Three bridges in three weeks

Minimise disruption by working fast. This has been the guiding principle at the locations where the UK’s new HS2 high speed rail line crosses existing railways.

In 2023 three major new structures were built – at Fulfen Wood, northeast of Birmingham, enabling HS2 to pass beneath the vital West Coast Main Line that links London to Glasgow; at Streethay, also northeast of Birmingham, crossing under the South Staffordshire Line, which is used mainly for freight; and at Kenilworth, southeast of Birmingham, which carries HS2 under the busy Coventry to Leamington Spa commuter railway.

Installation of all three was completed in a little under three weeks.

HS2’s visual impact has been a governing factor in its design, driving the requirement in many locations to dive underneath existing lines rather than leap over them.

These three new structures are bridges, buried within earthworks carrying the older railways and allowing HS2 to pass beneath them.

Race against time

Construction of all three bridges presented a common challenge: to minimise disruption to existing rail services.

In only a handful of previous instances have structures been built under operational lines without suspending services. It can be a complex and expensive process. The more practical solution for these three crossings was to close the existing lines while work was carried out. But the time windows allocated were mere days. What’s more, they were set far in advance, so there was no scope for renegotiation and any schedule overruns would incur stiff financial penalties.

We were engineering designer for joint venture contractor Balfour Beatty-Vinci (BBV) on all three projects. Together we tackled the challenge by performing nearly all the time consuming construction work ‘offline’.

The sequence started many months ahead of the agreed line closures: first the HS2 alignment was excavated to the desired formation level. The HS2 cuttings were advanced to within metres of the existing lines, then excavation paused.

At each location, the cuttings were then converted into prefabrication yards for the new reinforced concrete bridges. Construction time for each bridge varied, with the largest one at Fulfen Wood taking six months. Prefabrication was completed well ahead of the scheduled line closures.

As soon as the closures kicked in, BBV removed short sections of the existing tracks and dug out embankments and ground to through-connect the HS2 cuttings. The bridges were then moved into position.

Moving operations were substantially different for each bridge – the first two being literally picked up and wheeled into position, and the third being slid and hauled into place using powerful jacks.

With backfill placed and compacted around the bridges, track was then reinstated, allowing train services on the existing lines to be rapidly resumed.

Heaviest move of its type – Fulfen Wood

Fulfen Wood near Lichfield, in July 2023, was the first of the bridge moves.

A blockade of just nine days was permitted for the entire operation, within which BBV had to dismantle and remove the mainline tracks, excavate the embankment, install the new bridge and reinstate the track ready for use again.

It is believed that the bridge move itself was the heaviest of its type ever carried out in the UK. Weighing 6,200t and measuring 56m long by 19m wide, the mammoth structure was rolled into place over two hours using special wheeled transporters.

The transporters are rolled under the 6,200t bridge to slide it into position (Credit: HS2)

Offline construction introduces exceptional demands on structural engineers. They must design for temporary loading when the bridge is moved, which can be very different from permanent loading when the bridge is finally in place. Indeed, temporary loads can be the opposite of permanent loads.

What’s more, temporary loading is intrinsically tied to the construction methodology. A design solution can only be developed once the contractor has decided what approach to take.

A change of approach

Our project principal Hitesh Mistry was the engineering manager on HS2 contract SL8 for Fulfen Wood and Streethay. He explains that our team had to rapidly revise its design when BBV opted to change the installation method to mitigate the risk of overrunning the closure.

Originally it was proposed that the bridge would be built as a jacked box construction. This would have involved building the entire structure – base slab, abutment walls and deck slab – as a single unit and then pushing it into position with hydraulic rams or pulling it into place with strand jacks.

“We had finished the design in good time and were just about to issue the drawings when the installation method changed,” Hitesh recalls. “One of the big risks for the blockade was the time estimated to install the bridge using jacking – it would have been approximately 36 hours,” he says.

The alternative was to use huge, manoeuvrable wheeled platforms, known as self-propelled modular transporters (SPMTs). These vehicles, originally developed for the oil industry, can do the moving operation much faster, reducing the risk considerably. “We estimated it would take up to five hours, but in fact it took just two,” says Hitesh.

