Update of coal pillar database for South African coal mining
JN van der Merwe, M Mathey
Following the Coalbrook disaster in 1960, research into coal pillar
strength resulted in the adoption of the concept of a safety factor for
the design of stable pillars in South African coal mining.
At the time when the original statistical analysis was performed
by Salamon and Munro in the early 1960s, 27 cases of failed pillar
workings were considered suitable for inclusion in the database of
failed pillars. Pillar failure did not stop after the introduction of the
safety factor formula by Salamon and Munro (1967). In the ensuing
years, pillars that were created before the application of the formula
deteriorated and later failed, as did ones that were created after the
introduction of the formula. This means that over time, the database
of failed pillar cases increased in size, allowing ever more reliable
analyses to be performed.
The number of failed cases in the database had grown from the
original 27 in the 1960s to 86 by 2011. All the failed cases are
contained in the updated database. The database of stable pillars,
which is also used in the derivation of strength formulae, has now
been extended from 125 to 337 cases.
The new database of intact pillar cases is more complete as it
bridges the time gap between the Salamon and Munro (1967) and the
Van der Merwe (2006) databases. The original requirements for
inclusion into the database were satisfied in the compilation of this
latest collection.
The characteristics of the original database of intact pillars did
not change in a meaningful way. The mining depth and pillar
dimensions of the new database are largely as they were in the
original database.
Time-related trends with regard to pillar dimensions and depth of
mining could not be found, indicating that the geometrical parameters
of coal mining in South Africa have not changed meaningfully in
approximately a century of mining.
The characteristics of cases in the updated database of failed
pillars does not differ substantially from the one published by Van
der Merwe (2006). The same difference between that database and
the original Salamon and Munro database, namely that the average
safety factor of the failed cases had increased dramatically, from 1.0
to 1.5, is still apparent. This may be due to the inclusion of more
failures from specific areas that exhibit a disproportionate number of
failures at higher safety factors. These areas are the Vaal Basin, Klip
River, and Free State coalfields.
The new database confirms yet again that there is no correlation
between the safety factors of failed pillars and their time of failure.
The safety factor on its own is thus not a reliable predictor of longterm
stability of pillars.
Keywords: coal pillar, stable pillar cases, pillar collapse, pillar data base.