Page 455 - Rock Mechanics For Underground Mining

LONGWALL MINING IN HARD ROCK

hydraulic props should be installed with an initial or pre-set load. In order to maintain
the integrity of highly discontinuous hangingwalls, it may become necessary to in-
crease the areal coverage by supplementing the prop and/or pack support close to
the face with head boards, nets, link beams, cables or sprayed liners (Daehnke et al.,
2001).
The stope support design criteria proposed by Daehnke et al. (2001) include the
height of the potential instability; quasi-static and dynamic stope closure rates; the
compressive hangingwall stresses; discontinuity spacing, orientation and interface
properties; the effect of support length or stoping width; the anticipated consistency
of support performance; areal coverage; support element spacing; and the zone of
support influence. The stope support design methodology generally used in South
African gold and platinum mines uses a tributary area concept applied to the stope
hangingwall. A given weight of rock, determined by the area in the plane of the
reef and the height of a possible fall, is divided between a fixed number of support
elements. This approach is analogous to the tributary area theory of pillar design
discussed in section 13.3. For static or rock fall conditions, the maximum potential
tributary area that can be supported by a single support unit, A T ,isgiven by

A T = F/ gb (15.3)

where
F = maximum support unit load (N);
−3
= rock density (taken as 2700 kg m );
g = acceleration due to gravity; and
b = height of instability (m).
For a comparable situation under dynamic or rockburst conditions, the support unit
must be able to absorb both the kinetic and potential energy of the rock mass to be
supported. An energy-based calculation allows the maximum tributary area of a single
support unit to be expressed as a function of the unit’s energy absorption capacity and
the height of instability. For the example illustrated in Figure 15.6, the energy to be
absorbed is given by

2
1
E a = mv + mgh (15.4)
2
where
m = bA T = mass of rock to be supported by the unit;
−1
v = rock ejection velocity (often taken as3ms );
h = hangingwall displacement during the dynamic event (e.g. 0.2 m); and
E a = energy absorption capacity of the support unit (J).
Equation 15.4 may be re-arranged to give

1 2
A T = E a / b v + gh (15.5)
2
In their design approach, Daehnke et al. (2001) also use the zone of support in-
fluence, defined as the lateral extent of the vertical stress profile induced in the
hangingwall beam by a loaded support unit. Here, the term lateral extent refers to
the distance between the edge of the support unit and the point at which the stress

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