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And this is the easy bit. . .

The Channel Tunnel Rail Link is the UK's biggest civil engineering project. Construction of section one, between the Channel Tunnel and Ebbsfleet, is well under way, with section two set to begin in summer 2001. Ground Engineering examines the geotechnica

It could be argued that in geotechnical engineering terms, section one of the Channel Tunnel Rail Link is a simpler prospect than section two. The 74km long line, from the Channel Tunnel terminal near Folkestone to Ebbsfleet, just south of the River Thames, is mainly overland, whereas section two, between Ebbsfleet and London's St Pancras Station, is largely underground, throwing up a vast array of geotechnical challenges.

This does not mean that section one is a walk in the park. Concerns voiced by Kent residents have meant that environmental issues are high on the agenda, requiring schemes designed to minimise the impact of trains travelling at up to 300km/h. This, combined with the problems of building a high-speed rail line from scratch, has led to a number of interesting geotechnical solutions.

The strict environmental regime goes back to the project's environmental statement of 1994 and requirements set out in the 1996 CTRL Act. Most of the line is being moulded into the ground, with extensive landscaping further softening its impact. The CTRL is being built in cuttings where possible and the resulting surplus of material is being used for false embankments and bunds. Design of mitigation works is carefully matched to the local geology and landscape.

London & Continental Railways holds the franchise to build and operate the £5. 2bn link and to own and operate Eurostar (UK). LCR's shareholders are Bechtel, SBS Warburg Dillon Read, National Express Group, Systra (as subsidiary of French Railways SNCF), London Electricity, Ove Arup & Partners and Sir William Halcrow & Partners.

Engineering design and project management is being carried out by Rail Link Engineering, a consortium of the engineering shareholders in LCR: Bechtel, Systra, Arup and Halcrow. Client for the £1. 67bn (at 1997 costs) Section One is Union Railways (South), a LCR company controlled by UK rail network manager Railtrack.

Contracts for section one include financial incentives for contractors. If construction cost is below target, they get 25% of all savings. If cost comes in under 90% of target, they receive 50% of any savings. But there are corresponding disincentives. If cost exceeds target, the contractor will not be entitled to its fee and will have to pay Union Railways (South) 25% of the overrun up to 120% of the target and 10% of any excess beyond that. After 18 months, work is still under budget and largely on programme.

The first contracts for section two (£2. 25bn at 1997 prices) will be awarded in December this year, with work set to start in July 2001.

Section one is due to open in October 2003 and the St Pancras to Paris Gare du Nord service is due to start in December 2006.

Contract 330: East Thames, Medway Valley & Waterloo Connection

Contract 330 will be the first CTRL main contract to finish, in the middle of December this year.

A joint venture of Alfred McAlpine Construction and Amec Civil Engineering is working on the 16km long section which has a target cost of £80M and forms the eastern end of section one.

Earthworks, which began in March 1999, involve moving a massive 3M. m 3of mainly chalk to form the cuttings for the line and for modifications to the M2 motorway and other roads.

Near the west bank of the River Medway a huge cut has been created for the first stage of the remodelled M2 junction two interchange.

This will be finished later by M2 widening contractor Costain/Skanska/ Mowlem jv, but there was a large part including rail over- and underbridges that was more logically constructed with the CTRL.

Another huge slot has been sunk into the chalk at the west end of the contract where the Waterloo connection of CTRL is being built along the line of the abandoned Gravesend West Railway. One side of the original cuttings and embankments of this 3km dogleg to Fawkham Junction has been left as an overgrown, environmental screen. The other side and virtually every original structure has been removed.

The contract involves considerable excavation near upper levels of chalk and a major part of the contract has involved dealing with dene holes, the remains of shallow chalk mines dating back to before the 19th century.

While access shafts of up to 5m deep were backfilled, the old adits were not. These tend to collapse when the cover is stripped to expose the upper layer of chalk. Careful backfilling and compaction is needed to ensure that they do not interfere with the integrity of the railway formation. Compaction specification is demanding, with moisture contents of between 20% and 24%.

Sub-grade being laid on the top is recovered railway ballast which is washed, screened and blended for the contract by Foster Yeoman at the Isle of Grain.

