Page 385 - Rock Mechanics For Underground Mining

UNDERGROUND MINING METHODS

from the entrainment of barren country rock in the ore stream. The method produces
significant disturbance of the ground surface, imposing some possible limitations on
its applicability from considerations of local topography and hydrology. In the first
edition of this text, the authors observed that sublevel caving was then declining in
industrial popularity because of the low ore recovery (rarely greater than 65%) and
high costs of production (Brady and Brown, 1985). These high costs were seen to
arise from the relatively high development requirement per tonne produced and the
specific intensity of drilling and blasting required to generate mobile, granular ore
within a caving medium. As will be explained in Chapter 15, the spacings of sublevels
and of drawpoints have since been able to be increased significantly, reducing some
of the former cost disadvantages associated with the method and increasing the scale
and extent of its industrial application. Close control of draw is required to prevent
excessive dilution of the ore stream. Finally, geomechanics problems may arise in
production headings as a result of the concentration of field stresses in the lower
abutment of the mining zone.

12.4.9 Block caving (Figures 1.4 and 12.3)
The preceding discussion of sublevel caving indicated that the mining process in-
volved transformation of the in situ ore into a mechanically mobile state by drilling
and blasting, and subsequent recovery of the ore from a small domain embedded
in the caving country rock. In block caving, mobilisation of the ore into a caving
medium is achieved without recourse to drilling and blasting of the ore mass. Instead,
the disintegration of the ore (and the country rock) takes advantage of the natural
pattern of fractures in the medium, the stress distribution around the boundary of the
cave domain, the limited strength of the medium, and the capacity of the gravitational
field to displace unstable blocks from the cave boundary. The method is therefore
distinguished from all others discussed until now, in that primary fragmentation of
the ore is accomplished by natural mechanical processes. The elimination of drilling
and blasting obviously has positive advantages in terms of orebody development
requirements and other direct costs of production.
The geomechanical methodology of block caving entails the initiation and propa-
gation of a caving boundary through both the orebody and the overlying rock mass.
The general notions are illustrated in Figures 1.4 and 12.3. At a particular elevation in
the orebody, an extraction layout is developed beneath a block or panel of ore which
has plan and vertical dimensions suitable for caving. An undercut horizon is devel-
oped above the extraction level. When the temporary pillar remnants in the undercut
excavation are removed, failure and progressive collapse of the undercut crown oc-
curs. The ore mass swells during failure and displacement, to fill the void. Removal
of fragmented ore on the extraction horizon induces flow in the caved material, and
loss of support from the crown of the caved excavation. The rock forming the cave
boundary is itself then subject to failure and displacement. Vertical progress of the
cave boundary is therefore directly related to the extraction of fragmented ore from the
caved domain and to the swell of ore in the disintegration and caving process. During
vertical flow of rock in the caved domain, reduction of the fragment size occurs, in a
process comparable to autogenous grinding.
Block caving is a mass mining method, capable of high, sustained production rates
at relatively low cost per tonne. It is applicable only to large orebodies in which the
vertical dimension exceeds about 100 m. The method is non-selective, except that
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