Arroyo Vega flood relief tunnel design

This article aims to present structural and geotechnical design challenges of the flood relief project “Segundo Emisario del Arroyo Vega”, part of Buenos Aires city hydraulic master plan. Works comprised the execution of an 8.4 km-long tunnel for relief of the existing sewer network. Tunnel alignment develops in a highly populated area, with important interferences such as two metro lines, three railway lines and a main potable water tunnel. All tunnel sections extend mostly within firm, silty soils. Overburden varies between 15 and 25 m. The downstream section was executed with a 6.1 m-diameter Earth pressure Balance Tunnel Boring Machine (EPB TBM) launched from a 35 m-diameter shaft with water table pressures up to 200 kPa. Break-in was performed across a watertight, plastic diaphragm wall cell, which allowed safe shaft wall manual demolition, resulting in a very safe TBM launch. The tunnel upstream section, 2.85 m in diameter, was executed using pipe jacking technology, with the peculiarity of using bidirectional launch shafts along its alignment. To allow water entrance from existing system - through flow diversion chambers - into the new relief tunnel, openings were made in the TBM tunnel, with connection areas of 4 to 7 m2. Its design was particularly challenging, given that unbolted segmental lining was used.