At Scalers Hill the cut exposed the clay, silt and sand from the highest level of Woolwich & Reading Beds, which meant the sub-grade had to be beefed up from 500mm to 700mm thick.

Contract 350/410: Medway Bridge & North Downs Tunnel

At 8. 5km the combined £110M package of contract 350 and contract 410 is the shortest along the line but contains the two most significant structures of section one - the Medway Bridge and the North Downs Tunnel.

Work is being carried out by Eurolink, a joint venture of Miller Civil Engineering, Dumez-GTM and Beton und Monierbau.

Heading north, Eurostar trains w ill pass through the 3. 2km tunnel below Bluebell Hill before reaching their top permitted speed of 300km/h as they travel through the Nashenden Valley and across the River Medway.

Construction of the heavily reinforced concrete box girder bridge is the most programmecritical part of the contract and is slightly behind programme. It is being built just south of the original M2 crossing, now being strengthened by contractor Edmund Nuttall.

The structure will have a 152m main span, 12 spans on the east bank and nine on the west.

Piers are supported on 1. 5m diameter bored piles up to 35m below platform level, founded in the chalk. These were installed by foundation contractor Bachy Soletanche and were originally to be reamed out to 2m but the plan was abandoned in favour of straight-sided shafts.

Piling was delayed at the start of the contract when sheet pile installation for the temporary jetty out to the main pier site encountered problems.

Bachy used a new displacement pile technique to cope with an asbestos tip below four of the piers for the eastern approach viaduct. This allows piles to be formed without bringing significant amounts of the dangerous material to the surface. Once piles are complete the caps and bases for the viaduct piers will be cast just above ground level to avoid the need for open excavation.

The 3. 2km long North Downs Tunnel below Bluebell Hill is large by UK standards, with a 166m 2cross-section to allow for the aerodynamic effects of two Eurostars passing each other at 290km/h.

After the chalk is excavated to profile by roadheader and grouted spile pins installed above the heading, a 50mm layer of shotcrete is applied, followed by steel trusses and mesh reinforcement. Another 150mm of shotcrete is applied, 4m long rock dowels are installed and then a final layer of shotcrete sprayed on, giving a total thickness of 250mm. Final lining is 350mm thick unreinforced cast insitu concrete.

Tunnel support is not true New Austrian Tunnelling Method as it does not use the observational method. Support is designed in advance rather than being adjusted in response to ground movements. Monitoring data from the previous 24 hours is analysed every day and the support regime for the following day decided.

Swallow holes up to 40m deep hindered construction at the start of the contract but the tunnel is now back on schedule.

Contract 420: Mid Kent, Boxley to Lenham Heath

Contract 420 includes one of the most environmentally sensitive sections of the entire CTRL route, where it crosses Boxley Valley. The 20km long section runs mainly alongside the M20 motorway and is being built by a joint venture of Hotchief and Norwest Holst with a target cost of £85M.

Work began last June and so far earthmoving subcontractor John Jones has shifted 2. 1M. m 3of the total 4. 6M. m3. Earthworks are expected to finish this year. Value engineering lead to a reduction in temporary works on three cut and cover tunnels, including the Boxley Tunnel. Construction time is now estimated at only two-thirds of the original programme.

Four critical structures are the key to completing the civils work for this section on time in October 2001: the A249 crossing, the bridge over the Maidstone to Ashford railway, piling for the cut and cover tunnel at Harrietsham, where the line comes very close to the A20, and a nearby realignment and bridge taking the high-speed line over the A20.

These will allow sand from the east end of the contract to be moved to build the earthworks at the west end. Gault Clay that outcrops through the western half is unsuitable for any earthworks construction apart from landscaping bunds and spoil cannot be hauled along the A20 or M20.

At the A249, the road was temporarily realigned and a bridge deck built at ground level across two lines of contiguous piles installed for the CTRL. Once the deck is cast the road will be rediverted so the railway and its permanently strutted approach cuts can be burrowed out beneath.

