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Shotcrete support
Introduction
The use of shotcrete for the support of underground excavations was pioneered by the
civil engineering industry. Reviews of the development of shotcrete technology have
been presented by Rose (1985), Morgan (1993) and Franzén (1992). Rabcewicz
(1969) was largely responsible for the introduction of the use of shotcrete for tunnel
support in the 1930s, and forthe development of the New Austrian Tunnelling
Method for excavating in weak ground.
In recent years the mining industry has become a major user of shotcrete for
underground support. It can be expected to make its own contributions to this field as
it has in other areas of underground support. The simultaneous working of multiple
headings, difficulty of access and unusual loading conditionsare some of the
problems which are peculiar to underground mining and which require new and
innovative applications of shotcrete technology.
An important area of shotcrete application in underground mining is in the support of
'permanent' openings such as ramps, haulages, shaft stations and crusher chambers.
Rehabilitation of conventional rockbolt and mesh support can be very disruptive andexpensive. Increasing numbers of these excavations are being shotcreted immediately
after excavation. The incorporation of steel fibre reinforcement into the shotcrete is
an important factor in this escalating use, since it minimises the labour intensive
process of mesh installation.
Trials and observations suggest that shotcrete can provide effective support in mild
rockburst conditions(McCreath and Kaiser, 1992, Langille and Burtney, 1992).
While the results from these studies are still too limited to permit definite conclusions
to be drawn, the indications are encouraging enough that more serious attention will
probably be paid to this application in the future.
Shotcrete technology
Shotcrete is the generic name for cement, sand and fine aggregate concretes which are
appliedpneumatically and compacted dynamically under high velocity.
Dry mix shotcrete
As illustrated in Figure 1, the dry shotcrete components, which may be slightly predampened to reduce dust, are fed into a hopper with continuous agitation.
Compressed air is introduced through a rotating barrel or feed bowl to convey the
materials in a continuous stream through the delivery hose. Water is added tothe mix
at the nozzle. Gunite, a proprietary name for dry-sprayed mortar used in the early
1900's, has fallen into disuse in favour of the more general term shotcrete.
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Figure 1: Simplified sketch of a typical dry mix shotcrete system. After Mahar et al
(1975).

Figure 2: One typical type of wet mix shotcrete machine. After Mahar et al (1975).
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Wet mix shotcrete
In this case the shotcrete components and the water are mixed (usually in a truck
mounted mixer) before delivery into a positive displacement pumping unit, which
then delivers the mix hydraulically to the nozzle where air is added to project the
material onto the rock surface.
The final product of either the dry or wet shotcrete process is very similar. The dry
mixsystem tends to be more widely used in mining, because of inaccessibility for
large transit mix trucks and because it generally uses smaller and more compact
equipment. This can be moved around relatively easily in an underground mine
environment. The wet mix system is ideal for high production applications in mining
and civil engineering, where a deep shaft or long tunnel is being driven and whereaccess allows the application equipment and delivery trucks to operate on a more or
less continuous basis. Decisions to use the dry or wet mix shotcrete process are
usually made on a site-by-site basis.
Steel fibre reinforced micro silica shotcrete
Of the many developments in shotcrete technology in recent years, two of the most
significant were the introduction of silica fume, used as a...