Drilling and blasting

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Before the advent of tunnel boring machines, drilling and blasting was the only economical way of excavating long tunnels through hard rock, where digging is not possible. Even today, the method is still used in the construction of particularly long tunnels, where a TBM is unpractically expensive, such as in the construction of the Lötschberg Base Tunnel.

[edit] History

While drilling and blasting saw limited use in pre-industrial times using gun powder, it was not until more powerful (and safer) explosives, eg. dynamite, and powered drills were developed, that its potential was fully realised.

Drilling and blasting was successfully used to construct tunnels throughout the world, notably the St. Gotthard Tunnel, the Jungfraubahn and even the longest road tunnel in the world, Lærdalstunnelen, are constructed using this method.

[edit] Procedure

As the name suggests, drilling and blasting works as follows:

• A number of holes are drilled into the rock, which are then filled with explosive.
• Detonating the explosive will cause the rock to collapse and thus lengthen the tunnel.
• Rubbles are removed and new tunnel surface are reinforced.
• Repeating these steps will eventually result in a tunnel.

The positions and depths of the holes (and the amount of explosive each hole receives) are determined by a carefully constructed pattern, which, together with the correct timing of the individual explosions, will guarantee that the tunnel will have an approximately circular cross-section.