About ground freezing

What ground freezing is

Natural frost occurs many places around the world, in the top layer of the ground during wintertime. Far north and on the South Pole, there is permafrost. When the frost is created by humans, as a method for civil and earth works or in the mining industry, we use the term artificial ground freezing (AGF) or just ground freezing.

Ground freezing is a method utilizing property changes when the temperature is lowered below the freezing point. The ground becomes waterproof and the strength increases significantly. The method is normally used as temporary support and waterproofing, or just as a waterproof barrier against water flow through the ground.

Completely waterproof

When the temperature in the pore water in the ground drops below its freezing point, the water changes phase from liquid to solid. The free pore water freezes first, while the layer closest to the mineral grains has chemically bound water freezing last and that can remain unfrozen even at very low temperatures. Still, for all practical purposes, the ground is 100 % waterproof, also with respect to pore pressure and groundwater control.

Even above the groundwater level, frozen ground becomes waterproof as ice is built onto the frozen material, either from moist in the air or from entering water. It is the balance between added and removed heat that determines whether the frozen body expands or contracts.

Since ground freezing provides completely waterproof constructions, the method is excellent to preserve the groundwater level and pore pressure on the outside of for instance a construction pit or tunnel, and thus avoiding settlements caused by pore pressure reduction in the surroundings.

High strength

Frozen ground represents a complete multiphase system which constitutes solid mineral grains, viscoplastic ice in the pores, liquid as chemically bound water around the mineral grains and gas with different saturation.

The strength increase when the ground freezes is caused by the phase change from water to ice. After the free pore water has frozen, more and more of the bound water will freeze as the temperature decreases. Most materials get higher strength with lower temperature, including frozen soil. Since temperature partly determines the strength, it is a key design parameter.

Removing heat

Energy flows from higher to lower energy levels. To freeze the ground artificially, a temperature gradient must be created, so the heat can flow towards colder regions and get removed from the lowest level. This is normally done by putting freezing pipes in the ground and circulate a colder medium, which both creates the temperature gradient in the ground and continuously removes the heat entering the pipes.

Different materials have different thermal properties and the ability to conduct heat for different soil and rock types vary strongly. This affects the required initial freezing time. Temperature monitoring is part of the quality control to ensure the temperature criteria have been fulfilled according to the design.

Installation of freezing pipes

Because ground freezing often is done in unstable ground, it is common to install casing during drilling. The freezing pipes are fitted to the freezing system used and can be open single pipes, parallel or coaxial double pipes.

Open/direct freezing system

The simplest system is open freezing pipes down in the ground. A cold fluid with low boiling point is poured in the pipe where the heat from the ground makes the liquid boil. The evaporated gas transports the heat out of the system, and it is common to use liquid nitrogen, which is extracted from the air at air separation plants. Air contains 78 % nitrogen. As the liquid boils and evaporates, more liquid is added. The method is often called nitrogen freezing. In an open freezing system, the medium is both a refrigerant and coolant.

Nitrogen boils at -196 °C, and one liter liquid expands to 672 liter gas at atmospheric pressure and 0 °C. Nitrogen does not have any odor or color. The density compared to air at same temperature is 0.967, but because the gas is colder than the surrounding temperature when let out in the air, it will be denser. It must be planned where and how the cold gas is let out, so it will not concentrate too much at low points and displace air and offset the oxygen concentration, which is a potential suffocation risk. Oxygen meters are always used as a measure to mitigate this risk during nitrogen freezing.

Closed/indirect freezing system

In a closed system, the liquid circulated in the pipes will normally only transport heat from the pipes to a heat exchanger in a freezing plant. The liquid circulated must not freeze at working temperatures. A common liquid is brine, and the method is therefore termed brine freezing.

A brine often used is a mixture of water and CaCl₂. This salt is a main constituent in seawater and is regularly used on our roads for thawing ice during the winter and to bind dust during the summer. The brine is cooled down to a temperature in the range -20 °C and -40 °C. Freezing by means of a freezing plant heat exchanging with a coolant circuit is called indirect freezing.

