STREAMLINING SUCCESS
How a scanning total station saved vital time on HS2’s Chiltern Tunnel whilst providing the highly detailed documentation and as-builts that contributed to the tunnel’s successful delivery.
In July 2017, Align JV (a partnership of three international construction companies Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick) was awarded the Central 1 (C1) package of the UK High Speed 2 line (HS2) phase one. C1 consists of 21.6km of high-speed rail infrastructure and includes a 16.04km twin-bored tunnel. The Chiltern Tunnel, which passes beneath an area of outstanding natural beauty (AONB), is the longest on the HS2 route and was created using two identical tunnel boring machines (TBMs) named Florence and Cecilia launched in the summer of 2021, with Florence breaking through at the North Portal in February 2024 and Cecilia in March 2024. The C1 package includes the 16km Chiltern Tunnel and the 3.4km Colne Valley Viaduct which are two of the most technically challenging elements on HS2 phase one.
Aerial shot of the south portal site in spring 2021.
The Chiltern Tunnel, the longest rail tunnel in the UK, was designed to be constructed using two TBMs operating continuously without intermediate breakthroughs. This made the project particularly complex, especially survey team responsible for guiding the machines and maintaining the precise tolerances required to meet the design specifications for the high-speed railway. Responsible for determining the best methods for the highly technical requirements of the underground surveys and for coordinating the joint venture (JV) team and ensuring they are all fully trained is Align tunnel survey manager, Dimitris Dimitriou.
To further complicate things on an already extremely busy site with a lot of traffic and machinery, 38 cross passage tunnels connecting the two main ones were also being constructed creating a massive operation requiring many different specialist skills and different workflows.
The specification for Dimitris’s team included all the survey work centred around establishing the open-ended control, the constant checking of the misalignment of the TBMs and the creation of as-builts at each of stage of each construction element. These as-builts also included the precast concrete ring sections and in-situ cast concrete for the creation of the base for the track.
A number of challenges dictated how the team could work most effectively. Not only did This made the project particularly complex, especially for the survey team responsible for guiding the machines and maintaining the precise tolerances required to meet the design specifications for the high-speed railway. the survey work have to adhere to extremely strict principles including a requirement to document every structure with an as-built point cloud, but the window in which scans capturing the whole round profile could be undertaken was very tight.
HS2 Chiltern Tunnel TBM Cecilia breakthrough.
All scanning had to be completed after the TBMs had passed but before the invert concrete was in place. To further complicate things on an already extremely busy site with a lot of traffic and machinery, 38 cross passage tunnels connecting the two main ones were also being constructed creating a massive operation requiring many different specialist skills and different workflows.
Dimitris emphasises that establishing surveying control was one of the most challenging tasks compounded by the pressure to capture all data in a dynamic environment where everyone was focused on maximising efficiency. Following consultation with KOREC, a number of Trimble instruments were purchased for the project including a Trimble SX12 scanning total station.
Establishing surveying control was one of the most challenging tasks compounded by the pressure to capture all data in a dynamic environment where everyone was focused on maximising efficiencyThe SX12 functionality achieves 1” angular accuracy which combines with a very precise long-range scanner that produces exceptionally clean point clouds. This combination provideed Dimitris with the highquality scan data he needed along with the means to establish control far more efficiently than if the team were to use a separate scanner and total station.
Unlike a regular day on site, tunnelling and working with TBMs is a high intensity environment. If you lose one hour, you lose a lot, if you gain one hour, you gain a lot. You are constantly chasing a massive machine and being chased yourself, so the pressure is on to have no downtime. Extensive planning was therefore imperative and that included a very careful appraisal of workflows and survey equipment.
HS2 staff and contractors watch the breakthrough of Florence at the end of her 10-mile drive under the Chilterns.
The SX12 delivered in a number of key areas. Dimitris estimates that on the days that they were scanning, the SX12’s functionality as a hybrid scanner/ robotic total station meant that they saved at least 90 minutes. With just a conventional scanner and total station, the team would have to use a total station to tie the scan target to the existing network and then come back with a scanner meaning two different pieces of equipment and double the site time. With the SX12, existing prisms can be used, you don’t need to create a new system to register your point cloud.
With the tunnels being constructed with no intermediate breakthroughs, the team would be traversing down an openended tunnel with no fix. They were therefore always aware that once the TBMs have broken out at the surface, the raw data would be reprocessed with the new control network that would be established on completion.
With the tunnels being constructed with no intermediate breakthroughs, the team would be traversing down an open-ended tunnel with no fix. They were therefore always aware that once the TBMs have broken out at the surface, the raw data would be reprocessed with the new control network that would be established on completion. The ability to use the SX12’s hybrid functionality means that the scan is related to control. If the control is changed, there is no requirement to rescan – the scans are automatically aligned to any adjusted control in Trimble Business Center office software (TBC) for a true as-built. Therefore, the SX12 does not have to rely on Cloud to Cloud or complex point cloud adjustments.
This significantly cuts down on office time – a new control file can be simply dragged and dropped.
In the tunnels, the Align team used the SX12 to complete scans from one set up, 20m in each direction, with at least 10m overlapping with the next set up. Dimitris reports that even using the SX12 on medium density rather than the highest density, they were able to achieve more than sufficiently dense point clouds in less than 12 minutes, all with just a minimal amount of additional work because the existing control could be used with no extra scan targets or office processing required.
TBC view of the tunnel scans with SX12 resections using existing tunnel control highlighted.
Additionally, the ability to use the Trimble Access field software to set a maximum distance range on the SX12 greatly cut the amount of office time to clean the data, especially useful on a linear site such as this one with so much noise. Due to the fact you can use it as a total station, means that you can set the instrument up by resection, or over a known point, ensuring that when you bring the data into TBC, all scans will be at the correct position and correct orientation in global coordinates.
The TBC software enables surveyors to transform field data, from a range of sensors, into high-quality client deliverables. For the Align team, the software’s reporting functionality was key in providing the highly specific, customised reports cited by the client. Once formatted, the reports required just the addition of names and dates before submission. For Dimitris, the breakthrough of the first tunnel, well within the design tolerance, was a particularly proud moment which he credits to the talents of the Align survey team who were able to respond to the challenges presented to them.
Tunnel view prior to the invert concrete being poured. Survey control brackets are visible on the left/right of the tunnel.
The team were given a challenge, but it was feasible and they were able to deliver within the very strict parameters that were set.
The ability to do this has made the life of other teams working on the project far easier. The breakthrough was a date to remember, a lovely finish for the team’s journey over the three years.
The standout benefit of using the SX12 is the very detailed point clouds that were achieved in a very short time, with the minimum amount of additional work because of using the existing control.
All the data sets were collected without delays and the quality was excellent which meant no need for rework. Align JV is always looking to improve and the fact that this job has been successfully completed is a good message for future projects.
Chiltern Tunnels
- 16.04km twin-bore tunnels through the Chiltern Hills
- Two TBMs, Florence and Cecilia, built to handle chalk and flint bands
- 170m long and 2,200t each
- Excavation diameter – 10.26m
- Concrete segments 2 x 0.4m
- Seven segments per ring – 112,300 segments in total
- Three million m3 excavated material produced 24/7 and processed through the slurry treament plant at the south portal
- Five ventilation and access shafts