1. Introduction
Ore bodies come in every imaginable geometric shape. While surface hard rock mines apply the “Open Pit” method to almost any ore configuration, a large number of underground mining methods have been developed primarily in response to the requirements of differing geometry and the geomechanical properties of the host and surrounding rock.
The underground mining methods presented in this chapter are mainly applicable to hard rock mines. Softer, non-metallic ores, such as coal, salt, potash, trona (soda ash), and oil shale are often amenable to different methods than those described in this chapter.
A few of the underground mining methods, such as, Avoca? and Vertical Crater Retreat (VCR?), have been patented resulting in new names for variations (Modified Avoca and Modified VCR). Moreover, different names are often applied to the same mining method (Blasthole Stoping and Longhole Open Stoping are frequently used to refer to the same mining method). Other mining method names are merely variations or closer definition of others (Sublevel Stoping is typically Blasthole carried out with more than one drill drift per level interval and Sublevel Retreat simply defines the direction of the mining).
The many underground mining methods are difficult to categorize rationally because each application depends not only on ore body geometry, but includes other considerations, such as ground conditions, grade distribution, scale of operations, as well as the presence of structures (i.e. faults, dykes, etc.).
One logical procedure to categorize mining methods is to divide them into the following three generic classifications.
• Methods producing openings naturally supporting or requiring minimum artificial support (Room and Pillar, etc.)
• Methods requiring substantial artificial support (Cut and Fill, etc.)
• Caving methods where failure of the back (roof) is integral to the extraction process (Block Caving, etc.)
A more popular way to categorize mining methods is to divide the mining methods into bulk mining (Blasthole, etc.) and selective mining (Drift and Fill, etc.).
2. Rules of Thumb
Method Selection
• A flatly dipping ore body may be mined using Blasthole when the height of ore exceeds 100 feet (30m); otherwise, it is mined Room and Pillar. Source: John Folinsbee
Inclination
• Ore will not run on a footwall inclined at less than 50 degrees from the horizontal. Source: Fred Nabb
• Even a steeply dipping ore body may not be drawn clean of ore by gravity alone. A significant portion of the broken ore will inevitably remain (“hang”) on the footwall. If the dip is less than 60 degrees, footwall draw points will reduce, but not eliminate, this loss of ore. Source: Chen and Boshkov
Stope Development
• The number of stopes developed should normally be such that the planned daily tonnage can be met with 60% to 80% of the stopes. The spare stopes are required in the event of an unexpected occurrence and may be required to maintain uniform grades of ore to the mill. This allowance may not be practical when shrinkage is applied to a sulfide ore body, due to oxidation. Source: Folinsbee and Nabb
Ore Width
• Blasthole (longhole) stoping may be employed for ore widths as narrow as 3m (10 feet). However, this narrow a width is only practical when there is an exceptionally good contact separation and a very uniform dip. Source: Clarke and Nabb
Footwall Drifts
• Footwall drifts for blasthole mining should be offset from the ore by at least 15m (50 feet) in good ground. Deeper in the mine the offset should be increased to 23m (75 feet) and for mining at great depth it should be not less than 30m (100 feet). Source: Jack de la Vergne
3. Tricks of the Trade
• For an outcrop ore body (ore extends from surface of the bedrock), it is said to be best to mine by open pit down to the point where the cost of mining the last ton is equal to the cost of mining that ton from underground. This concept is not simple to apply. The last cut in the pit is highly profitable while the first production from underground is the most costly because it will take months to develop the sequence of stoping required to meet full production capacity. Moreover, the economical depth of an open pit is likely deeper if no underground mining is contemplated. Source: Tim Koniaris
• The fundamental distinction between underground mining methods is between those that employ pillars and those that seek complete extraction in the first pass. Source: RKG Morrison
• If the wall rock is considerably more competent than the ore, the stope should be mined by a top-down method. If the opposite is true, mining must be started at the bottom. In the latter case, it may be advisable to leave a thin layer (“skin”) of ore on the back. Source: Fred Nabb
• Whenever block caving is contemplated for a new mine, blasthole mining should be investigated as an alternative. Source: Bob Rappolt
• A flatly dipping ore body is usually mined using Room and Pillar. It requires little development since open stopes can be used for haulage ways. The method is comparable to drifting and slashing so the same equipment is used for both mining and development. In thicker ore bodies, the mining takes place in stages. The top lift is driven first and the back secured. The remainder is removed by benching. Source: Hans Hamrin
• In sulfide ore bodies, the stoping height may have to be reduced when retreat with shrinkage is employed due to oxidation of the broken ore. In severe cases, this method is simply not practical. Source: Fred Nabb
