Page 335 - Rock Mechanics For Underground Mining

ROCK–SUPPORT INTERACTION ANALYSIS

Concrete and shotcrete may creep as they cure, as may grouted rock bolts and dow-
els. The support systems with the poorest stiffness characteristics are those using
intermittent blocked steel or timber sets. Even if well installed, timber blocking pro-
vides a very flexible element in the system. Steel sets also suffer from the disadvantage
that they often fail by sideways buckling.
From these considerations of rock–support interaction mechanics, it is possible
to develop a set of principles to guide support and reinforcement practice. These
principles are not meant to apply to the case of providing support for the self-weight
of an individual block of rock, but to the more general case in which yield of the rock
mass surrounding the excavation is expected to occur.

(a) Install the support and reinforcement close to the face soon after excavation. (In
some cases, it is possible, and advisable, to install some reinforcement before
excavation. This case of pre-placed reinforcement or pre-reinforcement will be
discussed in section 11.4.)
(b) There should be good contact between the rock mass and the support and rein-
forcement system.
(c) The deformability of the support and reinforcement system should be such that
it can conform to and accommodate the displacements of the excavation surface.
(d) Ideally, the support and reinforcement system should help prevent deterioration
of the mechanical properties of the rock mass with time due to weathering,
repeated loading or wear.
(e) Repeated removal and replacement of support and reinforcing elements should
be avoided.
(f) The support and reinforcement system should be readily adaptable to changing
rock mass conditions and excavation cross section.
(g) The support and reinforcing system should provide minimum obstruction to the
excavations and the working face.
(h) Therockmass surrounding theexcavationshould be disturbed as little as possible
during the excavation process so as to conserve its inherent strength.
(i) For accesses and other infrastructure excavations under high stress conditions,
support and reinforcement performance can be improved by “closing the ring”
of shotcrete or a concrete lining across the floor of the excavation.

11.3 Rock–support interaction analysis

In section 7.6, a solution was given for the radius of the yield zone and the stresses
within the yield zone formed around a circular excavation in massive, elastic rock
subjected to an initial hydrostatic stress field. Extensions of analyses of this type
to include more realistic rock mass behaviour and to include the calculation of dis-
placements at the excavation periphery, can be used to obtain numerical solutions to
rock–support interaction problems.
Intheaxisymmetricproblemconsideredinsection7.6andillustratedinFigure7.20,
let the rock mass have a Coulomb yield criterion in which peak strength coincides with
yield and the stress–strain behaviour is as shown in Figure 11.5. Note that dilatancy
accompanies post-peak deformation of the rock mass. As before, the limiting states
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