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  Geological characterization and reduction of uncertain ties for mechanized tunnelling with mixed face coarse alluvium-granite for Lyon’s Metro B extension, France.

  Lyon’s Metro B extension tunnel will be shallowly excavated in a dense and sensitive urban context, through pre- and postglacial alluvium interfacing with old wells, erratic boulders and a granite substratum. Settlement control using confined TBM, which is essential for reducing the risk of damage to buildings, become highly sensitive in such context. Among the risks that have been identified during concept design, the following paper deals especially with the difficulties of TBM’s drive in very coarse materials with granitic mixed-face: excessive loss of TBM’s slurry in coarse materials, inadequate confinement pressure and over-excavation due to resistance heterogeneity, are indeed serious causes of uncontrolled settlements. The geological aspects of those risks are described, together with the geotechnical investigation and specific methods of interpretation. Hence, geotechnical investigations have been used in order to reduce uncertainties (geometrical uncertainties with boreholes and geophysical methods, granite characterization with UCS and Cerchar lab tests, and alluvium structure with particle size). The various investigation campaign contain around 260 boreholes for 3.5km of tunnel. Specific analysis have been used to add value to this large amount of data. A 3D geological model has been build, taking into account all borehole data and geophysical results. Using Kriging interpolation method with a variogram model, make possible to estimate uncertainties on the position of the alluvium-granite interface and thus the location of probable mixed faces. The particle size has been analysed along the alignment in order to locate the areas with coarser material and higher probability for excessive consumption of TBM’s slurry. Finally the results of these analysis have been presented on the geotechnical longitudinal profile, together with other critical data regarding tunnel excavation (such as building sensitivity, rock characterization results). Risk analysis; Uncertainties; Geotechnics, Investigation, 3D modelling.

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