Surveying the HS2 tunnels in London

As part of Britain’s new high-speed railway project (HS2), six tunnel-boring machines have been driving through clay, sand and ever-shifting ground beneath the streets of London. They are carving out the tunnels that will one day enable trains to reach the heart of the British capital faster. GIM International spoke to Matthew Baddeley, survey manager for the Skanska Costain STRABAG Joint Venture (SCS JV), about the challenges of guiding them there: in both line and level, and within millimetres of their intended breakthrough points. The precision engineering involved in this intriguing project includes gyroscopic control over distances of up to 8km without intermediate checks, laser scanning workflows feeding directly into design decisions, and a 21km surface traverse that reconciled two independent underground control systems: Northolt Tunnel and Euston Tunnel.

The geometry of the ‘London Tunnels’ section of the HS2 project dictates every surveying challenge. In early 2026, two tunnel-boring machines (TBMs) were launched from Old Oak Common, mining towards Euston Station for a 7.5km drive. Prior to that, in January 2024, two other TBMs had been launched from the Victoria Road Crossover Box, for a 5.5km drive to Greenpark Way Vent Shaft. This drive, called Northolt Tunnel East, was successfully completed in summer 2025. Another pair of TBMs had set out on a demanding 8km drive in October 2022 from West Ruislip back to Greenpark Way. These were for the Northolt Tunnel West drives that were completed in April 2025. At the Greenpark Way Vent Shaft, all four machines converged into two large vent shafts and had to break through into what Baddeley describes as a “steel can”: a pre-built reception structure with tight positional tolerances. “From a tolerance perspective, it was challenging for all the TBMs to meet into the shafts because of the steel can scenario and the high accuracy requirements,” he says.

Beyond the main TBM drives, the project has also encompassed extensive sprayed concrete lining (SCL) works for cross-passages and ventilation shafts, as well as two SCL tunnels connecting into Old Oak Common station from the Victoria Road Crossover Box, over a total of 350-400m. These all added their own surveying demands.

Non-uniform ground conditions

Due to London’s non-uniform subsurface, the ground conditions shape the survey efforts in ways that were difficult to predict at the outset. On the Northolt Tunnel East drives, conditions were relatively favourable, according to Baddeley; London Clay geology allowed the cross-passages to be excavated without ground treatment, keeping the survey workload manageable. However, for Northolt Tunnel West, the sandy soil required the ground around each cross-passage to be frozen before excavation could begin.

Cross-passage works, invert concrete pours and ground-freezing operations all compete for access and generated disturbance, making it extra difficult to maintain the control network while production activity continues behind the TBMs.

Shaft behaviour adds a further layer of complexity. Dewatering operations at Greenpark Way Vent Shaft caused the structure to sink and heave with the seasons, obliging the team to continually recalibrate the reference points at the shaft head. At the Westgate Vent Shaft, the passage of the TBMs caused approximately 30-35mm of settlement in a structure that had been largely built out, with steelwork and primary structural elements already in place. This created additional challenges for both the survey and design teams in terms of decisions on fitting the cladding and all the mechanical and electrical (M&E) works “in a shaft that has effectively just moved”, as Baddeley describes it.

8km without a check

The Northolt Tunnel West drive, running 8km from West Ruislip to the Greenpark Way Vent Shaft without an intermediate shaft to perform a check, presented the project’s biggest survey challenge. The survey team had to maintain confidence in their control network – and in their accumulated chainage and horizontal swing – through the full length of the drive. “We didn’t know for sure whether we were in the correct position or level until we broke through into that Greenpark Way shaft,” recalls Baddeley candidly.

In order to validate the control network across the longest distances, and to verify the positional relationship between Old Oak Common and Euston, the team carried out a rooftop traverse ahead of the recently launched Euston TBM drives. It is standard practice in major tunnel projects to prove the relativity on the surface before the underground connection is established. This confirmed that the Euston Tunnel independently controlled underground environment will meet within tolerance when the drive finishes its excavation works in summer 2027.

The traverse was performed using Leica TS60 half-second total stations with Leica GPH3 triple-array prism holders used in conjunction with GPR1 prisms. Leica GPH1P single prisms were also used as these are precisely machined for high-accuracy activities such as a rooftop traverse. Automatic target recognition (ATR) was possible to approximately 1.5km, after which the team switched to manual sighting. The combination proved workable and delivered favourable results, but lessons were also learned from the campaign. “The ATR on the Leicas is so much superior to manual sighting. It could be better to shorten the ranges to maintain the use of the ATR, because when you’re manually sighting over those distances, you’ve got problems with shimmer and low-level light due to observing in the autumn,” states Baddeley in hindsight. For any future tunnelling works where surface connection checks are required prior to launch, he intends to refine this approach.

