Abstract:
The paper deals with the design and construction of three horizontal ventilation adits, within the framework of the construction of the Naples Subway Line 6 in Italy. The ventilation system of the subway entails underground chambers connected to the TBM tunnel by a vertical shaft and a horizontal adit. The connection adits are below a water-table head of 18 to 23.5 m; for this reason they had to be excavated using the protection of preventive soil or rock improvement measures. At one of the three adits, penetration grouting through tubes à manchettes was adopted to seal the system of fractures within the Neapolitan Yellow Tuff (NYT). The result was successful: tunnelling was safe and almost dry, as well as avoiding any effect on the adjacent structures. At the other two adits a combination of vertical jet grouting columns, installed from the surface within the loose soil, and grouting, performed from the shaft within the NYT, was at first foreseen. But sometimes the design needs to be revised during the construction phase due to unexpected geotechnical conditions found. The two remaining adits at Naples Subway Line 6 are an example of the need of this design revision. After a field trial, the jet grouting activities started at the first adit. During the installation of the first 18 columns (out of 95), a phenomenon of systematic soil removal and consequent formation of chimneys was observed (the chimneys being backfilled by the contractor as they formed, to assure safety), leading to a high risk of instability of the drilling rig operating on the working platform. This occurrence led to seeking alternative design solutions. Among them, a solution entailing the preventive formation of a frozen soil shell all around the adit to be excavated was chosen and then applied with success.