World Tunnel Congress - Choice of Tunneling Method

I consider myself lucky that the World Tunnel Congress was in the country I live in.  Geneva is a beautiful city next to Lake Geneva (lac léman) in the south-west corner of the french part of Switzerland.

This meeting was just like all such meetings - full of excitement, business talks, a chance to see and be seen by colleagues from around the world, and to sit in on case study presentations.

The sessions, presentations, case studies and technical themes took place at the same time. So I could pick out what I wanted to see, and when to take a look around the exhibitions and talk to the exhibitors about their news, in which projects they are interested in and involved. 

My first step on Tuesday was to eat some lunch, meet my colleagues and go into the swiss sessions first. The theme of the swiss colloquium was the "choice of tunneling method".

At the beginning of a project, one of the main questions is how to go through the rock, how to excavate: drill and blast, with a tunnel boring machine (TBM), with or without a pilot tunnel, conventional excavation without blasting and how to excavate all the secondary elements like cross sections and niches.

The speakers talked about their chosen method, pros and cons, problems that occurred, the problem solving and suggestions which excavation methods could be applied in this situation too.

Here I will give you a summary of one speech:

The choice of driving method in soft ground - the Längholztunnel. This tunnel is situated in the western part of Switzerland in Biel / Bienne, not far from Berne, the capital of Switzerland. The tunnel goes through soft ground with a groundwater level of 3 to 10 m (9 to 33 feet) above the tunnel crown. First step, they did a risk analysis to figure out what kind of excavation method was appropriate for this kind of soft soil. After the first risk analysis they chose a Hydro-shield TBM out of EPB (earth pressure balance) TBM, compressed-air shield and conventional drive. The general analysis points were the standard cross-section, tunnel length, geology, overburden, close buildings and obstacles underground. The aspects for choosing the driving method were the evaluation of the ground conditions, process technology and budget considerations, surface settlements and reduction of time consuming measurements to improve the ground.

After taking a deeper look at the ground conditions, analyzing different tunnel cross-sections in soft ground - the project developer came to the interpretation, that an EPB drive was a better solution for the anticipated ground conditions.

The big advantage of an EPB shield is that a sudden intrusion into the extraction chamber isn't feasible with regard to the supported face. Also working conditions are better in terms of excavation noise. Among other advantages and disadvantages - like the difficulties with the earth pressure adjustments or the time it takes to enter the working area.  For reasons, the chosen excavation method was to go with EPB. Some criteria for the decision were the economy, face stability, cave-ins, dealing with boulders and blocks and the dumping capacity of the installation area.

The point of the speech was, that an enlarged and systematical risk analysis with detailed case scenarios and additional explorations of the geological conditions could result in a different driving method and should be considered in cases, in which the choice between similar kinds of driving methods are to be taken.