Page 457 - Rock Mechanics For Underground Mining

LONGWALL MINING IN HARD ROCK

followed by a zone of influence and a stability analysis, considering hangingwall
failure due to buckling, shear and block rotation. This gives a maximum safe spacing
of the individual support elements. The calculations are carried out for both static
(rock fall) and dynamic (rockburst) conditions. Daehnke et al. (2001) illustrate their
methodology with a worked example. Detailed design methods are also presented by
Ryder and Jager (2002).
Tunnel support and reinforcement. As well as carrying out the functions of support
and reinforcement systems discussed in Chapter 11, the support and reinforcement
systems used in the off-reef tunnels associated with hard rock longwall mining and its
variants, may also have to be dynamically capable to alleviate the rockburst hazard.
The support and reinforcement systems must be capable of reinforcing and retaining
or holding the rock mass in place, and surface support or containment of the broken
rock at the excavation boundary under both static and dynamic loading conditions.
It must be noted that rockburst conditions may also exist in mines that are less deep
and use different mining methods than the deep level gold mines of South Africa (e.g.
Li et al., 2003, Rojas et al., 2000, Simser et al., 2002). Under these conditions, the
reinforcing elements may consist of rock bolts, cable bolts, studs and tendons, often
with yielding capability. The surface support elements may include shotcrete or other
sprayed liners, mesh, straps, wire rope lacing, fixtures of a range of types, and energy
absorbing face plates. An example of the system used in the haulage level of a South
African gold mine is shown in Figure 11.30.
As in the case of stope support, the design methodology used for dynamically
capable tunnel support and reinforcement has been based on tributary area and en-
ergy absorption concepts (e.g. Wagner, 1984, Ortlepp, 1994). Recognising that it is
not practicable to attempt to prevent fracturing and displacement in the event of a
rockburst, tunnel support and reinforcement systems are designed to yield and absorb
energy, limit the amount of deformation occurring at the tunnel boundary, and contain
the fractured rock mass at the excavation periphery (Roberts and Brummer, 1988).
It is postulated that, in a rockburst, blocks or slabs of rock will be accelerated and
ejected from the tunnel boundary. In its simplest form, the energy released by the
rock mass and required to be absorbed by the support and reinforcement system, E,
is given by
2
1
E = mv + mgh
2
where m is the mass of rock ejected from the excavation, v is the ejection velocity
and h is the height through which the rock falls in coming to rest. The ejection
velocity may be up to 10 m s −1 in extreme cases but is often taken to be more
like3ms −1 for design. The use of this approach has directed emphasis towards the
development of a range of reinforcing elements that are able to yield or displace while
retaining significant load carrying capability. In the standard design methodology,
this approach is usually applied with a tributary area assigned to each reinforcing
element (Ortlepp, 1994). However, on the basis of results obtained in a large-scale
field experiment, Haile and Le Bron (2001) questioned the validity of the tributary
area approach under the complex dynamic loading conditions resulting from mining-
induced seismicity. Li et al. (2003) suggested that momentum change should be
used as an additional design criterion. They also demonstrated the practical value
of cable bolt plates made from conveyor belt material in absorbing incident energy
439