Introduction
The origin of sublevel caving has been attributed to a scaling- up of the now largely extinct top-slicing method. In this latter
method, a horizontal slice of ore was extracted from the top of an ore body of relatively large extent. A timber mat was
constructed on the floor of the opening, and the timbers
supporting the roof were either blasted or otherwise removed, allowing the overlying waste to fall down onto the timber mat. The miners would then mine beneath this timber mat and extract a second slice. The supporting timbers would be removed, the overlying waste would cave, and the process would be repeated.
The first modification of this system was to skip one drifting slice. Instead of extracting each slice by drifting, a layer of ore was left in the form of a roof between the horizontal slices. In retreating from the periphery of the ore body on the drifting slice, the ore in the slice above would be allowed to cave. It was then extracted from the drifts. In this way, miners would extract two slices from one development level. This retreating process would continue back to the main access, and a new development slice would be taken, leaving the slice to be caved in-between.
Since sublevels and ore caving were involved, it was logical to name the technique sublevel caving. A natural extension of this technique was to increase the number of slices removed by caving for every slice removed by drifting. Since a series of individual mining "blocks" were used to extract these large ore bodies, the method received the name block caving.
Sublevel caving was initially applied in extracting the soft iron ores found in the iron ranges of Minnesota and Michigan. The sublevel caving as practiced today is significantly different from this early version and should probably be given another name, such as sublevel retreat stoping, continuous underhand
sublevel stoping, or something similar that would better reflect the process. Sublevels are created at intervals of between 20 and 30 m (66 and 98 ft), beginning at the top of the ore body and working downward. On each sublevel, a series of parallel drifts are driven on a center-to-center spacing that is of the same order as the level spacing. From each sublevel drift,
vertical or near-vertical fans of blast holes are drilled upward to the immediately overlying sublevels. The distance between fans (the burden) is on the order of 2 to 3 m (6.5 to 10 ft).
Beginning typically at the hanging wall, the fans are blasted one by one against the front-lying material, which consists of a mixture of ore from overlying slices and the waste making up the hanging wall and/or footwall. Extraction of the ore from the blasted slice continues until total dilution reaches a prescribed level. The next slice is then blasted, and the process continued. Depending on ore body geometry, the technique may be
Today, the sublevel caving technique is applied in hard, strong ore materials in which the hanging wall rocks cave readily. The key layout and design considerations are to achieve high
recovery with an acceptable amount of dilution. The
uncertainties of fragmentation and ore cavability present in panel caving (discussed in the following section) are removed because each ton of ore is drilled and blasted from the
sublevels. The method has been used most for mining magnetic iron ores that can be easily and inexpensively separated from the waste. However, it has been and can be applied to a wide variety of other ore types.
Sublevel Caving Layout
As indicated, ore is recovered both through drifting and
through stoping. Because the cost per ton for drifting is several times that for stoping, it is desirable to maximize stoping and minimize drifting. This has meant that through the years, the height of the sublevels has steadily increased until today they are up to 30 m (98 ft). Whereas approximately 25% of the total volume was removed by drifting in the early designs, today that value has dropped to about 6% in the largest-scale sublevel caving designs. The sublevel intervals have changed from 9 m up to nearly 30 m (30 to 98 ft). The key to this
development has been the ability to drill longer, straighter, and larger-diameter holes.
Sublevel caving is an underhand method with all of the blastholes drilled upward. The ore moves down to the extraction and drilling drift under the action of gravity.
There are a number of factors that determine the design. The sublevel drifts typically have dimensions (W/H) of 5 by 4 m, 6 by 5 m, or 7 by 5 m (16 by 13, 20 by 16, or 23 by 16 ft) to accommodate LHDs. In the example used to illustrate the
layout principles, it is assumed that the drift size is 7 by 5 m (23 by 15 ft). The largest possible blasthole diameter (from the viewpoint of drilling capacity and explosive charging) is
normally chosen; today, this is 115 mm (4.5 in) based largely on the ability to charge and retain explosive in the hole. These large holes may be drilled using either in-the-hole (ITH) or tophammer machines. The large diameters and large drift sizes permit the use of tubular drill steel of relatively long lengths (thereby minimizing the number of joints and maximizing joint stiffness), so that the required long, straight holes can be produced. The largest ring designs incorporate holes with lengths up to 50 m (164 ft).
