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  Challenges in a great diameter tunnel excavation in sandy soil

  An 84 m long conventional tunnel with 117 m² tunnel cross section is under constructed in layered soil, which ranges from sandy soil with high permeability to clayey soil with low permeability. High groundwater pressures in the excavation of the crown and invert have been experienced. The tunnel concept had a pre-support in Jet Grouting executed from surface. This solution allowed a faster advance rate since it reduces the time spent with soil treatments from inside the tunnel. The tunnel is being executed in two stages. The tunnel top heading with 80 m² is being first excavated throughout the whole extension of the tunnel, followed by the invert execution. A vast range of groundwater control methods have been applied, going from deep wells, to horizontal internal drainage in conjugation with internal well points. Water control is the key to the success of the excavation. This paper presents unique design aspects and execution details of this project.

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