A MECHANISM FOR HIGH WALL-ROCK VELOCITIES IN ROCKBURSTS

Considerable evidence has been reported for wall-rock velocities durin g rockbursts in deep gold mines that are substantially greater than gr ound velocities associated with the primary seismic events. Whereas va ried evidence suggests that slip across a fault at the source of an ev ent generates nearby particle velocities of, at most, several m/s, num erous observations, in nearby damaged tunnels, for instance, imply wal l-rock velocities of the order of 10 m/s and greater. The common obser vation of slab buckling or breakouts in the sidewalls of damaged excav ations suggests that slab flexure may be the mechanism for causing hig h rock ejection velocities. Following its formation, a sidewall slab b uckles, causing the flexure to increase until the stress generated by flexure reaches the limit S that can be supported by the sidewall rock . I assume here that S ic; the uniaxial compressive strength. Once the flexural stress exceeds S, presumably due to the additional load impo sed by a nearby seismic event, the slab fractures and unflexes violent ly. The peak wall-rock velocity nu thereby generated is given by nu = (3 + 1-nu 2/2)(1/2) S/root rho E for rock of density rho, Young's modu lus E, and Poisson's ratio nu. Typical values of these rock properties for the deep gold mines of South Africa yield nu = 26 m/s and fbr esp ecially strong quartzites encountered in these same mines, nu > 50 m/s . Even though this slab buckling process leads to remarkably high ejec tion velocities and violent damage in excavations, the energy released during this failure is only a tiny fraction of that released in the p rimary seismic event, typically of magnitude 2 or greater.