Gyroscopic control and keeping the TBMs on course

Underpinning the alignment control for the Northolt drives, and allocated for the Euston tunnels, is a Gyromat 5000 gyroscope paired with a Leica TS60. Gyroscopic orientation provides an independent bearing reference that does not depend on surface observations or the continuity of the underground traverse, making it a critical safeguard when drives are long and connection opportunities are limited.

The guidance of each TBM depends on a chain of precision that runs from the surface control network through the underground traverse to the machine itself, and every link in that chain requires active management. Tunnel brackets – the mounting points for total stations working behind the TBM – are subject to movement due to settlement of the tunnel lining, temperature changes and vibrations caused by production activity. Refraction of the line of sight is a persistent concern at depth, particularly where temperature gradients exist between the ventilated tunnel air and the surrounding ground. Both these effects can introduce systematic errors that accumulate over the course of a drive. Therefore, both require regular monitoring and correction cycles to keep the guidance system within its required tolerances.

The Leica TS60 has been the instrument of choice throughout all TBM guidance and underground control work. Its half-second angular accuracy and robust ATR performance in tunnel conditions give the team confidence that the instrumentation is not the limiting factor. “The TS60 is rock solid and our surveyors all trust this instrument to deliver repeatable results,” Baddeley says. “We’ve been using it for all control work throughout this project.”

Traverse adjustment

The same discipline extends to the GNSS control network that was processed for Euston tunnels: six Leica GS18 receivers operating simultaneously across the Old Oak to Euston corridor, with the adjustment handled in Leica Infinity. Star*Net, the traverse adjustment package now within the Hexagon group, has been used throughout the project. It was pushed towards its limits on the 8km Northolt Tunnel West twin bore – a configuration that might have required the network to be split on some comparable projects, according to the survey manager.

The convergence monitoring and SCL monitoring programme is handled through Amberg, and the Amberg tunnel scan module is increasingly used to ingest laser scanning data and to generate cross-sectional profiles efficiently. Because this particular project exceeds the practical limits of such off-the-shelf software due to its scale and precision demands, SCS JV Tunnel Survey Manager Thomas King also developed an in-house coded tool specifically to analyse traverse data and control network results.

The laser scanning argument

If the control network keeps the TBMs on course, laser scanning keeps the rest of the project ‘honest’. Baddeley recalls early scepticism from project stakeholders. “People were asking, ‘Are you laser scanning too much? Why do we have to laser scan?’. I just had to push back and say: we are laser scanning all permanent works.” That stance has since been repeatedly vindicated, he says.

A well-established workflow was soon in place for SCL structures: scan after primary lining, scan again after secondary lining, compare against the design model with Cyclone 3DR to generate deviation heat maps, mill back anything out of tolerance, and rescan to confirm. For cross-passage underground freezing treatment on Northolt Tunnel West, the survey team conducted scanning to extract as-built information about the ring joint positions to feed them back to the design team for the inclined drilling layout. As a result of the freezing programme, the Northolt Tunnel West operation was significantly more resource-intensive than Northolt Tunnel East and generated an entirely separate data flow for the survey team. “We pulled in Cyclone 3DR for that,” says Baddeley.

For the team following the TBMs through the bored tunnels, the Leica MS60 multi-station was actually the only practical tool for capturing profiles: “That’s the only window we can laser-scan the tunnels to produce our ‘wriggles’ – effectively cross-sectional profiles – and that can be taken onto other projects,” he adds.

Data analysis and archive

Beyond deviation analysis, Cyclone 3DR has been used extensively for mesh generation, area calculations and volume assessments – tasks that extend the value of each scanning session well beyond a simple compliance check. “It’s like the Swiss army knife of point cloud processing; it’s revolutionary,” Baddeley says of Cyclone 3DR. Alongside 3DR, Cyclone REGISTER 360 PLUS has served as a robust platform to accurately merge the laser scans together and georeference onto the HS2 Survey Grid coordinate system.

The data has already proved its worth as a long-term archive. In one instance, drainage elements were scanned before being buried under a poured slab. When the engineering team later needed to connect new drainage into the bored tunnel, the point cloud provided the only available as-built geometry. “It happens almost weekly,” Baddeley says. “Engineers can understand the heat maps; the BIM team can adjust the design model if it’s out of tolerance, and that then becomes the as-built model.”

