Artificial Ground Freezing – A Typical Application for the Underpass of an Existing Metro Station in Rome-Paper

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Artificial Ground Freezing (AGF) temporarily strengthens soil and creates a groundwater barrier by freezing the ground around installed pipes. It can treat a wide range of soils, including those unsuitable or uneconomic for grouting, but requires thorough site investigation, laboratory testing, thermal modelling, accurate pipe installation and continuous monitoring. Groundwater flow, soil properties and pipe deviations can affect the formation and uniformity of the frozen wall.<br /><br />The paper describes AGF used to construct two mined Line C tunnels beneath the operating San Giovanni station on Rome’s Line A. Existing foundation piles prevented the use of EPB tunnel boring machines, while the work had to avoid disrupting the station above. The site presented additional challenges: restricted working space, piles obstructing the planned pipe layout, and variable ground conditions, including silty clay over sandy gravel.<br /><br />After preliminary cement and chemical grouting, freezing pipes and temperature sensors were installed from two levels within the new station. Liquid nitrogen was used to form the frozen wall rapidly; a brine-based closed circuit then maintained it during excavation and lining. The tunnels were excavated with steel ribs and sprayed-concrete support, waterproofed and given a final lining. Interfering foundation piles were removed by diamond wire cutting.<br /><br />Because freezing and thawing can cause ground movement, instruments monitored temperatures and deformations in the ground, station structures, slabs and tracks. The case demonstrates that, with careful design, accurate installation and comprehensive monitoring, AGF can provide a safe and effective means of tunnelling beneath sensitive infrastructure in difficult ground and logistical conditions.

Keywords

Artificial Ground Freezing

AGF

groundwater barrier

tunnelling

frozen wall

liquid nitrogen

soil stabilization

San Giovanni station

Rome Line A

ground monitoring