Civil Engineering Reference
In-Depth Information
20
NATM Tunneling
20.1
Introduction
The “New Austrian Tunneling Method” (NATM) nowadays is such a wide spread tunne-
ling method that it is often referred to as the “conventional tunneling method” in contrast
to mechanized tunneling (TBM tunneling) that is dealt with in Chapter 21. Because of the
outstanding importance of shotcrete (sprayed concrete) for the application of this method
the terms “sprayed concrete lining method”, or simply “shotcrete method”, are also used
(Wittke et al. 2002).
Interacting with the rock mass, the primary function of the shotcrete membrane is to
form an arch around the tunnel which is capable of carrying load. With a favorable
shape of the tunnel's cross-section and an adequate sequence of construction stages
it is possible to avoid or at least to minimize bending moments and shear forces in the
shotcrete membrane. Thus, large underground openings can be supported by relatively
thin shotcrete membranes. With an adequate design the subsidence on the surface can
also be limited to relatively small values.
Stability analyses in which interaction of rock mass and support are modeled in a realistic
way, however, are a prerequisite for a successful tunnel heading using this method. As out-
lined in Chapter 10, analyses according to the FEM are the most suitable for this purpose.
After giving an overview on the fundamentals of the NATM in Section 20.2, the case
history of a tunnel excavated underneath Stuttgart airport runway will be presented in
Section 20.3 as a practical example for the application of the anisotropic jointed rock
model (AJRM) to tunneling according to the NATM.
20.2
Fundamentals of the NATM
20.2.1 Excavation Classes
According to DIN 18312 (2012) the excavation and support works, which are decisive
for performance rates and costs, are defined by so-called excavation classes.
Excavation classes 1 and 2 are assigned to a heading which either requires no support or
only a support which does not hinder the excavation process. Excavation class 3 char-
acterizes a heading with a support for which the excavation has to be interrupted and
which follows the temporary face of the tunnel at a small distance.
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