ENGINEERS involved in planning the proposed dual carriageway linking Dublin Port with Whitehall have reaffirmed their decision to choose the "new Austrian tunnelling method" (NATM), despite a quite damning review of its use by Britain's Health and Safety Executive.
The HSE report "recites a tale of inadequacy and failure in concept, design, management and execution of NATM projects Worldwide", according to New Civil Engineer magazine, and it warns everyone involved to "take note of the advice given in this review and ensure that they act upon it".
Unlike conventional tunnels, where a concrete retaining structure is built as the tunnel is bored, NATM involves spraying concrete on the exposed face of a newly bored tunnel it sets and becomes the structure. It was chosen for the Dublin Port tunnel primarily because it is cheaper.
But the long list of safety requirements laid down by the British HSE could make the method uneconomical. Mr Stuart Nattrass, its chief inspector, said "anyone proposing to use NATM will now have to ask whether they have sufficient resources available to address the necessary functions."
"Tunnelling engineers now fear that the extra controls required will rule out NATM for all but the most complex underground structures," said few Civil Engineer. Contractors would also have to think carefully about recommending its use as a cheaper alternative to traditional techniques.
The HSE review was undertaken following the "catastrophic failure" during construction of an NATM tunnel at Heathrow Airport on October 21st, 1994 the day after the Government here sanctioned the Dublin Port tunnel. The Heathrow tunnel is for a new express rail link to central London. The technical term "catastrophic failure" appears several times in the HSE report.
The accident caused a crater at the surface and so destabilised an office building that it tilted like the leaning tower of Pisa. The HSE has since examined over 30 NATM projects. It concluded that "except for the smallest and simplest of jobs", they all had "significant deficiencies" from the outset.
The HSE review lists a total of 39 "significant NATM incidents" around the world, most within the past 15 years, but says the true figure may be "significantly" higher, as not all incidents have been reported. Japan alone had 71 NATM "incidents" between 1978 and 1991, most of them in hard rock.
Nearly all the collapses originated with the tunnel heading and were attributed to "unstable ground conditions". But the HSE report suggests that the collapses might not have happened if other construction methods had been used.
The particular hazard with NATM is "the absence of any ready means of supporting the heading should ground collapse into the tunnel", the report says. This could not be overcome simply by monitoring. "Every stage in the excavation of the heading and construction of the lining should be designed," it says.
The HSE report contains 97 recommendations on the use of NATM, covering legislation, risk management, organisation, control, communication, competence, planning, design, monitoring and construction. It says that NATM work could proceed safely "provided careful account is taken of all the issues in this review".
However, it also suggests that closely juxtaposed tunnels such as the twin bore envisaged for much of the Dublin port access route require particular review".
As New Civil Engineer noted, the formula recommended by the HSE for safe working of NATM "requires virtually every possible contingency to be planned for in advance", including design and construction techniques. The emphasis is on basic decisions about how to proceed with a project and its strategic management.
Mr John Higgins, of Ove Arup and Partners, consultant engineers for the Dublin Port tunnel, said they were still convinced that NATM was technically the right choice.
"All the recommendations made by the HSE will be taken on board in the design. It will affect the scheme's out turn cost, but in the interests of safety this money must be spent."
NATM is to be used for some 2.4 kilometres of the port access route, taking it under an estimated 190 houses in the Marino area of the city. Already, there have been vociferous complaints from local residents that the test borings have caused cracks in the ceilings and walls of some of the houses. This has been denied by the promoters.
The decision to use NATM was taken in 1994 after Ove Arup had priced the route, using conventional tunnelling, at £125 million. Geoconsult, an Austrian firm with expertise in NATM projects, was called in to review the proposal and, by straightening the alignment and using the NATM method, the cost was cut to £104 million.
The most recent estimate for the port access route mentioned by the newly established Dublin Transportation Office was more than £25 million higher, at £130 million. But this estimate was made before the HSE report was published last month and will now have to be revised upwards.
