Rockbursts mud and plastic

Abstract

The gold mines in South Africa are the deepest in the world and rockbursts (including tunnel collapse) are a common occurrence, endangering life, causing damage to rock support structures and resulting in costly delays. Rockbursts are violent outbursts of broken rock and concrete in tunnels primarily caused by the accumulation of elastic strain energy in the tunnel walls but often triggered by seismic events. During a rockburst hundreds of tonnes of pulverized rock may be expelled into openings accompanied by a sudden sharp sound, an air blast, and a dense cloud of dust. The air blast itself can propagate kilometers through the tunnel system and be of sufficient strength to overturn vehicles. Conventional wisdom is that rockburst activity is much less likely in tunnels with water or mud on the floor. We examine mechanisms that may explain such reduced activity using simple mechanistic models. The investigations suggest that lubrication effects due to the presence of water within cracks could inhibit, and in some cases prevent, the expulsion of rock lumps from tunnel walls. The effect of the water in a crack is to produce a suction force proportional to μω/h 0α with α ≈ 5, where h 0 is the crack thickness, ω is the frequency of the impacting earthquake wave and μ the shear viscosity of water. Thus strong suction forces occur for small cracks and high frequency impacting earthquakes. These forces may cause an almost detached rock lump to remain attached, or detach if the eruption time span is large enough and the attachment to the walls is weak enough. Estimates suggest that both scenarios are possible in practice. These results also suggest that coating the tunnel walls with moisture containing semi liquid pastes or spray on plastic materials may be effective for tunnel wall stabilization.