Assurances on tunnel subsidence queried

ASSURANCES that houses in the Marino area of Dublin would not suffer subsidence from the construction of the proposed port tunnel appear to be "unjustified", according to an expert report commissioned by the local action group.

Prof Chris Clayton, head of the department of civil engineering at the University of Surrey, said the absence of hard evidence about ground conditions along the route of the £130 million twin bore tunnel raised doubts about claims by its designers that there would be "no perceptible settlement".

Because the geological profile was "derived from only three boreholes", there was "no way of knowing that the tunnel construction would be within the underlying rock". As a result of this and the uncertainty about tunnelling methods, it was "impossible" to estimate the degree of settlement.

"The possible additional costs of tunnelling in the Marino area, arising from the need to protect against uncertainties in the position of the rockhead, the variability of the ground and the presence of water laden gravel pockets do not appear to have been considered," Prof Clayton said.

As the preferred route - known as A6 - would be constructed under a large number of buildings, he suggested it might no longer be the most economical route if the costs of repairs to damaged buildings were taken into account. However, this had not been addressed in the cost comparison of the six route options.

He also queried the proposed use of the New Austrian Tunnelling Method, saying it was "not supported by evidence to show - how the risks of tunnel collapse or uncontrolled ground loss - both of which could cause significant damage to overlying housing and, in extremis, would threaten life would be addressed."

There was evidence to suggest that the noise and vibration nuisance resulting from blasting the two tunnels "may be significantly greater than has been stated in the EIS (environmental impact statement) and that some structural damage may occur to those parts of the housing stock which are in a poor condition."

Prof Clayton's 28 page preliminary report for the Marino Development Action Group, which objects to the plan to tunnel underneath more than 270 houses in the area, also says the "large number of uncertainties" emphasised the critical importance of carrying out a risk assessment at the design stage.

The report quotes the EIS as saying the present bedrock surface is "highly irregular" and is overlain by a highly variable boulder clay containing gravel and sands. The area also contains some "made ground", where gaps in the surface had been filled in with rubble, and this was "very poor material" geotechnically.

Since it was envisaged as a "design and build" project, contractors would probably be allowed to submit tenders "based around any tunnelling method they prefer" - provided it was safe. To protect Marino residents and their property, the criteria for making this judgment should be spelled out, Prof Clayton said.

However, a spokesman for the Dublin Port Tunnel project team rejected the professor's thesis. "Over the period 1991 to 1996, approximately 100 boreholes have been drilled and 30 trial pits have been dug to investigate the geology of the A6 corridor," he said.

As a result, they had a good "preliminary picture" of its geological profile. The spokesman said this would be supplemented during construction by "advanced horizontal investigation" to a distance of up to 40 metres ahead of the tunnel face "so that ground conditions can be exactly confirmed" - in line with tunnelling practice recommended by Britain's Health and Safety Executive.