The New Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method[1] (SCL), is a method of modern tunnel design and construction employing sophisticated monitoring to optimize various wall reinforcement techniques based on the type of rock encountered as tunneling progresses. 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 earlier methods, with its economic advantage of employing inherent geological strength available in the surrounding rock mass to stabilize the tunnel wherever possible rather than reinforcing the entire tunnel.[2]
NATM/SEM is generally thought to have helped revolutionise the modern tunneling industry. Many modern tunnels have used this excavation technique.
The Sequential Excavation Method is very cost effective, even in karst conditions.[3]
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, where support requirements can traditionally be estimated using the RMR or Q System.[4] 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.
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 and so tend to use different support methods from the original NATM. That 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. That is partly caused by an increased use of the tunneling method in the United States, particularly in soft ground shallow tunnels.
Other designations are seen for this modern tunneling style; 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."[5]
Some engineers use NATM whenever they propose 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.[6]
Key features of the NATM design philosophy are:
When NATM is seen as a construction method, the key features are:
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.[7]