The distance between slices (burden B) depends both on hole diameter (D) and the explosive used. For initial design when using ANFO as the explosive, the relationship is B = 20D. For more energetic explosives (bulk strength basis), the
relationship is B = 25D. Assuming that D equals 115 mm (4.5 in) and an emulsion explosive is used, B would equal about 3 m (10 m). Typically, the toe spacing- (S) to-burden ratio is about 1.3. Hence the maximum S would equal 4 m (13 ft) in this case. To achieve a relatively uniform distribution of explosive energy in the ring, the holes making up the ring would have different uncharged lengths. Both toe and collar priming initiation techniques are used.
The sublevel drift interval is decided largely on the ability to drill straight holes. In this example it will be assumed that the sublevel interval based upon drilling accuracy is 25 m (82 ft)
(Figure 3.21). Once the sublevel interval has been decided, it is
necessary to position the sublevel drifts. In this example, the drifts are placed so that the angle drawn from the upper corner of the extraction drift to the bottom center of drifts on the overlying sublevel is 70&##176;. This is approximately the minimum angle at which the material in the ring would move to the drawpoint. The resulting center-to-center spacing is 22 m (72 ft). A one-boom drill is assumed to drill all the holes in the ring. The rotation point is shown in Figure 3.22. The inclination of the side holes has been chosen as 55&##176;, although holes somewhat flatter than this can be drilled and charged. The function of holes drilled flatter than 70&##176; is largely (1) to crack the ore, which is then removed from the sublevel below and (2) to reduce the maximum drill hole length. Holes flatter than 45&##176; are difficult to charge because of the
angle of repose of the ore at the extraction front. In Figure 3.23, the locations of the individual drillholes are shown. A buffer zone 1 m (3.3 ft) wide has been left between the ends of the blastholes and the boundary to the overlying drifts and outer fan holes.
In Figure 3.24, an extraction ellipse has been superimposed. The layout is very similar to that obtained using the theory of bulk flow as described by Kvapil (1982, 1992). The fans may be drilled vertically or inclined from the horizontal at an angle, typically 70&##176; to 80&##176;. Inclining the fans
improves brow stability and access for charging the holes. In the example, the inclination of the fans is 80&##176; and the burden is 3 m (10 ft).
To initiate mining a new sublevel, an opening slot must be made toward which the fans can be blasted. Several techniques are used: blind hole boring and slashing, fan drilling using an increasing inclination angle until the production fan inclination is reached, creation of an opening slot longitudinally along the hanging wall, to name a few. In transverse sublevel caving, upon reaching the footwall, the inclination of the fans is
to minimize waste extraction.
In Figure 3.23 the importance of drilling precision is easily
seen. If the forward or backward angular position caused by incorrect initial alignment or in-hole deviation exceeds
2&##176;, the ends of the longest holes find themselves in the wrong ring. Side-to-side angular deviations can mean that the fragmentation is poor because of too little explosive
concentration, dead-pressing of explosive, etc. Thus, careful drilling is of utmost importance for successful sublevel caving.
Recovery and Dilution
Sublevel caving lends itself to a very high degree of mechanization and automation. Each of the different unit operations of drifting, production drilling, blasting, and extraction can be done largely without affecting another
operation. Specialized equipment and techniques can and have been developed leading to a near-factorylike mining
environment. As indicated earlier, because every tonne of ore is drilled and blasted, there are not the same uncertainties regarding cavability and fragmentation present with block caving. However, a very narrow slice of blasted ore surrounded by a mixture of waste and ore must be extracted with high recovery and a minimum of dilution. As can be easily
visualized, the ore at the top part of the ring in the example is more than 40 m (130 ft) away from the extraction point, whereas the waste-ore mixture lies only the distance of the burden in front of the ring (on the order of 3 m [10 ft]). With care, recoveries on the order of 80% with dilution held below 25% can be achieved.