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Safe passages cross the Mersey

UNDERGROUND CONSTRUCTION

A Europe-wide review of tunnel safety has prompted the installation of new cross passages in the Mersey Kingsway Tunnel. Diarmaid Fleming reports.

Liverpool's Mersey tunnels are its arteries, linking the city centre with Wallasey.

The oldest is the Queensway Tunnel, built in 1934. The importance of a link beneath the river can be measured by the engineering challenge of building a 3,400m tunnel in those times.

As the city grew, two parallel tubes were added in 1971 and 1974 to form the 2,244m long Kingsway Tunnel. Today they carry 26M vehicles a year.

A series of fatal tunnel fires in Europe - most notably the March 1999 Mont Blanc tragedy in which 39 people lost their lives - prompted a safety review. Studies published in April 2002 by European motoring organisations classified some tunnels 'very poor'.

While the Kingsway Tunnel came top of the list in the UK, classified as 'good' (the older Queensway bore is 'acceptable'), work is already under way to improve safety.

Mersey Tunnels' chief engineer Peter Arch says: '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.' The twin-bored and busier Kingsway Tunnel was considered first. Its £1.20 toll supplied Arch with a war chest of £31.2M, enabling work to start early. He freely admits this puts him one step ahead of colleagues responsible for other tunnels that rely on public funds.

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

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

Consultant Mott MacDonald was asked to devise the engineering solution and began with a detailed geotechnical study to help determine the location of the new cross-passages.

Mott MacDonald project engineer Kevin Roberts says: 'We carried out a review of the original ground investigation studies.

However, the most reliable guide was the records of the actual construction of the road tunnels.' Cover above the tunnels under the Mersey varies from 7m to 15m above the crown. Bored in the Triassic Bunter Sandstone (also known as Sherwood Sandstone), the rock type posed difficulties during construction because of its fissile nature and the presence of silt bands.

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

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

AmcoDonelon suggested an alternative, using 3.35m internal diameter precast concrete standard bolted segmental lining with a cast insitu secondary lining to give a finished internal diameter of 3m. This was accepted.

Under the Engineering and Construction Contract (Option C), target cost with activity schedule, Mott MacDonald and Amco worked together on the design.

Peter Arch says the solution has turned out well and encouraged a partnership approach on a contract with difficult constraints.

'We did not set out to be prescriptive - we wanted something that was buildable, ' he says.

Further site investigations were then carried out. AmcoDonelon divisional director Mark Turner says: 'The cross passages were selected where we anticipated the best ground might be, based on the old records.

'But what was good ground 30 years ago may not be now after the two tunnel bores were driven, considering overbreak and the fissile nature of the rock.

'It's a moderately weak but competent rock, offering a compressive strength of about 5MPa.

The major risk in tunnelling would be hitting a major fissure with attendant problems, ' he says.

Arch adds: 'The rock is porous and there's plenty of the Mersey water in it. If preparatory work had not been carried out then the ground would have been a lot wetter - ground treatment reduced the water by about 50-60%.' Ground treatment was achieved by drilling and grouting through a series of grout holes and drainage holes around each cross-passage, working from the south tunnel to the back of the lining in the north tunnel in 6m lengths.

AmcoDonelon site agent Jim Yerkess says: 'We used an ordinary Portland cement 2:1 mix with pressures up of 1bar above hydrostatic to 4.5bar with about 60t of grout used.' Pressures varied as the Mersey's high tidal range can cause hydrostatic pressure variations of up to 1bar.

Before sections of the existing tunnel lining could be removed to allow excavation of the cross passages, temporary works had to be installed to support the tunnel lining beside the excavation.

Dowels socketed into rock hold the lining in place above the cross-passage opening, while a fabricated steel sill at the bottom of the cross passage excavation prevents movement of the segments below. Measurement systems were installed to monitor any unexpected movement.

The work is being done during night-time closures, but one lane of each dual carriageway tunnel must be kept open at all times for emergency vehicles, severely limits the working area.

This determined the choice of machinery, which had to be capable of working in a very confined space. Turner says: 'We are using a Brokk 250 and a 330, equivalent to a mini-excavator except with a pecker to break out the rock. The larger 3.5t 330 can develop the same force as a 20t excavator.' The machines are remote controlled and therefore reduce worker exposure to vibration, although some hand digging is needed to finish off before fitting segments.

Once the Brokks have formed a sufficiently large excavation, a larger Webster 2000 CR roadheader has enough space to continue excavation, which proceeds at about 2m a day, before three 0.61m six-segment precast rings are installed by telehandler.

Once they have reached halfway, tunnelling work will be completed from the other tunnel.

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

The tunnel possessions run from 7.30pm to 6am, and each shift has to replace a secondary lining to hide all evidence of the night's work so it does not distract motorists.

Turner says: 'The logistics are very important - when we come out we have to leave everything hidden. It's not like a greenfield tunnel.' Yerkess says the team of 30 manage 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 this month.

This is no mean feat as other work includes refurbishment of an enlarged section of the existing tunnel, and construction of two new passages under the deck for maintenance staff.

When completed, the new passages will provide an escape time of less than five minutes. Already planning has begun on work for the Queensway Tunnel, which is likely to involve an under-deck escape design.

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