Civil Engineering Reference
In-Depth Information
construction methods and sequence. The monitoring also takes into account
the geological information.
The thickness of the sprayed concrete lining, if used purely to seal the
excavation, is normally between 5 and 15 cm. However, if the lining is
to act as a structural element, then the thickness is of the order of 15 to
30 cm. The thickness of the lining can of course be greater, but this is not
considered to be a 'thin' lining according to principle 10.
If the long-term stability of the sprayed concrete is likely to deteriorate
due to aggressive water conditions and/or if there is a requirement for a
100% watertight lining, an inner lining is necessary (principle 19).
Many of the principles are undoubtedly key elements of most of the tunnel-
ling methods used today. Nevertheless, the principles have been heavily dis-
cussed since the strict adherence to some of them reduces the flexibility of
the method. Some proved not to be of practical use (principle 6), and some
did not consider the ground conditions (principles 12, 15, 16) or the size of
the tunnel (principle 16) (Schubert 1999). The principles were supported by
the official definition of NATM which was published in 1980 by the Aus-
trian National Committee 'Hohlraumbau' (translated from the German):
The New Austrian Tunnelling Method (NATM) follows a concept which
makes the ground (rock or soil) surrounding the void a supporting
construction element through the activation of a ground supporting arch.
Unfortunately, this definition did not prove to be unique to NATM since
nearly every excavation method tries to preserve the supporting ability of
the ground. Rokahr (1995) gave a definition which summarizes the original
intention of Rabcewicz, Müller and Pacher and makes it easy to distinguish
the NATM from other tunnelling methods: 'NATM is a support method
to stabilize the tunnel perimeter by means of sprayed concrete, anchors and
other support, and uses monitoring to control stability.'
According to this definition NATM involves:
•
support by sprayed concrete;
•
support by systematic anchoring if necessary;
•
using measurements to control the effectiveness of the support;
•
a flexible approach to support measures, i.e. increasing or decreasing
the support according to the geological conditions.
It is not NATM when:
•
excavation is by TBM;
•
support is by segmental lining (steel, SGI, concrete);
•
there is no support at all;
•
the full overburden is supported;
•
no flexible approach is adopted for the support.
