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  Ground Conditioning for the Excavation of a Rock Mass with High Hydrostatic Pressure Using an EPB: The Experience in the Toluca - Mexico Railway Tunnel

  The Interurban Train Line Toluca – Mexico crosses the mountain range of Las Cruces, which is part of the vast TransMexican Volcanic Belt, from east to west, with twin 4.7 km long tunnels, at depths from 15 to 136 m and with a slope of 4% in most of the alignment. Two 8.56 m. diameter EPB shields were used to bore the twin tunnels, facing the extremely complicated geology of the region composed of andesitic, spills with different types of alteration, heterogeneous breccias, lithic and ash tuffs. Several geological faults and fracture systems were faced, reaching hydrostatic pressure up to 6 bar. The geology at the tunnel faces used to change constantly and suddenly, so that continuous adjustments of the soil conditioning system were necessary to excavate the material and to create a sort of “plug” in the screw conveyor, able to the high hydrostatic pressure and to mitigate abrasion of the cutter head and screw conveyor. These adjustments were possible also thanks to a real-time monitoring of the TBM operative parameters and of the characteristics of the excavated material, together with the application of some dedicated technical solutions for the soil conditioning. This system implemented by the construction company improved the EPB performances ever under these extreme conditions.

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