ISSN : 2277-1328 (Online) - ISRM

4 ISRM (India) Journal
ratio, and therefore the material retained to bear cave loads
changes almost exponentially.
Fig. 1 : An example relation between crosscut spacing and
extraction ratio (assuming all other dimensions are fixed)
for a particular block cave extraction level design
The capacity of this ‘system’ of pillars is further diminished
by many factors. For deep caves critical considerations
include:
• Scale effects; the pillar scale strength is a small fraction
of the laboratory scale strength, probably closer to
the rock mass strength for most jointed rocks suitable
for caving.
• The very high extraction ratio. This well understood,
but sometimes not fully accounted for. Many models
do not simulate the extraction level excavations,
instead using an ‘equivalent material’ approach that
approximates the presence of extraction level tunnels
using a softer material for the entire horizon. The
problem with this approach is that it cannot account
for the loss of confinement that occurs when the
drawpoints and drawbells are mined nor the real impact
on pillar loads. As the confinement is much reduced,
so too is the strength of pillars even if there is no prior
damage and if this is not recognised the capacity of
the extraction level to bear load will be overestimated.
At even minor levels of rock mass damage (i.e., prepeak)
induced on the extraction level during
undercutting,the effects of the reduction in confinement
when drawpoints are extracted will be observable.
• The centroid between extraction level crosscuts is not
the location of the most important extraction level
pillars cores. The real extraction level drawpoint pillar
centroid is infact just a few metres from the extraction
level crosscut, within the zone of influence of the
extraction level crosscut if it is already in place as the
undercut is mined.
• Damage at the walls of pillars acts to diminish the
‘effective’ area of the pillar even though the ‘future’
pillar core may experience only minor or no damage.
Poorly conditioned models with an inadequate type or
insufficient number of elements will not capture this
effect, nor will models with over-sized extraction steps.
The most subtle of all of these points may be the loss of
confinement that occurs on the extraction level when the
drawpoints and drawbells are mined. It can explain why
cave loads can still affect pillars which show only minor
or moderate damage prior to extraction of the drawbells.
The reason for the initial lack of damage (i.e., in advance
of the undercut), will usually be the positive effects of
confinement up until this point and the large crosscut
spacing. Once the undercut has passed and drawbells
and drawpoints are extracted, the loss of confinement
can leads to an immediate loss in capacity. Since the
cave loads havenot developed at that time, this capacity
reduction may not even be noticed.
Too often it is assumed that as the peak strength has not
been exceeded prior to undercutting that the same confined
triaxial strength remains after drawbell and drawpoint
extraction, but the loss of confinement means that the
effective capacity of a pillar from that point forward will be
much reduced, apart from the effects of the very high
extraction ratio.
The effect of both combined is a step change reduction in
capacity of the horizon to bear loads after the draw points
and draw bells are extracted; the pillars are essentially,
or almost only uniaxially loaded from that point on and
the strength of a pillar, or its capacity can be halved or
more even though it has been ‘destressed’ by undercutting
and even though the rock is not in a residual state.
These considerations lead to several pillar scenarios that
must therefore be considered in any deep block cave:
• Low deformation should be expected where the rock
mass is sufficiently strong, only very minor or no
damage develops on the extraction horizon before the
undercut has passed, and the subsequent drop in
confinement and loss of capacity (when the
construction is completed) is small enough that future
cave loads will be managed despite the high extraction
ratio. The key positive factors are low enough cave
load, the extraction ratio and the peak rock mass scale
compressive strength.
• Higher deformation should be expected at low enough
strengths, or high enough abutment stresses,
thatdamage on the extraction level at future pillar core
locations occur during undercutting. The subsequent
losses of confinement and the increasing extraction
ratio does sufficiently reduce the capacity of the
extraction horizon to bear future cave loads.The actual
cave loads will determine the final state of damage.
• In some circumstances, the rock may be so weak
that it is damaged sufficiently at the abutment that a
Volume 1 No. 2 July 2012