Another benefit gained with the shift to wheeled transport was that working to the required tolerances became easier. “With jacking, your ability to fine-adjust position and alignment is limited – you’re pulling or pushing from one direction. You have only one chance to get it right,” Hitesh says. “With SPMTs there’s more manoeuvrability to get it into position correctly.”

But for our team, the benefits of moving to a faster installation procedure came with the challenge of having to redesign the structure, under tight time constraints, to suit different temporary loading.

Instead of being pulled from one end, the structure would be lifted from below. SPMTs would be driven under the bridge deck then would raise a row of beams to lift the structure off its temporary supports. They would then drive the bridge into position.

A consequence of this change of procedure was that the bridge could not be built as a complete box. The base slab could only be cast after the SPMTs had completed the move. “With a jacked box you are moving the full structure,” says Hitesh. “But with this process we had to omit the base slab when the bridge was built.”

In the original design, the base played an important structural role, with strong moment joints at the corners supporting and stabilising the walls. Without the base slab, “temporary props were required to support the walls of the portal during the move.”

Design for construction

Mammoet transported the bridge on four carrier beams assembled on SPMT units with a total of 840 wheels, controlling them remotely from a single unit.

We designed temporary reinforced concrete corbels into the walls to receive the SPMT carrier beams.

840 wheeled transporter

Critical communication

In the run-up to the closure, explains Hitesh, strong communication between all parties was a critical element of risk management. This included weekly and ultimately daily team meetings bringing together our staff with BBV, Mammoet and HS2, supplemented by instant messaging, to resolve any issues promptly and efficiently.

“We were working through everything you need to do ahead of the move,” says Hitesh.

“The main risk was not meeting the blockade deadline, so we had to be ready to react quickly to any changes that were required.

“In the months leading up to the closure we had dedicated teams and, when awaiting information from other sources to progress the design, we identified which members of our team we would need and made sure that they were on standby to act as soon as the information was received.”

Tight tolerances

Tilting trains operate on the WCML. “There is a safety case associated with these trains. The faster they travel the more they tilt as they negotiate bends, and the wider the working envelope that is required,” says Chris Mannion, our responsible engineer for track.

“This is managed using a fixed track position so that spatial constraints are known and can be maintained. It is referred to as absolute track geometry, or ATG.

“This had to be reinstated once the bridge was in position,” explains Chris. “Allowable track tolerances were just +/-10mm.”

Hybrid solution – Streethay

Because the South Staffordshire Line (SSL) is used mainly for freight, a 72-day closure was allowed for construction of the Streethay bridge. The methodology was tailored to take advantage of this, reducing complexity and risk.

Rather than prefabricating deck and walls as at Fulfen Wood, we worked with BBV on a solution using secant piled abutment walls. These were bored and cast before excavating the SSL embankment to connect the HS2 cuttings. The piles were then capped, ready to receive the deck – the only part of the bridge prefabricated offline and lifted into place.

Streethay bridge once moved into position (Credit: HS2)

Nonetheless, the as built design was not the solution first proposed. Hitesh says that “the original proposal was top-down construction, which would essentially have involved casting the deck slab on the ground in situ. It would have involved designing only for permanent loading.

“However, the sequence changed, requiring us to design the deck offline, supported above ground on temporary supports (so that SPMTs could be driven underneath) and then lifted and moved. The deck had to be designed to cope with several loading states.”

As much construction as possible was carried out before the closure, which began at the start of June 2023. The Streethay bridge sits within a long cutting with secant piled retaining walls (see below) that were installed right up to each side of the SSL railway embankment, enabling the ground either side to be excavated in preparation.

Once the SSL was closed and track and embankment fully removed, an additional 76 secant piles of 1.3m diameter were built to complete the retaining walls – which double as the bridge abutments.

The piles were capped with a reinforced concrete beam, allowing Mammoet to lift and move the deck using a remotely operated 344-wheel SPMT, redeployed from the earlier Fulfen Wood bridge lift.

