New Austrian Tunnelling method

The New Austrian tunneling method (NATM), also known as sequential excavation method (SEM), is a popular method of modern tunnel design and construction. This technique first gained attention in the 1960s based on the work of Ladislaus von Rabcewicz, Leopold Müller, and Franz Pacher between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from the old Austrian tunnelling approach. The fundamental difference between this new method of tunneling, as opposed to earlier methods, comes from the economic advantages made available by taking advantage of the inherent geological strength available in the surrounding rock mass to stabilize the tunnel .

NATM/SEM is generally thought to have helped revolutionise the modern tunneling industry. Many of the most famous modern tunnels have used this excavation technique.

The NATM integrates the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The NATM has often been referred to as a "design as you go" approach, by providing an optimized support based on observed ground conditions. More correctly it can be described as a "design as you monitor" approach, based on observed convergence and divergence in the lining and mapping of prevailing rock conditions. It is not a set of specific excavation and support techniques.

NATM has seven elements:

Based on the computation of the optimal cross section, only a thin shotcrete protection is necessary. It is applied immediately behind the excavated tunnel face to create a natural load-bearing ring and minimize the rock's deformation. Geotechnical instruments are installed to measure the later deformation of excavation. Monitoring of the stress distribution within the rock is possible.

This monitoring makes the method very flexible, even if teams encounter unexpected changes in the geomechanical rock consistency, e.g. by crevices or pit water. Reinforcement is done by wired concrete that can be combined with steel ribs or lug bolts, not with thicker shotcrete,

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