The construction of HS2’s longest green tunnel has entered a new phase following the realignment of the B4525 near Greatworth in West Northamptonshire. The temporary diversion of this key local route clears the way for the next sequence of excavations on the 2.7-kilometre cut-and-cover structure. For the UK’s digital construction and infrastructure sector, the scheme offers a detailed insight into how construction process innovation, offsite manufacturing, and data-led programme management are reshaping major rail delivery.
Shallow cutting
The Greatworth green tunnel is being built in a shallow cutting using a cut-and-cover method, with the excavated earth returned as a landscaped covering. This design reduces visual impact, lowers long-term maintenance requirements, and blends the high-speed railway into the surrounding topography. The structure is assembled from precast concrete segments delivered to site ready for installation, forming the distinctive ‘M’ shape that separates the northbound and southbound lines.
First kilometre
Engineers have already completed the first kilometre of the tunnel, and the realigned B4525 now enables teams to continue excavation without disrupting local traffic. The strategy demonstrates how digital planning, logistics optimisation and offsite component manufacturing are helping HS2 maintain progress at complex rural sites with limited working space.
Common design
The Greatworth and Chipping Warden tunnels share a common segmental design inspired by the French high-speed rail network. Work began first at Chipping Warden, where early productivity challenges highlighted the need for a construction process review. HS2 Ltd and EKFB, the main works contractor comprising Eiffage, Kier, Ferrovial Construction and Bam Nuttall, examined workflow sequencing, supply chain integration and installation logistics. By analysing data gathered during early tunnel assembly, the project team identified areas for optimisation and introduced a series of process changes.
Installation rate
These changes have more than doubled the installation rate. Teams are now assembling five to six concrete segments per day compared with the early rate achieved at Chipping Warden in 2022 and 2023. The productivity gains have been replicated at Greatworth, showing how construction insights captured through one site can transfer directly to another through structured digital reporting and shared methodologies.
Realignment
HS2 Ltd project manager Sam Arrowsmith noted that the B4525 realignment marks an important point for the programme, enabling excavation to continue at pace while keeping local travel routes open. He emphasised that lessons learned at Chipping Warden have resulted in a significant uplift in productivity across both sites.
Defined steps
The construction sequence at Greatworth involves a series of defined steps. First, the cutting is excavated and prepared with a layer of blinding to create a suitable base. To accelerate this phase, steel reinforcement is now delivered as a roll mat with bars connected by mild steel tape, allowing rapid deployment prior to concrete placement. This approach reduces manual handling and shortens preparation time.
Precast units
Each tunnel section is formed from five large precast units. Continuous improvements in delivery scheduling, lifting operations and alignment processes have reduced waiting times and increased accuracy during assembly. Once the segmental arch is installed, a concrete invert slab is poured to form the base for the track. Productivity on this phase has also increased through adjustments to concrete delivery routes and the introduction of prefabricated reinforcing cages.
Valley of the ‘M’
The upper section of the tunnel, formed by the central valley of the ‘M’, is being filled with an aerated concrete mix. This material is faster to install and offers consistent performance compared with traditional aggregate infill. To ensure improvements could be tested without delaying the main programme, EKFB constructed a dedicated test section at Chipping Warden. This controlled environment allowed teams to evaluate new methods before applying them to live works.
Interfaces
Optimising installation processes has reduced the number of interfaces at the tunnel face, where space is constrained and multiple teams must work in close sequence. Streamlined logistics and clearer task ownership have increased flexibility, helping the project manage variable conditions and maintain a steady construction rhythm.
Lessons
EKFB senior engineer Chris Barrett highlighted that the application of lessons from Chipping Warden has accelerated the installation programme at Greatworth. The completion of road diversions and other enabling works now opens access for winter construction activities.
Increase capacity
Once operational, HS2 will increase capacity between London and Birmingham and free up train paths on the West Coast Main Line for freight and regional services. However, while many sections of the route are advancing, some civil engineering packages remain behind schedule. HS2 Ltd’s chief executive, Mark Wild, is now leading a programme-wide reset to improve efficiency and cost performance.
Underground
Across Phase One, trains will run underground for 32 miles through major bored tunnels and several cut-and-cover green tunnels. Lessons from Greatworth and Chipping Warden are already influencing delivery strategies at Wendover in Buckinghamshire. Other green tunnels at Copthall and Burton Green use a different in-situ construction method suited to shorter structures.
The progress at Greatworth shows how digital construction, offsite manufacturing and process optimisation continue to evolve across HS2. As the project advances, the integration of learning across sites will remain central to delivering the railway in an efficient and predictable manner.
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