New Austrian tunnelling method

"NATM" redirects here. For other uses, see NATM (disambiguation).

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 .[1]

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.

Principles

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,

The measured rock properties suggest the appropriate tools for tunnel strengthening. Since the turn of the 21st century, NATM has been used for soft ground excavations and making tunnels in porous sediments. NATM enables immediate adjustments in the construction details, but requires a flexible contractual system to support such changes.

Variant names

NATM was originally developed for use in the Alps, where tunnels are commonly excavated at depth and in high in situ stress conditions. The principles of NATM are fundamental to modern-day tunnelling, and NATM fundamentally involves specifically addressing the specific soil conditions being encountered. Most city tunnels are built at shallow depth and do not need to control the release of in situ stress as was the case with the original NATM in the Alps. Projects in cities place a higher priority on minimizing settlement therefore they tend to use different support methods from the original NATM. This has led to a confusion in terminology in that tunnelling engineers use "NATM" to mean different things. New terms have arisen and alternative names for certain aspects of NATM have been adopted as its use has spread. This is partly caused by an increased use of this tunneling method in the United States, particularly in soft ground shallow tunnels.

Other designations are seen for this modern tunneling style, e.g. Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan the terms Centre Dividing Wall NATM or Cross Diaphragm Method (both abbreviated to CDM), and Upper Half Vertical Subdivision method (UHVS) are used.

The Austrian Society of Engineers and Architects defines NATM as "a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure."[2]

Some engineers use NATM whenever proposing shotcrete for initial ground support of an open-face tunnel. The term NATM can be misleading in relation to soft-ground tunnels. As noted by Emit Brown, NATM can refer to both a design philosophy and a construction method.[3]

Key features

Key features of the NATM design philosophy are:

When NATM is seen as a construction method, the key features are:

Safety

The 1994 Heathrow Airport tunnel collapse led to questions about the safety of the NATM. However, the subsequent trial blamed the collapse on poor workmanship and flaws in construction management, rather than on the NATM.[4]

See also

References

  1. ↑ Özdemir, Levent (2006). North American Tunneling 2006. Washington, DC: Taylor & Francis. p. 246. ISBN 0-415-40128-3.
  2. ↑ Tunneling: Management by Design, Alan Muir Wood, Taylor & Francis, 2002, ISBN 0203477669
  3. ↑ Jacobs & Associates Newsletter, Spring 2002, NATM IN SOFT-GROUND: A CONTRADICTION OF TERMS?, Victor Romero.
  4. ↑ http://www.tunneltalk.com/images/laneCoveCollapse/Ref5-Heathrow-failures-highlight-NATM-misunderstandings-Shani-Wallis.pdf

Further reading

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