will use Shotcrete because the cost of it is reducing. Initially, there was little demand for Shotcrete, so the cost to produce it was higher. As demand for Shotcrete has increased, economies of scale have meant that it can now be supplied at a lower cost making it an increasingly attractive option for mine stabilisation.