The magnitude and orientation of the in-situ
stress helps define the “direction and dimension of the undercut, fragmentation
of rock mass, caving rate, and the geometry and strength properties of
In the case of soft ground, it is recommended
to develop away from the principle stress as its orientation on the sides or
the rear of a cave opening is very important. Creating an undercut near the
principle stress can improve caveability and fragmentation, however, it could
also cause significant damage or rockbursts. Moreover, the application of
horizontal stress on the long face of a cave opening can lead to failure,
however, the opposite could be said for a circular cave opening.
Uniaxial Compressive Strength (UCS)
The UCS is regarded as one of the most
important parameters of rock masses. It is usually examined when looking at the
various rock classification systems. Characteristics of rock masses such as
weathering, micro-cracks, density, porosity, and internal fractures all influence
its respective UCS. As the UCS of a rock mass increases, its caveability
decreases and vice versa.
Before conducting a caving operation, during
the feasibility stage it is essential that the location of surface water paths
and storages, rainwater drainage, and groundwater hydrology are predetermined.
The reason for this is that any water located in potential caving zones can
enhance the caving process either by reducing friction around the joints or through
the effects of increased pore water pressure.
These properties are used to describe
discontinuities, specifically joint aperture, persistence, orientation,
filling, roughness, and spacing. When it comes to joint filling and persistence,
they have a major influence on rock mass caveability due to their role in
determining rock mass strength. In addition, factures that contain low shear
resistance, suitable incline and are closely spaced facilitate better
caveability of rock masses.
Furthermore, the orientation of discontinuities
is regarded as one of the most significant problems when it comes to rock mass caveability
analysis. Joints that are situated perpendicular to the direction of draw (usually
horizontal joints) encourage cave propagation and its rate at the mobilized
zone greatly exceeds that of the production draw rate. Joints that are situated
parallel to the direction of draw (usually vertical joints) are less likely to accommodate
cave propagation and there is little to no displace of rock mass above the
It is defined as the rate of upward advance of
the yield zone. Since block caving has low selectivity, the caving rate is the
only possible way of delaying dilution entry into the broken ore in the cave opening.
As a result, by controlling the caving rate, it is possible to greatly
influence the degree of caving and fragmentation in a rock mass. Furthermore,
characteristics of rock masses such as quality, induced stresses, and rate of
development of joints are all functions of the caving rate.
The block height, which is the vertical
distance between mining levels, is dependent on several factors – namely geometry
of the orebody, degree of fragmentation, and properties of the cap rock. It is
one of the key parameters that affect rock mass caveability. Secondary
fragmentation of caved material takes place due to the constant exertion of pressure
and stresses on the ore as it is drawn through the column. This, in combination
with the caveability of ore and cap rock, help determine the optimal block height
of an orebody that is most suited to the project at hand.
Undercut refers to the region that is initially
mined in order to initiate the caving process as a result of failure. Having a
poor undercut can lead to disturbances in the form of pillars, piping or large
blocks that halt the initiation of the caving process. The orientation of the
principle stress and the direction of advance of the undercut are related in
the sense that they influence the degree of abutment stresses. Therefore, in
order to reduce stresses on the back of the cave opening, it is crucial that
the undercut is extracted in the same direction of the maximum principal stress
as shown in Figure 2.