Development and field evaluation of a novel liquid CO2 artificial ground freezing system

Artificial ground freezing (AGF) is widely used in geotechnical engineering for soil stabilisation and groundwater control under complex geological conditions. This study introduces an innovative AGF system employing low-temperature liquid carbon dioxide (CO2) as a circulating refrigerant in a closed-loop configuration. Field experiments were conducted at a shield tunnel construction site (Line Z4) in Tianjin Binhai New Area, China, characterised primarily by silty clay, clayey silt, and silty sand strata. Comparative tests with a conventional brine-based AGF system demonstrated that the liquid carbon dioxide system achieved significantly faster cooling, stabilising refrigerant temperatures at ≈−43·8°C within 20 days, compared to −30·8°C after 30 days for the brine-based system. The carbon dioxide-based system exhibited higher cooling efficiency, evidenced by shorter latent heat plateaus and steeper soil temperature gradients. After 20 days, soil temperatures reached −25°C to −30°C using carbon dioxide, whereas the brine method achieved only −5°C to −15°C. In addition, the carbon dioxide system reduced the frozen wall closure time by nearly one-third and increased freezing wall formation rates by 40%–50%. These findings highlight liquid carbon dioxide as a sustainable and highly efficient refrigerant, offering significant potential for advancing AGF technologies in geotechnical engineering applications.