Rockburts

An Introduction to Rockbursts
When any underground excavation is made in an already weak rock, it often serves to further weaken the rock above it. This is due to a combination of 1) the excavation activity itself (primarily blasting) weakening the surrounding rocks as a result of shock waves and 2) the removal of supporting rock from underneath a large mass of rock. The net effect of this,weakening of rock and also giving it a place to go, is a movement toward the tunnel from above, which wraps around the tunnel and forms wedges of material which press in on the sides of the tunnel (Terzaghi, 1946) As Figure 4 illustrates, the rock in area abcd, loosened by the excavation, is attempting to move downward, and is being resisted by friction on surfaces ac and bd. The effect of this is thetransfer of a large amount of the overburden, W1, onto the abutments of the tunnel. This tunnel will require steel supports, and they will be supporting a load equivalent to Hp, which will depend on the characteristics of the rock mass and the dimensions of the tunnel.
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Figure 4. Movement of loosened rock toward a tunnel
As was mentioned earlier, tunnels are sometimes cut with flat roofswhen excavated in strongly laminated rock, in order to take advantage of the rock's tendency to separate on those planes. However, it is rarely the case that laminated rocks are found in a pristine state with their bedding planes parallel and horizontal. Inclined bedding planes in stratified rocks pose a great problem, in that there is a great tendency for the rock to move along the bedding planesand thus slide into the tunnel, as as indicated in Figure 5, modified from Terzaghi (1946). The steep angle of the bedding planes with respect to the tunnel will result in the wedge-shaped rock body (aed) sliding into the tunnel and putting stress on the support (ac). The lateral force, P, can be estimated from the mass of the sliding rock body and the angle which it makes with the tunnelsupport. According to King (1996), the load on the tunnel and supports depends on the strike and dip of the strata, and tunnels will have to be cut more narrowly in the event of steeply inclined stratified rocks.
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Figure 5. Forces acting on a tunnel in inclined strata
In general, rocks have a high resistance to crushing. King (1996) states that the walls of a tunnel will not fail as a result ofcompression except at great depth - more than 2000 feet (600 m) for softer sandstones and more than 19,000 feet (5800 m) for the strongest rocks. However, the rocks are still under an immense amount of stress, and the rock left standing after tunnel or cavern excavation must bear a greater load than before, as illustrated in Figure 4 from Terzaghi. This point is further illustrated in Figure 6,from Herget (1988). The grid lines represent the principal plane-strain stresses around a circular tunnel after excavation. The crowding of the trajectories at the sides indicates an increase in compression, and the widening at the top and bottom indicates a decrease in compressive stress. This implies that the walls of a tunnel, not the roof, would be more susceptible to failure. This is themechanical basis of the rockburst phenomenon.
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Figure 6. Deflection of stress around a circular opening
Of all the hazards associated with mining, rockbursts are perhaps the most terrifying. A rockburst is the sudden, violent dislocation of slabs of rock in a tunnel, usually from the walls, but also potentially from the roof or even floor. Considered to be a "mining-induced seismic event," arockburst can release enormous amounts of energy, and some have been measured at 4 on the Richter scale (Jha and Chouhan, 1994) and one rockburst was recorded by a seismological station 1200 miles distant. (Obert and Duvall, 1967) The danger is obvious and quantifiable: In a three-year period in the Kolar gold-field in India, rockbursts accounted for 50% of all fatalities. (Obert and Duvall, 1967)...