ABSTRACT
The mastery of tunnel boring in heterogeneous terrains, especially when using totally mechanised digging methods, is a delicate operation because the machine’s performance can be ruined by unexpected terrain. Preliminary reconnaissance can help to distinguish existing geotechnical conditions and plan adapted construction methods. However, due to the linear nature of tunnel projects, it is usually not possible to have a detailed vision of the geology of the site. One must use estimates obtained by interpolation of data. In these conditions, it is pratically inevitable that one will encounter geological risks.
The geological risk can be associated with an unexpected variation of the terrain (increase in permeability of the terrain, varying degrees of hardness, anomalies in the massif, presence of huge rocks, empty spaces). Whatever its nature, a geological risk causes technical problems which can impede the progress of the project either by raising costs and causing delays or by creating safety problems and affecting the quality of the works. In particular, unexpected geological conditions can cause diversions or momentary blockages of the machine, flooding, unstable terrain in front of the machine and settling of the terrain which can be harmful to existing constructions on the surface. Under these conditions, an early identification of problematic geological conditions can permit a better anticipation or even a preliminary treatment of the difficult areas and contribute to a better mastery of the works.
Information about the geologic conditions can be obtained in part from reconnaissances carried out in front of the machine during the works. In particular, the combined use of drilling and geophysic methods can provide a better detection of the different types of layers to be encountered. The methods of reconnaissance in front of the machine must be used as a complement to preliminary reconnaissance in an effort to obtain more precise information about the heterogeneities and anomalies which could cause problems in the works.
