Civil Engineering Reference
In-Depth Information
3.8.3
Conventionally reinforced shotcrete versus steel fibre shotcrete
Differences in the production process
The production process of a shotcrete layer reinforced with two layers of mesh consists of
the following working steps (ignoring further possible support elements like spiles, pipe
screen, top heading foot piles):
1. Sealing of the exposed sides (first shotcrete layer).
2. Installation of support arches.
3. Fixing of the first layer of mesh.
4. Spraying of the arch and the second layer of shotcrete.
5. Drilling and setting of the rock bolts.
6. Installation of the second layer of mesh.
7. Completion of the shotcrete layer.
In contrast, the process to produce a steel fibre shotcrete layer consists of the following
working steps:
1. Sealing of the exposed sides (first shotcrete layer).
2. Installation of support arches.
3. Spraying of the arch and the second layer of steel fibre shotcrete.
4. Drilling and setting of the rock bolts.
5. Completion of the steel fibre shotcrete layer.
The installation of the layers of mesh is a time-consuming manual activity, which can
scarcely be automated. It should also be considered that at least the first layer of mesh is
normally fixed in the first field next to the face, which is the immediate danger zone.
When steel fibre shotcrete is used, mesh does not have to be fixed, leading to an effective
reduction of the time taken to install support and thus the achievement of altogether faster
advance rates. If a shotcrete manipulator is used, only the setting of the arches demands
that the miners work in the immediate danger area at the face.
Quality differences. The reinforcing mesh represents an obstruction of the process of ap-
plying and compacting the shotcrete. Aggregate grains bounce from the steel and increase
the production costs as rebound. Voids can form in the shotcrete structure behind the
reinforcing bars, called the spraying shadow, and vibration of the mesh leads to a danger
of loosening and impairs the bonding between layers of shotcrete. Altogether, the result-
ing inhomogeneous shotcrete application leads to inconsistent strengths. Another negative
effect is the water-permeability, which is particularly significant for single-layer applica-
tions [147].
When steel fibre shotcrete is used, these diminishing effects on quality and strength no
longer apply. The danger of spraying shadows only applies to spraying around the arches.
If the nozzle is optimally guided, however, this can be minimised or even completely
prevented.
Finally, occurrence of loosening of the layer due to vibration from blasting should also
be mentioned. Mesh reinforcement leads to stronger connection between loosened and
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