RESIDENTS of the Marino area of Dublin are expected to experience noise and vibrations as blasting or "controlled detonation" is used to bore the proposed twin port tunnels through the limestone rock underneath their homes.
"It would be necessary to detonate explosive charges some four to six times per day for each tunnel, in order to fracture the rock" according to the environmental impact statement (EIS) on the scheme, which has been distributed to Dublin city councillors.
"Residents in houses above the tunnels would perceive vibration from these detonations over an estimated four week period, at any one location," says the EIS which was prepared by consultant engineers, Ove Arup and Partners, and Geoconsult, an Austrian tunnelling firm.
"Detonations would be restricted to the period from 7 a.m. to 8 p.m. in order to eliminate or minimise construction disturbance in the area," according to the EIS. "A system of local announcements would be put in place prior to blasting."
The tunnel would pass directly underneath 273 properties, at a depth of 16 metres (53 feet), on average. Most of these are houses in Marino, but they also include St Mary's High, Park Convent and St Joseph's primary, and secondary schools in Fairview.
Excavation of twin tunnels of approximately 10 metres (33 feet) diameter at a minimum 10 metre ground depth (ground level to the roof of the tunnel) would inevitably lead to some ground movements." But the study says the excavation work would be designed to ensure these movements would be minimal.
Structural surveys of the houses would, be carried out prior to construction and there would also be an ongoing monitoring programme, above and below ground.
Residents of Marino have already complained of cracks appearing in wall and ceiling plaster in their homes, blaming these defects on trial borings carried out to investigate ground conditions. There is also growing concern over the proposed use of the Austrian Tunnelling Method (NATM).
Unlike conventional tunnelling, which relies on building a concrete structure to retain a tunnel as it is bored, NATM involves spraying the newly exposed tunnel, with quick setting concrete, which then becomes the structure. It is, therefore, more heavily dependent on, the stability of ground conditions.
The EIS notes that Britain's Health and Safety Executive published a major report in May on the safety of NATM as a tunnelling technique, following a collapse at Heathrow Airport in October, 1994, when an office building was destabilised during tunnelling work for a high speed rail link to central London.
"The report concluded that it is possible to build NATM tunnels safely and, when finally completed and commissioned, they are as safe as those constructed by other means," the study says.
However, it concedes that major NATM collapses have occurred worldwide, prompting the HSE to recommend additional safety precautions.
The EIS says these recommendations would be taken into account where appropriate".
However, it notes the HSE report was written with reference to London clay, whereas the ground through which the proposed tunnel here would be bored was "predominantly limestone rock".
Although the cheaper NATM is still favoured for the scheme, the EIS says: As it is proposed to seek tenders for the project on a design and construct basis, alternative tunnelling methods may be submitted by contractors should they prove to be more economical, while also being safe and environmentally satisfactory."
Whatever tunnelling method is used, major relocations of utilities will be required - including sewers, watermains, gas mains, ESB cables and Telecom Eireann ducts Traffic diversions would also be inevitable.
Altogether, excavation works would yield some 819,000 cubic metres of spoil, much of it to be removed through the Fairview Park portals of the tunnel. Dublin Port has expressed an interest in acquiring surplus rock from the excavation to reclaim 50 acres of land from the inner bay, east of the existing port area.
The EIS concedes that the proposed tunnel would result in a deterioration of air quality in the vicinity of its portals, and along the M1 motorway, with a threefold increase in the levels of nitrogen dioxide - though this would still be "well under the EU standard".
On the other hand, there would be significant reductions in traffic on other routes, including Drurncondra Road (17 per cent). Griffith Avenue (between 11 and 14 per cent), Grace Park Road (17 per cent), Richmond Road (21 per cent) and Phillipshurgh Avenue (31 per cent). This would also result in a "slight improvement" in air quality.
But the scheme would lead to a 39 per cent increase in traffic on East Wall Road, north of its junction with Sheriff Street, and a 17 per cent increase in traffic using the East Link bridge. This in turn would result n pressure to provide another Liffey crossing, with adverse implications for the rejuvenation of the Grand Canal Docks area.
While this problem "would emerge in any event without the implementation of the scheme", the port tunnel would make these pressures "even more urgent" the EIS says. It suggests that a land, use planning study is required to deal with this issue, linked to the work of the proposed Docklands Development Authority.
There has always been concern that the port tunnel would create such congestion in the north port area that the construction of another bridge would be needed to relieve it. Alternatively - or, perhaps, in addition a full Eastern Bypass, motorway, linking Whitehall with Booterstown, may become inescapable.