Streaming across the project

At the project data-management level, Hexagon GeoCloud provides the ability to stream point cloud and panoramic imagery to any authorized user across the full site, without the system degrading as data volume grows. This is a valuable new capability, according to the survey manager: “We’ve got thousands of projects in there now, and it’s still a smooth system to operate.” This is not the case, he emphasizes, with some competing platforms that slow significantly as data accumulates.

This has led to a measurable shift in how non-survey disciplines engage with spatial data. Engineers from the design house used the solution to carry out ground-movement assessments on all the bridges along the alignment from Euston to West Ruislip, a task that previously would have required site attendance. It has served as a data review and collaboration platform, giving engineers and project managers access to scanning data without requiring specialist processing skills. Rather than having to go underground, site engineers can extract coordinates without leaving their office. “There’s a health and safety gain as well. It reduces the footfall on the site,” he continues. The combination of Hexagon GeoCloud for streaming and Cyclone 3DR for processing has, in his view, been the most transformative pairing on the project.

Building a surveying culture

The project has required a total of 56 surveyors, ranging from apprentices to chartered professionals, to operate as a coherent team across numerous active fronts simultaneously. The user-friendliness of the tools have certainly contributed to their success. Cyclone 3DR’s intuitive interface and Leica Infinity’s clear learning resources are proving genuinely useful, lowering the barrier for junior staff to engage with data processing tasks that once required dedicated specialists. Baddeley contrasts this with earlier generations of software. “Back in 2007, I went on a five-day training course for Cyclone CORE, which I would say was also necessary. Today, you probably don’t need a five-day training course for Cyclone 3DR or Cyclone REGISTER 360 PLUS, because the software is so intuitive.”

For the most complex tasks, senior staff are drawing on their broad base of project experience. SCS JV’s Head of Survey William Archibald has previously worked on both Crossrail and the Channel Tunnel. Baddeley and Tunnel Survey Manager Thomas King benchmarked against published accounts of the Gotthard Base Tunnel and conducted a site visit to the Align JV’s 16km project to directly observe control network configuration and bracket systems.

But he emphasizes that the team structure matters just as much as the tools. “We’re not just trying to have one dedicated person sitting there doing the Star*Net processing. On Northolt Tunnel East, we let the surveyors embrace the tunnel adjustments themselves. They’ve all been having a go at the data processing and developing as tunnel surveyors, not just gathering data on site.” The goal here has been a team where basically every surveyor can complete a full data cycle: gather, process, adjust, report. “I think that level of knowledge sharing has been a big positive in this project,” he states.

Traditional foundations, new skills

The recently launched Euston tunnel drives will benefit directly from lessons accumulated across Northolt tunnels. The GNSS network has been re-observed with Leica GS18 receivers. The Gyromat is ready. The team is revisiting tunnel bracket configurations and gyro observation protocols, drawing on Gotthard documentation alongside the Northolt Tunnel experience. New instrumentation is also on the horizon – technology that arrived after the Northolt Tunnel drives commenced and which Baddeley is keen to evaluate for future applications.

When asked which skills the next generation of tunnel surveyors will need, his answer spans traditional and contemporary expertise. “They still need the grounding at university to learn the bread-and-butter survey elements like geodesy. If you really want to drive TBMs over many kilometres, you’ve got to understand about scale and geodesy if you want your tunnel to close correctly at the other end.” But alongside that foundation, coding is now essential, he says, as illustrated by the SCS JV team’s experience developing in-house traverse analysis tools. Skanska is actively putting staff through programming courses in recognition of this. BIM literacy, GIS integration and awareness of AI tools complete his list of skills needed by a future senior tunnel surveyor.

Baddeley is proudest of the team’s achievements at the Greenpark Way Vent Shaft, where four TBMs arrived from different directions and proved that 8km of surveying, through unpredictable ground and without intermediate position checks, had worked. The breakthrough itself was, he admits, less cinematic than in the movies; the machines came into the enclosed steel reception structure, and the moment of revelation came when the lid was lifted. But the engineering behind it was anything but understated.

About Matthew Baddeley

Matthew Baddeley is a chartered civil engineering surveyor who has worked in industry for 20 years, 13 of which have been at Skanska. He joined the HS2 Main Works Civil Engineering Contract in 2020. Operating as a survey manager for SCS JV, he leads the route-wide survey control and as-built delivery.