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Sprayed concrete is used to provide temporary support for tunnels through weak rock, stabilising the rock by filling crack and openings. It also seals the surface, preventing water ingress. Sprayed concrete is also used in the New Austrian Tunnelling Method (NATM) to provide temporary support in soft or weak ground. (Details of the technique are outside the scope of this guidance.)
In all cases the tunnel can only remain unsupported for a limited time and hence a permanent, load-bearing lining (either concrete cast in situ or precast concrete units) will be required.
However, in some countries, particularly Scandinavia, but increasingly elsewhere, fibre-reinforced sprayed concrete (FRS) is used to provide both the primary and long term support of tunnels. In many cases the sprayed concrete may be left exposed and visible; alternatively non-structural linings may be provided in, for example, some road tunnels. The suggested overall approach to the design of both steel and macro-synthetic FRS is first to specify the concrete, usually in terms of its characteristic cube or cylinder compressive strength and toughness parameters, and then to determine the thickness (or thicknesses), and the type and amount of fibre required, to comply with these specified material requirements and to achieve the particular design objectives. This approach allows FRS material(s) to be specified in the first instance which the designer can be reasonably confident will achieve the specified requirements consistently during construction, and at reasonable cost. The design requirements, and the method by which the thickness and amount of fibre are determined, may differ between projects depending on factors such as ground conditions, excavation geometry, construction sequence, type of support proposed and design method etc.
The design may be based on semi-empirical, toughness characterisation, or deterministic and analytical approaches, depending on the particular project requirements. Further information may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete and Technical Report 65, Guidance on the use of macro-synthetic-fibre-reinforced concrete.
Acknowledgement: The Concrete Society
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