New Austrian tunneling method

Did You Know?

• The 1994 Heathrow Express tunnel collapse led to questions about NATM safety, but the subsequent trial blamed the collapse on poor workmanship and flaws in construction management, not on the method itself.
• NATM is generally thought to have helped revolutionise the modern tunneling industry and is very cost effective even in karst conditions.

Principles

The NATM integrates principles of rock mass behavior under load with monitoring of underground construction during building. It is often called a "design as you go" or more accurately 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. The method has seven elements. It exploits the strength of the native rock mass by conserving its inherent strength as the main tunnel support component, with primary support directed to enable the rock to support itself. Shotcrete protection is applied as a thin layer immediately after face advance to minimize loosening and excessive rock deformation. Measurement and monitoring of potential deformations is done through sophisticated instrumentation embedded in lining, ground, and boreholes; additional supports are installed only when needed based on observed movements. Flexible support uses a thin primary lining with active support from rock bolts, wire mesh, and steel ribs rather than a thicker concrete lining. Closing of the invert quickly creates a load-bearing ring, especially crucial in soft ground. Contractual arrangements must allow changes in support and construction method based on monitoring. Rock mass classification, ranging from very hard to very soft, determines minimum support measures to avoid economic waste.

Variant names

The New Austrian Tunneling Method was first created for deep Alpine tunnels dealing with high natural ground pressures. In contrast, most urban tunnels are shallow and focus on controlling ground movement rather than managing stress release, which has caused confusion among engineers who now use the term NATM in different ways. As the method spread, especially for soft ground and shallow tunnels in the United States, new names emerged. For shallower work, Sequential Excavation Method or Sprayed Concrete Lining are common alternatives. In Japan, engineers refer to Centre Dividing Wall NATM, Cross Diaphragm Method, or Upper Half Vertical Subdivision method. The Austrian Society of Engineers and Architects defines NATM as an approach where the surrounding ground becomes part of a ring-shaped support system. Some professionals apply the term NATM whenever shotcrete is used for initial support in an open-face tunnel. This range of meanings means NATM can describe both a design philosophy and a construction technique, making its use in soft ground particularly misleading.

Key features

The NATM design philosophy focuses on maximizing the use of the surrounding ground’s own strength. This is achieved by permitting controlled ground movement, which mobilizes that strength. Primary support is then installed with load-deformation properties suited to the specific ground, and its timing is coordinated with ongoing ground deformations. Monitoring instruments track deformations in this initial support, providing data to adjust the support design and excavation sequence as needed. As a construction method, NATM involves excavating and supporting the tunnel in stages, with the sequence adaptable to changing rock conditions. Initial support consists of shotcrete reinforced with fiber or welded-wire fabric, steel arches (often lattice girders), and occasionally ground reinforcement like soil nails or spiling. The permanent lining is typically cast-in-place concrete placed over a waterproofing membrane. A key step is quickly closing the invert—the tunnel’s bottom—to form a continuous structural ring, which leverages the natural arching effect of the rock or soil above the tunnel section.