  Salini-Impregilo is building part of the largest sanitary sewer system in the
history of Argentina in the suburbs of Buenos Aires City, to serve a population
of almost five million people. The project is an outfall TBM tunnel 12 km long,
starting from a reception shaft in the river margin, and transporting the
sewage 35 meters below the \textit{Rio de la Plata} riverbed to the point of
discharge. Within the final kilometer of the tunnel, a set of 36 standing pipes
so-called \textit{risers} are constructed by driving steel tubes upwards and
passing through dense sands, sandy clays and soft clays. Risers are linked-up
with the launching lining segment using flange unions.
  Driving of risers upwards will generate excess pore pressure and disturbance
in fine soils and, once the pipe is placed in its final position, negative skin
friction due to reconsolidation and creep. A risk assessment of the downdrag is
presented in this paper, based on the estimation of the force and/or
displacement in the riser-tunnel union generated by this effect. The issues of
whether it is desirable to instalock the riser-tunnel union at an early age
after installation of the riser and the time lapse required to reduce negative
skin friction effects are discussed. Results are validated by comparing the
model results with field measurements in prototype models.
