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View from the valley

Contract 350/410

Contract 350/410

Medway Bridge & North Downs Tunnel Eurolink JV Miller Civil Engineering/DumezGTM/Beton und Monierbau 8.5km Target cost £110M Key features: Ultimately the most visible part of CTRL with Eurostar trains streaking through the Nashenden Valley and across the Medway at 300km/h right beside the M2 motorway. Most programme-critical is the elegantly slender, heavily reinforced, concrete box girder bridge at the Medway. A slow start was made with piling. It will have insitu balanced cantilever main and back spans and push launched approach viaducts. Innovative screw-displacement piling is now going in to deal with an asbestos tip beneath the eastern viaduct. The huge chalk tunnel being bored under Bluebell Hill is weeks ahead of schedule. But an unfortunate rockfall and serious injury accident late last year put a dampener on the spirit of the entire project.

The combined contract 350/410 package is shortest along the line. But RLE contract manager Alan Myers boasts: 'We've got the two significant structures of section one'.

CTRL's Medway Bridge is shaping up as a very slender and pristine white concrete box structure beside the mature form of the original M2 motorway crossing. This lies just to the north and is presently being strengthened by contractor Edmund Nuttall.

At the east end of the CTRL contract is the huge North Downs Tunnel which will carry both tracks of the railway 3.2km beneath Bluebell Hill.

This stretch of the route is about to become a very busy place. M2 widening contractor Costain/Skanska/Mowlem JV is soon to start work on Medway Bridge number three which has to be tucked in between the existing two. Eurolink and CSM have an intimate interaction right through the Nashenden Valley up to the portal of the CTRL's North Downs Tunnel.

The long climb of the M2 with its stepped dual two lane carriageways is being widened to dual four, concurrently with work on the railway.

Nashenden Valley ceased to be a peaceful spot with the arrival of the motorway some four decades ago. Now it is set to offer a spectacular journeydefining view for travellers by both rail and road.

But for the moment attention in the valley is focused on access arrangements for CSM, whose site is sandwiched between the M2 and the nearly completed earthworks of CTRL for more than 5km.

There is also some debate as to the extent to which chalk arisings from the tunnel can be re-used as structural fill beneath the new motorway alignment.

Site access sliproads from the motorway have been built by Eurolink using 80,000m 3was compacted in a berm on the line of the widening using material dug from the country portal. Shifting the spoil involved a fleet of 45 muck wagons each turning in 12 round trips a day lugging the chalk over the top of the hill by road.

Originally this delivery to the Highways Agency motorway project was to account for 250,000m 3of excess chalk.

But because the tunnel excavation is now way ahead of programme the Pushed to the limit Medway Bridge No 2 will soon be the most visible expression of the CTRL.

Just over half way from Folkestone to the high speed railway's eventual terminus at St Pancras, it is near the bottom of a vertical curve where Eurostars will be running at their 300km/h maximum permitted speed. In consequence it has to resist enormous dynamic braking forces imparted in the extreme case of two Eurostars passing in opposite directions and both making emergency stops.

Talk to the engineers involved with the bridge and very soon they mention the huge amount, and complexity, of the reinforcement that is involved in its prestressed concrete superstructure.

Talk to the engineers working on plans for Section Two of the CTRL and very soon you get the impression that the bridge illustrates how things are going to be done differently next time.

The original plan for CTRL allowed a period for a 'constructibility review' by the chosen contractors. But following the project suspension in January 1998, and its restart the following June, this period was lost.

'When the project was frozen it pushed the bridge design behind, ' says RLE's C350/410 contract manager Alan Myers.

On Medway Bridge Eurolink and RLE had to do the constructibility review on the run. It is not the most efficient way to develop a design, especially with a push launch which involves fundamental decisions on what are temporary works and what are permanent works.

On CTRL Section Two the chosen contractors will be involved in a full six month exclusivity period when they can comment on and come up with suggestions to improve the buildability of RLE's designs.

Eurolink's Dumez-GTM partner has a strong background in push-launch and balanced cantilever techniques, and has also brought in specialist Tony Gee & Partners to advise on construction engineering.

The structure will have a 152.4m main span to match the 500ft span M2 motorway crossing alongside. This span and the back spans will be formed as cast insitu, stressed balanced cantilevers built out from 13m deep hammerheads on top of the massive piers piled deep below the river. The 4.1m long segments will be cast using a 100t travelling form on each span.

