24-06-2013, 04:11 PM
Shotcrete support
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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 for the 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 conditions are 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 and
expensive. 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.
Shotcrete technology
Shotcrete is the generic name for cement, sand and fine aggregate concretes which are
applied pneumatically 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 to the 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.
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 cementitious admixture, and
steel or polypropylene fibre reinforcement.
Silica fume or micro silica is a by-product of the ferro silicon metal industry and is an
extremely fine pozzolan. Pozzolans are cementitious materials which react with the
calcium hydroxide produced during cement hydration. Silica fume, added in
quantities of 8 to 13% by weight of cement, can allow shotcrete to achieve
compressive strengths which are double or triple the value of plain shotcrete mixes.
The result is an extremely strong, impermeable and durable shotcrete. Other benefits
include reduced rebound, improved flexural strength, improved bond with the rock
mass and the ability to place layers of up to 200 mm thick in a single pass because of
the shotcrete's 'stickiness'. However, when using wet mix shotcrete, this stickiness
decreases the workability of the material and superplaticizers are required to restore
this workability.
Design of shotcrete support
The design of shotcrete support for underground excavations is a very imprecise
process. However, one observation, which is commonly made by practical engineers
with years of experience in using shotcrete underground, is that it almost always
performs better than anticipated. There are many examples (very few of which are
documented) where shotcrete has been used as a last act of desperation in an effort to
stabilise the failing rock around a tunnel and, to most people's surprise, it has worked.
The complex interaction between the failing rock mass around an underground
opening, and a layer of shotcrete of varying thickness with properties which change
as it hardens, defies most attempts at theoretical analysis. It is only in recent years,
with the development of powerful numerical tools, that it has been possible to
contemplate realistic analyses, which will explore the possible support-interaction
behaviour of shotcrete. A clear understanding of shotcrete behaviour will require
many more years of experience in the use of and in the interpretation of the results
obtained from these programs. It is also important to recognise that shotcrete is very
seldom used alone and its use in combination with rockbolts, cablebolts, lattice
girders or steel sets further complicates the problem of analysing its contribution to
support.