Introduction
As discussed in the previous section, the room-and-pillar mining system is applied to subhorizontal ore bodies of relatively uniform thickness. A portion of the ore body is removed in the form of rooms, and pillars are left to support the overlying strata. Mining may be done in a checkerboard pattern of rooms and pillars or long rooms may be created with rib pillars left between. The strata making up the roof and floor are competent, as is the ore. The deposits mined range from thin to moderately thick. As the dip of the strata increases and/or the thickness of the ore body increases, other methods must be employed.
Consider a moderately thick deposit that would be mined by the rib pillar system if flat dipping. Now increase the dip to 90&##176;. In this case, loading on the pillars would come from the horizontal direction, and the blasted ore would fall downward to be collected at bottom. Although the general geometry is the same as for the room-and-pillar method, the generic name given the system is sublevel stoping. Blasthole stoping, vertical crater retreat, and vein mining also fall under this general heading. The shrinkage stoping method is a special
form of sublevel stoping. In general, the method is applied to ore bodies having dips greater than the angle of repose of broken material (greater than about 50&##176;), so that material transport to collection points occurs by gravity. For massive deposits, stopes with vertical walls are created, and the overall dip of the deposit is immaterial.
The criterion for applying the method is that the openings created remain open during extraction. These openings may be later filled or left open. The pillars left between stopes may be extracted at a later time or left in place. In this section, some typical layouts used for extracting ore will be briefly presented. It is assumed that mobile equipment is used with ramp access.
Extraction Principles
Consider an ore block of width W, length L, and height H as shown diagrammatically in Figure 3.4. For simplicity, it has been drawn vertical. This block will be mined using a number of sublevel stoping techniques. The blasted ore will fall to the bottom of the block and be removed using LHDs. There are various designs for the extraction level. Here it will be assumed that a trough is created using fans blasted toward an opening slot. The LHDs travel in a footwall haulage drift running parallel to the trough. Access to the trough is from the side. The
location and number of access points (drawpoints) are such as to provide full extraction coverage.
Blasthole Stoping. Blasthole stoping will be the first method considered for mining the block. From the drilling level at the top of the block (Figure 3.5), rows of parallel blast holes are drilled down to the top of the extraction trough. A raise is driven at one end of the block and slashed to full stoping width to form a slot. The rows of blastholes are now blasted as one row or as several rows at a time toward the open slot. The blasting design and layout are very similar to that used in bench blasting.
Hole diameters vary widely, but typically lie in the range of 76 to 165 mm (3 to 6.5 in). For wide blocks, 165-mm (6.5-in) in diameter holes are often used. Hole straightness is an
important design consideration that affects fragmentation, ore loss, and dilution. In general, one would select the largest hole diameter possible for the stope geometry since straight hole length is strongly dependent on hole diameter. The specific development (amount of development required to exploit a certain volume of ore) is inversely proportional to block height. Since the cost of development is significantly higher than costs for stoping, one wants to have the highest possible number of extraction blocks associated with a given extraction and a given drilling level.
Sublevel Stoping. If geomechanics studies indicate that very high blocks (heights exceeding the straight drilling length from one drill location) can be extracted using the same extraction level, then several drilling levels at various heights within the block must be created. Because of the multiple drilling levels, or sublevels, this method is called sublevel stoping. The layout is very similar to blasthole stoping with an extraction level and an opening slot, but now there are multiple drilling levels.
Mining can take place overhand, in which the lower drilling blocks are extracted before the upper, or underhand, in which the extraction of the upper drilling blocks precedes those beneath. Here it is assumed that overhand stoping is employed.
The simplest approach is to repeat the drilling layout for one- level blasthole stoping. This is shown in Figure 3.6. The ore body thickness is assumed to be such that the full width is undercut and becomes available for drilling access. Parallel holes can be drilled in this case. An alternative is to drill fans of holes (Figure 3.7) rather than parallel holes from the sublevels. Furthermore, there may be one or multiple drill drifts on each sublevel, and the rings may be drilled downward, upward, or in full rings. Selection is based upon a number of factors, a full discussion of which is beyond the scope of this chapter.
As indicated, the application of the sublevel stoping technique assumes good stability of the openings created. Stability
surprises can mean the partial or even full collapse of partially extracted stopes. Production may be stopped completely because of the presence of large blocks in the drawpoints. In the best case, there is ore loss and dilution. Reinforcement of the footwall, hanging wall, and roof can be done prior to or during mining. These extraction blocks can be oriented along the strike of the ore body (longitudinally) or transversely.
Vertical Crater Retreat. In the cases just discussed, rings of holes were blasted toward a vertical slot. In vertical crater retreat or vertical retreat mining systems, the need for a slot to connect the drilling and extraction levels has been eliminated, thus simplifying development. The vertical slot is replaced by a horizontal slot (undercut) created at the bottom of the block on the extraction level. Although a real trough may be created, it is not necessary.
