Page 361 - Rock Mechanics For Underground Mining

MATERIALS AND TECHNIQUES

Figure 11.29 Some support mech- Some of the support mechanisms developed by shotcrete on the peripheries of
anisms developed by shotcrete: (a) a excavations are illustrated in Figure 11.29. The support functions, modes of failure and
single block; (b) a beam anchored by
methods of design of shotcrete as a component of hard rock support and reinforcement
bolts; (c) a roof arch; (d) a closed ring
systems are discussed by Holmgren (2001) and by Kaiser and Tannant (2001). Hoek
(after Brown, 1999b).
et al. (1995) provide a set of detailed recommendations for the use of shotcrete in a
range of rock mass conditions likely to be encountered in hard rock mining.
Shotcrete is prepared using either the dry-mix or the wet-mix process. In the dry-
mix process, dry or slightly dampened cement, sand and aggregate are mixed at the
batching plant, and then entrained in compressed air and transported to the discharge
nozzle. Water is added through a ring of holes at the nozzle. Accurate water control
is essential to avoid excessive dust when too little water is used or an over-wet mix
when too much water is added. In the wet-mix process, the required amount of water
is added at the batching plant, and the wet mix is pumped to the nozzle where the
compressed air is introduced. A comparison of the dry- and wet-mix processes is
given in Table 11.2. Until the last decade dry-mix method was more widely used,
mainly because the equipment required is lighter and less expensive, and because the
dry material can be conveyed over longer distances, an important advantage in mining
applications. However, wet-mix methods have important advantages for underground
mining applications in terms of reduced dust levels, lower skill requirements and the
need for less equipment at the application site. They have now become the industry
standard (Brown, 1999b, Spearing, 2001).
Shotcrete mix design is a difficult and complex process involving a certain amount
of trial and error. The mix design must satisfy the following criteria (Hoek and Brown,
1980):
(a) Shootability – the mix must be able to be placed overhead with minimum re-
bound.
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