---
title: Artificial ground freezing where principles matter
url: "https://www.systra.com/uk/expert_insights/artificial-ground-freezing-where-principles-matter/"
type: expert_insights
date_published: 2024-03-06
date_modified: 2024-07-09
schema:
  @type: WebPage
language: en-GB
word_count: 1177
reading_time: 6 min
canonical: "https://www.systra.com/uk/expert_insights/artificial-ground-freezing-where-principles-matter/"
featured_image: "https://www.systra.com/uk/wp-content/uploads/sites/11/2024/03/freezing-of-soil-with-liquid-nitrogen_1-edited.jpg"
type-insight:
  - Sustainable Transport
---

# 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.

![freezing-of-soil-with-liquid-nitrogen_1](https://www.systra.com/uk/wp-content/uploads/sites/11/2024/03/freezing-of-soil-with-liquid-nitrogen_1-edited.jpg)

_Image source: Paris Metro Line 12 Reaches Aubervilliers._

### 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’_](https://www.youtube.com/watch?v=cpuzFZxN-DY)
(The British Tunnelling Society, London 18^(th) January 2024).

![british-tunnelling-society-christian-gilbert-lecture-jan-2024-12](https://www.systra.com/uk/wp-content/uploads/sites/11/2024/03/british-tunnelling-society-christian-gilbert-lecture-jan-2024-12-edited.jpg)

![british-tunnelling-society-christian-gilbert-lecture-jan-2024](https://www.systra.com/uk/wp-content/uploads/sites/11/2024/03/british-tunnelling-society-christian-gilbert-lecture-jan-2024.jpg)

_Images showing Dr Christian Gilbert and SYSTRA colleagues at the British Tunnelling Society lecture, London 18th January 2024._

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