Ground Freezing

This report provides a detailed review of Artificial Ground Freezing (AGF) as a technique to improve site conditions for civil engineering projects.

Artificial Ground Freezing (AGF) is a ground improvement technique in which a soil mass of a certain geometry is frozen using a refrigeration process involving a coolant, either chilled brine or liquid nitrogen, which is circulated through freeze pipes embedded in the ground. AGF is typically used for ground stabilization and groundwater control for a wide variety of applications involving all soil types.

This report is based on a review of the available literature on ground freezing and provides a brief history of ground freezing and its effects on typical geotechnical engineering properties.  It goes on to discuss considerations for implementing ground freezing in the field, as well as the advantages and disadvantages of the process.  Finally, two case studies of AGF implementation in the field are reviewed.

History

Artificial ground freezing (AGF) is a soil stabilization technique involving the removal of heat from the ground to freeze a soil’s pore water.  The concept of ground freezing was first introduced in France, and industrial applications date back to 1862 where it was used as a mine shaft construction method in South Wales (Schmidt 1895).  The method was eventually patented by German mining engineer F.H. Poetsch in 1883 (sometimes termed Poetsch Process).  The method involves a system of  pipes consisting of an outer pipe and concentric inner feed-pipes where a chilled coolant (calcium chloride brine, typically) is circulated.  The coolant is pumped down the inner pipe and back up the outer pipe.  It is then cooled again through a refrigeration process and returned through the pipe system.  A further development on the AGF technique occurred in France in 1962, when liquid nitrogen (LN2) was pumped into the freeze pipes instead of chilled calcium chloride brine.  This allows for much faster ground freezing if necessary.  The liquid nitrogen runs through the freeze pipes and is allowed to evaporate into the atmosphere (Sanger and Sayles 1979).

Currently, AGF has been applied to a wide variety of engineering projects where stability, groundwater conditions, and containment are an issue.  Example situations include: vertical shaft construction for mining or tunneling, stabilization of non-engineered earth fills (large obstructions), sites that require horizontal access (e.g. a TBM canopy for cross passage construction), lateral and vertical contaminant containment, contaminant redirection, groundwater cutoff (can be tied into bedrock), and emergency support/stabilization using LN2 (Schmall and Braun 2006).  

During the process, heat is removed from the soil in a cylindrical pattern around freeze pipes.  This produces columns of frozen soil. The columns continue to expand until they intersect.  From here, the frozen mass will expand outwards creating a wall or solid ring of frozen soil (Sanger and Sayles 1979).

The following sections describe the effects of AFG on the engineering properties of soils, namely hydraulic conductivity, stiffness, shear strength, and volume change capacity.  In addition, laboratory testing and classification of frozen soils is introduced per JGS and ASTM standards.

Hydraulic Conductivity of Frozen Soils

When applied to civil engineering projects for containment or control of groundwater, frozen soil is practically impermeable.  Ice fractures also have the propensity to heal themselves by refreezing. Problems with permeability arise when freezing procedures are not performed correctly, and soil does not freeze completely as one mass, leaving “windows” of unfrozen soil which could compromise the ability of the frozen barrier to contain and control groundwater or isolate a contaminant within the soil.  Windows of unfrozen soil are often found and sized using the ultrasonic measurement method (Jessberger 1980).

Strength Behavior of Frozen Soil

The strength behavior of frozen soils, as with any other soil, is dependent on a number of factors, including soil type, temperature, confining stress, relative density, and strain rate.  Frozen soils exhibit higher strength than unfrozen soils.  In general, frozen soil strength increases as temperature decreases and confining stress increases.  

Da Re et al. 2003 performed a study on the triaxial strength characteristics of frozen Manchester Fine Sand (MFS), in which specimens were prepared over a variety of relative densities (20 - 100%), confining stresses (0.1 - 10 MPa), strain rates (3 x 10-6 - 5 x 10-4 s-1), and temperatures (-2 to -25°C).  

The results, shown graphically in Figure 1, show two distinct regions of strain over which the frozen soil acts differently.  Small strains (less than 1% axial) result in a linear strength increase which has a slope (modulus) independent of relative density or confining stress.  The magnitude of the initial yield point (at 0.5-1% axial strain in all cases) increases with increasing strain rate and decreasing temperature.  Large strain behaviors include strain softening, exhibited by specimens prepared at low relative density and under low confining stress, to strain hardening, exhibited by specimens prepared at high relative density and high confining stress.

