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Fast and loose

Tunnelling - A patch of loose ground has stalled tunnelling on Iceland's big power scheme, reports Adrian Greeman.

Five months after it ground to a halt at a fault in recent volcanic basalt, the second of three Robins tunnel boring machines working on Iceland's big Kárahnjúkar hydroelectric scheme has got going again.

'And we seem to be moving reasonably well at last, ' says Richard Graham, chief engineer for Italian contractor Impregilo, which is building both the big dam for the power project and the 53km of headrace tunnels which will feed water to an underground generating station (NCE 9 September 2004).

More than 300m has been driven since the machine restarted towards the end of November, with a best result of 46m on one day.

'It is still quite variable and we have hit one or two smaller faults with loose material that have needed rock bolting and rib support, and that has slowed things, ' says Graham.

But the work crew and engineers are hoping they will not encounter anything like the 10m long void and its loose filling that stopped the 7.2m diameter machine entirely last May.

The current area being traversed has patches of so-called Scoriacious basalt and a number of faults with conglomerate in the otherwise hard solidified volcanic flow layers of basalt that make up much of Iceland's geology.

It has proved to be very loose.

As the machine tried to advance it brought down material from the roof, which started to unravel, creating a significant void more than 10m above the tunnel line.

'These are very good TBMs' says Sigurdur Arnalds, spokesman for the project client, the national power company Landvirkjum. 'But they are not designed for that kind of ground.' It has not been easy or pleasant to work a way out of it, not least because of the 100litres/sec water flow through the fault, washing out early attempts to simply plug the gap.

'At least the water is at around 5-6- and not the 1-2- melt water that we find on the TBM 3, ' says Graham. 'That is numbing and gets to you very quickly.'

First attempts to solve the problem followed methods used further back along the tunnel where a number of smaller voids and patches of loose material had been encountered.

Closely spaced steel ribs and infill grouting consolidated these sections and let the machine press on, even if they did slow progress somewhat.

'But when we tried grouting ahead of the machine at this point, we found it was just being washed out again with no time to set, ' says Graham.

After dwelling on the problem for a while it was decided to build a bulkhead in the tunnel that could hold back the flow and allow it to be pumped off in a controlled manner while consolidation grouting was carried out. A series of pipe and valve holes was cast into the 600mm concrete wall to allow both the tunnel water to be drawn off and grout and foamed concrete to be pumped in.

This needed substantial preworks. As much of the roof as possible was consolidated with grout. Access through the blades of the cutter head allowed the tunnellers to build two timber formwork stop ends with rebar socketed into drill holes in the tunnel walls. Pipe void formers were added between them before the space was filled with concrete.

Some attempts to see what might lie ahead were made with probes but they hit 'fresh air', so the only way to proceed was by trial and error, pumping a thin cement grout primarily into the void, some 600m 3 in all.

A forepoling system was used for a 'campaign of consolidation grouting' around the tunnel line ahead; then the machine was tentatively restarted, moving up through the new artificial ground.

The TBM is still in the patch of variable ground, says Graham, and a number of smaller soft areas have been encountered since, although the machine is now moving reasonably consistently. The hope is that the series of faults in the rock will end and that better progress can be made further on. Just under 4km has been completed on an overall 11km drive section, so there is a way to go yet.

The tunnel will carry around 75% of the water for a new 690MW underground power station from a 2,100M. m 3 capacity reservoir held back by a new 193m high rock fill dam - also being built by Impregilo.

First power is due to be generated in spring 2007.

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