Inside the freezing plant the refrigerant is circulated in a separate circuit where the cooling process takes place: Evaporation, compression, condensation and choking. The compressors and pumps are powered by electricity.

The heat from the freezing plant must be removed and happens by direct water cooling, cooling tower or dry cooler. In the end the heat is let out to the water or air in the environment.

Refrigerants

The freezing plants can use a variety of refrigerants, depending on temperature range and purpose. In the past, refrigerants based on chlorine fluoride compounds were commonly used, but due to their negative climate effects (several thousand GWP), but they are now banned in more and more countries.

The best refrigerant for the climate is the natural media ammonia, NH₃, which does not affect the climate or the ozone layer (Ozon depletion product, ODP=0, Global warming potential, GWP=0). In high concentrations in air it is toxic to humans. If let out in water, too high concentrations is toxic to fish and other sea creatures. However, the media is naturally present in nature and small concentrations do no harm. The refrigerant has high efficiency and is commonly used for both ground freezing and other commercial purposes.

Another natural refrigerant which has gained popularity recently is carbon dioxide, CO₂, which also has zero ozone depletion, and a global warming potential of one (ODP=0, GWP=1).

When and why

Excavation below groundwater table

If water leaking into the excavation is a problem or water ingress will make the ground unstable and unsafe, then ground freezing should be considered for waterproofing or stabilization. Likewise, if the surroundings are sensitive to settlements and cannot sustain groundwater draw down. Ground freezing provides a safe and stable construction pit or soil tunnel with full groundwater control. The groundwater influence area is limited to the construction area itself.

Temporary support

Ground freezing is a temporary aid, used to get something done or built. The fact that the ground thaws and goes back to its original unfrozen state after use is a big advantage in all cases where it is desired to remove the construction after use and is not part of the permanent construction. This can for instance be temporary support extending into a neighbor property, blocking of natural groundwater flow, temporary underpinning of parts of a building on direct foundation or closing of waterfilled tunnel for maintenance and refurbishment.

Permanent as part of a heat pump

More and more heat pumps have been installed throughout the last decades, collecting heat from the ground. The needs of ground freezing and heat production can be combined, reducing overall costs. In arctic areas ground freezing is used to ensure a permanent stable foundation, utilizing the heat drawn from the ground to heat the buildings. Same principle is used for instance when artificially frozen ice rinks are combined with heating of swimming pools or soccer fields.

Comparison of methods

According to the textbook "Ground engineering equipment and methods", published by Frank Harris in 1983, sheet piles, injection, groundwater lowering, and compressed air are the preferred choices among Europe's contractors. But sheet piles were reported to be too expensive for depths greater than ca. 20 m and specially if the area exceeded 200 - 300 m2. Compressed air was only practically feasible with water pressure less than 35 m head. Groundwater lowering and injection were only possible in a limited range of grain sizes. Such limitations are not applicable to the artificial ground freezing method.

Comparison of methods depending on the soil's permeability was done by J.S. Harris in 1995. The figure below shows artificial ground freezing works irrespective of grain size, soil type and permeability.

Technical Designbasis for InterCity, BaneNor, 2019-08-15, says in chapter 8.4.1 about soil tunnels:

"Tunnels in rock with very low Q-values are normally excavated with rock bolts, pipe umbrella or spiling. Often a similar concept is tried when overburden is low or lacking. A big difference for tunnels in soil is that the soil usually has to be stabilized in advance of the tunnel excavation and that the stabilized mass cannot be part of the permanent construction."

Normally, and always below groundwater table, a waterproof construction is needed in the permanent condition. In soil injection is generally not suitable as waterproofing method and unsuitable as stabilization method.

Part of Bane NOR's soil tunnel definition is tunnels where only the upper part lacks overburden.

Feasibility of different ground improvement methods for different ground conditions. Ground freezing can be used for all conditions.

Geofrost's assessment

Geofrost's assessment of various methods' feasibility for tunneling and construction pits is shown below.

