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Going Underground | NATM explainer

The New Austrian Tunnelling Method (NATM) is the generic shorthand for an open face tunnelling technique.  

It is better described as the sequential excavation method (SEM) or sprayed concrete lining. Now in use all over the world in a wide variety of ground conditions, the technique was first developed in the 1960s as a development of conventional hard rock tunnelling in the Austrian Alps.

Its main innovation was to take advantage of the inherent strength of the surrounding rock strata. As the open face of a tunnel advanced, a thin layer of sprayed concrete was applied immediately to stabilise the exposed surfaces and allow the rock to deform and release internal stresses.

It was particularly important to close the invert of the lining to complete the load-bearing ring.

Movement and settlement was carefully monitored and strengthening measures adopted as determined by sensors. Strengthening usually took the form of rock bolting, wire mesh or steel ribs. It could vary as work progressed, leading to NATM being characterised as “design as you go”.

heathrow collapse

heathrow collapse

The 1994 collapse of the Heathrow Express station tunnels sent shock waves through the tunnelling world

A second sprayed concrete lining could then be applied. The final lining, if required, was usually insitu concrete.

Such tunnels were normally at depth where there were high insitu stresses in the rock. Since then, however, a developed version of the technique has been widely used to construct shallow tunnels in soft ground, often below city centres.

Sprayed concrete linings are seen as the optimum solution for underground stations in particular, where very large diameter tunnels with complex junctions and interfaces would be difficult to construct by traditional methods.

Real challenge

Here the real challenge is usually the control of subsidence of the ground above. Monitoring is even more important. The length of unsupported ground behind the open face has to be kept to a minimum and the first sprayed concrete layer applied without delay.

These days the concrete is likely to contain steel fibres and to be produced by the “wet process” method. Bags of dry mix containing cement, fine aggregate and fibres are mixed with water in a concrete mixer on site and then pumped to the spray gun. High pressure air is introduced at the nozzle, and the mix is blasted onto the clay surface.

A more consistent mix and less reliance on the nozzleman’s expertise is said to be the wet process’ main advantages over the older dry process alternative. In this the dry blend is blown to the nozzle, where water is added at a rate varied by the nozzleman to ensure consistent performance.

Mesh or steel lattice arches

Whichever option is adopted, mesh or steel lattice arches can be deployed as required, supplemented by soil nails when indicated by the sensors. In London Clay it is normal practice to use sprayed concrete for primary and secondary linings. Concrete that fails to adhere and falls into the invert is known as rebound, and if not cleared away quickly can seriously impair the strength and integrity of the lower sections of the lining.

Clay can also contain natural inclined joints. These are more of a risk to operatives, as large lumps of clay known as ‘greasybacks’ can fall from the exposed face without warning.

What was then universally known as NATM was received with enthusiasm in the tunnelling world towards the end of the last century. Unfortunately, understanding and expertise were somewhat lacking.

70 significant NATM failures

Between 1973 and 2007 there were nearly 70 significant NATM failures worldwide, of which the best known was the 1994 collapse of three parallel tunnels at Heathrow. These were being constructed as part of the Heathrow Express Rail Link, and though the primary sprayed concrete lining was in place the main insitu concrete lining was still missing,

Contractors building the new Jubilee Line Extension stations near Waterloo and London Bridge immediately suspended all NATM work as a precaution. The Health & Safety Executive (HSE) launched an immediate investigation into both the causes of the Heathrow collapse and the safety of NATM as a soft ground tunnelling technique.

Five years later main contractor Balfour Beatty was fined £1.2M for failing to ensure the safety of its employees and members of the public. Specialist Austrian consultant Geoconsult, whose responsibility it was to monitor the safety of the tunnelling works, was fined £500,000.

High consequence event

Overall, the HSE concluded that “there is no intrinsic reason….why NATM work should not proceed in safety.” However, it warned that a failure in a shallow NATM tunnel in an urban area “may result in a high consequence event, especially for the larger diameter tunnels, which are commonly constructed using NATM.”

In such cases, crown holes, or craters, may open up. Despite the HSE’s warnings, seven people died when a massive crown hole appeared above a section of NATM tunnel on the Sao Paulo Metro project in January 2007. The 18.5m diameter section that collapsed was intended for use as a metro station and was being excavated in three stages.

This was the third collapse of a NATM tunnel on the Sao Paulo Metro. There were other high profile collapses in Kwachon and Seoul, South Korea, both in 1991, and Munich in 1994.

Historically, the most common cause of NATM failures is unforeseen ground conditions – either perched water tables, undetected boulders, pockets of softer material or similar. Collapses nearly always occur close to the face. Failures of the sprayed concrete lining are much rarer.

However, the HSE warns that “NATM tunnel collapses in urban areas can result in major consequences not just to those working in the tunnel but to members of the public, the infrastructure and the built environment.”

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