How Channel Tunnel was built? : TBMs - the Real Heroes
How Channel Tunnel was built? : TBMs - the Real Heroes 
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Connecting the UK and France by excavating seabed soil might seem like a simple project. But to understand the reality, please review a cross-section of the soil. The Chalk Marl Layer is important for drilling the tunnels using a tunnel-boring machine.
How does a TBM work?
Please have a look at the TBM's cutterhead. TBM has many robust cutting tools. You can also notice port holes on the cutterhead. Hydraulic pistons ensure that the cutterhead is pressed against the ground with high pressure.
The cutterhead then spins, carrying the excavated muck into its portholes and filling the mixing chamber, where a screw conveyor moves it away. Note that these pistons have a fixed stroke length.
Pushing the cutterhead, breaking the rock, and removing the muck occur simultaneously. After one stroke, the machine reaches the precast assembly position and begins assembling the precast concrete rings as shown. After assembly, the hydraulic pistons are fitted to the concrete rings and the TBM begins the next cycle of boring and muck removal.
Assembly of the TBM
The TBMs, manufactured by The Robbins Company and Kawasaki Heavy Industries and were transported in pieces and assembled on-site at Shakespeare Cliff near Dover (UK) and Sangatte (France).
At both sites, the large underground caverns or open-cut portal areas served as assembly halls, sized to fit the full length of the TBM. Inside the launch chamber, a skilled team of engineers and technicians spent weeks precisely to assembling a TBM. Once assembled and tested, the TBM began its slow crossing beneath the English Channel.
Survey and determination of the Tunnel Axis line
The engineers first had to create a path under the seabed with known coordinates. This coordinate is known as the tunnel axis line and the TBM had to strictly follow this line. This required the clever surveying techniques and laser guidance system of the Channel Tunnel.
Surveyors initially used traditional triangulation techniques for creating a web of interlocking triangles across the English Channel. They measured angles from high points on the cliffs of Dover to points on the coast of Calais. For more details, please check our article - Channel Tunnel Surveying | A Mammoth effort to determine the Tunnel Axis.
The geometry of the Channel Tunnel
The Channel Tunnel project required three tunnels—two main tunnels and one service tunnel. We already know the role of the piston relief duct valves between the main tunnels. In the event of an incident or obstruction in one of the running tunnels, the maintenance staff enter the main tunnel from the service tunnel by walking through cross passages.
TBM Drilling Process
The Chalk Marl was easy to bore, its fissures risked collapse under high hydrostatic pressure, which could cost lives and trap the TBM. To prevent this, engineers reinforced the ground beforehand, using grouting.
The TBMs could drill probe holes from the tunnel face up to 100 - 250 meter ahead of the machine. If probe drilling showed unfavorable conditions, grout was injected through the probe holes to consolidate the ground, reduce permeability, and strengthen the chalk before the TBM physically bored through it.
Did you know engineers drilled the service tunnel before the main ones? Can you explain how they performed the grouting for the main tunnels?
The image shows that, because the service tunnel was already drilled, grouting the main tunnels using these radial drills was straightforward.
Despite extensive surveys, true ground conditions beneath the English Channel were only revealed by excavation. Drilling the service tunnel first reduced risk and simplified grouting of the main tunnels.
What is soft docking?
Using smart engineering techniques, both TBMs stayed on course. When they were 100 meters apart, they stopped and only one resumed tunneling—this is called soft-docking. Continuing to operate both TBMs until their meeting point was clearly unsafe. Another concern was possible misalignment. On October 30, 1990, engineers successfully drilled a narrow 5‑cm probe from the English side to the French side, confirming the tunnels were correctly aligned. A huge relief for the engineers.
The Historic Handshake
After the probe succeeded, workers hand-excavated a short pilot tunnel, enabling the historic "handshake" on December 1, 1990, when British and French teams met deep beneath the English Channel. This handshake marked the first land connection between Great Britain and mainland Europe since the end of the last Ice Age.
Finally, the French TBM was carefully advanced to break through the remaining ground, completing the main service tunnel excavation.
Did you know a total of Eleven TBMs were used for the project—five from the French side and six from the English. Although TBMs were sufficient. Engineers deployed the extra five because of varying geological conditions.
The geology between the coast and tunnel portals was more complex than the deep-sea chalk, so the same TBMs couldn't be used; three different TBMs were employed on the UK side and two on the French side.
The TBM burial
Surprisingly, two of the eleven machines were deliberately driven into the rock and buried — a procedure known as a TBM burial. Both were British TBMs. TBMs cannot move backward. Dismantling and transporting them over such distances was difficult and costly. Five land-based TBMs were completely taken apart and removed; the other four were partially dismantled, with their most valuable components salvaged.
A Journey Through the Channel Tunnel
The service tunnel was completed, but it isn’t straight. Engineers used a more complex design with two crossovers, which divide the 50.5-kilometre tunnel into six manageable sections. For more details, please check our article - The Genius Geometry of the Channel Tunnel.
I hope this article clarifies how the Channel Tunnel was built.
Published On : November 24, 2025
The information shared, was really great & help lay people understand easily.
Thank you Donwin for the encouraging words.