Work to create CTRL's international terminus at St Pancras station means dealing with a Pandora's box of underground obstructions, says Julian Wallace.
St Pancras station, with its landmark neo-Gothic façade, is to be the CTRL's London gateway. The aim is to create a magnificently extended and refurbished station blending the 21st century railway into the Victorian architecture and engineering.
The area has a rich history of commercial and industrial use, associated particularly with the canals and later the railways. The legacy underground is a Pandora's box of infilled canal basins, railway viaducts buried beneath existing embankments, locomotive turntables, disused gasholder tanks and all manner of building foundations.
There is also a large burial ground in the west around the original St Pancras church and a selection of working Victorian brick sewers several metres wide crossing the site. Ground contamination is an important consideration.
The site extends north across the King's Cross Lands to the North London Line (NLL) and is bounded to the west by the Midland Main Line (MML) corridor leading into St Pancras station and to the east by the East Coast Main Line (ECML) running into King's Cross station.
The Regent's Canal - part of the Grand Union Canal - winds east-west across the site; the MML crosses it on a bridge leading to platforms above ground level in St Pancras station, whereas the ECML goes under the canal in tunnels into King's Cross.
Beneath the MML corridor and also passing under the canal is another Victorian brick tunnel carrying the Thameslink line, which further south curves eastwards to go under St Pancras station.
By far the greatest challenge is working around the rail and road networks, in essence, superimposing a new rail 'spaghetti junction' on top of an existing one.
CTRL will emerge from the London Tunnels at the north east corner of the site, immediately crossing the ECML on a new bridge and sweep south west across the site on new embankments and bridges towards the MML corridor and into the station. There will be new viaducts and embankments for CTRL connections to the WCML, via the NLL, both from the station and directly from the London Tunnels, bypassing the station.
The works are phased, involving a series of temporary rail movements to keep the trains running during staged construction of bridges and other structures.
The project also includes a new 360m long underground station to replace part of the existing Thameslink tunnel, to allow passenger interconnection with the MML and CTRL. Part of the station box will be beneath the St Pancras station extension. Connecting to the north end of the box will be the twin bored Thameslink 2000 Tunnels, running north east beneath the King's Cross Lands and rising to the surface in cut and cover tunnel to provide a crosssite rail link to the ECML.
Most of the major structures are on piled foundations and the many underground obstructions have led to highly irregular pile layouts and large pile caps straddling sewers, former gasholder walls and other historic structures.
Site geology is typically 1m to 2m of made ground (but up to 17m where the gasholders were located) over London Clay, the Lambeth Beds, Upnor Formation, Thanet Sand and Chalk. There are perched water tables in the made ground maintaining nearhydrostatic piezometric pressures in the London Clay, although the ground beneath has been drained by long-term pumping.
But already, with construction only just started, there have been surprises from unknown buried structures - despite a lot of ground investigation. The possible presence of unexploded Second World War bombs may be an added hazard, and this is being examined using specialist magnetic surveys.
There are a number of thirdparty interfaces and consents to be cleared before work can proceed - including Railtrack, London Borough of Camden, English Heritage, the Environment Agency and utility companies. Protection of buried services has greatly influenced foundation solutions.
Design of the new structures in the St Pancras area is being carried out jointly by RLE and 'work-order' teams in the member consultants' offices. Most of the structural design is outsourced to these teams, while the geotechnical engineering input is provided mainly by RLE.
All the designs are subject to independent checking before drawings are issued for construction. Once contractors are appointed, a process of value engineering takes place with all parties collaborating in a review of the base design to look for alternatives and refinements offering reductions in cost, programme and risk.
Work inside the St Pancras train shed involves a general lowering of the ground level in much of the undercroft to accommodate an underslab void for air circulation to the new station concourse down at ground level.
Since there are increased design train loads from the platform level above, careful assessment has been carried out of the bearing capacity and potential settlement of the existing pad footings supporting the train deck, as these will be retained in the refurbished station.
