Channel Tunnel Surveying | A Mammoth effort to determine the Tunnel Axis

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The Channel Tunnel is linking England and France beneath the English Channel. The discovery of a consistent chalk marl layer was a huge relief for the engineers. The TBMs could easily penetrate through it as shown.

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.

How did engineers achieve the path?

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.

1. The Primary Survey

The primary surveying task was to create a single, unified grid system connecting the British and French coasts. Before tunneling could begin, surveyors had to know the precise three dimensional relationship between the starting point in Folkestone UK, and the starting point in Sangatte, France. On a clear day, anyone can see England from France.

2. Traditional triangulation techniques

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.

3. Accurate distance measurement

The distance was too great for highly accurate direct measurement across the water. To bridge the Channel with the required accuracy, engineers use high-precision electromagnetic distance measurement instruments. These tools send out light or microwave beams to reflectors on the opposite coast to measure the distance with pinpoint accuracy. The advent of GPS further helped with surface surveying. It also confirmed their surveying measurements.

ELECTROMAGNETIC DISTANCE MEASUREMENT INSTRUMENT

4. Selection of Initial baseline

On this known coordinate plane, the engineers select a few points to create the initial baseline for the tunnel.

5. Shaft plumbing technique

The surface grid was locked. The technique used for the next task is called shaft plumbing. Shaft plumbing accurately transferred the surface-level survey coordinates down to the seabed. A specialized surveying instrument called a zenith plummet was set up with great precision directly over a known survey point at the top of the shaft.

This instrument is designed to look perfectly straight up or down. This tube isolates the plummet from air currents. For the Channel Tunnel, optical plummets were also used. At the bottom of the shaft, there is another plummet that looks perfectly straight up to align with the laser or the wires coming down from the surface. Once this alignment is achieved, that spot at the bottom of the shaft becomes the new coordinate on the seabed.

From the detailed geophysical survey, the engineers knew the depth of the Chalk Marl. Combining these two pieces of information, the engineers established the tunnel axis with many known coordinates on it.

How do TBMs follow the tunnel axis?

Now, the only question was how to make the TBMs follow this tunnel axis? This is where one of the smartest techniques of the Channel Tunnel project came in - the laser guidance system for the TBMs.

A laser theodolite was mounted in the tunnel behind the TBM as shown. They also arranged a few control points on the tunnel wall. The coordinates of the theodolite and control points were known to the engineers. A photosensitive target was mounted on the frame of the TBM; its position was also known.

Suppose after moving forward a few meters, to align with the tunnel axis, the TBM had to be steered as shown. The control points were arranged such that the laser would fall exactly in the middle of the target point upon perfect alignment. Suppose the driver didn't steer that much while moving forward.

This would obviously cause the laser to fall off-center on the target. The computer in the driver’s control cabin would immediately inform this error, and engineers could take remedial action.

As the TBM moved forward, the laser station was periodically leapfrogged ahead along with more recently established control points. This painstaking process ensured that the path never deviated from the master plan. I hope you understand how engineers find the tunnel axis of the channel tunnel and its detailed survey.