Strong ground motion and site response in deep South African mines

The most widely used support design criterion for rockburst-prone mines takes into account the kinetic and gravitational potential energy of keyblocks1,2. The criterion for effective rockburst-resistant support systems is to absorb the kinetic and potential energy2 associated with the hangingwall moving with an initial velocity of 3 m/s and brought to rest within 0.2 m of downward movement3. The kinetic energy is proportional to the square of the Peak Particle (or ground) Velocity (PPV). The dynamic resistance required is dominated by the kinetic energy component for a PPV greater than 1 m/s. Therefore, a comparatively small decrease in PPV results in a large decrease in the energyabsorption requirements of a rockburstresistant support system. As a result, a decrease in PPV requirement would allow for considerably lower energy-absorption demands on rockburst-resistant support systems. Conversely, an increase in PPV requirements would considerably increase the support requirements. Considering that mine-wide seismic systems are installed at most rockburst-prone mines, it is expected that these systems would provide sufficient data for reliable PPV estimates in stopes. The use of such data for obtaining PPV estimates is suggested by Jager and Ryder (pp. 25, 26, 303 and 304)3. A number of previous studies on peak particle velocities and site response have shown that the PPV on the skin of the excavations may be larger by four to ten times the PPV at a point in solid rock4,5 at a similar distance from the source. In addition, points less than a metre apart show differences in amplitude and phase6, which can only be accounted for by large strain across fractures. A simulated rockburst experiment was conducted on the wall of an underground tunnel. PPVs with maximum of 3.3 m/s and ejection velocities with maximum of 2.5 m/s were measured on the blasting wall7. The existing support comprised rockbolts. The mesh and lacing, comprising part of the original support of the tunnel wall, has been removed to allow ejection of the rocks during the experiment. The rockbolts were affected but not even a single rockbolt failed8 despite the severe dynamic loading. A similar effect was also observed in numerous rockburst investigations9. This can be explained by the rapid attenuation of the PPVs from the skin to the more competent rock mass in the solid. On the other hand, a severe failure of the rockbolts is also quite frequently observed10. Most probably, in these cases the stronger site effect, due to fractured ground, was contributing to the damage5. Strong ground motion and site response in deep South African mines