Ground Freezing for the Suburban Rail Loop in Melbourne 

In the eastern section of the Suburban Rail Loop – known as the Suburban Rail Loop East – the line will be constructed entirely underground. Two parallel tunnel tubes are being built there, connected at regular intervals by cross-passages. These cross-passages serve, among other things, as escape routes. Due to the challenging ground conditions, these connections must be constructed safely. This is where ground freezing comes into play. It enables the ground to be temporarily stabilised and prevents groundwater from entering the excavation – and this is done exclusively from within the existing tunnel tubes, without any intervention at the surface. CDM Smith is responsible for the planning and design of the ground freezing for this project and is providing technical support throughout its implementation. Originally, ground freezing was only intended for one cross-passage. However, due to its compelling advantages, the concept was extended to a total of 29 cross-passages.

The project area consists predominantly of water-bearing unconsolidated soils. This combination of soil with low bearing capacity and nearby groundwater poses a considerable challenge during the construction of the cross-passages. Ground freezing allows the excavation area to be selectively stabilised and, at the same time, made watertight by freezing the water contained within the ground. The freezing process is carried out from within the existing tunnel tubes. A stable frozen mass forms between the two tunnel tubes, creating a protected zone in which the cross-passage can be safely excavated. The key advantage is that no surface intervention is required. Traffic, built-up areas and public life remain unaffected.

Ground freezing does not require any permanent alterations to the ground or groundwater. No substances are injected, and no additional excavation pits or surface installations are required. Once the work is complete, the ground thaws again, groundwater returns to its original conditions, and only the steel pipes of the freezing system remain underground. The system operates in a closed circuit: the refrigerant circulates continuously and is neither consumed nor replenished. The main energy input consists of electricity for the freezing units. Depending on the local energy mix, the method can be operated in a correspondingly climate-friendly manner. Furthermore, the early integration of ground freezing enables the optimisation of structures and processes. This leads to reduced material usage, particularly of concrete, and thus to a significant reduction in CO₂ emissions.

‘World First’ recognises the outstanding achievements in project sustainability. The ISGF (International Symposium on Ground Freezing) has defined the criteria for this award. The primary aim is to minimise the carbon footprint of the entire project through the early optimisation of structures and processes. In the case of ground freezing, this includes, for example, the electricity consumption of the freezing systems or the design optimisation.