The main span will be approached across prestressed concrete box girder viaducts push launched across the nine 40.5m spans on the west bank and 12 spans on the east.

Difficulties in obtaining site access and delays with the piling effectively bought the design team extra time for the superstructure. Construction troubles began with the jetty out to the main pier site.

Initial expectations that 17m long Larssen HB sections would do the job were frustrated and 33m sheets had to be driven instead. Bored pile subcontractor Bachy Soletanche then made a slow start with the foundations for the piers. Among other problems, the design for 1.5m bores reamed out to 2m diameter in the chalk 35m below platform level was abandoned in favour of straight sided holes.

Piling is now completed on the eastern main pier which is following on behind the western one. The balanced cantilevers are the most critical item on the programme.

Along the line of the approach spans, the slender fingers of the plasterlike finish, dazzling white, concrete piers are now all in place, except at Factory Farm where special foundations are required to cope with an asbestos tip.

Push-launching of the approach viaduct deck began on 31 January at the western abutment. In charge is Eurolink's engineering manager Gerard Garcia, who recently ran a push launched tunnel for the 0resund Crossing.

'All bridges are different and pushlaunch is demanding, ' he says. A special difficulty on the Medway, apart from the extremely confined launch site, is that the vertically curved, 25.5km radius, deck is running steeply downhill. 'You have to make sure that the bridge does not move under its own weight, ' cautions Garcia.

Stainless steel sliders under the casting bed and the Teflon faced Neoprene pads fed in to ease the deck's passage across the stainless steel bearers on the pier heads are all designed to minimise friction. Brakes are needed just in case the bearings are too effective. The 800t capacity hydraulic pushing frame is bolted to the rear segment of the deck so it can also hold the structure back. Flat jacks on top of the abutment can be inflated to lift the deck and act as a sprag to stop movement.

Each 40.5m span is being poured in the steel shutter as four match cast segments. The segment that will be the piertop unit is particularly heavily reinforced. Axial launch stressing tendons run through ducts in the box flanges and overlap at the pier segments. They are separate from the continuity stressing tendons which will be exposed within the deck box passing over deviator combs formed at the intricate piertop units.

An 85t steel girder nose reaching out from the front of the first unit avoids the need to deal with the huge bending moments that would be induced by a sagging 600t concrete span leading the way. The intended cycle time is two weeks per span. Site prefabricated reinforcement cages help speed the construction process for the four pours needed in that time.

A 'very strict' control regime is imposed during the pushing cycle of the cured and stressed deck span, says Garcia. All communications are brought to a focal point. Data monitored by the central computer includes pushing force, deflection of pier tops and rotation of the pilecaps. Staff on the piers feeding in the Teflon pads are in constant communication with Garcia so that if anything untoward is happening the launch can stop.

When the launch is completed the deck will be stressed to the abutment.

Long term shrinkage of the deck structure will then tend to reduce the deflection of the piers induced by the launch.

Attention is now focused on the balanced cantilever construction over the river. Questions still being resolved relate to craneage air rights when M2 widening contractor Costain/Skanska/Mowlem starts work on the motorway bridge running tight alongside CTRL.

The remaining 140,000m 3will be quarried directly from the London portal adit which has reached the Middle Chalk, a sufficiently good quality raw material.

Precisely when it is available and how it can be stored may affect the intention to use it as structural or landscaping fill.

There is a particularly demanding material at the bottom of the valley beneath four piers of the eastern approach viaduct to the Medway Bridge - asbestos in an old licensed tip at Factory Farm.

Bachy Soletanche is now attacking this with a specially developed rig. A massive screw auger mounted on the bottom of a kelly bar is being used to displace the material in the pit so that piles can be cast without bringing any significant amount of spoil to the surface. A metal shroud covering parts of the rig that run into the bore ensures that the asbestos is confined.

Once the piles are completed the caps and bases for the viaduct piers will be cast just above ground level to avoid the need for open excavation.

David Hartshorne Union Railways implementation director 'We are the client, ' says Union Railways' David Hartshorne firmly.

He describes himself as 'new railway' compared with his colleagues - Chris Jago, managing director of URS, and Alan Dyke, chief engineer. Hartshorne is seconded from Railtrack and joined the project eight months ago from Kvaerner.