• A slot raise is normally required when employing sublevel retreat. This can be avoided by placing Styrofoam? blocks suspended with wire ropes against the ore wall when backfilling with cemented rock fill (CRF). For paste fill, we use a production rig to drill a hole near the wall soon after placement and back ream a cut hole 24 inches in diameter. The latter procedure is even less expensive than using Styrofoam@. Source: Jacques Perron
• In low-grade (low oxidation) massive sulfide ore bodies of competence and vast extent, a 200-400 foot stoping height has been successfully employed using big-hole, in-the-hole (ITH) rigs and a wide drill pattern. However, problems such as re-drilling plugged holes, hole squeeze, bottom collar (“toe”) loss, hole misalignment, sulfur dust ignition, wall craters, sloughing and sliding can result in delays and dilution that are not acceptable when the ore width is finite. In this case, better over all results have been obtained using top hammer drills, traditional patterns, and a stope height of 150 feet. Source: Ken Lowe
• The ore from a flatly dipping ore body too steep for mechanized equipment may be successfully extracted by “cheating.” This method is simply to drive the entries and cut the rooms at an angle from the dip thereby reducing the effective gradient. Source: Fred Nabb
• When the ore dip turns shallower than 51 degrees, plan an extra footwall hole to steepen the slope and prevent muck buildup. Source: Olav Svela
• When mining a steeply dipping narrow vein with small-scale LHD equipment, it is helpful to have a cross slope in the floor of a drift in a narrow vein of ore so that the drive is carried out as if the vein were vertical. Source: Brian Robertson
• Ongoing stope development for sublevel caving will produce waste rock in the amount of 25% of the ore extracted and there is no place to leave any of it underground when this mining method is employed. (This factor should be considered when sublevel caving is contemplated as a mining method.) Source: John Folinsbee
• It should be considered that a high back is difficult to scale and keep safe. It may be better to dispense with arching and drive with a flat back when ground conditions permit. Source: Douglas Duke
• If the back of a heading is secured with split-set friction rock bolts, screen is easily applied by using smaller diameter “utility” split sets that are driven inside the existing bolts to a depth of 18 inches (0.45m). Source: Towner and Kelfer
4. Mining Method Selection
To select a mining method, certain data describing the ore body is required.
• Geological cross sections and a longitudinal section
• Level maps
• Block model (grade model)
• Geomechanical characteristics of the host and surrounding rock.
One approach is to find one or more comparable ore bodies that are being or have been mined successfully and use that mining method(s) to determine the most likely methods to investigate further. Another approach is to determine applicable mining methods and develop a short list for detailed consideration through a process of rationalization.
Following are typical considerations to be weighed in selecting a mining method (listed roughly in order of importance).
• Maximize safety (integrity of the mine workings as a whole or in part).
• Minimize cost (bulk mining methods have lower operating costs than selective extraction).
• Minimize the schedule required to achieve full production (optimize stope sequencing).
• Optimize recovery (80% or greater recovery of geological reserves).
• Minimize dilution (20% or less dilution of waste rock that may or may not contain economic minerals).
• Minimize stope turn around (cycle) time (drill, load, blast, muck, backfill, set).
• Maximize mechanization (trackless versus track and slusher mining).
• Maximize automation (employment of remote controlled LHDs).
• Minimize pre-production development (top down versus bottom up mining).
• Minimize stope development (selective versus bulk mining methods).
• Maximize gravity assist (underhand versus overhand).
• Maximize natural support (partial extraction versus complete extraction).
• Minimize retention time of broken ore (open stoping versus shrinkage).
Maximize flexibility and adaptability based on size, shape, and distribution of target mining areas.
• Maximize flexibility and adaptability based on distribution and variability of ore grades.
• Maximize flexibility and adaptability to sustain the mining rate for the mine life.
• Maximize flexibility and adaptability based on access requirements.
• Maximize flexibility and adaptability based on opening stability, ground support requirements, hydrology (ground water and surface runoff), and surface subsidence.
Following is a list of mining methods most often employed underground listed roughly in the order of increasing cost (direct mining cost, including backfill where applicable). The order is generally true, but can be deceiving because some methods, such as blasthole can have a wide range of costs.
Bulk Methods
• Block Caving/Panel Caving – columns of rock are undercut wide enough to cave under the weight of the column. Caving is initiated by undercutting the ore zone. Block Caving involves a significant capital investment in pre-production development and may be especially risky. It should only be implemented in consultation with a block-caving expert.
