Measuring 1,075m long, 55m wide and 16m to 22m deep, the Stratford box is a mammoth structure.
Excavating the hole posed a major challenge, matched by the difculty of constructing a base slab and walls that were absolutely watertight, and of providing a dewatering solution that would prevent the box oating on the high water table.
But it is not its dimensions alone that make the Stratford box a key feature of the highspeed line.
The Channel Tunnel linking the UK and France consists of three bores, notes RLE technical director Mike Glover - two train running tunnels and a route for escape and use by emergency services. Such is the length of the London tunnels that a third bore looked essential. 'That was until we proposed breaking the tunnel in two. By creating the Stratford box the emergency services were able to cut evacuation and access times, enabling us to dispense with the third bore.' The box also contains one of section two's three stations, cross-overs to allow reverse running on the main line when required, and the single line track that will take the Eurostar trains to their vast new depot at Temple Mills (page 27).
When construction of the London tunnels got underway, it was at Stratford that work began, with tunnel boring machines (TBMs) driving forward from either end of the box, east towards the Thames and west towards the heart of London.
This placed box construction on the project's critical path.
End walls were constructed with soft 'eyes' - areas reinforced with glass bre rather than steel - to allow the TBMs to break through at the start of their journeys. Working back from the TBM launch pits, the contractor installed regular cross walls across the station box as it went, creating water-tight cells so that excavation could start well before the whole of the perimeter diaphragm wall had been completed.
Constructing the box involved some of the largest diaphragm walls ever built in the UK. It is 1,075m long and 55m wide at the central section where the platforms are located, tapering at either end to 37m. Steve Dyson, lead engineer for design of the box, describes it as 'cofn-shaped'.
The ground in which it sits consists of alluvium and river gravels overlying Lambeth Group clays, followed by dense Thanet Sands and the Upper Chalk. The eastern end of the box is bounded by the River Lea and the western by London Underground's Central Line.
To avoid these obstructions and still locate the platforms at a reasonable depth the base of the box rises from 22m at either end to 13m at the middle. The diaphragm walls themselves extend down to a maximum 31m, taking them well into the Thanet Sands.
For the excavation of the diaphragm walls, Cementation Skanska used rope-suspended hydraulic clamshell grabs for the upper layers, switching to state-of-the-art reverse circulation hydromills in the lower, denser, strata. Spoil from the diaphragm walls and from the excavation of the box has been used to raise ground levels around the international station in preparation for the new Stratford City development.
The panels are a 'chunky' 1.5m thick in the central section, says Dyson, and 1.2m thick in the tapering end sections. The 50t reinforcement cages were tandem lifted into place by a pair of cranes. Concreting was carried out with a tremie pipe inserted into the excavation.
Central sections of the diaphragm wall, where height above the base slab is least, are cantilevered. However, where the box tapers at either end, permanent props have been installed.
Up to 40m long, these are supported on plunged columns into short, straight-shafted, 1.5m diameter base-grouted piles. Base grouting was used to increase the bearing capacity of the piles, which range in length between 25m and 31m.
Longer piles were ruled out as they could not be allowed to penetrate the chalk aquifer.
Because the propped and cantilevered walls behave differently, it was not possible to use a conventional continuous capping beam. The differential movement was estimated to be in the region of 70mm.
Instead specially manufactured movement joints were used.
These were pre-compressed for installation so they would remain in compression over the full designed movement range.
A particular concern for the designers was the behaviour of the base slab. The problem was to avoid oatation on rising groundwater. At the start of detailed design the water table at Stratford was 7m above the lowest point of the base slab and rising annually. The designers could anchor the slab or design a permanent dewatering system.
After investigation they opted for dewatering and a series of wells and pumps was installed around the perimeter of the box.
These pull the level of the lower aquifer to 10m below that of the base slab, well within the safety zone. The decision enabled huge savings on rebar in the slab, which is a simple mass concrete structure, as well as avoiding the cost of installing anchors.
Water from the pumps discharges into the River Lea.
RLE engineers have taken advance measures to ensure that Stratford International station and the development due to be built around it will have no effect on the river or its flood plain.