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What is NATM Method in tunneling?

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The term New Austrian Tunneling Method Popularly Known as NATM got its name from Salzburg (Austria). It was first used by Mr Rabcewicz in 1962. It got world wise recognition in1964. This method has been evolved as a result of experience gained in Austrian Alpine tunneling condition. The first use of NATM in soft ground tunneling is done in Frankfurt metro in 1969. The basic aim of NATM is for getting stable and economic tunnel support systems. This method has been very useful in complex diversified geological condition where forecasting of the rock mass is difficult due to rapidly changing geology.

NATM broadly based on the following principles: • Mobilization of the strength of rock mass - The method relies on the inherent strength of the rock mass being conserved as the main component of tunnel support. Primary support is directed to enable the rock to support itself. • Shotcrete protection - Loosening and excessive rock mass deformation should be minimised by applying a layer 25-50mm of sealing shotcrete immediately after opening of the face.

Measurements - Every deformation of the excavation must be measured. NATM requires installation of sophisticated measurement instrumentation. It is embedded in lining, ground such as load cells,extensometers and relectors

Primary Lining - The primary lining is thin.It is active support and the tunnel is strengthened not by a thicker concrete lining but by a flexible combination of rock bolts, wire mesh and Lattice girders. • Closing of invert – Early as far as possible closing the invert so as to complete the arch action and creating a load-bearing ring is important. It is crucial in soft ground tunnels • Rock mass classification - The participation of expert geologist is very important as the primary support as well as the further designing of supports etc during the excavation of rock requires the classification of the rock mass. • Dynamic Design – The deigning is dynamic during the tunnel construction.Every face opening classification of rock is done and the support are selected accordingly. Also the design is further reinforced based on the deformation as noiced during the monitoring.

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