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A tale of love and Mersey

Technical feature Mersey Tunnels

New cross-passages are being built between the Mersey Kingsway tunnels to improve safety measures following a review after recent fatal tunnel fires in Europe. Diarmaid Fleming went to see the work.

Arecent series of catastrophic tunnel fires in mainland Europe - most notably in the Mont Blanc tunnel between France and Italy where 39 people lost their lives in March 1999 - prompted a review of safety in tunnels across the continent. Failings were confirmed by studies, published in April by European motoring organisations, which classified some tunnels as 'very poor'.

While the Mersey Kingsway tunnels topped the safety list in the UK, being classified as 'good', and the older sister Queensway bore 'acceptable', work was already under way to improve safety.

The Mersey tunnels are essential arteries for Liverpool, forming a link under the River Mersey between the city and Wallasey across the water. The Queensway Tunnel was built in 1934 - the importance of a link equalled by the engineering feat of building a 3,400m tunnel in those times. Two more tubes were added in 1971 and 1974 to create the parallel 2,244m long Kingsway tunnels.

'Following the Channel Tunnel Fire in 1996 and Mont Blanc in 1999, we were concerned about emergency evacuation in the event of a fire in one of the tunnels,' says Mersey Tunnels chief engineer Peter Arch.

Finding a solution to the twinbored and busier Kingsway tunnel was looked at first. The £1.20 a time toll provided by the 26M vehicles using the tunnel supplied Arch with a significant war chest, an advantage which Arch freely admits places him one step ahead of colleagues responsible for tunnels reliant on public funds.

Merseyside Fire Brigade and Mersey Tunnels both identified the need for additonal cross-passages to allow motorists a quicker route of escape from one bore to the other as well as providing better access for emergency services in the event of fire.

The 9.63m diameter tunnels - approximately 27m centre to centre apart and at the same level - were built with only two cross-passages linking the ventilation shafts, between 450m and 470m from the Liverpool and Wallasey portals.

Consultant Mott MacDonald began a detailed geotechnical study to determine the location of the new cross-passages. 'We carried out a review of the original ground investigation studies. The most reliable guide however was from the records of the actual construction of the existing road tunnels,' says Mott project engineer Kevin Roberts.

Cover above the tunnels under the Mersey varies between 7m and 15m above the crown. Bored in the Triassic Bunter Sandstone, the rock type posed difficulties during construction due to its fissile nature and presence of silt seams.

Mott's preliminary design opted for three new cross-passages at 325m apart and located away from areas of difficult ground.

Tunnelling specialist AmcoDonelon was awarded the contract in September 2001 for £2.2M. The preliminary design envisaged the passages to be constructed with 3m internal diameter bolted segmental spheroidal graphite iron linings.

But AmcoDonelon's alternative of 3.35m ID precast concrete standard bolted segmental lining with a cast insitu secondary lining to give a finished internal diameter of 3m was accepted.

Mott MacDonald and Amco worked together on the design, which Arch says has encouraged a partnership approach. 'We did not set out to be prescriptive - we wanted something buildable,' he says.

Further site investigations were carried out after the award.

'The core passages were selected where we thought the best ground might be, based on the old records, but what was good ground 30 years ago may not be now, after the tunnel was driven and with overbreak,' says AmcoDonelon divisional director Mark Turner. 'It's a very soft but competent rock, offering strengths of around 5MPa-6MPa. The major risk in tunnelling would be hitting a soft spot with a flood of water coming in,' he says.

'The rock is porous and there's plenty of Mersey water in it. But ground treatment reduced the water by around 50% to 60%,' says Arch.

This was achieved by drilling and grouting a series of drainage holes around each cross-passage, working from the South Tunnel to the back of the lining in the North Tunnel in 6m lengths.

'We used an ordinary Portland cement 2:1 mix with pressures up of 1 bar above hydrostatic to 4.5 bar with around 60t of grout used,' says AmcoDonelon site manager Jim Yerkess. The Mersey's high tidal range can cause hydrostatic pressure variations of up to 1 bar.

Before sections of the tunnel lining could be removed to enable excavation of the crosspassages, dowels were socketed into rock for temporary support, while a fabricated steel cill at the bottom of the cross-passage excavation prevents movement of the segments below. Measurement systems were also installed to monitor movement.

One lane of the dual carriageway tunnel must be kept open at all times for emergency vehicles, limiting the working area to the other lane. This confined space determined the choice of machinery. 'We are using a remote controlled Brokk 250 and a 330, similar but more powerful than a miniexcavator but with a pecker to break out the rock. The larger 3.5t 330 can develop the same force as a 20t excavator,' says Turner.

Inside the excavations, these formidable beasts break off the rock almost like lumps of chocolate yet, being remote controlled, reduce exposure of workers to vibration. Some hand digging is however needed to sculpt the rock before fitting segments. Once the Brokks have formed a sufficiently large excavation, a larger Webster 2000 CR roadheader takes up the task.

Excavation proceeds at around 2m per day, before three 0.61m six-segment precast rings are installed by telehandler.

Once at the half-way point, tunnelling work will be completed from the North Tunnel.

Complicated surveying work is needed to ensure that the new tunnels, named Tom, Dick and Harry after those in The Great Escape, meet up.

All the work has to be completed in a tunnel posession between 7.30pm and 6am, with replacement of secondary lining to hide evidence of the night's work a requirement to prevent distraction to motorists. 'The logistics are very important. It's not like a greenfield tunnel,' says Turner.

Yerkess says the team of 30 manages to set up and clear up in half an hour at the end of each shift, in time to allow traffic through and avoid substantial penalties which could otherwise accrue. Sometimes the wish to keep going has had to be tamed. 'There has to be a strict regime and tight discipline,' he says. Five nights a week has been enough to maintain progress, with the works due for completion in August.

Completion in this time is no mean feat as other work has included refurbishment of an enlarged section of the existing tunnel, and construction of two other new passages under the deck for maintenance staff.

Once completed, the new passages will provide an escape time of less than five minutes.

Already plans are under way for work on the sister Queensway Tunnel - likely to involve an under-deck escape design.

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