How value engineering finetuned complex piling works around York’s new railway overbridge

Tricky ground conditions around a key railway overbridge in York have necessitated no less than 5,669 piles, but careful planning and value engineering meant this tally was far smaller than it might have been. Railway overbridges are typically complex engineering projects, requiring specialised solutions to handle dynamic loads, strict safety clearances and difficult construction phasing over live rail lines. The new East Coast Main Line (ECML) overbridge in the historic city of York has an added distinction, partly because it’s the primary point of access to the £2.5bn York Central scheme. Billed as England’s largest brownfield urban regeneration project, York Central is transforming 45ha of underused land next to the city’s railway station into a new residential, cultural and commercial quarter. But another key distinguishing feature of the overbridge is the challenging nature of the site on which it is situated. It is bounded by active rail lines and transversed by a deep alluvial channel, the Holgate Beck. Complex subterranean obstructions are another difficulty, in the form of a network of live underground utilities, 19th-century railway dumping and foundations, and ancient Roman archaeology, not to mention occasional unexploded WWII ordnance. Its topography includes a 4m level variation across the site towards the east/southeast – an approximate 1:50 gradient. Ground conditions involve variable thicknesses of made ground up to 4m deep, soft Glaciolacustrine deposits of interbedded clay and sand at up to 15m depth – which at its base has a layer of sand indicated to be around 1m thick, glacial till (firm clay) up to 30m depth, and Sherwood Sandstone bedrock. Groundwater at shallow depths represents a further challenge. The site sits between the River Ouse and Holgate Beck, with high river levels forcing the groundwater table upward, causing waterlogging and making sub-surface construction difficult. Geotechnical solutions The ground around the new ECML overbridge – as well as other structures such as the existing Severus overbridge – has been a focal point of vital soil stabilisation and deep foundation work, as Alan Rodger – managing director of York Central’s lead contractor Sisk Infrastructure – explains. “The ground itself isn’t very structural,” Rodger says. “So there’s been a significant amount of ground remediation, which included reinforced embankments, particularly adjacent to the new overbridge. “In order to strengthen it, you either do reinforcement, which is what the embankments are, or where we had structures like the bridges, they all had to be piled.” Bauer Technologies was engaged on the scheme in March 2024 to deliver geotechnical design and piling solutions design, including the installation of large piles to support heavy new infrastructure – including the foundational bases for the ECML overbridge – without disturbing the surrounding active rail network. Coffey Geotechnics was engaged with a focus on specialist piling and geotechnical works. Other key project participants include City of York Council, Network Rail, lead engineering and planning consultant WSP, Tony Gee and RoC Consulting. Piling rigs were deployed next to Millennium Green – a public nature space adjacent to the ECML and overbridge – to install reinforced concrete viaduct piers and secure the earthworks. Soil reinforcement was mandated to safely elevate the new access road and support landscaped terraces running down toward Holgate Beck. Bauer Technologies’ early activity included rigorous test piles before work started on site, leading to a range of solutions selected for specific end uses, Bauer Technologies project manager Steve Owen confirms. These included Full Displacement Columns (FDC), Continuous Flight Auger (CFA) piles and Full Displacement Piles (FDPs). Ultimately the total number of piles deployed on the area around the new overbridge was 5,669. The majority, 3884, were deployed on the north embankment – which runs between the entrance to key access road Water End and the new ECML overbridge. Of the 3884, 2,367 are FDCs, 826 are FDPs, 269 are CFAs and 422 are CFAs at Millennium Green which were 600mm in diameter driven 12m into the ground. A total of 1,785 were deployed on the south embankment, comprising 1,004 FDCs, 405 FDPs and 376 CFAs. Owen notes that FDCs – key to ground improvement to mitigate settlement and increase the bearing capacity of soft, compressible soils around deep foundations – were used for the area around the main abutments and approach embankments of the overbridge. For the transfer at the back of the abutments, CFAs were selected as appropriate for the restricted space. FDPs (Full Displacement Piles) were used to transfer heavy structural loads directly into deeper, competent soil or bedrock. With work being undertaken close to Network Rail infrastructure and restrictions on the road network around the site – as well as the proximity of housing nearby, Owen adds that noise-friendly solutions such as the full displacement method were a priority. “The full displacement piling method also densifies the ground and produces minimal spoil, which can be dealt with on site, so that was another benefit,” he says. Both FDP and CFA piling were also critical to groundwater management. Displacement piling pushes the soil outward into the surrounding ground as it drills down, compacting the ground and sealing the wall of the hole, stopping groundwater from flooding the site. CFA piling was designated for use on deeper foundations, and the technique – which retains the drilling tool in the ground – also prevented water ingress. Settlement concerns Coffey Geotechnics geotechnical associate David Stone observes that the presence of compressible Glaciolacustrine drying deposits had necessitated “strict settlement tolerances” at the site during and after construction, including across a number of interfaces around the embankment. “The embankment itself was only allowed to settle by about 50mm post-construction and where we had interfaces, obviously there’s a differential settlement issue between existing structures and the new bridge and new embankment. For most of the interfaces, the differential requirement was somewhere around 25mm, which is quite a big ask for such a thickness of compressible deposits. That’s