Soil Improvement for Ventilation Adits at the Naples Subway Line 6-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes ground improvement for ventilation adits on Naples Subway Line 6, where excavation had to proceed beneath groundwater heads of approximately 18–23.5 m. The design called for a low-permeability, 3 m-thick improved ground shell around each adit, using methods suited to local geology: penetration grouting in Neapolitan Yellow Tuff (NYT) and jet grouting in loose sands and volcanic deposits.<br /><br />At Santa Maria in Portico, the adit was in NYT and was treated with staged, selective injection through multi-packer sleeved pipes. Cement-based grout was used in primary pipes and silicate grout in subsequent stages. The treatment performed successfully: excavation was safe and nearly dry, with no reported effects on nearby structures.<br /><br />At Vittoria and Torretta squares, jet grouting was planned for loose soils, with penetration grouting where the adit entered NYT. A field trial at Vittoria was positive. However, at Torretta, the first jet-grouted columns caused substantial soil removal, forming underground chimneys and surface depressions. Aggregates were used to backfill these features, but the soil loss posed a potential risk to the drilling rig and nearby buildings.<br /><br />After further investigation, the team replaced the jet-grouting approach with ground freezing. A frozen-soil shell at least 1.2 m thick was formed around the adit using horizontal or inclined pipes installed from the shaft. Compensation grouting was also planned to mitigate settlement during thawing. The alternative enabled safe, nearly dry tunnelling while avoiding adverse effects on adjacent structures. The project illustrates how unexpected ground behavior can require design solutions to be revised during construction.

Keywords

Naples Subway Line 6

ground improvement

ventilation adits

groundwater head

Neapolitan Yellow Tuff

penetration grouting

jet grouting

ground freezing

sleeved pipes

compensation grouting