Environmental Engineering Reference
In-Depth Information
more common in deeper mines and tunnels, especially those about 2000 to 3000 ft in
depth, although they have been noted in quarries and shallow mines. They can occur
immediately after the opening is made or at any future time. Their activity has been cor-
related with earthquakes (Rainer, 1974).
Occurrence
There are a number of conditions particularly favorable to the occurrence of high
in situ
stresses and rock bursts including:
Massive rock such as granite or gneiss, with few joints, located at substantial
depths.
●
Geologically complex conditions characterized by rock fracture and anisotropy,
such as existing where highly competent, brittle rocks are interbedded with less
competent, relatively plastic rocks.
●
Concentrations of large stresses accumulated near fractures. Several conditions
that create forces to cause rock bursts in tunnels and mines are illustrated in
●
6.6.4
Anticipating Unstable Conditions
Instability Factors
The extent of rock instability in excavation from loosening and overall displacement is a
function of the nature of the jointing, foliation, and schistosity (Paulmann, 1966), the mag-
nitude of the residual stress (Denkhaus, 1966), and the significance of the
in situ
stress in
terms of rock strength (Hawkes, 1966).
Stress Ratio Criteria
Deere (1966) stated that tensile stress will not prevail in the vicinity of an opening unless
the ratio of lateral to vertical stress falls outside the range of 1
/
3 to 3 for circular or
FIGURE 6.65
Some geologic conditions creating forces resulting in rock bursts. (a) Load of overlying rock in deep tunnel in
brittle granite causes semiplastic movement in a weak lens of mica-rich schist. Rock burst (hatched) occurs in
tunnel at point closest to lens. (b) An igneous dike which was forcefully injected into a hard quartzite is
accompanied by residual stresses promoting a rock burst in the wall of the tunnel. (c) Forces that have caused
elastic strain in strong igneous rock are directed toward the tunnel opening to cause a rock burst. (From
Wahlstrom, E. C.,
Tunneling in Rock
, Elsevier, New York, 1973. With permission.)
