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Going underground

The Medway bridge is not the only star of Contract 350/410.

But M2 motorists probably do not appreciate the contract's other big construction site, as it lies beneath the North Downs in the form of a 3.2km tunnel.

This is separated from the bridge by a 4km overground stretch of earthworks, recently completed and awaiting ballasting.

Excavation of the 14m high, 13m wide tunnel through the North Downs chalk strata is now complete, a process carried out in three stages due to the scale of the tunnel, explains Myers.

'The tunnel was too big to do in one excavation, ' he says.

'So first of all we excavated a 6m deep top heading.'

Excavation began at the North Downs portal in 2m drives, using a road header excavation machine. In areas where the chalk was expected to be weathered, 20 steel bars or 'spiles', 4m long and 20mm in diameter, were inserted into the chalk to prevent rocks from falling during the excavation process. But Myers explains that part way through the project a rock collapse occurred in an area where the chalk was thought to be sound, resulting in a permanent injury to site worker Shane Halliday.

'Then we used spiles for every excavation, ' he says.

Following the drive, the exposed chalk was immediately sprayed with a sealing layer of concrete and a lattice girder attached to the tunnel walls together with a reinforcement mesh. The whole section would then be re-sprayed with concrete and the process repeated to a thickness of 250mm.

This five hour process to create a 2m length of primary tunnel lining was sometimes extended, however, if a section of rock was particularly weathered and needed 350mm thickness. Every other drive would also see the insertion of 14 rock bolts in the crown.

Once the top heading had been completed to the centre of the tunnel, the same technique was employed on the second 4m deep bench layer of excavation. The final invert excavation was carried out using a continuous miner machine - normally used on road schemes - to produce a final, blinded flat surface.

'We excavated and blinded 1,500m of tunnel in nine days, working 24 hours a day, ' says Myers. The resulting 0.5Mm 3ofspoil for the total excavation works was re-used over the project as structural fill, a fifth of which was placed in an embankment for the nearby M2 widening project.

Current work includes laying the final 600mm reinforced concrete surface over the blinding, and placing the tunnel's secondary lining, due for completion in June next year.

'Originally, we had designed the secondary lining to be a reinforced layer over a full waterproofed membrane, ' says Myers, 'but we redesigned it as unreinforced, assuming that the primary lining was actually rock.'

This reduced the secondary lining thickness from 500mm to 350mm, increasing the internal volume of the tunnel. It also helped to reduce the eventual air pressure differentials created when two trains passed each other at speed in the tunnel. The waterproof membrane ensures there is no infiltration, which is instead redirected around the tunnel through a drainage fleece and is then drained by gravity.

So far 700m of secondary lining have been completed.

Initially the fleece and waterproofing layers go up using a custom-made lining gantry. This is eventually followed by a 12m long concrete spraying gantry.

'It takes 6 hours to spray a section and 8-10 hours for it to go off, ' says Myers, adding that one 12m section is completed in a day. The gantry then moves along to the next 12m section, dragging a 24m curing gantry behind it that ensures the concrete is cured over two days to achieve an eventual 40N/mm 2compressive strength.

The curing gantry holds a plastic membrane over the concrete to help prevent cracking and plasticisers have been added to the wet sprayable mix to prevent segregation.

Completion of the Medway bridge and the North Downs tunnel is expected by the end of 2001.

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