Chalk Marl Layer : A surprising blessing for the Channel tunnel

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Linking 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 Importance of Chalk Marl method

After a detailed geological study, the engineers decided to drill through Chalk Marl, which was ideal due to its low permeability and good stability.

The detailed diagrams of the different layers of soil below the seabed were not available before the mid-1950s. To understand the nature of the seabed, engineers had to drill hundreds of boreholes into the seabed, and extensive geophysical surveys were conducted from the 1950s to the 1980s.

The discovery of a consistent chalk marl layer was a huge relief for the engineers. The TBMs could easily penetrate through it as shown. Unfortunately, the chalk marl layer in the French side was weak and prone to formation of fissures. Due to this the engineers had to deploy special TBMs called Earth Pressure Balance TBMs in this region.

The TBMs had to precisely pass through the layer and eventually meet in the middle without a significant offset. If the tunnels met with an offset, the project would have ended in a disaster. How could the TBMs maintain such a complicated course? Remember, satellite mapping like GPS won’t work at such a great depth. For more details, please check our article - Channel Tunnel Surveying | A Mammoth effort to determine the Tunnel Axis.

Interestingly, Napoleon Bonaparte once planned to build a tunnel connecting France and England under the English Channel. In 1802, during a brief peace with Britain, the idea of an oil-lamp-lit tunnel for horse-drawn carriages was formally presented to Napoleon. Unfortunately, the war between Britain and France resumed in 1803, and the project was quickly abandoned. Obviously, Napoleon had no idea about the existence of the Chalk Marl layer or the techniques required to drill through it. If he had attempted the tunnel, the effort would have resulted in disaster.