Page 371 - Rock Mechanics For Underground Mining

OREBODY PROPERTIES INFLUENCING MINING METHOD

12.3.3 Size
Both the absolute and relative dimensions of an orebody are important in determining
an appropriate stoping method. A large, geometrically regular deposit may be suitable
for mining using a mechanised, mass-mining method, such as block caving. A small
deposit of the same ore type may require selective mining and precise ground control
to establish a profitable operation. In addition to its direct significance, there is also an
interrelation between orebody size and the other geometric properties of configuration
and disposition, in their effect on mining method.

12.3.4 Geomechanical setting
Specific geomechanical issues determining an appropriate mining method for a de-
posit have been defined, in part, in preceding chapters discussing the properties of
rock materials and rock masses. The response of a rock mass to a particular mining
method reflects the mechanical and structural geological constitution of the orebody
rock and the surrounding country rock. Rock material properties include strength,
deformation characteristics (such as elastic, plastic and creep properties) and weath-
ering characteristics. Rock mass properties are defined by the existence, and geomet-
ric and mechanical properties, of joint sets, faults, shear zones and other penetrative
discontinuities. The pre-mining state of stress in the host rock is also a significant
parameter.
In addition to the conventional geomechanical variables, a number of other rock
material properties may influence the mining performance of a rock mass. Adverse
chemical properties of an ore may preclude caving methods of mining, which gen-
erally require chemical inertness. For example, a tendency to re-cement, by some
chemical action, can reduce ore mobility and promote bridging in a caving mass.
Similarly, since air permeates a caving medium, a sulphide ore subject to rapid oxida-
tion may create difficult ventilation conditions in working areas, in addition to being
subject itself to a degradation in mechanical properties.
Other more subtle ore properties to be noted are the abrasive and comminutive
properties of the material. These determine the drillability of the rock for stop-
ing purposes, and its particle size degradation during caving, due to autogeneous
grinding processes. A high potential for self-comminution, with the generation of
excessive fines, may influence the design of the height of draw in a caving op-
eration and the layout and design of transport and handling facilities in a stoping
operation.
In some cases, a particular structural geological feature or rock mass property
may impose a critical mode of response to mining, and therefore have a singular
influence on the appropriate mining method. For example, major continuous faults,
transgressing an orebody and expressed on the ground surface, may dictate the ap-
plication of a specific method, layout and mining sequence. Similar considerations
apply to the existence of aquifers in the zone of potential influence of mining, or
shattered zones and major fractures which may provide hydraulic connections to wa-
ter sources. The local tectonic setting, particularly the level of natural or induced
seismic activity, is important. In this case, those methods of working which rely at
any stage on a large, unfilled void would be untenable, due to the possibility of local
instability around open stopes induced by a seismic event. A particular consequential
risk under these conditions is air blast, which may be generated by falling stope wall
rock.
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