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Awfully big venture


Only four months in and with the first of two three-month back-to-back rail blockades under way, work on the Channel Tunnel Rail Link's Stratford Box is progressing apace.Dan Simpson reports.

The UK's largest diaphragm wall, the £29.6M Stratford Box, is taking shape in east London.

Designed to house Stratford International Station, it is a key part of the Channel Tunnel Rail Link section 2 (GE March 2001). By 2007 the station will form an important interchange on the high speed railway from the Channel Tunnel to central London.

Foundation contractor Cementation Foundations Skanska is building the wall, which will have perimeter of 2434 linear metres and a total area of 70,000m 2.The giant box will be 1073m long, up to 50m across and 25m deep.

Several project deadlines rely on the construction programme for the box. Blockades of two rail lines intersecting at the east and west ends - planned and approved months in advance - have to be strictly adhered to and the tunnel launch chambers, for contract 220 in the west and contract 240 in east, have to be ready on time, explains Cementation project director Julian Crawley. His firm is working for main contractor Skanska Construction.

Construction began in the deepest excavation areas at each end of the box and is working towards the shallower centre, producing an arched profile. This will allow tunnel boring machines to be lowered in at each end and start work on the tunnelled links while the remainder of the box is built. 'A healthy competition has developed between the east and west end teams, ' jokes Crawley.

Rail Link Engineering (RLE) is responsible for engineering design and project management on the entire CTRL scheme.

'About 120m of the box structure at the east and west ends has to be competed on schedule to allow access for the TBMs, ' confirms RLE construction engineer Kevin McFarlane.

Four tunnel 'soft eyes'will be built to allow the TBMs access from the box to sections 220 and 240.

These are formed by 31m deep and 14m wide sheet pile walls embedded in a sand/cement bentonite slurry. When the TBMs are ready to bore through the diaphragm wall the sheet piles will be removed.

Cementation is working against the clock to install sections of the diaphragm wall where the two rail lines cross the box's footprint.

'During the blockade we have to install 40 panels in four weeks, ' says Crawley.

McFarlane says: 'To achieve the programme on the three-month blockade, we worked up as close as we could get to the railway line so we could start working on this section as soon as the blockade started.

'Everything must be complete within the three months: the track must be taken up, diaphragm walling and excavation carried out, the overbridge built and the track reinstated.'

Logistics on the site are critical as it is crammed with hydromills, rope suspended grabs, cranes, piling rigs and earth moving equipment. Limiting delays by moving the right machinery at the right time takes a lot of planning, says Crawley.

Equipment includes Skanska's flagship diaphragm wall rig, a spectacular, state-of-the-art, Bauer reverse circulation hydromill worth £1M (Ground Engineering news August 2001).

The rig is operated by a touch-screen computer that gives the exact depth, speed, verticality and bentonite pressure of the diaphragm walling, allowing detailed monitoring of progress.

The mill is supported by two reverse circulation rigs, one configured to work in low headroom. The machines are capable of digging 1200mm and 1500mm wide trenches more than 60m deep to accuracy greater than 1:1000 and speeds of up to 25m 2/hr.

Cementation also has six rope suspended grabs on the site; four 1200mm/1500mm 13t grabs and two 1200mm 11t grabs for low headroom working.

'The west end of the box is the most crucial with the first of the two blockades taking place there, ' explains McFarlane.

Cementation has two hydromills and four grabs working here.

Excavation for the diaphragm wall is being carried out in two phases. This is dictated by the four main geological layers.

Up to 4m of fill and made ground overlies alluvium and the Thames gravel, followed by the mixed clays, silts and sands of the Lambeth Group.

The geology then changes to dense Thanet Sand about 18m below ground level with chalk bedrock beneath.

The upper layers are excavated using the giant grabs which bite out chunks of the soft ground. Once into the dense Thanet Sand, between halfway and two thirds down the wall, the hydromills, supported by 350m 3/hr to 700m 3/hr bentonite handling and cleaning plants, are brought in. These excavate using two cutting wheels rotating in opposite directions, flushing the cuttings up the centre of the head with the supporting bentonite, which is then pumped away for recycling.

Bentonite provides excavation support before the concrete and the massive steel reinforcement are placed.

