46 BLOCK CAVING Keeping up with caving Caving mining techniques are experiencing a surge of interest, thanks to a combination of favourable metal prices, high forecast demand and the need to exploit deeper, more complex orebodies at the lowest cost possible. Carly Lovejoy investigates the current market June 2012 www. .com 46-48,50-52,54-60,62-64MM1206.indd 46 23/05/2012 16:37 BLOCK CAVING 47 oth surface and underground mass SRK’s caving mining methods have generated experts at Bincreasing interest in recent years, Codelco’s as mining companies look to exploit large Chuquicamata orebodies faster and more economically. mine, Chile Underground mining methods such as block, panel and sub-level caving continue to be the premier choice for deeply situated massive orebodies, thanks to the high potential production rates and low operating costs involved. Recent technological developments and improved solutions for designing, planning and modelling caving operations mean that these techniques can now be applied at greater depths, in more competent rock masses with greater geotechnical challenges than ever before. GLOBAL INTeReST A combination of factors that include relatively higher metal prices, high projected supply and demand forecasts and a lower discovery rate of significant new near-surface deposits have helped to drive global interest in cave mining methods. Simon Hanrahan, principal consultant (mining) based in SRK’s Perth office, explains: “In addition, a number of large open pit mines across the globe, some “Caving is producing in excess of 50,000t/d, are coming to the end of their lives. These the only make up the bulk of world copper underground production, and many mining companies bulk mining are examining the feasibility of shifting to low-cost, large-scale underground method operations in order to continue exploiting able to offer these orebodies. “Caving is the only underground bulk continued mining method that is able to offer production continued production at a comparably at a com- high rate and with low operating costs.” Aside from strip ratios, other factors parably high influencing the economics of open pit rate with low projects include the increasing escalation of energy prices, together with a focus on operating reducing carbon emissions. costs” A number of large-scale, low-grade underground operations are also considering expanding in order to access areas of the orebodies previously thought to be uneconomic to exploit. These are challenging applications that will require careful front-end investigation and analysis, innovative engineering design and a strict approach to development and operation. Sourcing of skilled labour is also a significant problem in the mining industry today, so a mining method such as caving that has a low reliance on skilled labour pools can be an interesting factor for mining companies to consider. Otto Richter, principal consultant at Snowden, says: “The recent interest in www. .com June 2012 46-48,50-52,54-60,62-64MM1206.indd 47 23/05/2012 16:37 48 BLOCK CAVING An LHD at work costs than many open pit mining methods at El Teniente and, in many cases, lower environmental mine in Chile impacts. However, it is very capital- Photo: Bloomberg Finance intensive, and one of the primary disadvantages is that it removes much of the supporting rock from under the overburden, which can lead to subsidence at the surface. Caving-induced subsidence might endanger mine infrastructure and is a major concern for operational safety. Changes to surface landforms brought about by subsidence can be dramatic and may lead to a pronounced environmental impact. Therefore, the ability to predict subsidence has become increasingly block caving comes down to two factors destroys its ability to support overlying important for operational hazard and envi- – less access to large orebodies near the rock, and fractures spread throughout the ronmental impact assessments. surface and an improvement in the rock mass, forming a cave. There are numerous variations of the knowledge of caving mining methods. Following extensive preparation and block caving technique that use different “It is becoming increasingly difficult to development work in order to access the layouts and extraction patterns. The find large, near-surface orebodies that are orebody at the correct depth, a set of choice will depend largely on the orebody suitable for open pit mining methods parallel, horizontal tunnels is created on characteristics, the nature of the host which will also yield high production rates the upper level, known as the undercut rock, and the associated development and low operating costs. Those open pit level. A set of holes are then drilled into and production costs. mines that are currently operating in these the roof, which are filled with explosives Panel caving allows the extraction of conditions are becoming deeper, and now and blasted. The tunnel collapses in very large orebodies by dividing them up have to consider closing operations or selected