Just to the east is a deep pile retained cut through the ancient Horish Wood. Originally, the line was to run in a wide cut in the Weald Clay but instead massive contiguous pile walls are being used to minimise land take and reduce damage. Replacement woodland will be planted to minimise the damage caused by construction, part of 230ha of 'new' ancient woodland that is being created along section one.

Stent Foundations has finished piling for all the critical structures except the Harrietsham tunnel. In all, the firm will install 2,700 bored piles on the contract.

The cut and cover Boxley Tunnel was originally going to be built by excavating between contiguous piled walls, placing three levels of props, then casting the invert and roof before backfilling and landscaping. Instead, the roof will now be cast first, the fissured Gault Clay dewatered and the tunnel excavated before the invert is cast, reducing the propping.

Contract 440: East Kent, Ashford to Cheriton

For most of its 16km length, the £75M Contract 440 runs alongside, straddles and travels above the existing route taken by the Eurostar trains.

This interaction is a concern for main contractor Balfour Beatty Major Projects, with punitive damages of £75,000 per hour for stopping the Eurostar service.

Where the new railway leaves the old one, it is surrounded by the complex of motorway slip roads, services and communications networks leading out of the Channel Tunnel terminal at Cheriton. Work is being carried out on a number of isolated sites to fit in the foundations and superstructures of new bridges for the CTRL.

The tracks then fork away either side of the existing line and climb up past Dollonds Moor high security goods yard.

South of the yard the London line will be carried on a seven-span, 25m high steel composite structure between the sidings and a thick wood at Grange Alders. Each bridge pier is supported by nine 1500mm diameter bored piles, base grouted to improve end bearing capacity.

On the north side, the line rises steeply through a deep cut in the hill above the 150year-old brick-lined Saltwood tunnel. The lining will become part of the foundation for the flying junction carrying the CTRL 'up' line over from one side of the original London to Folkestone railway to the other.

The 960m long tunnel's integrity is paramount because at the moment it carries all the Channel Tunnel rail traffic. But removing 14m of material from above the tunnel is likely to cause heave and other ground movements that could disturb and distort the brick lining.

Future dynamic loading from the high-speed traffic could also affect the structure.

There is no clearance inside the tunnel for an additional reinforcing lining, so intensive survey, grouting and monitoring is being carried out, aiming to guarantee its stability.

Hourly checks on tunnel movement are made using total stations to read targets at 25m intervals along the tunnel.

Some 2,200 holes have been drilled through the 600mm to 900mm thick lining to allow inspection. The lining is proving to be in much better shape than expected with a minimum of voids behind it, despite the old timber lagging left in place by the original miners. Areas needing the most attention are around additional refuges cut through the lining in the 1960s, which caused washouts of the sand and left large voids behind.

Sand excavated from the cutting will be used as structural fill at the western end of the contract where soft silty clay is unsuitable.

Earthworks contractor CA Blackwell has been testing lime stabilisation of the clay so it can be used for landscaping. Some 0. 5M. m 3of the total 2. 5M. m 3material has been moved since work began in May 1999.

West of the tunnel, the new line runs very close to the old line at about the same level.

There are two pinch points, total length 2km, where new embankments have to be keyed in to the existing ones to ensure the integrity of both.

For overlapping structures up to 3. 5m high, the toe of the old embankment is cut in 1m long strips and new fill compacted in place. For structures over 3. 5m, a wide platform of new material is compacted to form the base of the CTRL earthworks first and underpinning is performed by trenching down again at the intersection with the old embankment.

CTRL section two

Section two of CTRL is roughly half the length and, at £2. 25bn, nearly twice the cost of section one. Its construction will involve extremely complicated urban underground work, undoubtedly requiring innovative geotechnical solutions.

The section will link CTRL to the rest of the UK rail network. It begins near Pepper Hill in Kent, runs through Ebbsfleet and into 3km long twin tunnels under the Thames at Dartford, crosses the Essex marshes on a viaduct and a 16km long piled slab and runs into 19km long twin tunnels beneath east London via Stratford before emerging north of King's Cross to terminate at St Pancras Station.

