A well-functioning shotcrete system is like the air we breathe: you take it for granted when it is there, but problems escalate quickly if it is not.
The focus of mining companies is on extracting ore from a mine. But that extraction can stall if shotcrete is not available where and when it is needed. It is therefore essential to achieve a no-surprises shotcrete system for underground mining operations.
Design your shotcrete system for the life of the mine
Reliable shotcrete supply begins at the mine’s design stage. It is important to determine how much shotcrete will be required throughout the life of the mine, and how far and deep it must be transported, so that choices in the design of the shotcrete system along with choices for the rest of the mine are made in tandem. Similar to other components of a mine’s infrastructure, such as a backfill system, a shotcrete system must consider available resources, constraints, opportunities and synergies with other systems. For example, the work of building a shotcrete-delivery system must be done at times that avoid conflict with ore extraction. The shotcrete, however, must be available when it is needed, so as to not delay the mine’s expansion.
The next issue is how to deliver the shotcrete to where it is needed. In the early stages of a mine’s life, and for places where only small volumes of shotcrete are needed, it may be best to use bags of dry-mix shotcrete, as the product can be carried on mine vehicles to where it is needed, without a pipeline system.
But if higher volumes are required, the mine operator must determine the best method of moving wet shotcrete produced on the surface to the necessary location. One way is through pipes called slicklines that run down the mine’s shaft, fastened to the shaft walls. The other is to drill boreholes lined with piping through the rock from the surface down to the stopes.
In-shaft slicklines may cost less to construct, but a break or a leak could shut down the shaft’s skips or cages until the slickline leak is fixed. Boreholes generally have a higher construction cost but avoid risk to the operation of the shaft, which is the mine’s lifeline.
Budget time and money for monitoring and maintenance
In deciding how to transport the shotcrete, designers must clearly understand the difference between the higher capital cost of drilling shotcrete-specific boreholes from the surface versus the higher risk and operating costs of an in-shaft slickline.
Budgeting enough resources for monitoring and maintenance is vital for two reasons. Firstly, it is important to provide reliable supplies of shotcrete to the right parts of the mine. Secondly, the potential for a breakdown in the shaft can be extremely detrimental to safety and ore production operations.
Due to the abrasiveness of the product and its velocity in the pipeline, there is also a strong potential for wear on any pipe carrying shotcrete. Studies indicate that by falling through a vacuum, a mass of shotcrete can reach speeds of 50 to 100 metres per second. This results in serious differential wear on the inside of the pipe, on about 10 per cent of its circumference, and can cause pipeline failure.
The greatest erosion occurs at the interface between the freefalling shotcrete and the shotcrete that has filled the lower portion of the pipe that is restricted by the outlet design. Envision bedrock at the bottom of a waterfall that has been eroded into a bowl, for instance. The high turbulence at this interface zone, where the shotcrete changes speed, can be the highest erosion wear part of the shotcrete system.
Frequent inspection of the pipe using technologies similar to those used to inspect underground backfill system such as pipeline cameras and ultrasonic thickness testing can help avoid problems. Replacing vulnerable sections of the pipe using high-tensile and harder steels and possibly adding ceramic liners can similarly improve reliability.
Use vendors and consultants effectively
Many mining engineers are well versed in matters related to the effective application of shotcrete in order to achieve a safe and reliable mine. But they naturally focus on extracting ore, rather than on developing a strategy to ensure the right formulations and volume of shotcrete is available when needed. Because shotcrete systems are not core to mining operations, mining companies often need to look outside for expertise on shotcrete strategy.
Vendors of equipment, concrete and admixtures can be a useful source of information for their products, but it is important to remember the limitations under which vendors work. They are restricted to solutions made by their company and thus are understandably motivated to sell those solutions.
Therefore, it is important to also employ consultants who can provide a wider view of the situation. That said, vendors and consultants are not as familiar with the mine as are the staff employed by the mine. As with much of mining, success is achieved through the right balance of wisdom from vendors, consultants and employees of the mine.
Paul Rantala, P.Eng., is principal consultant with Kovit Engineering Limited in Sudbury, Ontario, which recently merged with Outotec. Paul is also owner of PAR Innovations Inc. that developed the unique Pipeline Pressure Pill test to troubleshoot backfill, slurry and shotcrete piping systems. firstname.lastname@example.org (email@example.com), 1.705.523.1040 ext. 203.