Installation of large underfloor utility ducts running the length and breadth of the station involve deeper excavations down to founding level over the whole width between rows of existing footings. This means that integral construction with the footings and column stumps is needed to achieve combined action, so preventing settlement of individual footings.
There are plans to add a sixstorey hotel block above the twostorey, west side of St Pancras station and piles for this will be installed as part of the contract.
London Underground Northern Line tunnels lie beneath the site and the building layout requires piles to be bored close to the tunnels, founding below them.
Studies are under way to develop a safe construction method that meets time constraints on pile excavation and concreting, while avoiding the need for suspension of underground services.
The train deck extension to the north of the station is 315m long and 100m wide, and is needed to accommodate the 400m long Eurostar trains. The deck is of reinforced concrete, elevated above ground and supported on piles. Pile layout is complicated by the sewers and former gasholders beneath the site. Lateral loads on the piles arise from train braking and acceleration forces and also temperature-induced loads due to expansion and contraction of the deck.
Structural analysis has therefore required assessment of vertical, horizontal and rotational spring stiffness for all the foundations, which include single piles, regular groups and large irregular shaped caps straddling obstructions.
Up to 7m of overburden is to be excavated from the deck extension area, therefore the effect of heave on the piles due to swelling of the London Clay has had to be considered. This has involved assessing heave magnitudes between pile installation and the programmed application of superstructure loads. In some areas, pile reinforcement cages have been extended to limit potential crack widths in the piles.
Crossing the site obliquely at 30m depth is the London Water Ring Main, and piles above this are to be under-reamed so they can be founded at a high level to keep them clear of the pipeline.
The box for the new Thameslink underground station is 360m long and 22m wide. The concrete box superstructure will be built before the tunnel inside is broken out during a 25-week blockade of train services. This closure period must include all dismantling and subsequent recommissioning of the trackwork, overhead electrification and signalling.
The station box will have secant pile walls (1,200mm diameter) installed before the blockade.
The dig depth to the underside of the base slab varies from 10.5m to 11.5m. For maximum construction speed, the walls are being designed to allow excavation to 6m without propping. Permanent roof beams near ground level will then be installed at 15m centres and excavation continued. There will be no temporary propping between roof and base slab.
Design studies are exploring the option of a slender perforated ('leaky') base slab to relieve uplift water pressures. Reducing the slab thickness offers potential savings both in the excavation depth, which would speed construction and reduce bending moments in the wall, and in reinforced concrete quantities in the slab itself. The vital benefit would be the reduced risk to programme arising from less excavation, less steel to fix and less concrete to place during the critical blockade period.
Finite element analyses using the 'Brick' soil model, which incorporates strain-dependent soil stiffness, are modelling the stress history of the ground, including historical overburden and the excavation and construction of the Victorian tunnel. The results are being used to calibrate the simpler retaining wall program FREW based on the traditional Mohr-Coulomb soil model, which is the primary analysis tool adopted for the many design sections around the box.
In the King's Cross Lands, many new retaining walls and two CTRL crossover box structures are founded on shallow ground bearing bases in the London Clay.
However, piles are needed for the several large bridges, retaining walls and other structures where there is insufficient space for shallow base widths or where there are limits on loads that can be transmitted to adjacent or underlying structures.
Many existing structures also have to be assessed and sometimes strengthened to cater for increased loads or the effect of nearby construction.
A typical example is the underpinning needed for the abutments for the bridge carrying the NLL over York Way to accommodate lowering of the road beneath the bridge by up to 2m. As built details of the abutments are not available; modern codes require current design standards to be met when modifying existing structures; there are severe physical constraints imposed by services beneath the road; and the work is to be carried out without disruption to rail services and without closing more than one road lane.
These issues combine to create a challenging task. The solution adopted in this case is underpinning with the toe supported on bored piles installed using a lowheadroom rig.
Between the structures in the King's Cross Lands, up to 6m high railway embankments are being built. This means surcharge effects on adjacent structures, potential differential settlements and negative skin friction on piled foundations all have to be considered.
Julian Wallace is RLE lead geotechnical engineer at Area 100, St Pancras.