Temporary tunnel support using jet-grouted cylinders

A 750-m portion of the 7.1-km “Monte Olimpino 2” tunnel was located in glacial-fluvial silty sands and gravels. The 12-m-wide tunnel was advanced rapidly through the granular deposits by forming, in radial planes ahead of the excavation face, an arch support of slightly outward inclined and overlapping jet-grouted soil-cement cylinders. Measurements of strain at the surface and in the ground through vertical boreholes placed at two different sections demonstrate that the construction method adopted caused small ground movements without appreciable variation of the state of stress. The excavation was carried out without incidents over a 1.5-yr period. The tunnel was completed in time with savings both to the contractor and the client and with safety to men and equipment.