He was responsible for the Queen Elizabeth Hospital PFI redevelopment at Greenwich. Before that he was technical director of Lusoponte, client for the second Tagus crossing in Lisbon.

'The reality of contracting is that you must get people to buy into the design. If you don't, and if things go wrong, they will say the design was cr*p in the first place.'

Leo Falkner Eurolink project manager - tunnels The North Downs Tunnel beneath Bluebell Hill is rated by Leo Falkner as 'in the top quartile' of the many tunnels he has worked on in 10 years with Beton und Monierbau. 'It's gone really well.'

Falkner is from Innsbruck - a good pedigree for a NATM specialist - and has driven tunnels in the Tyrol for water, road and power station projects, and in Germany for high speed railways.

When asked to go to the US on secondment to a joint venture, he jumped at the opportunity: 'a challenge!'. Falkner (pictured wearing the JV tie) helped drive the first two NATM tunnels in the US, for the subway in Pittsburgh, Pennsylvania.

Andy Sindle Eurolink project director C350/410 Andy Sindle shows off his tie because he is proud of it. The logo is Eurolink Joint Venture's which was created on site to symbolise the bridge and tunnel and the partnering with RLE and URS.

'The logo helps emphasise the joint venture and sinks the identity of people's original employer.' Sindle has about 100 staff from Miller, 15 from Dumez-GTM and six from Beton & Monierbau. Labour is mostly directly employed but he says: 'We treat our two labour-only subbies Tunnelcraft and Clipfine as partners.'

Sindle only relatively recently merged his own identity as a Miller man. He has been three years with the company following 23 with Balfour Beatty - some of that time working for Alan Myers, now his opposite number on the contract for RLE.

It is typical of the coming together throughout CTRL reuniting teams of people who worked together on the Channel Tunnel and the first batch of large design, build finance and operate road schemes.

'I've got 117 key dates. The client wants to be in operation on the final key date. And I want a happy client.'

Sindle espouses the 'softly, softly approach' as the best way of managing his £110M contract. But he emphasises: 'You need to get what you want!'

Under Bluebell Hill

In tunnelling nothing is ever certain until the last muck is out and all the permanent lining is in place. But with only about 700m of chalk left to explore with its top heading, the huge 3.2km long North Downs Tunnel is running well ahead of schedule and looking less and less critical to the CTRL.

Every day and night, seven days a week, two 12 hour shifts working from the London portal are gouging out another 8m to 10m of Bluebell Hill. They are driving to meet the top heading from the country portal which was sealed off after 1,470m - when it was 12 weeks ahead of programme - so that work could concentrate at that end of the tunnel on pulling out the main bench.

This task of opening the bore up to its full 166m 2cross-section is currently running twice as fast as planned.

The scale of North Downs Tunnel is big by normal UK tunnelling standards, dwarfing the few large machines and very few people involved in the excavation and shotcreting work. Each tunnel face is worked by just six miners and a shift boss backed up by a fitter, electrician and engineer. RLE's tunnel team is a section manager, field engineer for each shift, tunnel designer, an assistant and a field geologist.

By the end of 2003 the tunnel will carry both tracks of the CTRL with trains cresting a switchback and passing each other at 290km/h. Size of the tunnel allows plenty of space for the aerodynamic effects.

The primary rock support of shotcrete was originally to be lined with a 500mm thick tube of reinforced concrete cast insitu around a travelling shutter.

Value engineering based on experience of actual chalk excavation and a revised structural analysis proposed by contractor Eurolink led to the decision to slim the lining to a 350mm thick unreinforced insitu concrete shell. It will save £5M against the target cost: earning £1.25M for Eurolink and saving the client £3.75M.

The tunnelling support is not true New Austrian Tunnelling Method as it is 'not the observational method', says RLE contract manager Alan Myers. The arch truss spacing, rock reinforcement and shotcrete is fully designed in advance rather than being adjusted in response to measurements of ground movements.

A mass of instrumentation is included, from rock bolt pullout tests to surface settlement. Every day at 4pm the NATM engineer, production team and RLE's designer review all data from the previous 24 hours, agree the support regime for the next 24 hours and sign up to it.

Typical truss spacing, and hence the advance cycle, is between 1m and 2m depending on the ground. Now that the London portal drive has reached the middle chalk it is up at the higher end.

