Artificial ground freezing where principles matter

Ground freezing is a solution utilised in saturated ground when other methods of groundwater control and temporary support are unfeasible. It’s vital to understand the principles for early optioneering – and for when things don’t go to plan on site, says Dr Christian Gilbert, Director of Civil Works and Structures, SYSTRA

Artificial ground freezing is rarely used, because of its relatively high complexity and cost, but when other techniques, such as grouting, won’t work in saturated soils, engineers are forced to consider it.

In simple terms, ground freezing involves installing pipes in the ground and passing very cold liquid – either brine or nitrogen – through them so that temperature of the material around the pipes lowers and the water freezes. As well as making the area watertight, this increases the strength of the soil temporarily so that excavation can be carried out safely.

Typical applications could be for excavating the cross passages that run between two parallel large-diameter tunnels, for instance on a city’s new metro project. The frozen ground around the pipes is stronger than when it is saturated and creates a temporary structure within which to excavate.

Ground freezing is not an off-the-shelf solution. It is important to understand the principles behind ground freezing so that early design decisions can be taken and then later so that adjustments can be made on site.

Principles of ground freezing: the basics

To work out whether ground freezing will work for a particular project, it’s important to understand how different soils behave when frozen, the impact of different levels of water saturation, groundwater flow conditions and whether brine, nitrogen – or both – would be the best form of refrigerant.

The strength of the frozen ground varies depending on the soil type, with the unconfined compressive strength (UCS) decreasing as the particle size of the soil decreases. So, frozen sand could have an UCS of, say, 8-to-10 MPa, clay 5 MPa and silt 2-to-3 MPa.

However, there’s more to this story, since the capabilities of soils subject to artificial ground freezing are heavily dependent on their reaction to creep. This means that after a material has been loaded at a constant level for a longer period, it can fail suddenly, at a lower stress than its UCS would suggest. This must be tested for and then factored in during calculations for ground freezing so that the critical strain at which creep begins is not reached, since ground freezing may have to support an open excavation for several months.

Early calculation should estimate energy requirements too. This will help decide whether nitrogen, which can lower the temperature of the saturated ground faster, or brine should be used. It may be best to conduct initial freezing with nitrogen and then maintain it with brine.

Changing water from its liquid state to its solid state – ice – and then lowering it to, say, -10 or -15 degrees C takes plenty of energy, the greatest proportion of this, around 60%, goes into the change of state.

The higher the water content, the more energy is required, the longer freezing takes and the higher the cost of construction. Therefore, it makes sense to reduce the amount of water in the ground before the freezing operation. This could be achieved by jet grouting at certain points.

It should also be noted that above certain groundwater flow rates, the water will not freeze because it is moving too fast to be cooled sufficiently to freeze by the liquid in the ground freezing pipes. When using brine in the freezing pipes, the maximum flow rate of ground water at which freezing can still be achieved is 5m per day. For nitrogen it’s 15-to-20m per day.

Trouble shooting

Anyone involved in a ground freezing project should be ready for surprises and to adapt accordingly. For instance, on a project in North Africa, where ground freezing was being deployed to excavate 20m-long cross passages between two 15m-diameter main tunnels in very fine sand with a 50m head of water with a high saline content, the full area would not freeze. Appropriate instrumentation and monitoring established that the problem was a pumping well at the end of the tunnels which was causing the ground water to flow too quickly for freezing to take effect.

During a complex ground freezing exercise on the extension of Paris Line 14 at Porte de Clichy, there were several complex design challenges. This project saw the creation of a box frozen around the area where a new tunnel had to be excavated in interlayered strata of very fine sand and clayey sand, under an existing metro tunnel, through existing barrettes and under 18m head of water.

As well as cutting through barrettes supporting the old tunnel, the roof of the new tunnel had to be supported off the walls on jacks and adjusted when the ground thawed and its volume decreased as ice turned to water again. Unexpected issues on this project included some of the supplementary grouting not working, and sections of ground failing to freeze. Again, the use of observational methods allowed for adaption of the temporary works as any problem arose.

In conclusion

Ground freezing is a powerful method with a large reliance on observational methods, which is relatively rare in civil engineering. It depends on a good monitoring regime and good levels of engineering competence and experience so that theoretical calculations can be compared with the reality of execution. It can be an effective and safe way to excavate below ground in saturated sands or silts. But those deploying it must expect the unexpected, be prepared to work out what is happening and react dynamically as the situation requires it.

To view Christian Gilbert’s lecture, visit ‘Artificial Ground Freezing: a versatile solution for water tightness and temporary soil structure’ (The British Tunnelling Society, London 18th January 2024).

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