sections and broken rock is into ‘panels’, which are mined progres- changing to an underground method that removed via sections of the tunnel not yet sively, although, as with block caving, the will produce at a similar rate with low affected by blasting. This process initiates panel can in some cases cover the full operating costs.” the development of the upper cavern of footprint of the orebody. The fundamental “It is He adds: “In recent years a lot of effort broken rock, ie the block cave. difference between the methods is that has been applied to understand how the A second series of tunnels is then block caving produces from the full becoming internal mechanics of the caving process developed beneath the ore block, on the orebody footprint, while in panel caving increasingly works, thereby removing a lot of the production level, in which a series of the active caving zone moves across the difficult to uncertainty previously associated with tighter, vertical drill hole patterns are panel – ie while one end is being these mining methods. As caving has drilled into the broken rock above. These undercut, the other end is producing. find large proven to be a safe and effective mining allow the formation of funnel-shaped Once a panel becomes depleted, the orebodies method, its popularity has increased structures called drawbells, which act as adjacent panel is mined in sequence until around the world.” conduits for the broken ore and connect the orebody is exhausted. Like block cave suitable for the block to the production level. mining, panel caving offers low operating open pit BLOCK ANd pANeL CAVING From the drawbells, the ore is funnelled costs, and lower development costs due mining that Block caving is an underground mining into drawpoints. The broken ore is then to its progressive approach. method that uses gravity to exploit loaded by load-haul-dumpers (LHDs) and will also massive, steeply dipping orebodies delivered to ore passes or an under- SuB-LeVeL CAVING yield high located at depth, particularly those with ground crusher, prior to hoisting to the Sub-level caving is sometimes used where disseminated mineralisation or that are surface in skips or by conveyor. large open pit mining operations production low-grade in nature, but too deep to be Ore in the column is diluted by the transition to underground extraction rates and extracted using open pit methods. Block material in adjacent columns and methods, although it is often used for low oper- caving also depends on insitu stresses and ultimately by overburden and lateral independent underground projects too. the host rock’s ability to fracture in the waste. When the column drawdown is Once the orebody is developed, it is then ating costs” correct fashion, although this can now be complete and the drawpoint grade drops drilled and blasted on progressively lower overcome using preconditioning. below a minimum value, the drawpoint is levels until it is depleted. The waste rock The technique is commonly used to abandoned. above the orebody caves gradually extract low-grade copper, copper-gold, Technically, once the drawbells have upwards as the ore is extracted. iron and molybdenum ores, as well as been created, no further drilling is Sub-level caving is typically used where asbestos diamond-bearing kimberlite and necessary. The speed at which ore travels the orebody has a smaller footprint and/ lamprolite, although there are other through the drawbells is effectively or more competent rock mass that applications. controlled by the speed at which it is prevents the continuous caving required The term ‘block’ refers to the mine removed from the drawpoints. As broken for a block cave. Blasting of the entire layout, which usually divides the orebody ore from the block exits via the drawbells orebody is required to produce a into large sections, although in some and drawpoints, the roof of the cavern production ore flow. cases the block can represent the full gradually collapses under its own weight As sub-level caving uses a ‘top down’ footprint of the orebody. Upward caving to create more broken rock, forming a approach, it requires less upfront capital of the rock mass is induced by under- continuous process. than a block cave operation and much cutting the block and blasting it. This Block caving offers lower operating less time to reach full production. This June 2012 www. .com 46-48,50-52,54-60,62-64MM1206.indd 48 23/05/2012 16:37 50 BLOCK CAVING could be also advantageous where sufficiently weak to allow caving and have high-grade ore is located near the top suitably fine fragmentation, yet strong of the orebody. enough to ensure that the excavations will Mr Hanrahan explains to MM: “A last the 10- to 50-year life usually significant difference between sub-level associated with caving operations, caving and other caving techniques is that although preconditioning can be used to multiple levels of development are weaken rocks that would otherwise be required, many times more than a block unsuitable.