The tendering process for the main contracts began last month, with the first contracts let in December. Already £300M is committed to design, land acquisition and preliminary works, ready for a 1 July 2001 construction start. Right of way for 18% of the route is secured and this should rise to 80% by the end of the year.

Some utility diversions in east London are already under way. Archaeological packages are being let for Swanscombe and Ebbsfleet. Bids are being invited for three bridges at Dagenham. Two of them are essential to the Ford Motor Works, which will be bisected by CTRL, and one is worth about £6M on its own.

While the Thames tunnel is on the critical path, the biggest challenge is the new King's Cross/St Pancras Thameslink station. This will be in a 500m long, 25m wide box built underground diagonally across the north end of St Pancras Station. It will also carry West Anglian and Midland lines.

The box will be the bottom layer of a complex structural sandwich which has to be built in and around existing rail traffic. The project involves slewing the Midland lines to the west side of St Pancras, constructing the east side of the high-level deck extension to the station, moving the trains back onto the deck, digging out the box and building the west half of the deck. In addition a tunnelled burrowing junction has to be built across railway land to bring the West Anglia lines into the Thameslink box.

Thameslink services will have to stop for 25 weeks to allow this work to go ahead and box construction cannot start until the end of 2003, only three years before the line opens.

Despite its size - 1km long by 50m wide and 25m deep - Stratford's station box will be relatively straightforward and doubles as the worksite for the twin 7. 15m diameter running tunnels driven both to east and west.

A six-month exclusivity period will allow contractors to procure tunnel boring machine designs. Earth pressure balance machines are likely to be chosen to cope with groundwater and settlement through the London area. Much of the western tunnels run beneath the North London Line (NLL) cuttings, so rotation of the brick walls and abutments of the old bridges has to be controlled. At two places the tunnels have to pass through the piles beneath modern replacement bridges over the NLL. The eastern drives pass into Thanet Sand where dewatering will have to be used to control the dense low permeability material.

The twin bores of the Thames tunnel are likely to be driven with a slurry machine. Water from the chalk, and possibly the river, was a major difficulty when the Dartford road tunnels were driven using compressed air at up to 3bar.

The marshes east of London will be crossed on a 16km long piled slab. Ground conditions are a 4m thick peat and alluvium layer overlying Thames Gravel. Pile tests are being commissioned this summer to help bidders.

Involvement of contractors in refining the design is seen as particularly important because of the thousands of piles involved.

Timetable

1994 Channel Tunnel opens. London Waterloo to Paris Gare du Nord in 2hr 54min. Eurostar trains run on dedicated track in France but in England they must share Victorian-built track with commuter trains

1996 February: Government and London & Continental Railways sign development agreement obliging LCR to build CTRL December: Channel Tunnel Rail Link Act passed by Parliament

1997 April: First construction tenders issued

1998 January: Crisis as LCR announces it cannot raise finance for CTRL June: Government announces deal with LCR and Railtrack to build CTRL in two sections. Railtrack to buy section one, has option on section two. Tendering process restarts September: First civils contracts let for section one October: Construction starts on site

1999 February: LCR restructuring completed 2000 January/February: System-wide track/power and system-wide signalling/comms contracts let June: Tenders invited for civils work on section two, St Pancras to Ebbsfleet July: Rail Regulator's determination on track access charges (CTRL remains unregulated) Autumn: Railtrack and shareholders'decision on purchase of section two. Bond issues for LCR to finance section two December: Completion date for first civils section one, Contract 330 December to January 2001: Civils contract awards section two

2001 June: Track and power insta l lat ion beg ins on site section one July: Section two construction starts, Ebbsfleet to St Pancras 2002 Order must be placed if 200km/h commuter trains are to be ready for section two

2003 July: Railtrack's opt ion to buy Sec t ion Two expires October: Section one opens, London Waterloo to Paris Gare du Nord in 2hr 35min

2006 December: St Pancras to Paris GdN service starts,2hr 20min Circa

2010 SNCF completes Roye deviation; St Pancras to Paris Gare du Nord 2hr 3min CTRL section one

contracts

Contract 330: East Thames, Medway Valley & Waterloo Connection Alfred McAlpine/Amec JV Alfred McAlpine Construction/Amec Civil Engineering £80M,16km

