Engineering certainty over the M42

Industrialised Construction Journal spoke to the HS2 site team about the DfMA, precast and installation strategy behind the M42 twin box - and how it is reducing risk at a live motorway interface.

Installing 175 precast beams above one of the Midlands’ busiest motorway corridors is the most visible part of HS2’s M42 twin box programme. Each beam is 23m long, weighs between 56 and 92 tonnes and must be lifted into position using a 300-tonne crawler crane.

The operation is being completed across four weekend carriageway closures, with around 44 beams installed during each possession. Every closure has a fixed endpoint: the motorway must reopen on schedule.

The lifting operation is the final stage of a strategy shaped years earlier around the constraints of working beside and above live traffic. Design, manufacture, logistics and assembly have been planned as a single production sequence.

Building above a live motorway

Located near Solihull, the 300m twin box will carry four HS2 tracks over both M42 carriageways, between the Packington and Pool Wood embankments and close to the future Interchange Station.

The structure comprises two 10m-high external walls, a central pier formed by 46 supporting columns and a 9,800-tonne beamed roof. Its scale is considerable, but the principal challenge has been the motorway interface.

Working within the operational highway boundary without increasing risk to road users was complicated by a central reservation only just over 11m wide. That constraint drove the move away from traditional in-situ methods, which would have required more labour, deliveries and vehicle movements close to live traffic.

Designing work out of the central reservation

The DfMA approach replaced significant site-based fabrication with precast and prefabricated elements.

The supporting walls use hollow concrete shell units averaging 14 tonnes, lifted into position before being reinforced and stitched together. The central columns were also precast, while reinforcement cages were assembled away from the restricted workface and lifted into position.

Factory production improved quality control, reduced weather exposure and removed the need to assemble extensive formwork or fix reinforcement bars individually within the central reservation.

The delivery benefit extended beyond productivity. Every lorry entering or leaving the central reservation introduced an additional interface with motorway traffic. Reducing the number of deliveries therefore reduced public risk as well as pressure on the site logistics operation.

The approach concentrated site work on installation, connection and completion, transferring labour-intensive preparation to locations offering greater space and control.

Manufacturing to the installation sequence

The structure was sequenced from the beginning of on-site activity. Each DfMA element was identified during production and allocated to a predetermined position within the twin box.

That allowed factory output to be coordinated with the construction programme, ensuring the correct components were completed, transported and available ahead of each installation phase. It also reduced the risk of units arriving in the wrong order or occupying limited holding space before they were needed.

Expanded Structures developed the modular structural solution, with WSP delivering the design. Explore Manufacturing produced the precast elements at its Worksop facility, while the wider supply chain included precast beam supplier FTB. Balfour Beatty VINCI coordinated the overall construction programme and site delivery.

The method depended on continuity between permanent works design, temporary works, manufacturing, transport and lifting. Each decision affected the next stage of production and assembly.

Turning closures into production windows

The weekend installation strategy was developed at least a year before work began and refined into four closures, allowing HS2 and its partners to secure agreement with National Highways and other stakeholders.

Each possession is controlled through an hour-by-hour programme, with activities tracked against the motorway handback deadline.

Around 180 BBV and Expanded engineers and specialists work in shifts throughout each closure, completing a predetermined production cycle within a fixed operating window.

Tim Rowe, Project Leader at Expanded, said: “Our modular bridge solution offers a quicker and safer installation than traditional bridge construction, leveraging the benefits of off-site manufacture to take risk and disruption away from the site and public interfaces.”

The prefabricated beams also avoid the need for temporary supports beneath the deck, which would not be compatible with keeping the motorway operational between possessions.

One team across the interface

Delivery certainty depends on coordination between HS2, BBV, Expanded, the manufacturing supply chain and National Highways. The organisations have worked around a common requirement: complete the planned scope within each closure and return the motorway on time. That has required agreement across design, factory production, component transport, crane operations, traffic management and stakeholder approvals.

Oliver Shore, Project Director at Balfour Beatty VINCI, said: “This ambitious structure, spanning a busy stretch of the M42 motorway near Solihull, is a huge engineering challenge which has required precision planning and close collaboration with many stakeholders.” DfMA reduced the amount of work carried out beside live traffic, but it increased the importance of early decisions and reliable information. A late component, unresolved interface or change in lifting sequence could affect the entire closure programme.

Our modular bridge solution offers a quicker and safer installation than traditional bridge construction, leveraging the benefits of off-site manufacture to take risk and disruption away from the site and public interfaces."

Find out more at:
www.hs2.org.uk

Learning between installations

The use of four repeated installation weekends also created an opportunity to improve the process while work remained live.

Following the first closure, the joint team held a lessons-learned session that identified more than 60 individual items for implementation before the second weekend. The findings were also shared with HS2 and National Highways.

That feedback loop is central to the industrialised approach: performance from one cycle informs the next, strengthening safety, sequencing and productivity.

The M42 twin box demonstrates industrialised construction across design, manufacture, logistics and assembly. Its value lies not only in the precast components, but in the delivery system around them: early decisions, work transferred away from the public interface and installation planned against a defined production sequence.