ABSTRACT
In order to be able to execute ground freezing works, the construction of 4 shafts was required. Due to the geometrical set up of tunnels and geological conditions, the construction of the shafts is particularly challenging because they are wide (800-1600 m2, ellipse with main axis 35-55 m), deep (up to 28 m) and subject to high water pressure (up to 18 m of water table above bottom). Each shaft was built by means of ground consolidation with 700-1000 jet grouting columns (d = 2000 mm) and underpinning excavation with final r.c. lining, in order to obtain a reasonably waterproof structure able to adequately counter the huge forces acting especially on temporary structures during excavation phases. The following paper will present an analysis of the experience acquired during the excavation of the first shaft, with particular reference to comparisons between design predictions, actual findings and critical issues encountered.