Two main panel sizes, 2.6m and 7.4m wide, are being used at the ends of the diaphragm wall.This allows the 1200mm thick wall to be propped every 10m down in the middle of the 7.4m wide panels where the box is at its deepest.

The larger panels are constructed in three phases. First one side of the panel is excavated, then the other, and finally the middle section, carried out with 2.8m wide cutting tools. Panel depth varies from 25-31m.

Profiled stop ends form the construction joints at each end of the panels, with in-built slots to accommodate a 150mm rubber water bar. This forms a watertight construction joint with the adjoining panel.

Props supporting the ends of the box are up to 40m long, spanning across the box and need support to prevent them from sagging.This is achieved by 100 base-grouted plunge column piles, 1.5m in diameter and 25-31m deep with a plunge depth of 12-24m.

Concrete is tremmied in to the required low casting level. Cementation's Cemloc guide frame is then lowered down the hole to guide in an H-section column, 14-21m long. The Cemloc system is a long frame, which locks into position in the borehole and has series of moveable rollers on the inside. The rollers position the columns to obtain a tolerance of up to 1:500 verticality and +/- 5mm accuracy. The Cemloc is left in position until the concrete has gained enough strength.'At Stratford we have developed a way of positioning the Hcolumns under blind conditions in the bentonite, ' says Crawley.

To increase pile capacity, base grouting is carried out one to two weeks after casting. Grout is pumped in via tubes-à-manchette at the pile base.

Piles are monitored throughout - the plan is to induce minimal upward lift at the base, mobilising skin friction and increasing pile capacity.

The central section of the box, comprising mainly wall sections 7m wide and 1500mm thick, acts in cantilever propped only by a lightly reinforced basement slab. The wall extends 25m below ground level with a retained height of 10-15m.

'The basement slab is prevented from buckling upwards by its own self weight and prevented from buckling downwards by the ground, ' explains RLE senior field engineer Neil Rigby.

Difficulties on site include headroom of as little as 12.7m in areas where there are overhead power lines. Cementation has not only had to use its low headroom rig but gigantic reinforcing cages for the heavily reinforced diaphragm wall slabs, up to 28m deep, had to be sliced into four sections. The sections are installed separately and joined as they are sunk into position.

About 2.5M. m 3of soil will be excavated from the box and adjacent tunnels. Only highly contaminated soil is removed from the site.The rest will be treated as necessary and landscaped, with ground surface levels in the surrounding area raised by about 7m as a result.

The diaphragm wall is due to be complete by July 2002, with the box construction due to end by October 2003. The land raise is programmed to be finished by October 2005 and trains are due to be running by 2007.

Dewatering joint venture

Dewatering for the Stratford Box is being carried out by a joint venture of Project Dewatering and Drilcorp.

Project Dewatering is undertaking the design and operation of the dewatering and Drilcorp is drilling and installing the wells.

The project involves installing 22 deep dewatering wells and a number of monitoring wells and piezometers. The wells are designed as permanent feature with a design life of 120 years.

During construction of the box they will be attached to a temporary dewatering scheme which will later be replaced with a permanent system.

Each well is drilled to 65m and lined with stainless steel 300mm casing to a depth of 35m, 3m to 5m into the underlying chalk. A stainless steel wire wound well screen lines the production section of the well in the chalk.

Geophysical logging and CCTV are used to check the integrity of the well and each is tested for verticality. Well cleaning is done by airlifting and acidizing with hydrochloric acid.

The wells are controlled automatically using a flow meter and water level probes. When the water level gets too low, the pump automatically cuts out and then restarts when the water levels rise.

The control panel contains a series of alarms which alert the mobile phones of staff to any problems. The alarms identify pump or power failures and provide a high water warning. This is crucial because the design and construction of the Stratford Box depends on the water level in the Thanet Sands remaining at or below the design level. The system includes a 10m buffer zone beneath the base slab of the box to allow time to fix any problems.

Water from the wells is discharged into four 9m 3settling tanks.

Before each well is put into production, a five-day continuous pumping test is carried out at 12 litres a second. During the pumping tests and throughout the works, the water levels in the Thanet Sands and the chalk are measured using a series of piezometers and monitoring wells.

When all the wells are completed there will be a 30-day group test pumping to identify the best combination of wells to operate in the long term.

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