Key Features
According to E.Brown (Weblink 2), the key features of the design philosophy refer to:
- The strength of the ground around a tunnel is deliberately mobilized to the maximum extent possible.
- Mobilization of ground strength is achieved by allowing controlled deformation of the ground.
- Initial primary support is installed having load-deformation characteristics appropriate to the ground conditions, and installation is timed with respect to ground deformations.
- Instrumentation is installed to monitor deformations in the initial support system, as well as to form the basis of varying the initial support design and the sequence of excavation.
When NATM is seen as a construction method, the key features are:
- The tunnel is sequentially excavated and supported, and the excavation sequences can be varied.
- The initial ground support is provided by shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails, spiling).
- The permanent support is usually (but not always) a cast-in-place concrete lining.
Some experts note that many of these construction methods were used in the US and elsewhere in soft-ground applications, before NATM was described in the literature.
In an article of 2002 Romero states the major difference between the viewpoints of design and of construction: The deformation of the soil (rem.: at soft-ground tunnels) is not easily ‘controlled’. Therefore it can be concluded that the excavation and support planned for sequentially excavated, shotcrete-lined tunnels .. utilizes NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.
Read more about this topic: New Austrian Tunnelling Method
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