Page 260 - Rock Mechanics For Underground Mining

9 Excavation design in

blocky rock

9.1 Design factors

A blocky or block-jointed rock mass presents a more complex design problem than the
cases considered previously. The complexity arises from either the number (greater
than two) of joint sets which define the degree of discontinuity of the medium, or the
presence of a discrete structural feature transgressing such a simply jointed system
as a cross-jointed, stratified mass. The condition that arises in these types of rock
masses is the generation of discrete rock blocks, of various geometries defined by
the natural fracture surfaces and the excavation surface, as illustrated in the simple
schematic of Figure 9.1. Because the blocks exist in the immediate boundary of an
excavation whose surface has been subject to the removal of support forces by the
mining operation, uncontrolled displacement of a single block or collapse of the block
assembly is possible in the prevailing gravitational and local stress fields.
The issues to consider in the design of an opening in a blocky medium are a natural
extension of those proposed for the structurally simpler media considered previously.
That is, it is necessary initially to determine the likelihood of induced fracture in the
rock mass, in the total stress field after mining. For continuous features, such as faults
or bedding planes which persist over dimensions exceeding those of the excavation,
it is necessary to examine the possibility and consequences of slip under excessive
shear stress. Also, since joints have effectively zero tensile strength, a jointed rock
mass is unequivocally a no-tension medium. Any part of a jointed or blocky medium
which is notionally subject to tensile stress will, in practice, de-stress. The process
of de-stressing a discontinuum implies loss of control, and possibly local collapse of

Figure 9.1 Generation of a discrete
prism in the crown of an excavation
by the surfaces of defined geological
features and the excavation boundary.

242