Page 497 - Rock Mechanics For Underground Mining

BLOCK CAVING

is drawn and the residence time in the draw column. In general, the mechanisms of
secondary fragmentation can be expected to include:
extension of pre-existing discontinuities;

opening of filled of healed discontinuities;

opening along bedding or schistosity planes;

crushing under superimposed weight;

compressive (shear) failure of blocks under the influence of arching stresses within

the cave;
failure of individual blocks by induced tension produced by point or line loading

at inter-block contacts within the caved mass;
bending failure of elongated blocks; and

abrasion or “autogenous grinding” of block corners and edges to reduce block

sizes and produce fines.
It is desirable that fragmentation models be developed to provide reliable estimates
of fragmentation for use in mine planning. The basic requirement of any such model
is to provide a measure of the range and distribution of the sizes of the rock blocks
expected to be produced at the various stages of fragmentation and, in particular, those
finally reporting to the drawpoints. Since the production equipment and the drawpoint
layouts must accommodate the resulting blocks, knowledge of their shapes as well
as their sizes will be of value. It is generally accepted that because of the limited
understanding of the mechanisms involved and, for practical reasons, the lack of
availability of sufficient data, the development of a complete, mechanistically based
fragmentation model is not currently plausible. The most widely used method of
predicting fragmentation is the expert system program, Block Cave Fragmentation or
BCF which depends on a large number of assumptions and empirical rules (Brown,
2003).
One of the major difficulties encountered in developing and validating caving
fragmentation models is the difficulty (or impossibility in some cases) of sampling
the caved mass and measuring the fragmentation distributions at the different stages of
fragmentation. These difficulties can extend to measuring the complete fragmentation
distributions of the ore finally reporting to the drawpoints. Digital image processing
systems are now being developed and used for this purpose (Brown, 2003). The finer
fragment sizes produced following secondary fragmentation are particularly difficult
to measure by methods other than sieving. A particular problem is caused by large
block sizes which can cause hang-ups in drawpoints and may require secondary
breakage.

15.5.6 Draw control
Although draw control lies outside the normal scope of the discipline of rock me-
chanics, it is so important to the successful operation of block and panel caving mines
that it merits some consideration here. Draw control has been defined as the practice
of controlling the tonnages drawn from individual drawpoints with the object of:
minimizing overall dilution and maintaining the planned ore grade;

ensuring maximum ore recovery;

avoiding damaging load concentrations on the extraction level; and

avoiding the creation of conditions that could lead to air blasts or mud rushes.

479