METRO ROME LINE "C" - ARTIFICIAL GROUND FREEZING APPLICATION FOR THE UNDERPASS OF THE EXISTING METRO STATION SAN GIOVANNI

Paper

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The paper describes the use of artificial ground freezing (AGF) to excavate two Line C tunnels beneath the operating San Giovanni station on Rome’s Line A. The tunnels had to pass through water-bearing, heterogeneous soils beneath sensitive existing structures, with a hydraulic head exceeding 10 m. Maintaining Line A train services and limiting structural movement were key requirements.<br /><br />Chemical and cement grouting was first used to consolidate and homogenise the ground. Subhorizontal freeze pipes, approximately 40 m long, were then installed around the tunnel perimeters to create a temporary support and watertight barrier. Liquid nitrogen rapidly initiated freezing, after which circulating brine maintained the frozen ground while limiting further ice-wall growth and associated movement. Additional pipes provided potential waterproofing if needed. A roughly one-metre layer beneath the existing station slab was deliberately left unfrozen to accommodate deformation. Steel bridges supported the Line A rails during construction.<br /><br />Drilling was complicated by large soil particles, unexpected pile locations and interference from railway electrification, which prevented reliable use of magnetic guidance. Optical guidance and down-the-hole-hammer drilling were used instead, but deviations and obstructions required changes to the freeze-pipe layout. Temperature sensors and structural monitoring—including level gauges, survey targets and track measurements—tracked freezing and the response of the station and tunnels in real time.<br /><br />Monitoring recorded maximum slab heave of about 20 mm near the station diaphragm wall and up to 50 mm in the central alignment. Despite the challenging ground and drilling conditions, additional planned freeze holes were not needed to repair gaps in the frozen wall. The paper concludes that AGF, supported by careful sequencing, adaptable drilling and integrated monitoring, enabled the underpass construction while the existing metro station remained operational.

Keywords

artificial ground freezing

Line C tunnels

Rome metro

San Giovanni station

freeze pipes

watertight barrier

chemical grouting

brine circulation

structural monitoring

tunnel excavation