CrossPassages GroundFreezing TunnelConstruction UndergroundInfrastructure

Artificial Ground Freezing as a Tool in Urban Tunnelling

*Ground freezing is a process of converting pore water or pores into ice by continuously refrigerating the soil. Image source: The Constructor*

The construction of cross passages along High Speed 2’s (HS2) London tunnels represents the most extensive application of artificial ground freezing (AGF) in the UK. These cross passages, typically 500 metres apart, form essential links between the twin running tunnels, enabling safe evacuation routes in the event of an emergency. By circulating brine through freeze pipes at approximately -32°C, the surrounding soil mass is transformed into a solid ice body, allowing safe excavation without uncontrolled inflows or collapses. In many cases, this method was chosen over jet grouting and permeation grouting due to limited surface access and the fine nature of the soils.

Ice body formation behaviour of different geological formations. Source: Snapshot of ICE video (lecture on cross passage construction including the use of artificial ground freezing)

The tunnel alignment cuts across complex ground conditions including the Thames Group, the variable Lambeth Group, and the Seaford Chalk Formation with its abrasive characteristics and high groundwater pressures. To address these challenges, cross passages were categorised into four types, ranging from those entirely in clay to those directly influenced by lower aquifers. Each category required tailored treatment, highlighting the critical role of geotechnical investigation.

Geological profile of the tunnel alignment. Source: Snapshot of ICE video (lecture on cross passage construction including the use of artificial ground freezing)

Laboratory tests were performed to assess frozen soil properties, including compressive strength and frost heave pressure. This data supported finite element modelling to verify the stability of the frozen walls. In practice, a 2m thick freeze wall maintained at -10°C was sufficient to provide stability during excavation. Bespoke freeze pipe layouts were developed for each location, accounting for TBM deviations of up to 150mm.

Excavation of the cross passages followed a systematic sequence involving soil freezing, excavation with remote-controlled plant, sprayed concrete lining, waterproofing and secondary lining. Although AGF extended programme duration compared to depressurisation methods, it offered a safer and more predictable outcome in challenging geology. Monitoring data showed target soil temperatures and freeze body closure were consistently achieved, ensuring stability throughout construction.

Notably, shotcrete application on frozen ground required specific measures to ensure adhesion. By correlating early-age strength development of sprayed concrete with in-situ testing, the project team was able to safely reduce entry requirements from 10MPa at 24 hours to 2MPa within three hours. This adjustment improved productivity without compromising safety.

Freeze closure, segmented steps and resin grouting highlighting connection works. Source: Snapshot of ICE video (lecture on cross passage construction including the use of artificial ground freezing)

By August 2025, 19 of the 20 cross passages on the Northolt Tunnels West section had been completed, with 11 constructed using AGF. These works demonstrate how robust geotechnical investigation, innovative design, and continuous monitoring can deliver safe underground infrastructure in highly variable conditions. The lessons learned will inform future projects, establishing a valuable benchmark for ground freezing applications in tunnelling.

Sources: ice.org.uk, geplus.co.uk, hs2.org.uk, theconstructor.org