Seismic tomographic imaging has been applied to determine the blast
induced damage zones in bauxite pillars in underground mining. At the
retreating stage, when pillar dimensions are reduced from 8 x 8 m to
approximately 5 x 5 m, the' seismic tomographies reveal, through
measurements of the P-wave velocities which is a measure of the degree
of fracturation of the pillar rock mass, the presence of three zones within
each pillar. In the external zone, where velocities recorded are below 750
m/s. extensive pillar damage occurs which is attributed to both the growth
and coalescence of pre-existing cracks and even the creation of new
cracks in the rock mass. This effect is in agreement with literature, when
considering the peak particle velocity as a criterion for rock failure in
tunnels. In the intermediate zone, where velocities range between
7S0-1000 m/s, damage to a much lesser extent occurs which is due
mostly to growth of pre-existing cracks and widening of their apertures.
The inner zone, where velocities exceed 1000 m/s, is considered to be
unaffected by blasting. Simple remedial measures of low cost, which are
based on blasting mechanics are proposed to minimise the blast induced
damage zone and consequently the transition zone.