The construction of the tunnel for Metro Line 12 represents a breakthrough and a challenge to Mexican engineering as this is the first great diameter mechanized tunnel to be bored under urban area and in the special soft soil conditions typical of Mexico City clays. It is a tunnel excavated by EPB TBM under very low covers and also subjected to regional subsidence phenomena. Along its way, the EPB shield runs below and next to important underground structures such as sewerage tunnels and river conductions, pile foundations for bridges, superficial Metro lines and underground crossways. The present paper discusses the excavation process of the first three sections of the tunnel, the first of which served as the learning curve of the process.
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