Figure 1. Strength behavior of MFS (Da Re et al. 2003)

The strain softening behavior of the MFS shown in the Da Re et al. study is explained by Kornfield and Zubeck 2013.  They state that a decrease in stress past the initial yield point is due to an increase in crushing and pressure melting of the frozen pore water.  Yang et al. 2009 and Xu et al. 2011 also showed that as confining pressure increases, shear strength reaches a peak then decreases due to ice crushing and pressure melting.  Generally, at -10°C frozen sands and frozen clays have compressive strengths of 15 MPa and 3 MPa respectively (Klein 2012).

The compressive strength of frozen clay was analyzed by Li et al. under variable temperatures, strain rates, and dry densities.  The clay was compacted  to three different dry densities and had a liquid limit of 28.8 % and plastic limit of 17.7%.  Uniaxial compressions tests were performed at different temperatures (-2 to -15°C) and different strain rates (approximately 1 x 10-6 to 6 x 10-4 s-1) at each dry density.  Results of the study showed similar strength behavior to the study performed by Da Re et al. for frozen MFS.  The compressive strength of the clay tested increased with increasing strain rate, decreasing temperature, and increasing dry density, similar to the behavior of the MFS tested in the Da Re et al. study.  Additionally, frozen clays exhibited both strain hardening and strain softening behavior after an initial yield stress was reached, which was highly dependent on the time to failure, which itself depends on strain rate.  The results of the study showed that frozen clay samples loaded at low strain rates reached a low uniaxial compressive strength (approximately 2 MPa defined at 10% strain if no failure was reached) at a longer time to failure but exhibited strain hardening behavior.  Conversely, frozen clay samples loaded at high strain rates reached a much higher uniaxial compressive strength (approximately 6 MPa at failure) but exhibited strain softening (Li et al. 2004).

Stiffness of Frozen Soil

In general, frozen soils are stiffer than unfrozen soils.  Da Re et al., in their frozen soil strength study on MFS, conducted a study on Young’s Modulus.  They found that frozen MFS had a Young’s modulus of approximately 23 GPa to 30 GPa.  Because the small strain strength behavior of the frozen MFS was similar throughout the variables tested, Young’s modulus was independent of the variables tested (relative density, confining stress, strain rate, and temperature).  

Figure 2.  Normalized stress-strain behavior of MFS (Da Re et al. 2003)

Figure 2 from the Da Re et. al., 2003 study shows the independence of Young’s modulus of frozen sands, by normalizing the shear stress with the initial yield stress.  Figure 2 also shows the different volumetric strains due to the strain hardening or softening behavior of the frozen MFS post initial yield stress, as denoted by Type A, B, C, or D stress-strain behavior.

Volume Change Characteristics of Frozen Soil

During a phase change from liquid to solid, water increases in volume by approximately 9%, which translates to soil heave at the ground surface (Lackner et al. 2005).  Heave due to volume expansion may damage nearby structures (tunnels, surface structures) during freezing and thawing, therefore understanding the soil properties and how they contribute to soil heave is important during AGF.  Soil that has exhibited heave will also experience settlements upon thawing, which must be considered. Soil may also observe volume changes due to creep when loaded.  

Soil heave occurs in soils where ice lenses form within voids.  The soil structure must promote transfer of water from surrounding void spaces to the freezing front of the ice lense through capillary forces.  For this reason, silty soils are particularly frost-susceptible (Widianto et al. 2009).

It is also important to note that clays may exhibit low frost susceptibility in some cases.  As the freezing front moves outward, clays exhibit heave due to volume expansion of the ice lense, however consolidation may occur ahead of the freezing front, where negative pore pressures are being generated by the movement of water into the freezing zone.  The net effect of heave and consolidation below the ice lense may be small or negligible at the surface (Han and Goodings, 2006).  Despite this, site specific soils must be tested for frost-susceptibility if frost heave is expected to be an issue to nearby structures.

General Laboratory Testing for Frozen Soil

With respect to frozen soils, both ASTM and JGS have some standards for laboratory testing.  However, many of these tests either apply to pavements, multiple freeze-thaw cycles, or only provide information in the direction of heat flow.  JGS 0171-2003 is a test method for predicting the frost heave of a soil.  This standard utilizes Takashi’s equation for frost heave in the direction of heat flow.  Kanie et al. 2013 has proposed the use of a three-dimensional evaluation method involving a unique laboratory apparatus and finite element method modeling.

