Introduction
In this section, the term "panel caving" will be used to represent both "block caving," suggesting the mining of individual blocks, and "panel caving," indicating a laterally expanding extraction. There are a great number of variants of this system, and it is impossible to do them all justice in a very short discussion such as this. The intention is to provide the reader with an introduction to some of the more important layout considerations. The emphasis will be on development
and extraction.
In panel caving, the three most important elements of the extraction system are the undercut level, which removes the support from the overlying rock column, the funnel through which the rock is transported downward to the extraction level, and the extraction level itself.
The basis for system design and performance is the degree of fragmentation present as the rock blocks enter the top of the funnel. The impact of fragmentation will be discussed in more detail as the section proceeds. An overview of the panel-block caving system has been presented in Chapter 1 of this book. In the early days of block caving, the materials were soft and caved readily. Today the trend is to use cave mining on ever harder and tougher ores. The result is that an engineer must thoroughly evaluate the ore body and tailor the design so that a successful extraction will result. This is the least expensive of the mining systems as measured on an extracted-tonnage basis.
Extraction Level Layout
Assuming the use of LHDs, the major development on the extraction level consists of extraction drifts, drawpoints, and extraction troughs and bells. To simplify the discussion, it is assumed that all drifts have the same cross section.
Design is an iterative process, and it is always a question as to where design begins. In this case, it is with knowing or
assuming the size of the material to be handled. The physical size of the loading equipment is related to the required scoop capacity, which, in turn, is related to the size of the material to be handled. If fragmentation is expected to be coarse, then a larger bucket size and a larger machine are required than if fragmentation is fine. Knowing the size of the machine, one arrives at a drift size. In sizing ore passes, it is expected that ore pass diameter should be three to five times the largest block size to avoid hang-ups. If this same rule is applied to the size of extraction openings, then the size of the extraction opening should be of the order of 5 to 7 m (16 to 23 ft) for block sizes with a maximum dimension of 1.5 m (5 ft).
Depending upon density and shape, such a block would weigh 5 to 10 tonnes (5.5 to 11 tons). A large piece of equipment is
required to be able to handle such blocks. It is typical for extraction drifts to be sized (W:H expressed in meters) according to the ratios 4:3, 5:4, and 6:5. For the machine in the example used here, drift size would be on the order of 5 by 4 m (16 by 13 ft) or larger.
To begin the design of the extraction level, a grid of extraction drifts that will accommodate the LHDs and the lines of
associated drawpoints is created. The actual caving and draw behavior is quite complicated. The simplified geometry shown
in Figure 3.25 is assumed to be representative. In practice, a
series of circles of radius R corresponding to the draw radius of influence on the undercut level is drawn first. Figure 3.26
shows one such pattern for staggered coverage with the
locations of the extraction and drawpoint drifts superimposed. It has been found that the value of R depends upon the degree of fragmentation. If fragmentation is coarse, the radius will be larger than if fragmentation is fine. This presents a design problem since in the initial stages of draw, fragmentation will generally be larger than at later stages.
The degree of desired coverage is one of the design factors. The "just-touching" case is shown in Figure 3.27 and the "total coverage" case in Figure 3.28. In the example, it is assumed that R = 7.5 m (25 ft, and a square "just-touching" drawpoint pattern is used. Shown in Figure 3.29 are the locations of the extraction drifts and the drawpoint drifts on the extraction level. For extraction drift 2, drawpoints 1 and 2 are associated with drawpoint drift 1, whereas drawpoints 3 and 4 are
The orientation of the drawpoint drift with respect to the extraction drift must then be decided. Figures 3.30 and 3.31
show two possibilities involving the use of a 45&##176; angle. A careful examination of these figures reveals that the choice affects both loading direction and the ease with which the openings can be driven.
A drawpoint entrance made at 60&##176; to the axis of the extraction drift is very convenient angle from the loader operator's point of view. Some designs involve the use of 90&##176; angles (square pattern). In this case, loading can be done from either direction. The 90&##176; pillars provide good corner stability, but the loading operation is more
difficult. When considering the different drawpoint design
possibilities, LHD construction must be taken into account. It is important for the two parts of the LHD to be aligned when
loading to avoid high maintenance costs and low machine availability.
As indicated, the design of the extraction level is made in response to the type of fragmentation expected. For coarse fragmentation, the openings have to be larger to permit extraction of the blocks. However, larger openings present possibilities for stability problems, and since these openings must last for the time required to extract the overlying column of ore, the design, creation, and reinforcement of the openings must be carefully made. Fortunately the type of rock in which one expects coarser fragmentation is also stronger, providing a better construction material. In softer rocks yielding a finer fragmentation, openings can be smaller. The need to protect the integrity of the openings is of highest importance. This will be discussed in more detail under the undercutting heading.
