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CIVIL ENGINEERING

New Austrian Tunnelling Method

Tunnellers had been developing new techniques for difficult ground conditions for many years before the New Austrian Tunnelling Method came on the scene in the 1960s. NATM's long genesis really began back in the late 19th Century, when the builders of the great rail tunnels under the Alps were first confronted with the challenge of variable, weak and fissured rock.

To deal with this unpredictable 'squeezing ground' the tunnellers had little alternative but to install massive support structures. This was a slow and expensive operation on contracts like the 15km St Gotthard or 20km Simplon tunnels and, as the 20th century advanced, the search was on for more economic and practical alternatives.

Two early developments that were to be crucial to the ultimate development of NATM were a new theory of rock mechanics and the invention of sprayed concrete. The former, first mooted in 1881, began to describe how weak strata would deform after tunnel excavation. The latter is credited to a Chicago taxidermist, who in 1907 developed the technique to protect the bones of dinosaurs against pollution.

By the 1920s sprayed concrete was appearing on American tunnelling contracts, and in 1954 it was used to stabilise squeezing ground for the first time.

This was on an 8m diameter diversion tunnel for an Austrian power plant. But when Professor von Rabcewicz first used the term 'new Austrian tunnelling method' in a 1962 Salzburg conference presentation, the technique he described could be used without a sprayed concrete lining.

The distinctive feature of NATM as defined by Rabcewicz was the way the excavated tunnel bore was allowed to deform in a controlled fashion to redistribute and relax the internal ground stresses, mobilise what inherent strength the ground possessed, and achieve a stable state. This deformation was to be constantly and accurately monitored, and decisions on the degree of reinforcement and support the ground needed taken on the job on the basis of the monitoring.

Reinforcement could come from rock bolting alone, from rock bolting in conjunction with a temporary sprayed concrete lining, or from a sprayed concrete lining alone. Crucially, this lining had to be capable of deforming with the ground as it settled. Once stabilised, a permanent lining could be installed, usually with either insitu or pre-cast concrete.

Incorporating the inherent strength of the ground into the tunnel design cut down lining thickness. Open face excavation speeded construction, insitu concrete linings slashed the cost of complex junctions and the like. Several Alpine tunnels were built by this method, and by1968 NATM was accepted for use in classic soft ground conditions on the Frankfurt Metro project.

From the British point of view the first major NATM application was the vast Channel Tunnel crossover cavern, completed in 1992. But even before NATM was selected for the high profile Jubilee Line Extension and the ill-fated Heathrow Express, some of the inherent limitations of the technique were being discovered.

According to the Health & Safety Executive report published after the Heathrow collapse, at least 39 cases of collapse or serious deformation had occurred on NATM projects between 1973 and 1994. The worst occurred in Munich only four weeks before the Heathrow disaster.

There, four people died when a bus plunged into an enormous water-filled crater above a Metro tunnel. The crater had appeared when the open face of the NATM tunnel below collapsed, like 17 other NATM tunnels before it. Temporary lining failure, as at Heathrow, is a much rarer event. Nevertheless, the HSE insisted that temporary lining thickness on the JLE project be significantly increased - at vast cost.

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