  The Mantanza-Riachuelo basin recovery is one of the most ambitious
environmental projects under construction in Argentina. In this context, the
sanitary bureau of the metropolitan area of Buenos Aires (AySA) is building a
sewage collection network to transport the waste water of the population in the
southern area of the city, composed by almost five million people. The most
complex tunnel in this big project is named \textit{Lot 3}, an outfall EPB-TBM
tunnel starting at a shaft located at the \textit{Rio de la Plata} margin and
running under the river 12 km to a discharge area.
  The tunnel runs through soft clay belonging to the \textit{post-pampeano}
formation and dense sands of the \textit{Puelchese} formation. In operation, it
will be pressurized by a pumping station which will produce a piezometer head
that, in the first 2000 m, might be eventually higher than the confining
pressure around the tunnel.
  This paper presents the numerical analysis of the structural forces acting on
the tunnel rings using a risk-oriented approach that considers the stochastic
nature of materials, stratigraphy and tunnel-ground interaction. The
compression of the lining is evaluated and compared with field measurements in
order to predict the structural forces and the risk of the rings going into
tension beyond the structural capacity of the system.
