The Wessex Capacity Alliance (WCA) is responsible for an £800 million investment to provide extra capacity on services to and from London Waterloo. Most of the work is centred around Waterloo itself, with additional work at key outlying stations.
Three aspects of the upgrade project, which make a significant use of concrete, are described in outline here. When complete, they will have used nearly 2,100 tonnes of in-situ concrete and 570 tonnes of pre-cast concrete units to create the new facilities that will provide the additional passenger capacity. It is worth noting that, had it not been for an imaginative approach to the design on one of these projects, the quantity of concrete required would have been even greater.
A crucial stage in the redevelopment of Waterloo station to provide the extra capacity needed was the refurbishment of Platforms 20 to 24, the former International Terminal, which had been largely out of use since November 2007. Convertion of this part of the station for use by domestic services would need an operationally more flexible track layout on the approach to the platforms to permit up to 18 trains per hour in and out of these five platforms, compared to the six per hour capacity in the days of Eurostar.
It gets more complicated. The optimal track layout for this new service capacity and flexibility would not ‘fit’ on the approach viaduct which was installed for the introduction of the international services. It is a very carefully considered series of structures, which threaded the new international tracks above busy streets, over the existing Waterloo masonry arches and with piled foundations avoiding London Underground infrastructure and many utility services.
Mott MacDonald, one of the design consultants for the structural aspects of the work at Waterloo, had been the independent checker of the original terminal in 1992. This was to prove very useful in developing the alterations needed to the approach viaduct.
The existing viaduct arrangement consisted of three separate parallel decks with gaps between them. The original proposal was to construct two further, structurally independent, decks to infill the gaps and provide continuity for carrying the new track layout. These additional decks would have required a further 65 piles to carry them, the piling itself being an immensely complex part of the whole work, having to take place with only five metres of headroom under the existing viaduct, needing to avoid many services and requiring disruptive road closures.
Some thorough value engineering generated an ingenious alternative proposal. Could infill decking be designed such that it would act integrally with the existing decking? If it could be shown that this was feasible structurally, then there would be no need for the new piles, which would generate a significant saving in time, construction risk and cost.
Some further slight modifications were made to the proposed track layout in order to minimise the longitudinal extent of deck infills needed. Many different loading combinations were analysed to confirm the design. It was found that, under certain combinations, the existing bearings would be overstressed, or unsuitable in other ways, for their new loading conditions, so would have to be replaced.
Confirmation that the existing substructure of piles, pile caps and leaf piers could take the new loading patterns was also established. Soil investigation and pile loading test results from the original work in the early 1990s were invaluable in completing an assessment of the substructure’s capacity. Even with that information, it was essential to carry out accurate modelling of the soil-structure interaction to confirm that the reuse of the existing substructure was feasible with the new loading conditions.
Having confirmed that this was possible, the design for the new arrangement consisted of the partial demolition of the edges of the existing viaduct slabs, the modification of the structural articulation, and the casting of new in-situ concrete infill slabs to join the existing decks.
The connection between the new and the existing decking, by reinforcement bar lapping, was designed to minimise the amount of edge demolition required. The design for the new decking infills and for the bearings both required complex finite element analyses. These had to consider many different loading situations, according to the position of the live train loads, for which four different situations were analysed with associated vertical and horizontal loads (traction, braking, centrifugal and nosing) in addition to all the usual structural load parameters to be considered.
Two hydro robots were used for exposing the reinforcement in the areas where the lapping was to be carried out. 1,050 tonnes of C40/50 concrete was used for the infill slabs, with a CIIIA mix with shrinkage reducing admixture, to reduce the risk of cracks in the dry joints.
The collaborative approach to this work between the Wessex Capacity Alliance partners produced a very effective solution to the modification of the approach viaduct with estimated savings of 17 weeks in the construction period, 1,480 tonnes of CO2 and £5 million in cost. This work was completed during 2016 in preparation for the temporary use of Platforms 20 to 24 during the major closure of the other side of the station in August 2017.
Platform rebuilding and extension
Rebuilding and extension works to provide capacity for 10-car trains on Platforms 1 to 8 at Waterloo was completed this August. This work had to be achieved in a very intensive construction period during the recent 24 day closure of the whole east side of the station.
There were six key elements for the success of this whirlwind installation:
- The use of pre-cast concrete units to the fullest extent possible;
- Specification of rapid hardening concrete for the platform wall foundations;
- In-situ concrete placement by pumping;
- Delivery of pre-cast units by rail;
- Placement of platform units by Road-Rail Vehicles (RRVs);
- The use of sacrificial shuttering for the platform reinforced concrete slab decking.
A total of 161 pre-cast ‘C’ section platform wall units, manufactured from C45/55 concrete and each 2.5 metres in length, weighing 2.2 tonnes and with an associated over-sail unit weighing 1.1 tonnes, were delivered to the site by engineering train and unloaded and positioned using RRVs.
To permit the various stages of the work to follow one another as closely as possible, a rapid hardening C32/40 concrete specification was used for the wall foundations, with a required strength of 20MPa gained in only two hours. The 140 tonnes of in-situ concrete for the foundations was all placed by pumping, up to 75 metres distance, from a delivery point just outside the station.
Kingspan sacrificial shuttering units were used to form the new platform decking and, all the in-situ concrete, another 750 tonnes of C32/40 specification, was again pumped to create the mesh-reinforced slabs.
New lift shaft at Vauxhall station
Vauxhall station is the first station after leaving Waterloo and has four island platforms built atop a long masonry arch viaduct.
Congestion relief on Platforms 7 and 8 is needed and the WCA is constructing an additional set of stairs at the country end of these platforms. To accommodate these stairs it is necessary to relocate the lift shaft.
The existing station viaduct structure relies on continuity and buttressing for structural stability. The horizontal thrust at the spring line from one arch is balanced by forces from the adjacent arches.
Works to Platforms 7 and 8 necessitated breaking out a three-metre-wide section of arch to allow for the construction of the new lift shaft, which would affect the existing thrust flow. As the removed section would be small when considering the overall width of the viaduct, it is possible that the thrust would have redistributed.
However, arches are complex and difficult to analyse and the WCA designed structural elements to provide a defined and alternative thrust path to ensure that the stability of the arches was not compromised. These elements are the new in-situ reinforced concrete walls of the lift shaft and a transfer beam keyed into the pier. Strain gauges installed on the new concrete elements confirmed that the thrust was now taken through them.
The in-situ part of the lift shaft is constructed up to the level of the arch intrados. Once the three-metre length of the arch had been demolished to make way for the lift shaft, four pre-cast units were installed above the in situ work to complete the shaft. The in-situ part of the new shaft did all the arch propping and so no temporary works were needed for overall stability. The pre-cast units were made by Shay Murtagh and the work will be completed in 2018.
Generating extra capacity from Waterloo station is an enormously complex programme involving all engineering disciplines, of which concrete engineering is perhaps the least well-known but vital part of this work. 2,700 tonnes of new concrete used in the Waterloo completed approach viaduct and platform works, with more required for forthcoming Vauxhall lift shaft works, demonstrate this in no small measure.
This article was written by Mark Phillips.