Time-dependent tensile strengths of Bushveld Complex rocks and implications for rock failure around mining excavations
D Nyungu, TR Stacey
Despite observations of spalling and damage of mine excavation wall rock in
the Bushveld Complex (BC) over the passage of time, there have been very
few time-dependent or creep tests carried out in South Africa on rock, particularly
on BC rock types. The research described in this paper deals with the
investigation of stress and strain conditions influencing spalling of wall rock
in BC mine excavations, and the influence of time on the tensile strength of
several BC rock types.
Time-dependent laboratory testing of BC rocks was carried out in indirect
tension. The results show that the magnitude of the tensile strength of BC
rock types is approximately 5% of their uniaxial compressive strength
magnitudes. The average long-term uniaxial compressive strength of the BC
rocks, interpreted from the axial stress-volumetric strain graphs, is 56% of the
UCS value. The long-term tensile strength is shown to be less than 70% of the
normal tensile strength. Extension strains at tensile strength failure ranged
between 0.16 and 0.21 millistrain. Values corresponding with the long-term
tensile strength are less than 70% of this range, namely, 0.11 to 0.15
millistrain. These results represent new knowledge, since such rock testing
and analysis does not appear to have been carried out previously on BC rock
types.
Elastic numerical modelling was carried out to illustrate the extents of
tensile stress zones and extension zones around of typical BC mine
excavations. The models showed that large zones of extension strain can
occur around BC excavations, and that the magnitudes of the extension strain
can substantially exceed the critical values determined from the laboratory
testing. The implication of this is that there are substantial zones surrounding
BC mine excavations that will be prone to time-dependent spalling conditions
and perhaps more significant failure.
Keywords: tensile strength, creep, time-dependent failure, rock strength, Bushveld
Complex.