Environmental benefits

CO₂ and climate

Ground freezing has generally a low CO₂ footprint. For instance, using Geofrost retaining walls in Norway for one year, the CO₂ footprint will be reduced by 80 % compared to a AZ-48 sheet pile wall.

Energy and use of resources

Production of construction materials requires energy, both for frozen ground and steel or concrete. A comparison between sheet piles and Geofrost retaining walls shows equivalent energy consumption for freezing half a year and using a light GUGN sheet pile. Similarly, freezing almost 2 years equals the energy needed to produce the steel for an AZ48 sheet pile wall, while almost 3 years freezing equals the energy used to produce the steel for a wall of Ø406 x 10 mm piles.

Material transport is reduced to transportation of freezing pipes since the ground itself is the construction material. The freezing pipes constitute a small share of the earth support volume.

Noise and vibrations

Noise and vibrations are limited to drilling of holes for freezing pipes. Freezing plants and equipment are designed to meet applicable requirements and standards.

Pore pressure and influenced area

There is no groundwater draw down, pore pressure reduction and associated risk for settlements because the frozen body and support is completely waterproof. The influence zone does not reach further out than the frozen soil, limiting potential settlements to the volume that has been frozen.

The coolant circulates in a closed circuit. No chemicals are left in the ground. Groundwater outside the frozen region is unaffected.

Other aspects

Warm climate

Plenty of freezing plants are working in tropical areas. Ground freezing can be executed during the summer and in warm climate as well as in cold climate. The temperature must be lowered from a higher start temperature for initial freezing in warm climate than in cold climate, but it is the phase change from water to ice that requires most of the energy. There will also be more heat to remove during maintenance freezing. Down in the underground, the annual mean temperature is more important than the peak daytime temperature.

Hong Kang has an annual mean temperature of 28 °C. They were skeptical to ground freezing in warm climate. First time ground freezing was done in Hong Kong was in 1995, when Geofrost participated in the first phase of The Strategic Sewage Disposal Scheme in Kowloon. The summer climate is warm and humid with a daily average temperature often above 32 °C.

High water pressure

High water pressure does not delay the freezing process. Both below high mountains and deep below fjords the water pressure is high. These pressures are not a problem for ground freezing. Largest water pressure Geofrost has frozen so far is 1400 kPa, at the bottom of the frozen construction used for the Oslofjord subsea tunnel.

Running water

Moving water represents extra heat added, which must be removed in connection with ground freezing. If water flows through the volume to be frozen, one must ensure sufficient freezing capacity to be able to remove the additional heat.

Insulation of exposed frost structure

To keep the frozen body intact if exposed to warm environments, it must be covered by a layer where the temperature gradient goes from sub-zero to the surface temperature. The layer can be thawed soil, shotcrete, or insulation.

Concreting against frozen structure

It is fully possible to use the frozen body directly as formwork. Already during the 1950s this was studied thoroughly both in laboratories and in situ during construction of many shafts in Great Britain.

Two things to consider:

• Heat balance at time of casting:
  The volume of the concrete must be large enough to generate sufficient heat - long enough - to start the curing process. Then it is not of importance if a minor part of the concrete freezes, as the curing will continue when the concrete thaws and will eventually develop full strength. Casting directly toward the frozen body will usually make a layer thaw. This layer may eventually freeze again and the thawing will normally have insignificant effect on the frozen body stability.
• Concrete temperature at casting:
  Too high start temperature together with the curing top leads to an unnecessary large temperature gradient during refreezing. Too low temperature will delay the curing process and may lead to reduced concrete quality. 19-20 °C is typically an ideal concrete temperature.

Frost heave

Volume expansion during phase change from water to ice is per definition not frost heave and is often negligible since the excess water is pressed out of the pores during water-ice transformation. A parallel to this is water pipes bursting due to freezing. It is the water pressure building up, if the water cannot escape, that bursts the pipe, not the ice itself.