There are, as indicated, a great number of different design possibilities for the extraction level. All involve the basic components of fragmentation, radius of influence, draw
coverage, machine size, and drift size examined roughly in that order
Undercutting and Formation of the Extraction Trough In the undercutting process, a slice of ore forming the lower portion of the extraction column is mined. As the drilled and blasted material is removed, a horizontal cavity is formed beneath the overlying intact rock. Because of the presence of this free surface, subhorizontal side stresses, and the action of gravity, the intact rock undergoes a complex process involving loosening, crushing, and caving. The ease with which the intact rock transforms into a mass of fragments is reflected in its characteristic "cavability."
One approach to addressing a material's cavability is to describe the size and the shape of the area that must be undercut to promote caving. The other, and more important, part of cavability is the description of fragment size
distribution. This is much more difficult to predict, but ultimately of more importance from a design viewpoint.
The simplest design is to combine undercutting and the trough- formation process into a single step. As described in the
previous section, a series of parallel extraction drifts is driven. The center-to-center spacing of these drifts is determined by the size of the influence circles. In this example, the plan layout of Figure 3.29 is used. The center-to-center spacing of the extraction drifts is 30 m (100 ft) (4R). A series of parallel trough drifts is driven between the extraction drifts. Starting at the far end of the extraction block, fans of holes are drilled and then blasted toward opening slots. In the case shown in Figure 3.32, the side angles of the fans have been chosen as
52&##176;, and the resulting vertical distance between the extraction level and the top of the undercut is 20 m (66 ft).
Note that the trough drifts and the troughs can be created either before or after driving the extraction drifts. The latter case would be termed "advance" or preundercutting. An
advantage with this design is that all the development is done from one level. An example of the use of this design has been presented by Weiss (1981).
Most mining companies using panel caving have separate undercut and extraction levels. Figure 3.33 shows the same cross section as shown in Figure 3.32, but now a separate undercut is constructed. As seen in Figure 3.34, the undercut level has been designed as a rib pillar mine. The rooms are 5 by 4 m (16 by 13 ft), and the room center-to-center spacing is 15 m (50 ft). In step 2 of this design, the interlying pillars are drilled and blasted. In step 3, the extraction troughs are created to complete the undercut-trough development. It is possible and often desirable to develop the undercut level first and then do the development on the extraction level.
Figure 3.35 is an alternative design for the same basic
extraction level layout. A separate undercut level is used with the undercut drifts spaced on 30-m (100-ft) centers. From these drifts, fans of holes are drilled to form a trough. The angle of the side holes is 52&##176;. As can be seen, the undercut drifts are positioned directly above the underlying extraction drifts. Once the undercut has been created, a sublevel caving type of fan pattern is drilled from the trough drifts on the extraction level. This completes the development. The total height of the undercut in this case is 40 m (132 ft), which has some advantages in the caving of harder rock types.
Figures 3.36 and 3.37 are the plan and section views of a more
traditional undercutting and bell layout for panel caving. In the previous examples, an extraction trough is used primarily to demonstrate the principles involved. A trough has the
advantage of simplicity of construction, but the disadvantage that additional rock is extracted during the development process. This rock, if left in place, could provide extra stability to both the extraction drifts and the drawpoints. Drawbells are created rather than troughs.
The first step in drawbell construction is driving a drawpoint drift connecting adjacent extraction drifts. A raise is driven from this drift up to the undercut level. Fans of drillholes are then drilled from the drawpoint drift around the opening raise to form the bottom of the drawbell. Fans of holes are also drilled from the undercut drifts to complete the bell formation. A disadvantage with this design is that the amount of
development and the level of workmanship required is higher than if the trough design is used. As a result, it is more difficult to automate.
In all designs, it is important that a complete undercut be accomplished. If this is not done, then very high stresses can be transmitted from the extraction block to the extraction level, causing major damage. Traditionally, the extraction level has been prepared first, followed by creation of the undercut and completion of the drawbells. This procedure does have a
number of advantages.
Unfortunately, very high near-vertical stresses are created just ahead of the leading edge of the undercut. These stresses are transmitted through the pillars to the extraction level and can induce heavy damage to the newly completed level. The result is that repairs must be made before production can begin. The concrete used for making the repairs is generally many times weaker than the rock that has been broken, and structural strength can never be completely restored.
