| Summary: | PhD, Faculty of Engineering and the Built Environment, University of the Witwatersrand, 2010 === The aim of this investigation was to provide a proper design procedure for
Merensky crush pillars, based primarily on underground measurements. Three
sites with a variety of geotechnical conditions were selected. An interaction
between the pillars and the rock mass around the stopes was shown by the
literature, relevant laboratory tests and numerical modelling. During the
investigations, nonlinear rock behaviour was observed at one of the sites. Further
studies revealed that nonlinear behaviour also occurred in samples extracted
from high stress conditions at the other sites, but the rock mass was not
nonlinear at these sites. A methodology for determining stress from strain
measured in nonlinear rock was established.
The research also established that there is an approximately linear relationship
between peak pillar strength and w/h ratio at ratios between about 1.2 and 8. The
so called ‘squat’ effect is not observed because pillar failure is not contained
within the pillar but extends into the foundations. A linear peak pillar strength
formula was established from back analyses of underground pillar failures and
was confirmed by numerical modelling. Pillar behaviour was established from
underground measurements on one stability pillar and six crush pillars, which
included peak and residual strengths. Also, stable and unstable loading
conditions were established from an analysis of pillar bursts and the minimum
strata stiffness for stable pillar failure was determined. This stiffness is only
achieved near the advancing face and pillars that fail in the back areas are likely
to burst. For this reason, pillar design needs to include the peak strength as large
pillars may be too strong and fail in the back area. The residual strength also
needs to be considered as the load-bearing capacity of these pillars needs to
satisfy the criterion of 1 MPa across the stope to prevent back-breaks. This
translates into a pillar stress of between 8 MPa and 13 MPa if the pillar lines are
spaced 30 m apart. The peak and residual requirements have been included in a
design chart, and the relationship between w/h ratio and residual strength is
provided in a graph for easy design.
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