Swallow holes at the portal put work six weeks behind programme at one time. Under Buckmore Park kart track there was just 6m cover to the bottom of a filled valley, and there was 40mm surface settlement. In the good chalk deep beneath Bluebell Hill surface settlements are down to 4mm to 6mm at the most.

As soon as the chalk is cut to profile, a 50mm thick sealing layer of shotcrete goes on. The arches are fixed along with mesh reinforcement followed by 150mm thickness of shotcrete. Rock dowels are then drilled and grouted through the fresh concrete shell in arrays of 14 or 15 holes. The 4m deep holes are filled with thixotropic grout and 25mm diameter rebars pushed in. A second layer of shotcrete follows to give an overall thickness of about 250mm.

A cocktail of admixtures ensures that the shotcrete goes off and hardens very fast.

But a puzzling concrete failure occurred during the country portal drive.

For no obvious reason, the shotcrete changed behaviour and began to set and harden more like ordinary concrete.

Detailed investigations discovered that the cement was arriving on site coarser ground. The material was still within its manufacturing specification but the small change had a substantial effect on the specialised mix.

The unrelated accident last year in which a miner was crushed and severely injured prompted a safety shutdown for a week. A large lump of rock fell from the face during placing the trusses - the time when miners are most vulnerable to unexpected ground behaviour. Falls are more likely during excavation, with boulders dislodged by the roadheader, not when the face sealing layer is in place.

Alan Myers RLE contract manager on C350/410 The name Alan Myers is synonymous with Balfour Beatty and large scale tunnels - under the Channel, under Heathrow, under lots of other places.

After 28 years he crossed the divide and arrived on site heading up RLE's team on contract 350/410. Who better to be involved with the socking great NATM tunnel slicing beneath Bluebell Hill? 'It's not true NATM, ' he protests:

'Not the observational method.' But Myers made sure that everything was in place to keep a very close eye on how the ground and the shotcrete lining is behaving. He applied 'what we learned from Heathrow - no one could explain to them better than me!'

Having a large bridge to build is an added delight: 'Tunnelling has lots of rewards but everyone can see a bridge.'

Myers zealously espouses CTRL's special form of partnering, challenging visitors to guess by appearance and behaviour which of the team works for the contractor and which for RLE.

Time for history

Kent has been the principal transport corridor between mainland Europe and the British Isles for thousands of years and so it is not surprising that the linear trace of the CTRL cut across more than 60 known archaeological sites.

Some of these are being left undisturbed beneath mitigation earthworks, but 50 sites have been excavated - including unexpected discoveries made as work began. The overall investigation has become the UK's largest current archaeological programme.

Nationally important discoveries have included remains of a Neolithic Long House at Bluebell Hill, A Roman cemetery of 500 graves at Pepper Hill, Anglo Saxon cemeteries, and a Roman Villa at Thurnham - where the M20 had already partially destroyed parts of the complex. A great number of artefacts have been discovered ranging from swords to a range of domestic items and, inevitably, a large volume of pottery.

Unfortunately there has been a constant need for secrecy and sometimes 24 hour security to prevent illicit exploitation of the exposed sites. This has meant that RLE has not been able show off its archaeological activities to the general public during the course of excavation.

Excavation of known sites that would inevitably be disturbed by the railway involved a large amount of preliminary trenching to check what might be discovered. A priority has been to allow sufficient time for the archaeological investigation and at the same time minimise the risk of surprises that might hold up construction.

RLE's own team of archaeologists and its contractors Oxford Archaeological Unit, Museum of London, Canterbury Archaeological Trust and Wessex Archaeology worked closely with English Heritage and Kent County Council.

This work is now almost complete and efforts are concentrated on collating the data and working out the best way to publish it. 'We have to decide what to publish and how, ' says RLE's senior archaeologist Helen Glass.

There is a shortage of storage space in Kent for the material that has been discovered. And a suitable museum in which to keep it and display a selection for viewing has yet to be identified.

Plans for Section Two include careful examination of the route down through Ebbsfleet to the Thames crossing which was a busy place in Roman times.

Glass says that there may be areas of interest on gravel banks within the Stratford area but that on the final surface approach to St Pancras anything of interest is likely to have long gone with the large holes sunk for the gas holders in the area.

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