Page 322 - Rock Mechanics For Underground Mining

ENERGY, MINE STABILITY, MINE SEISMICITY AND ROCKBURSTS

The dynamic resistance to slip, d , is taken to be described by
(10.92)
d =
d n
where
d is the coefficient of dynamic friction.
Evaluationof equations 10.91 and 10.92 indicates a‘stress drop’, given by ( s − d ),
in the transition from static to dynamic conditions on a fault subject to frictional
sliding. Stress drops of 5–10% of the static shear strength have been observed in the
laboratory. Applications of these notions of variable fault shear strength in rockburst
mechanics have been discussed by Ryder (1987) and are considered in section 15.2.
An alternative treatment of dynamic instability, due to Dieterich (1978, 1979),
Rice (1983), and Ruina (1983), among others, has been based on explicit relations
between sliding velocity and fault shear strength. The analysis involves empirically
derived expressions which describe the temporal evolution of shear resistance on a
fault surface when it is subject to a step change in shear velocity. However, successful
application of the various relations has yet to be demonstrated in practical seismic
analysis.

10.10 Characterisation of seismic events

In most cases, episodes of joint slip or rock material fracture in a rock mass result
in the radiation of some of the energy released in the form of seismic waves (Cook,
1964). With their history of concentrated research in mine seismicity, real-time, mine-
scale seismic monitoring systems have been developed in South Africa (Mendecki,
1993) and Canada (Alexander et al., 1995) mainly for the purpose of monitoring
seismicity for management of rockburst hazards. Contemporary seismic monitoring
systems record the complete waveforms resulting from the propagation of acoustic
energy from a seismic event, and the waveforms are analysed, interpreted and applied
in measuring the main parameters which characterise the event. The parameters of
interest are the location of an event and the size and strength of the source. Information
derived from the waveforms can also be used in characterising the failure mechanisms
occurring at the seismic source.

10.10.1 Seismic source location
Accurate determination of source locations relative to mining activity is essential for
spatial analysis of seismic events. The source location is calculated by assessing the
P- or S-wave arrival times at sensors in an array surrounding the volume of the rock
mass of interest in the mine. By way of example, Figure 10.26 shows a waveform
of a seismic event with the P-wave and S-wave arrivals marked. The waveform was
recorded using a triaxial accelerometer and the (International Seismic Services) ISS
XMTS software package (ISS Pacific, 1996).
Several methods can be used to calculate the location of the source hypocentre from
a set of P- and S-wave arrival times such as are shown in Figure 10.26. The earliest
and simplest was a string model, which is a three-dimensional physical analogue of
a geophone array. Scaled lengths of string are used to invert the wave arrival times
to construct the distances between geophones and the seismic source. Although the
model gives a fast estimate of the location of the seismic source, it is the least accurate
of all methods, and has been superseded by developments in computational methods.

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