EIA-REP-325d-12 _BB_ - 201.09.14.pdf - SAHRA

sudden and a ration above 3 indicates that the failure will be more gradual, meaning the signs of pillar failure will
precede the actual failure.
To date no pillar has been known to have failed in South Africa with a width to height ratio of above 3,75.
The safety factor that is used to calculate a stable pillar geometry for underground bord and pillar mining is
derived by dividing the pillar strength by pillar load. Because it is a safety factor, there will statistically be a certain
probability that a given pillar with a certain safety factor will fail. The probability of failure is a function of the
safety factor. The probability of failure for a safety factor of 2 is 0,0006%, indicating that less than 1 pillar would
fail in 1 000 000 (one million). The probability of pillar failure for different safety factors are illustrated below:
Table 4: Probability of pillar failure for a given safety factor
EIA-REP-325d-11
October 2012
SAFETY FACTOR PROBABILITY OF FAILURE (%)
2 0.0006
1.8 0.02
1.5 0.5
1.0 50
0.8 99.34
0.6 99.4
The Safety Factor formula is based on the assumption of the Tributary Area Theory (TAT) which assumes that:
• Each pillar carries the weight of overburden directly above it;
• The panel width exceeds the depth to the workings; and
• All pillars in the panel are uniform size.
The biggest concern undermining watercourses and wetlands is the subsidence or displacement that may occur.
Subsidence can be described as a downward movement of the surface that overlies an underground excavation,
or adjacent to the surface excavation.
The effect of subsidence or displacement can be divided into various types of subsidence namely:
Tilt:
During active subsidence tilting may cause for example buckling of the building. Windows can crack and doors
can be stuck. After the area has stabilized the structure will return to the vertical. In the case of watercourses and
wetlands drainage or inlet course might be slightly affected.
Strain:
Two types of strain can be identified, tensile strain and compressive strain. Tensile strain can cause elongation of
a piece of land whilst compressive strain can cause the surface to buckle.
Horisontal displacement:
This type of displacement can cause a section of land to be displaced with x amount of meter. This type of
displacement usually form cracks on the edges.
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