An alternative to this procedure is to create the undercut first (advance undercutting), thereby cutting off vertical stress. The extraction level is then created under this stress umbrella. Where this has been done, conditions on the extraction level have been markedly improved over those in which undercutting has been done afterward. There are pros and cons with both techniques, but advance undercutting will be the way of the future for most mines.
Size of Block
The size of the block refers both to the height of the extracted column and to the plan area. In the early days of block caving, the height of the blocks was on the order of 30 to 50 m (98 to 164 ft). Over time, this height has progressed to the point where extraction heights of several hundred meters are being used or planned. Obviously, as the specific development is inversely proportional to the height of the block, there are pressures to make the extraction units as high as possible.
Naturally, there are limits imposed by ore body geometry, mineral types, etc. There are also limits imposed by the life of the extraction points. If the reasonable life of the extraction point is, for example, 100,000 tonnes (110,000 tons), there is no point in selecting a block height yielding 200,000 tonnes (220,000 tons) per drawpoint. Drawpoints can and are rebuilt, but it is best if they can last the life of the draw.
As indicated in the introduction, most caving today is done in the form of panel caving rather than the caving of individual blocks. Once the initial cave is started, lateral dimensions are expanded. Cavability is an issue affecting the size of the undercut that must be created to get a sustainable cave.
Relationships have been developed relating rock mass
characteristics, hydraulic radius (area/perimeter), and ease of caving.
It is possible, unfortunately, to have initial caving followed by the formation of a stable arch. The undercut area must then be expanded and/or other techniques, such as boundary
weakening, must be used to get the cave started once again. With a large enough undercut area, caving can be induced in any rock mass. Although necessary, it is not sufficient for successful block caving.
The other factor is the degree of fragmentation that results. As the method is being considered for application to ever stronger rock types, both of these factors, cavability and fragmentation distribution, must be satisfactorily addressed prior to selection of any method. Unfortunately, the database upon which such decisions are made is very limited.
Cave Management
Cave management refers to keeping control over how much is extracted from each drawpoint each day. It involves a number of different factors. The rate of draw is an important parameter in planning the required area under exploitation. As loosening of the fragments appears to be a time-dependent process, this must be recognized in planning the draw. The rate must not be so rapid that a large gap is formed between the top of the cave and the bottom of the block. A sudden collapse of the rock above can result in disastrous air blasts. In high-stress fields, it has been observed that too rapid a draw can result in the
creation of rock-bursting conditions.
In section, there is a zone in which the height of the column under draw increases from near zero (where extraction is just beginning) to the full column height. This is followed by a zone in which the height of the ore column decreases to near zero where extraction is complete. It is important to maintain the proper height of draw versus distance slopes in these two sections to avoid the early introduction of waste from above. Poor cave management can also mean the build-up of high loads in various areas and subsequent stability problems. Typical rates of draw as taken from the available literature are
on the order of 0.3 to 0.6 m/d (1 to 2 ft/d).
The proper sequencing of undercut and extraction is a very important aspect of cave management. Unfortunately, design guidelines are difficult to obtain from the literature in this regard.
An important design consideration for the extraction level is the means by which oversized material will be handled. There are a number of different problems to be addressed. The first
concern is management of true hang-ups at the extraction points. Sometimes these can simply be blasted down by the careful placement of explosives. At other times, boulders must be drilled first. This is not a simple procedure and involves dangers to men and machines. The second concern is where and how to handle the "movable" oversize. These blocks can be (1) handled at the extraction points, (2) moved to a special gallery for blasting, (3) moved to an ore pass equipped with a grizzly and handled there, or (4) directly dumped into an ore pass for later handling. All variations are used, and each company has its own philosophy in this regard.
Initially, the sizes of the blocks arriving at the drawpoints are the result of natural jointing, bedding, and other weakness planes. As the blocks separate from the parent rock mass, they displace and rotate within the loose volume occupying a larger volume than the intact rock. The swell volume is extracted from the extraction points, thereby providing expansion room for the overlying intact rock. Loosening eventually
encompasses the entire column.
As the column is withdrawn, the individual blocks abrade and split, resulting in finer fragmentation than that in the early part of the draw. The initial fragmentation, corresponding to that resulting from initial fractures in the rock, has been termed primary fragmentation. As the column moves downward and new breakage occurs, the resulting fragmentation has been termed secondary fragmentation. Data concerning this
transition from primary to secondary fragmentation are very difficult to obtain.
SUMMARY
aspects of the major mining systems used in underground mining. With this background, it is hoped that the reader will better understand the mining systems described in some detail in the case studies portion of this book.
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