Geokon 4850, Instruction Manual

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1. INTRODUCTION
1.1 Theory of Operation
The “New Austrian Tunneling Method”, or N.A.T.M., calls for the support of a tunnel by the
rapid application of shotcrete to the freshly exposed ground. The theory behind this method of
support, particularly useful in weaker ground, is that if the inherent strength of the ground can be
preserved, it will be almost self-supporting and will require much less artificial support in the
form of concrete or steel. To preserve the inherent cohesion of the ground it is necessary to
prevent it from breaking up in the first place and, hence, the need for a rapidly applied layer of
shotcrete.
Support Deformation
Ground Characteristic
In-Situ Stress Level
Radial
Support
Pressure
Failure
A
B
C
D
(A,B,C,D)
Figure 1 - Ground Reaction Curve
The above figure graphically shows the ground reaction curve, i.e., the amount of support
required versus the amount of inherent support and ground deformation. Thus, to prevent any
support deformation (or tunnel closure) at all, would require a support pressure exerted on the
tunnel walls equal to the original in-situ ground stress.
A strong lining with characteristics of curve A would allow only a small amount of ground
deformation, but might, because it is too strong, be uneconomical. A thinner lining which would
allow more deformation would have characteristics of curves B or C. However, a lining which is
too thin, with characteristics shown by curve D, would allow too much deformation of the rock
allowing it to weaken and ultimately fail.
The task of the N.A.T.M. stress cells is to provide a measure of the support pressure which, when
coupled with a measurement of tunnel closure, using a tape extensometer, will allow an
assessment to be made of the adequacy of the shotcrete lining, indicating the need for perhaps
more or less shotcrete to maintain stability. It is this ability to monitor the performance of the
shotcrete lining that can lead to significant reductions in tunnel support costs.