• Blasthole/Sublevel/VCR@ – the ore is drilled in rings or by long hole and the ore is drawn off (“mucked”) as it is blasted. A common variation is to pull only the swell and leave most of the broken ore temporarily remaining in the stope to support the walls (deferred pull).
• Sublevel Caving – the ore is drilled in rings and drawn off (pulled) after blasting in successive lower lifts. Unless the ore dips steeper than 70 degrees, a great deal of ore may be left behind as production losses. One difficulty with sublevel caving concerns grade control. A gradual dilution occurs toward the end of the draw and it can be difficult to determine when it is best to stop pulling. Recovery may be improved if the draw point layout is staggered from one level to the next. One large sublevel cave operation in North America has reduced dilution dramatically. It calculates the ore tonnage in the first ring. Then, it then pulls only 70% and leaves the remainder (“deferred pull”) to be drawn along with 70% of the ore tons calculated in the subsequent ring beneath it, etc.
• Room and Pillar/Post Pillar – a grid of rooms is developed on a near horizontal plane, leaving pillars of ore to support the back (roof). The pillars left in the Post Pillar method are undersized (posts) and designed to fail in a controlled manner. Typically, a zone of low-grade mineralization or host rock (“barren”) must be mined with pay grade ore to maintain access and control stress distribution. On the other hand, Post Pillar (and even Room and Pillar) may be considered to be selective when the pillars can be arranged in zones of lower grade material, as opposed to a regular geometric pattern.
• Modified Avoca – the ore is drilled by long hole and drawn off in retreating vertical slices, followed closely by placement of rock fill dropped “over the bench” or “over the fill” (via access to the back of the stope from the footwall drift).
Selective Methods
• Shrinkage (narrow vein) – the ore is sliced off in successive horizontal lifts (overhand). Only the swell is drawn off leaving broken ore to support the walls and provide a working platform for the next lift. Narrow vein shrinkage stoping is classed as selective because it permits mining to variations in the horizontal contour of the vein and even remove pockets of ore extending into the wall rock. It is not selective with respect to the fact that once initiated, a shrinkage stope has to “take it all.” Normally, a barren portion of the vein cannot be left behind.
• Cut and Fill (Overhand/Underhand) – Access is provided by first ramping down from a crosscut access and then taking down back in successive slices. After mucking, the stope is filled but enough space is left to mine the next slice. Mining equipment is captive unless an access ramp is employed. If underhand (undercut), slices are taken from the top down under cemented backfill or a concrete matte.
• Drift and Fill (Overhand/Underhand) – a modification of Cut and Fill in which drift sized cuts are taken adjacent to one another and, upon completion, packed with cemented backfill. The process is repeated next to the backfill once it has consolidated.
Many additional mining methods exist; the foregoing are the most commonly employed. The selection of a mining method and its application to a new ore body may be simple in some cases, but it is more often a challenge requiring not only logical and practical reasoning, but creative minds working in three dimensions.
5. Dilution
Modern bulk mining methods reduce direct operating costs and facilitate management of the mine operations, but a common drawback is often increased dilution. For ore bodies of vast expanse, dilution is not a problem; however, most mines deal with ore zones of finite width and many experience dilution as high as 20%, or even 25% when bulk-mining methods are employed.
Dilution is the great nemesis for miners because the cost of dilution is not only the obvious direct cost (dilution tons displace ore tons in the ore handling and process circuits), but also includes significant indirect costs. For example, each ton of sterile rock or backfill that circulates through the mill carries mineral values with it to the tailings. The minimization of dilution should be given weight in the selection and subsequent application of a mining method. The causes of excess dilution include using the wrong mining method and related factors. The causes can be illustrated in the following fish-bone chart (Figure 3-1).
Figure 3-1 Fishbone Chart
6. Mine Planning
Once a mine is producing, a series of mine production schedules based on current reserves and proposed mining methods should be prepared and updated on a regular basis. These schedules form an integral part of the ongoing reconciliation that must be performed to monitor the success of the selected mining method. Most active mining operations in North America have the following production schedules readily available for review and audit by senior staff or consultants.
• Three month (updated every three months)
• Six month (updated every six months)
• One year (updated every year)
• Two year (updated every year)
• Five year (updated every two to three years)
• Ten year (updated every five years)
7. Mining Methods for Large Capacity Underground Mines
Table 3-1 Mining Methods for some of the World’s largest Capacity Underground Mines (Sorted alphabetically by geographical location)