Frost heave is caused by the growth of ice lenses from water flowing to the freezing front. The frost heave's direction of force is perpendicular to the freezing front, while the ice lenses are parallel to the freezing front. To obtain frost heave, or ice lens growth in the ground, all the following three conditions must be satisfied:

1. 1. The temperature must be below the freezing point.
2. 2. There must be free access to water.
3. 3. The ground must be frost heave susceptible.

To which extent the third condition is satisfied, frost heave susceptible ground, depends on:

• Amount of fines, that is particles smaller than 0.02 mm.
• Velocity ratio between freezing front progression and water suction to the freezing front.
• The pressure on the soil in the freezing front's direction.

The winter gives slow freezing from the surface where the overburden load is minimal. Ice lenses grow in frost heave susceptible ground and lifts the surface.

Seasonal frost and artificial ground freezing have different effect on the ground.

Prof. Williams (1967) has published a comparison for how deep it is possible to get ice lens growth, depending on groundwater level and soil type expressed by the following formula:

pi – pw = 2 σiw / rc
pi	= ice pressure = total weight of soil above
pw	= water pressure = pore pressure
σiw	= boundary level energy air-water (surface tension)
rc	= radius corresponding to the size of the largest continuous opening through the pore system

Frost heave susceptibility classes have been made for Norwegian soils and climate.

The temperature gradient during artificial ground freezing is much larger than during seasonal frost, and hence the freezing velocity is much faster and the permeability in the normally frost susceptible fine grained soil classes is too small to feed the freezing front with water for ice lens growth. The frost rarely reaches deeper than 2 m from the surface in Norway, while artificial ground freezing is used much deeper, where the confining stresses are much larger. Hence, seasonal frost and artificial ground freezing have different effects on the ground and the Norwegian frost heave susceptibility classes are not applicable for artificial ground freezing.

Seasonal frost - winter

• Slow freezing
• Small surface load
• Ice lenses grow in frost heave susceptible ground and lifts the surface.
• Frost heave susceptibility classes are made for Norwegian climate and temperature gradients.

Ground freezing

• Quick freezing
• In fine grained soil (clayey), the permeability prevents water suction for ice lense growth.
• In coarse grained soil (frictional), the ice develops from the pore centers, displacing the excess pore water.
• Only at the frozen body's outer region, ice lenses may grow during long lasting freezing.

Without ice lens growth (frost heave), there is no frost heave settlements, neither by seasonal frost nor artificial ground freezing.

By artificial ground freezing the initial freezing is so fast that the permeability in the normally frost heave susceptible fine grained soil types effectively prevents water supply to the freezing front, so ice lens growth is minimal. The soil pressure increases with depth and the possibility for ice lens growth decreases similarly. Penner (1978) points out that the water suction from the surroundings to the freezing front decreases with increasing earth pressure and that the limits depend more on soil type than freezing temperature.

Thaw consolidation

Seasonal frost and artificial ground freezing have different impact on the ground. Settlements due to thawing requires both that frost heave has occurred during the freezing period and that the excess water from the thawed ice lenses is prevented from being absorbed in the ground in step with the thawing. The resulting increase in pore pressure reduces the effective stress and settlements occur if the loading exceeds the bearing capacity. The settlements will further increase if the loading is dynamic, as is the case with traffic loading on roads in the frost thawing period. For seasonal frost, the excess water in the top layer is common because there is a frozen waterproof layer below preventing drainage of the excess water.

Difference in freezing velocity and boundary conditions during thawing makes the pore system behavior different for seasonal frost and artificial ground freezing, and hence different impact on the ground.

Generally, very little ice lenses occur from artificial ground freezing and the thawing takes place at a slow rate from the outer surfaces without any drainage barriers. Thaw consolidation is rarer and generally less for ground freezing than for seasonal frost. Still, it should always be considered which order of magnitude and which consequences potential thaw consolidation may cause.

Quick clay and soft clays where natural water content is higher than the liquid limit are exceptions that require special attention. Such soils require pretreatment with moderate amounts of lime/cement to avoid thaw consolidation.