Integral joints

To minimise long term maintenance needs, Network Rail required bridge joints to be integral.

“From a bridge design point of view, it’s a complex detail,” says our design coordinator, Tom Quirk. “We had to run reinforcement down the back of the capping beams to create structural continuity between the deck and the abutment walls.”

Tom says the primary difficulty was caused by reinforcement congestion resulting from the quantity of steelwork required to control the forces transferred through the construction joint.

Concrete was poured in situ to complete the joint between deck and pile caps.

Track transitions

With the deck in place, site workers placed around 2500t of backfill on each side of the bridge.

“We had to manage the transition between the existing SSL embankment and the new structure, which have different dynamic properties. We had to avoid differential settlement and achieve consistent stiffness,” says Chris Mannion, our responsible engineer for track.

The bridge deck was designed to support a deep layer of backfill material underlying the track ballast.

Box jack – Kenilworth

The third of the bridge moves took place at the end of July 2023: a 46m long structure weighing 5600t was put into place as part of a 10-day closure of the Coventry to Leamington Line near Kenilworth.

Strand jacking was chosen rather than a ‘lift and shift’ using SPMTs, because of the expertise of key personnel in BBV’s team. It is a method that employs powerful jacks pulling on cables – known as strands. The bridge was hauled into position, with its base sliding along the ground.

The Kenilworth bridge being hauled into place (Credit: HS2)

Although the existing line is currently single track, the bridge was designed to accommodate two tracks, in anticipation of increased traffic in future. It is a rectangular tube measuring 46m long by 21m wide and 10m high, with integral joints at all corners: it is monolithic reinforced concrete. Its sides – the abutment walls – extend to form short, splayed ‘wing’ walls at the northern end, giving the structure an overall width of 30m.

Despite being a relatively simple structure, its design took more than a year. “The specialist subcontractor, Freyssinet, provided us with layouts of where the jacks were going to be positioned for the move, and identified elements that we needed to incorporate,” says our project director Tim Akers.

“They needed to understand the design of the box, and we needed to understand the jacking forces. The exchange of information was an iterative process and we had fortnightly meetings for a year to coordinate the design.”

Tough ground conditions

Early in the project, trials were carried out to determine if the ground could be excavated without using explosives. “We are in Kenilworth sandstone, which is very strong, and initially there was concern that we might have had to blast down to formation level – which could have been a problem so close to the railway,” says Tim.

Fortunately, test excavation to the north of the crossing point showed that mechanical diggers would do the job. They were used to prepare a flat, smooth ‘raft’ on which to prefabricate the bridge and slid slide it into position.

Five hour operation

Four days of the 10-day line closure were spent removing about 100m of railway track and excavating to formation level. The box is founded directly on rock.

8.5m/hr speed of pull

The structure was pulled into place at an average speed around 8.5m/hr by five jacks, each with a capacity of 1000t. A spare jack was available in case any one of the jacks failed, and design redundancy meant the operation could be completed with only four jacks if necessary. The slide took just five and a half hours.

The remaining six days were devoted to backfilling and reinstating the Coventry to Leamington Line.

Digital detail

Tim says that building information modelling (BIM) was used to achieve a high level of detail early in design process.

“I’d not worked on a project where we’ve had so much design certainty at such an early point,” he explains. “For example, drainage on earthwork drawings is usually marked as a simple line for the location, with information about ditch dimensions, falls and so on only becoming available with the survey. Here, we knew all these things at scheme design stage.”

Modelling in 3D from project outset is still unusual because it is relatively slow and expensive – though Tim sees advances in technology rapidly reducing those barriers. He says the scale of the Kenilworth project justified the investment, and that it paid off.

At the heart of the model was detailed site survey data. Then, “each discipline – geotechnics, structures, earthworks, drainage – created its model with reference to that. As the model for each discipline matured, it was made available to more people within the design team,” he says.

Before the full federated model was issued, each individual discipline model was checked for clashes with others.

BBV used the model to guide construction equipment such as motor scrapers, resulting in faster, safer and more accurate construction.