Big on rock mechanicsMatthew Pierce leads research that can help large cave mines achieve better safety and production
Matthew Pierce leads research that can help large cave mines achieve better safety and production
By Virginia Heffernan
November 11, 2014
Matthew Pierce, a specialist in geomechanical characterization of rock masses and underground mine design, was recently named director of the Rio Tinto Centre for Underground Mine Construction (RTC-UMC) at the Centre for Excellence in Mining Innovation (CEMI). Pierce is a principal engineer with Itasca Consulting Group in Minneapolis and a recipient of the International Society of Rock Mechanics Rocha Medal.
RTC-UMC was created in 2010 to conduct research in support of Rio Tinto’s Mine of the Future programme and its underground mining operations, and Pierce is now leading the Centre into new territory.
CIM: What makes Rio Tinto’s underground mine construction centre unique?
Pierce:From an organizational perspective, Rio Tinto has given CEMI the mandate to find the right researchers by recruiting several different players including other mining companies, consultants and academics. CEMI does not conduct its own research but can organize the appropriate parties to carry out the research because it’s not associated with any one university.
From a technical perspective, we recognize that characterization, excavation and support design need to consider brittle spalling as the main rock mass failure mechanism, not shear failure, and that traditional rock mass failure criteria are likely to underestimate the rock mass strength under high confinement.
CIM: Why are you stepping in to lead the Centre?
Pierce: Peter Kaiser was the director of the Centre for the first four years and set the technical vision to help Rio Tinto improve how they excavate and support their caving operations. In a step towards retirement, he wanted to remain involved but find someone to start developing a vision for the next five years that would add even more value. The solution was to have the directorship shared between me and associate director Erik Eberhardt from UBC, with Peter’s continuing involvement.
CIM: The Centre has made rock mass characterization a priority. What further work is needed in rock mass characterization research to improve on what has been done so far?
Pierce: In rock mechanics, there is an increasing recognition that we don’t completely understand how rock masses respond to mining under one particular combination of conditions: when the rock is massive (sparsely jointed) but densely veined and under high stress – for example copper porphyries at depth. Traditional rock mechanics has focused on how more heavily jointed rocks behave at shallow depth. But if we want good designs for deep underground mines in massive rocks, we need to change the way we think about spalling as a failure mechanism. The Centre will bring cutting-edge rock mechanic theory into practice rather than discuss it purely in academic terms.
CIM: Footprint reliability is a big focus for the Centre; what is it and why it is important?
Pierce: In this case, the word “footprint” means the bottom of a caving mine. In order for a huge volume of fragmented ore to get out of the mine to the mill, it must flow through the footprint or extraction level. Recent experience at a number of operations shows that if you keep the extraction level stable and the ore flowing freely through the drawpoints, you will have a more productive mine. If, on the other hand, the tunnels begin to deteriorate or the support turns out to be inadequate, you can quickly lose drawpoint availability. If only half of your drawpoints are available, you start to pull in waste, stress the extraction level causing further damage, and create the knock-on effects of lost recovery, low productivity and higher rehabilitation costs. The key driver for footprint reliability is keeping drawpoint availability high and a large part of that is proper ground support for spalling ground conditions.
CIM: What are some of the solutions you are working on to minimize delays and maximize speed in underground mine construction?
Pierce: To speed things up, you need to pull as much ore out as fast as you can. That comes down to drawpoint availability, but perhaps an even more important consideration is the lead-up to developing a cave. The faster we can excavate the tunnels while still being safe, the quicker we can start getting the ore out. One way is to put in fewer drawpoints. Another way is to design the ground support so that it takes less time to install and acts more effectively, minimizing rehabilitation.
CIM: Can you elaborate on how the Centre incorporates lab, in situ and numerical studies?
Pierce: Any engineering study should have a basis in these three areas. If we can take the field data and use it to validate a numerical model, we have a tool to predict a wider range of scenarios. For example, the Centre will be monitoring the pillars at three different mines in the coming year. We’ll take that data and use it to calibrate numerical models of those pillars and hopefully validate their utility. The third leg is physical modelling that allows us to scale down the problem in the lab. There is a lot of value to that for cave mining because instead of having to model the flow of giant boulders we can model the flow of gravel or sand. It’s another way to either study the problem directly or collect data for calibration of numerical models.
CIM: One of the Centre’s aims is to develop efficient and effective ground control measures. Are there any new measures either in the works or ready to be deployed?
Pierce: A number of these caving operations are not in a position to take on a new type of support element and incorporate it into their support design philosophy. We’re aiming to combine the existing types of support including bolts, cables and shotcrete in new ways and change the timing of installation. So our focus is not necessarily to come up with a new type of support, but rather to understand how existing support tools should be installed, in what combination and when.
CIM: The Centre is planning to deliver a support selection guide founded on deformation-based support design principles. Can you explain those principles? What is novel about them?
Pierce:You can think about ground support in terms of surface pressure (applying pressure to the wall of the pillar or tunnel) or in terms of reinforcement (tying the rock together internally with bolts and cables). Those are two ways to achieve the same result: allowing the rock to stay together and support itself. We are questioning which combination of these two methods is appropriate for the spalling mode of failure. This forms the basis for a lot of our support design studies.
CIM: What is the “Mine of the Future” and can you explain the importance of the concept?
Pierce:From the Centre’s perspective, it’s a mine that can be more reliably developed and operated. That has been a struggle for caving mines for a long time. We have to change the way we construct, support and monitor these operations.
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