Well drilled in the wrong place breaches railway tunnel

What happened?

At a large construction site in a big city, contractors drilled an exploratory well in slightly the wrong place and as a result drilled into the wall of a busy railway tunnel, closing the railway for three days.

The incident occurred during the construction of an extension to an underground railway line in Paris.  The well was drilled for civil engineering/geological test purposes by the consortium of contractors in charge of the project.  The well was drilled through the water table, and when the drill bit penetrated the railway tunnel, there was an inrush of ground water and mud into the tunnel.  Within a few hours, this had reached a volume of 40 cubic metres.  The RER A ­ the busiest urban railway line in Paris ­ was closed for three days as a result.  There were no injuries or casualties.

What went wrong?  What were the causes?

• The decision to drill the well was made by the contractors without the approval of the client;
• The positioning of the well was conducted without the control and validation of a surveyor. The official report noted: [translated from the French]

The lack of intervention of a surveyor resulted in an error which was sufficient to lead to the accident

What lesson was learned?

• Ensure that appropriate technical and professional advice is sought from surveyors for all operations requiring any type of positioning.

The French government report into the incident (in French) Expertise sur la maitrise des risques des travaux en souterrain, premier rapport suite a l’incident de chantier du 30 octobre 2017* is available here.  

*[Risk management in underground works – initial report following construction incident of 30 October 2017].