a key reason why we went down the piled route.” A major interface challenge lay in the sweeping curve traced by the ECML around the western edge of York; if the ground settles beneath a curve, the track can tilt or twist out of tolerance, creating a derailment risk – clearly of concern to Network Rail. “The East Coast Main Line itself is closer to the north embankment, so the load from the north embankment is closer to the line,” says Stone. “It’s the twist in the track, it’s the differential across the track that’s the problem, and obviously we’re loading closer to one side of it than the other, so Network Rail was more interested in what the different track levels were going to be across the track width.” He also observes that “moment capacity” was a related consideration, which led to most of the FDCs being located under the main embankment more than 20m from the back of the main bridge. The rationale for this was the potential for “a moment” to be generated – in other words the creation of a twisting or rotational force around a pivot point or axis – in which moment capacity is the maximum bending moment a structural element can withstand before failing. “Because the FDCs are unreinforced, they don’t really have much in the way of moment capacity,” Stone explains, “so in the centre of the embankment where you’ve got an equal load either side of the FDCs, you don’t generate a great deal of moment. Towards the edges of the embankment, we had to reinforce the columns so they would have a bit of moment capacity just at the edges. That’s where the FDPs come in. The FDPs – essentially reinforced FDCs – were used along the perimeter of the embankment.” Value engineering Value engineering was key to optimising the rationalisation of settlement issues – along with a range of other considerations – and in fact resulted in changes in approach, Stone says. “The post-construction settlement requirement was originally uniform across the embankment, but we undertook a value engineering exercise to reduce the settlement requirement at the shoulders, where the embankment was less sensitive to settlement and you didn’t have to worry about damaging the road or the serviceability of the embankment. There, we managed to reduce the number of piles and in some areas, we were able to remove them where the settlements were sufficiently low.” Finite element analysis (FEA) – which breaks forms into millions of tiny, simple elements – was critical to understanding three-dimensional source structure interaction analysis across multiple points, confirms Coffey Geotechnics operations director Basil Ogunmakin. In short, the method was used to model how the foundations for the overbridge would impact the surrounding earth and railway. Creating virtual models of soil-structure interactions reduced the need for physical prototypes and over-engineered foundations, he says. “We wanted to ensure that we had a robust solution that was also cost effective. The key was choosing a solution that was appropriate,” Ogunmakin explains. “What would you typically do in this sort of ground environment and ground conditions? You could pile the whole thing, but we chose FDCs – which falls within the family of rigid inclusions. “The technique is a cost-effective solution, both in terms of material cost programme and carbon, because when you compare that to the alternative, which is CFA piles, they could generate spoil and you may need to include reinforcement. For rigid inclusions, you don’t generate same level of spoil and for the most part, you don’t need reinforcement.” FDCs also offer engineering benefits, Ogunmakin adds. “They only work where you’re primarily applying compression loads; they’re not good in tension. And here, the infrastructure that we’re building is embankment for the most part, with the two bridges at one end. The load of the embankment is primarily acting in compression, which is why this solution is good from a technical point of view.” Performance validation With York Central positioned as a sustainable development – from low carbon energy planned for its residential plots to a masterplan that supports non-vehicular transport – the ECML overbridge has been designed and planned accordingly as lightweight and slender, minimising the height of the approach embankments and reducing the overall embodied carbon of the structure. Ground engineering was executed in line with this thinking. Re-engineering the original structural designs for maximum material efficiency – along with zero-spoil piling techniques – were key, while other sustainable inputs included the use of hydro-treated vegetable oil (HVO) fuel in all heavy site machinery. Considered approaches, for example to load testing, also enhanced project sustainability, optimising pile foundation designs to prevent the over-ordering of steel and concrete. In fact, by substituting static load testing with dynamic load testing, faster, cost-effective results were achieved, says Stone. “With static load testing, you need to put a kentledge [deadweight] on site and it has to sit and be loaded for a period of time, whereas with the dynamic method you just have to strike the top of the pile with a machine and it gives you a response. Where it would take about two days to do a static load test, we were doing nine in a day with dynamic testing. So that was a big saving for the project as well.” Such efficiencies were made possible through controlled planning and gradual execution, driving a project whose solid implementation was demonstrated when Sisk Infrastructure completed the critical phase of the ECML overbridge installation over the May 2026 Bank Holiday weekend. In the wake of the installation, work quickly got underway to deliver new roads, footpaths and cycleways, with the bridge expected to open in 2027, unlocking the site for development. The challenges entailed in preparing the site for these stages were not small, but Ogunmakin’s summary comments indicate they have also been a galvanising force for his organisation – and by implication the whole team. “As a specialist geotechnical consultancy, the more difficult the scheme, the more excited we get,” he says. “And York Central is a particularly difficult site, so it’s been really exciting to work on it.” Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here. Have your say or a new account to join the discussion.