Contract 350/410: Medway Bridge & North Downs Tunnel Eurolink JV Miller Civil Engineering/Dumez-GTM/Beton und Monierbau £110M,8. 5km

Contract 420: Mid Kent, Boxley to Lenham Heath Hochtief/Norwest Holst JV £85M,20km

Contract 430: Ashford Kvaerner Construction £150M,14. 4km

Contract 440: East Kent, Ashford to Cheriton Balfour Beatty Major Projects £75M,16km

Contract 434: Railway Infrastructure Modifications J Mowlem & Co £60M

Contract 552: Ashford Re-signalling Westinghouse Signals £6M

Contract 570: System wide track and power Amec/Spie £120M for 75km of high speed track

Contract 550: System wide signalling and communications CCA Consortium CSEE Transport/Corning Communications/Amey Rail £56M

Contract 430: Ashford The contract to run the CTRL through the heart of Ashford is undoubtedly one of the most important on section one. Any hold-ups here could cause problems for the entire programme.

Kvaerner Construction is building the 14. 4km section with a target cost of £150M. It includes a 2km long cutting and two short tunnels through the town centre, which have complex interfaces with live railways including Eurostar's current line, the M20 motorway, utilities and signalling systems. Flying junctions with the cutting and a 1. 5km long viaduct to the east of the town are also being built so Eurostar trains can be switched onto a link to Ashford International Station.

The cutting is formed between pile retaining walls made up of between 900mm and 1500mm diameter bored piles from 20m to 42m deep. Much has been excavated and work is now concentrated on forming the shear connection joint between the base slab and the piled walls. Dewatering allowed retrofit formation of the shear key, enabling Kvaerner to get the central section of the thick base slab in place early. But dowelling into the piles using robotic drills and avoiding the cast-in reinforcement cages is proving to be more of a challenge than expected.

One of the most complicated sections of the trench is where Godinton Rroad used to cross the Maidstone railway. Here, CTRL tracks will go under the railway in concrete box, 18m below ground level. First, the brick arch carrying the road was demolished in a programmecritical 24-hour possession. Construction of the box is in two parts, with the eastern section built first, allowing the railway to be moved on top. The tunnel will then be completed and the lines moved to their final alignment in November.

Continual dewatering by dozens of wells is needed to stabilise the soft silty clay here and a massive gravity retaining wall is being built along the north side to support the ground behind residential properties.

Just to complicate the stack of intersecting lines and roads this is also the point where the two CTRL tracks switching to and from Ashford Station will curve around the Maidstone line and ramp down to meet the high-speed line.

At the eastern end of the trench, the CTRL will rise to the surface, running alongside existing lines before ramping up onto a 1. 5km long viaduct, being piled by Kvaerner Cementation Foundations. Here, an antique level crossing at Aylesford Place has been replaced by a 1000t box that was jacked under the line, just over 1m below track level, during a 33-hour Christmas maintenance closure last year.

Some 0. 6M. m 3of clay will be excavated from the cutting, used in mitigation works on the 10km country section of the contract.

While less complex, this section involves construction of a dozen bridges, a short tunnel and a major bridge crossing of the M20.

S2 Contracts St Pancras:

C102 Gasholders and governor C103 Civil works on King's Cross railway lands C104 Trackwork staging C105 Deck extension St Pancras Station and train shed refurbishment St Pancras Station, including Thameslink tunnels London Tunnels & Stratford:

C220 Stratford to London portal C230 Stratford box C232 Stratford Station C240 Stratford to Barrington Road C250 Ripple Lane to Barrington Road C260 Precast concrete tunnel linings Thames & Ebbsfleet:

C302 Ford service diversions (let to Alfred McAlpine/AMEC) C303 Ford and Choats Manor Way bridges C310 West Thames C320 Thames Tunnel and route civil works C340 Ebbsfleet Station C342 Highway works connecting A2 and Ebbsfleet Station C360 National Grid cable diversions C361 Gas and sludge mains diversions C364 Supply of pressure reduction stations C365 pipe jacking for utilities at Ripple Lane (let to AMEC)

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