Rock reinforcement
76 and rock support
1 tL1 The stabilization system
When an excavation is made in a rock mass, there may be a need
to stabilize the excavation to reduce inward movement of the rock
mass. The amount of such movement will depend on the quality and
continuity of the host rock mass, and the significance of the movement
will depend on the engineering objective. For a rail transport tunnel,
there will be strict constraints on the allowable movement; for example,
the design may require the rock displacements to be constrained to 10
mm or less over 120 years. In a mine stope, there may not be any
constraints; and in the case of longwall coal mining the roof should
collapse fully until the roof is in contact with the floor for the mining
system to be effective.
The term rock reinforcement is used to indicate methods of enhancing
the rock mass strength and hence improving the ability of the rock mass
to contain the engineering excavation without deforming excessively.
Rockbolts, wire mesh and shotcrete (mortar sprayed onto the excavation
surface) are reinforcement methods because they strengthen the rock
mass. The term rock support is used to indicate methods of applying
supporting loads or displacement constraints as additional structural
elements, so that the engineering excavation retains its integrity. Steel
ribs, pre-cast concrete segmental lining and cast in situ concrete lining
are support methods.
There can be some overlap between the two terms. In the case of a thick
layer of shotcrete, for example, the shotcrete acts as a reinforcement in
the sense that it inhibits rock block rotation and weathering, but it can
also act as a structural element containing tangential stresses (Fig. 16.1,
from Wilson, 1991 I). Alternatively, the use of pre-cast segments for sup-
port also has a reinforcing effect because the rotation of rock blocks and
weathering is inhibited (Fig. 16.2, from Wilson, 1991) I. The term 'rock
stabilization' refers to the use of either method, or the two in conjunc-
' Wilson D. (1991) Bredfhmugh. Random Century Group, London, 144pp.