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Engineering geology 'is key to tunnelling success'

ENGINEERING GEOLOGISTS must be involved in every stage of tunnelling projects, from conception to construction and operation, Professor Paul Marinos told the Engineering Group of the Geological Society last month.

Marinos, who is director of geotechnics at the National Technical University of Athens, was delivering this year's Glossop Lecture: 'Ongoing challenges in engineering geology for tunnelling in difficult ground'.

Tunnelling was 'a fascinating game between the ground and humans', Marinos said.

'Demands to improve infrastructure have pushed engineers to handle or try to handle difficult ground, ' he said.

'The plethora of tunnels being built worldwide, particularly in Europe, makes the subject particularly relevant today.' Marinos charted the involvement of engineering geologists from the Eupalinos Tunnel on the Island of Samos in Greece, built in 530BC, to present-day construction of the Gotthard Base Tunnel in Switzerland.

He drew on a range of high profile case studies to illustrate the importance of good quality engineering geology in tunnelling and how it made possible difficult projects such as the Athens Metro and tunnels on the Egnatia Highway in Greece.

He stressed the importance of the geological model in tunnel design.

Underlying developments in site investigation, numerical analysis, risk assessment, construction, innovations in TBMs and monitoring was the need to assess ground quality.

'As John Burland said: 'There is no sense in going further if you do not have the full picture of the ground'.' One field where engineering geologists could contribute greatly was in developing rock mass models.

'Estimating rock properties and selecting appropriate parameters for design is very important, but the challenge is to quantify geological reality using numbers without misinterpreting it, ' Marinos said.

'Geologists can become trapped by numbers and forget the geology and engineers may then be misled, ' he warned. 'The problem is not the rock mass classifications but their misuse.

Always use laboratory testing for parameters.' He advocated the use of the Geological Strength Index, which is based on geological description. But he advised against its use in structural failure analysis (in very deep tunnels where structures are under high stress, for example) and in intact soft rocks, adding that water content changed behaviour and hence classification.

Engineering geologists were often misunderstood at the design and construction stages of tunnelling projects, Marinos said.

But they could be involved and contribute.

He emphasised the importance of past experience in assessing ground response and controlling deformation when choosing tunnelling equipment and construction techniques.

'Tunnelling is at the borderline between geology and engineering, and engineering geology is at the interface between geology and ground engineering, ' he said. 'It is very important never to forget basic geological knowledge.'

lAt the event Fleur Loveridge from Mott MacDonald presented a paper on the engineering geology of a landslide in Gault Clay that won her this year's Glossop Award for young engineering geologists (GE October 02 News).

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