Page 362 - Rock Mechanics For Underground Mining

ROCK SUPPORT AND REINFORCEMENT

Table 11.2 Comparison of wet- and dry-mix shotcreting processes (after Spearing, 2001).

Wet Process Dry process
Little dust Considerable dust
Low maintenance cost High maintenance cost
High capital cost Low capital cost
Low rebound, typically about 5% to 10% High rebound, usually more than 25%
Moderate to high placement rate, between
3
3
4 and 25 m /hr Low to moderate placement rate, up to 6 m /hr
Low transport distance, up to 200 m High transport distance
Moderate to high placed quality Moderate placed quality

(b) Early strength – the mix must be strong enough to provide support to the ground
at ages of a few hours.
(c) Long-term strength – the mix must achieve a specified 28 day strength with
the dosage of accelerator needed to achieve the required shootability and early
strength.
(d) Durability – adequate long-term resistance to the environment must be achieved.
(e) Economy – low-cost materials must be used, and there must be minimum losses
due to rebound.
A typical basic mix contains the following percentages of dry components by weight:

cement 15–20%
coarse aggregate 30–40%
fine aggregate or sand 40–50%
accelerator 2–5%
The water : cement ratio for dry-mix shotcrete lies in the range 0.3–0.5 and is adjusted
by the operator to suit local conditions. For wet-mix shotcrete, the water : cement
ratio is generally between 0.4 and 0.5.
The efficacy of the shotcreting process depends to a large extent on the skill of the
operator. The nozzle should be kept as nearly perpendicular to the rock surface as
possible and at a constant distance of about 1 m from it. A permanent shotcrete lining
is usually between 50 mm and 500 mm thick, the larger thicknesses being placed
in a number of layers. The addition of 20–50 mm long and 0.25–0.8 mm diameter
deformed steel fibres, or plastic fibres, has been found to improve the toughness,
shock resistance, durability, and shear and flexural strengths of shotcrete, and to
reduce the formation of shrinkage cracks. Fibre-reinforced shotcrete will accept larger
deformations before cracking occurs than will unreinforced shotcrete; after cracking
has occurred, the reinforced shotcrete maintains its integrity and some load-carrying
capability. However, fibre-reinforced shotcrete is more expensive and more difficult
to apply than unreinforced shotcrete.

11.6.5 Wire mesh
Chain-link or welded steel mesh is used to restrain small pieces of rock between
bolts or dowels, and to reinforce shotcrete. For the latter application, welded mesh is
preferred to chain-link mesh because of the difficulty of applying shotcrete satisfac-
torily through the smaller openings in chain-link mesh. For underground use, weld
mesh typically has 4.2 mm diameter wires spaced at 100 mm centres. In some mining
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