Quick clay

The water content of quick clay is higher than the liquid limit and causes settlements if it is released and the clay reconsolidates. Therefore, the quick clay's sensitivity with respect to deformations has to be included in the geotechnical assessment for ground freezing.

Tests show that mixing in a moderate amount of lime/cement in quick clay before freezing will prevent settlements from thawing.

Guidelines

There is no own standard for ground freezing, neither in Norway, Europe or the rest of the world. Ground freezing is however referred in The Norwegian Public Roads Administration's guidance V221 and in Bane NOR's Technical design basis for Intercity.

All geotechnical design follows the Eurocode with its national annexes. Eurocode 7 applies for geotechnical design for buildings and constructions. This includes for instance foundations of buildings, support constructions, fillings, and slope stability.

Construction pit guide

After a large research project (BegrensSkade) revealed the causes of many of the damages to adjacent buildings after earth and foundation works, the Norwegian Geotechnical Society started the works on making a construction pit guide. This has been an enormous effort of voluntary work by the industry which has taken time to publish. Ground freezing is treated in chapter 6.20 and 7.11.

CEN

Since there is no international standard for design and execution of ground freezing works, a CEN-group (CEN TC288 WG20) has been created with the assignment to get such a standard in place. Geofrost is represented in this group.

Ground freezing history

Mining industry in the 19th century

Already in 1852 French engineers discovered the useful properties the ground gets when it freezes. During the very cold and harsh winter that year, they were about to make a mining shaft through a layer of soil usually saturated and unstable, but which now were possible to pass relatively easy.

First known planned artificial ground freezing project to utilize the strength and waterproofness was in 1862 in connection with a mining shaft in Wales. Brine was circulated through freezing pipes that were installed through the instable soil layer. The principles were the same as those that were patented in 1883 in Germany and which still is used today: Freezing pipes are installed through the layer to make stable and/or impermeable, then it is frozen and finally safely excavated.

Shafts dominating also in the 20th century

In 1950 the depth record for ground freezing from surface was 900 m for a shaft in Canada.

Experience was gradually built, but originally ground freezing was done by trial and error. Design methodology and computational models were lacking.

Frozen ground is a complex material where the properties in addition to stress, depend on both temperature and time. Based on much experience from permafrost, Vyalov et. al. published in 1962 an empirically based model. Later many more arose.

In the mining industry ground freezing proved being both reliable, safe and cost efficient, and is therefore often chosen as method early in the design phase.

In civil works however, the method has been regarded as being costly. The known qualitative benefits are often overlooked and the method is often considered a last resort when the problems arise. A contributing reason to this may be that the responsibility for temporary works and choice of method has been by the contractor. In tenders it is then unavoidable that the contractor chooses the method that appears to be the cheapest.

Geofrost

Geofrost was founded in 1986, as the first specialized ground freezing company. With its background within geotechnics and engineering geology, they realized what fantastic tool ground freezing would be for civil works. By containerizing plants and modularizing the freezing system both costs and time for mobilization are reduced.

International Symposium on Ground Freezing (ISGF)

To exchange experiences and further develop the ground freezing method in a more efficient way, Prof. H. L. Jessberger arranged the first international symposium on ground freezing in Bochum, Germany in 1978. The conference gathered approximately 100 participants from all over the world within all disciplines related to ground freezing.

ISGF was set to be arranged every third year, but eventually stopped. The symposiums held were:

• ISGF´78: Bochum, Germany
• ISGF´80: Trondheim, Norway
• ISGF´82: Hanover, USA
• ISGF´85: Sapporo, Japan
• ISGF´88: Nottingham, UK
• ISGF´91: Beijing, China
• ISGF´94: Nancy, France
• ISGF´97: Luleå, Sweden
• ISGF´00: Louvain-la-neuve, Belgium

21st century

Ground freezing gets more used as the projects get more complex, areas more constrained and the need to utilize areas with challenging ground conditions increases. Groundwater control has also become more important with deeper basements nearby old buildings with direct foundation.

Contact

Geofrost holds courses and seminars about ground freezing. If you have questions related to ground freezing or would like more information, you are welcome to contact us.

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