Jump into this series and get an overview of the factors affecting the calculation of a mineable ore reserve computation for a block cave mine. In our previous posts​​​​​​​, we introduced the list of parameters to consider when starting a new project. Discover in this post the following parameters of your checklist: fragmentation, economics, max/min hod, haircut and time.

By Tony Diering, Ph.D. VP Caving Business Unit Dassault Systèmes, GEOVIA

EXCAVATION GEOMETRY

In any mining method, the excavation geometry plays a very important role in the conversion from a mineral resource into a mineable reserve. Block cave mining is probably the least selective mining method in use today. The following factors are discussed in this section:

• Draw point spacing vs. ore body recovery. Hoped for vs. real draw cone. (Effect of erosion).
• Maximum panel or undercut width.
• Minimum span and Hydraulic Radius.
• Flat vs. inclined caves.
• Block cave vs. Panel cave.
• Peeling the potato! (Converting a numerical footprint into a practical footprint (analogous to Whittle pit to design pit conversion).
• Geometric resolution and detail at base of draw cone. The nitty gritty around the draw bells and undercut.

DRAW POINT SPACING

The draw point spacing is one of the trickiest and most contentious design considerations in block caving. Of course, draw point spacing is very closely related to the expected fragmentation so the range of options open to a planner may be limited. Effects of being too close or too far apart are discussed, below. The ideal is in the middle.

Too close

If the draw points are spaced too closely, one positive aspect could result but it is associated with many negatives. The potential will exist to recover the whole ore body above the layout since the draw cones or ellipsoids of draw from each draw point will overlap and the ingress of dilution material can be effectively managed.

There are many negative side effects from a draw point spacing which is too close:

• There are more draw points to be developed, which will be more expensive.
• The extra draw points will take longer to construct, which will in turn limit the maximum production potential. In a large panel cave, the maximum production potential is proportional to the number of new draw points developed per period. (Diering 2008)
• There will be significant risk of the pillars around the draw points being too weak. In that case, the pillars fail and the result is loss of recovery of the ore body and loss of production capacity as less draw points become available.

Too far apart

Positives in this case are:

• As the spacing increases, the potential for increased production (in a panel type cave) increases If the draw point spacing is too big, there are several positives and potential negatives: Larger equipment can be put into larger service excavations.
• Capital cost per ton of rock mined can decrease.
• Pillar strength is increased.
• Pillars can absorb more brow wear.

Negatives are:

• Not all the ore is recovered. Cave behaviour is altered and early dilution ingress is likely with parts of the ore body remaining intact or unrecovered above the major pillars for the production tunnels.
• Irregular cave propagation can result with adverse stress conditions on the layout.
• Fragmentation may deteriorate and there may be reduced life for draw points (due to early dilution ingress) causing early draw point closure and reduced production rate.

Figure 2 shows output from a 2D Cellular Automaton tool in PCBC representing a vertical cross section for a test case. The left side of each image represents eight closely spaced draw points with regular draw, while the right side represents four points, which are more widely spaced with higher tons being extracted from each draw point. The horizontal (coloured) bands represent different zones or rock. These are preserved more effectively with closer spaced draw points and the incomplete extraction from the right side is clear on the bottom row of images. The upside down “V” shapes to the lower right demonstrate the potential for loss of reserves if the draw point spacing is too large.

MAXIMUM PANEL OR UNDERCUT WIDTH

One may wonder why mineable reserves from a block cave would be limited by maximum panel width, but it turns out that this can be significant. In larger block caves, there is likely to be a need to split the ore zone into panels. In this case, the maximum width can be limited by cycle time or, in the case where electric loaders are being used, then the maximum length of the electric cable. A typical value would be around 300m. So, if you have an ore body, which varies from 280 to 350m in places, it may be impractical to split this into two smaller panels, but if the maximum panel width is limited to 300m, then some of the edges of the ore body would likely end up being trimmed.

Another limitation can be linked to the maximum length of the undercut, which in turn depends on the undercut sequence or direction. If the undercut face gets too long, then the rate of advance of the undercut face can become too slow and geotechnically problematic. So, the maximum panel length would be limited by the undercut face in this situation.

The end result from either of the above situations is the excavation geometry will result in lower recovery from the ore body.

MINIMUM SPAN AND HYDRAULIC RADIUS

Minimum span is determined by the Hydraulic Radius (Laubscher). This is the minimum span, which should be used in the block cave to ensure that the material will cave after undercutting. Thus, the hydraulic radius will limit the ability to recover some smaller parts of the ore body or alternatively, additional low grade draw points would be required to be developed to ensure cavability.

Having a computer tool such as PCBC is very useful in this situation. The alternate footprints can quickly be evaluated to see which of the two alternatives would be more favourable.

A related constraint is that it will often make sense to ensure that the ore body is mined as a single footprint rather than being split into several independent footprints. The reason for this relates to the added difficulty of starting up a new cave.

FLAT VS. INCLINED CAVES

Most of the above rules relate to a flat cave layout; the tunnels are almost flat, but may have a slight drainage gradient. If an ore body has a footwall inclination of around 45 degrees, then there is merit in considering an inclined cave. In either case, flat or inclined, there will be ore losses and dilution inclusions due to the planar nature of the bottom side of the cave. Unless the ore body is very large and is to be mined out in several lifts, one will usually end up with some draw points, which are initiated in low grade ore and have to mine the low grade before reaching the economic material. In these cases, the inclination of the draw points has to be matched up reasonably with the base of the ore body.

BLOCK CAVE VS PANEL CAVE

Block caves and panel caves are each variants of the block cave mining method. In general, a panel cave would apply to a larger ore body and the shapes of the panels would be closer to rectangular in shape. If using a block cave, there would be more flexibility in the shape of the footprint, but this would still be limited by minimum and maximum widths and overall curvature.

FOOTPRINT OUTLINE

This generally needs to be created without sharp or concave corners. Ideally, it should be convex in shape. If the ore body perimeter is irregular, then the process of smoothing the footprint edges is analogous to peeling a potato. Bits sticking out need to be trimmed off and indentations cause trouble. A peeled potato reduces the original reserve due to the loss of tonnage, which was required to be removed. This seems obvious, but often the process of smoothing the footprint outline can cause considerable loss of value. This loss of value needs to be quantified and compared against the operational convenience of the simpler outline.

The process of smoothing the outline is quite similar to what is done in open pit evaluations when an optimal pit outline, such as is generated using GEOVIA Whittle™ Pit Optimization, is converted into a practical minable pit with haul roads and benches, etc.

GEOMETRIC RESOLUTION AND DETAIL AT BASE OF DRAW CONE

The final adjustment to the ore reserve occurs when detailed adjustments are made around the base of each draw cone to reflect the accurate volumes and tonnages of each draw point (or half draw bell). This usually requires manual calculation of what the true volume will be and then adjusting the computed volumes in PCBC at the base of each cone (Figure 3). Often, the potential reserve tonnages lost at the base of the footprint can be recovered later using pillar retreat or from a second mining lift. For larger lift projects (500m), the tonnage in the bottom 15m is relatively insignificant, but still needs to be accounted for in the overall reserve calculations.

Care should also be exercised to ensure that material from tunnels (on production and undercut levels) is not double counted between production and development tonnages.

In our next post we will cover the "mining sequence and history", the "non linear material flow" and the residual material in multi-lift situations" parameters. Stay tuned!

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