New Austrian Tunnelling method
The New Austrian Tunneling method (NATM) was developed between
1957 and
1965 in
Austria. It was given its name in
Salzburg in
1962 to distinguish it from old Austrian
tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller, and Franz Pacher. The main idea is to use the geological
stress of the surrounding
rock mass to stabilize the tunnel itself.
The NATM is an approach of
philosophy integrating the principles of the behaviour of rock masses under load and monitoring the performance of underground construction during construction. The fact is that the NATM is not a set of specific excavation and support techniques.
There are seven features on which is NATM based:
* Mobilization of the strength of rock mass - The method relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support. Primary support is directed to enable rock support itself.
*
Shotcrete protection - Loosening and excessive rock deformation must be minimised. This is achieved by applying thin layer of shotcrete immediately after face advance.
* Measurements - Every deformation of excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground, and boreholes.
* Flexible support - The primary lining is thin and reflects recent strata conditions. Used is rather active than passive support and strengthening is not by thicker concrete lining but by a flexible combination of rock bolts, wire mesh and steel ribs.
* Closing of invert - Important is quickly closing of invert and create load-bearing ring. It is crucial in soft grounded tunnels where no section of tunnel should be left open even temporarily
* Contractual arrangements - Since the NATM is based on monitoring measurements, changes in support and construction method are possible. This is possible only if the contractual system those changes enables
* Rock mass classification determines support measures - There are main rock classes for tunnels and corresponding support. These serve as the guidelines for tunnel reinforcement.
Based on the computation of the optimal
cross section, just a thin
shotcrete protection is necessary. It is applied immediately behind the
Tunnel boring machine, to create a natural load-bearing ring and therefore to minimize the rock's
deformation. Additionally,
geotechnical instruments are installed to measure the later deformation of
excavation. Therefore a
monitoring of the stress distribution within the rock is possible.
This monitoring makes the method very
flexible, even at surprising changes of the
geomechanical rock consistency during the tunneling work, e.g. by
crevices or
pit water. Such (usual) problems are not solved by thicker shotcrete, but the
reinforcement is done by
wired concrete which can be combined with steel ribs or lug
bolts.
The measured rock properties lead to the appropriate
tools for tunnel
strengthening. Therefore in the last decade NATM was also applied to
soft ground excavations and to tunnels in porous
sediments. The flexible NATM technique enables immediate adjustments in the construction details, but this requires a flexible
contractual system, too.
When tunneling engineers talk on NATM, they often mean different things because some of them define it as a special technique, but others as a sort of
philosophy. Recently the scene has been complicated by new
terms and even alternative names, when discussing certain aspects of NATM. This is partly caused by an increased use of the method in the
USA, particularly in
soft ground conditions (see External links).
Besides the official name
New Austrian Tunneling Method other designations are used in the last years, e.g.
Sequential Excavation Method (SEM), or
Sprayed Concrete Lining (SCL). In
Japan sometimes other names were used, e.g.
Centre Dividing Wall NATM, or
Cross Diaphragm Method (both abbreviated as
CDM), and even
Upper Half Vertical Subdivision method (UHVS). Evidently, the scientists and the tunneling
industry cannot find an unified name for this wide-used method.
As defined by the
Austrian Society of Engineers and Architects, the NATM
constitutes 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.But many engineers already refer to as NATM, when
shotcrete is proposed for initial ground support of an
open-face tunnel. Especially with reference to soft ground, the term NATM can be misleading. As noted by Emit Brown, NATM can refer to both a
design philosophy and a
construction method.
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 mobilised to the maximum extent possible.
* Mobilisation of ground strength is achieved by allowing
controlled deformation control 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 .. utilises NATM construction methods but not necessarily NATM design methods. These details are less essential at tunnels in solid or fair rock.
*
Tunneling*
Geotechnical engineering* Johann Golser,
The New Austrian Tunneling Method (NATM), Theoretical Background & Practical Experiences. 2nd
Shotcrete conference, Easton (USA), 4-8 Oct 1976.
*
Insight into the New Austrian Tunnelling Method (NATM)*
NATM in Soft-Ground: A Contradiction of Terms?*
19 NATM-Tunnels in Europe and Japan (length 1-13 km)