From the drilling level, large-diameter (on the order of 165 mm [6.5 in]) parallel holes are drilled downward to the undercut level (Figure 3.8). Short explosive charges (length = six hole diameters) are lowered to positions slightly above the top of the undercut. These "spherical" charges are detonated, dislodging a crater or cone-shaped volume of rock into the underlying void. As each layer of charges is placed and detonated, the stope retreats vertically upward, hence the name "vertical crater retreat" mining.
The design of the blasting pattern is based upon full coverage of the block cross section by the adjacent craters. Normally, the blasting pattern is tighter (holes spaced closer) than would be the case in large-hole blasthole stoping, and hence the powder factor is larger. When blasting under these confined conditions, fragmentation is generally finer than with blasthole stoping.
the location of the free surface. Special tests are performed to determine crater dimensions. In this system, the level of
broken rock remaining in the stope can be controlled to provide varying levels of support to the stope walls. If the stope is kept full except for a small slot to provide a free surface and room for swell volume for the blasted rock in the slice, it is classified as a shrinkage method. In this case, the remaining ore would be drawn out at the completion of mining.
Vein Mining. Another approach to extracting the ore block is called vein mining. At the highest level of the block to be extracted, a connection is made to the ore body. It will be assumed that the access is located on the footwall side and the connection is made in the middle of the extraction block. On the extraction level, an undercut or an extraction trough is prepared. A raise is driven between the extraction level and the upper access point using the Alimak technique (Figure 3.9). Here, the raise will be assumed to also be located in the footwall a short distance from the ore-footwall contact.
The next step in the process is the drilling of subhorizontal fans of blastholes from the Alimak platform in such a way that the plan area of the extraction block is fully covered. Hole diameter is determined by the capacity of the drilling machine, but
should be as large as possible since the toe spacing and the burden (distance between fans) is determined by hole diameter and the explosive used. Once the drilling of the entire
blasted one or more at a time working off the Alimak platform. Access to the block is now only from the upper level, and the Alimak guides are removed as the stope is retreated upward.
Ore in the stope can be removed after each blast or it can be left in place, removing only enough to provide for swell volume for the next slice(s). If required, rock reinforcement can be installed in the hanging wall from the Alimak platform during drilling the production holes.
This method allows the extraction of very high ore blocks with a minimum of development (upper access point, the extraction level, and the connecting raise). The overall length of the extraction block is determined by the straight-hole drilling length of the available drilling equipment. The disadvantage of the method is that drilling and charging must be done from a raise environment, which traditionally has not been pleasant. Major advances have, however, been made in the
mechanization and automation of the rigs used for drilling.
Shrinkage Stoping. The final method to be considered under this category is shrinkage stoping. Although normally
considered as a separate method, it is logical to include it here. The method is generally applied to very narrow extraction blocks that have traditionally not lent themselves to a high degree of mechanization. Here, a very simplified layout (Figure 3.10) is presented to illustrate the steps.
The extraction block is laid out longitudinally because of the very narrow nature of the ore body to be recovered. An extraction drift is established in the footwall with loading
crosscuts positioned at regular intervals. Raises are driven at each end of the extraction block connecting to the above-lying level. An initial horizontal extraction slice is driven across the block from raise to raise.
Extraction troughs are created by drilling and blasting the rock between the extraction level and the underlying extraction points. When the extraction system has been created, short vertical holes are drilled into the roof of the first extraction slice using the raise access. The miners stand on the broken ore, which forms the working floor. Jackleg or stoper drills are used for drilling small-diameter holes. The holes are charged, and then ore is extracted from the stope to provide room for the blasted material. The blast is initiated, and the miners reenter the newly created void to drill out the next slice. The process continues working upward one slice at a time. Upon reaching the upper end of the extraction block, the ore is drawn out. Until that time, the stope is filled with broken ore.
Summary
In summary, depending upon the geometry of the ore body, several varieties of sublevel stoping can be employed. The ore bodies must have strong wall rocks and competent ore either naturally or helped by the emplacement of reinforcement, since large openings are created in the process of ore removal.
The extraction block used to illustrate the layouts for the
different mining systems can now be duplicated and translated laterally and vertically in the ore body, leaving pillars to
separate adjacent blocks. The size and shape of the extraction block can be adjusted to fit ore body geometry and mine infrastructure. The openings created during primary mining may be filled with various materials or left unfilled. Filling materials may be cemented or uncemented, depending on the next stage of recovery envisioned. Various methods are used to recover the remaining reserves tied up in the pillars. These secondary recovery methods should be studied at the same time the primary system is designed. Although for simplicity the basic extraction block was considered to be vertical, the process could obviously be repeated for ore bodies having various dip conditions.