Currently, standards exist for the determination of strength properties at constant strain (ASTM D7300-11) and creep properties (ASTM D5520-11).  Both of these tests are performed in uniaxial compression.  Standards for triaxial testing of unfrozen soil do not apply to frozen soils, and new standards are needed to generate comparable results.

There are many unstandardized laboratory and field tests currently used for frozen soils including (Oestgaard and Zubeck 2013):

• Direct Shear (Bennett and Nickling 1984, Yasufuku et al. 2003).
• Triaxial Compression (Baker et al. 1984, Arenson et al 2004).
• Uniaxial Tension (Zhu and Carbee 1987, Erckhardt 1981).
• Constant Creep (Andersland and Ladanyi 2004).
• Relaxation Test (Andersland and Ladanyi 2004).
• Thaw Consolidation (Morgenstern and Nixon 1971).
• Pressuremeter Creep (Ladanyi 1982).
• Pressuremeter Relaxation (Ladanyi 1982, Ladanyi and Melouki 1992).

Classification of Frozen Soil

The classification and description of frozen soils is currently documented by ASTM D4083-89 (reapproved in 2007).  This involves the description of both the soil phase and the ice phase of the material.  Description for the soil phase is the same as an unfrozen soil, ASTM D2488.  The frozen phase is then classified into one of two groups: N for soil with no ice visible and V for soil with significant visible ice.

These groups are subsequently broken down into subgroups described in the standard.  Figures 3 and 4 show visual representations of both the visible ice and no visible ice classifications, per the ASTM D4083-89 standard.

Figure 3.  Visible ice structured in frozen soil (ASTM D4083-89)

Visible ice is represented by the color black in Figure 3. Visible ice may exist within the soil structure as separate pockets of ice (Vx), coatings around soil particles (Vc), irregular formations (Vr), or stratified formations (Vs).

Figure 4.  Structure of frozen soil with no visible ice (ASTM D4083-89)

As in Figure 3, ice is represented by the color black in Figure 4.  When there is no visible ice within the structure of a frozen soil, the frozen soil is classified by how well the sample is bonded by the ice.  Frozen soil with no visible ice may be poorly bonded (Nf), well-bonded with no excess ice (Nbn), or well-bonded with excess ice (Nbe).

Sayles et al. 1987 provides several recommendations for the complete description of a frozen soil.  These include unfrozen soil USCS symbol and description, frozen soil symbol and description, grain size distribution, Atterberg limits, as well as physical properties such as ice content (frozen), water content (unfrozen), unit weight, specific gravity of soil, saturation percentage, and salinity.  These parameters have a strong influence on the frozen strength and behavior of the soil.  For artificial ground freezing applications, it is recommended that the system described in Andersland and Anderson 1978 be used (Sayles et al. 1987). Still et al. 2013 has proposed developing standardized index testing for use in frozen soil classification.

Ground freezing implementation in the field may be performed with a variety of equipment, coolants, and procedures.  Described in the following sections is a general overview of ground freezing implementation.

Equipment

Ground freezing requires the use of a mobile refrigeration plant.  The plant may run on coolants such as ammonia or CO2, and works to remove heat from the circulating fluid, which is typically calcium chloride or magnesium chloride brine (Jessberger 1980).

Figure 5. Mobile refrigeration plants during AGF (SoilFreeze)

Brine temperatures of -25°C or less are typically sufficient for most projects.  Commercial brines designed specifically for use with AFG are also available.  It is important to investigate the properties of these coolants to ensure compatibility with other equipment (e.g. corrosion of pipes).  The coolant used may depend on the temperature requirement of the project, magnesium chloride brine freezes at -34° and calcium chloride brine freezes at -55°C.  

LN2 boils at a temperature of -196°C and may be used in the place of a generic coolant.  Due to the extremely low temperature of LN2, freezing soil in contact with LN2 occurs much more quickly.  Therefore, complete freezing can be accomplished much faster using LN2 instead of chilled brine.  However, due to its higher cost, this is usually reserved for emergency stabilization, short-term freezing, and small-volume projects.  In this case, LN2 is transported to the site in specialized storage tanks and inserted directly into the freeze pipes.  It is not circulated through a refrigeration plant.  Rather, it is allowed to evaporate at the surface, as shown in Figure 5, after it has removed heat from the soil (Jessberger 1980).

Figure 6.  Liquid nitrogen evaporation during AGF (“Ground Freezing”)

Table 1 provides a basic summary of relative comparisons between chilled calcium chloride brine and liquid nitrogen (LN2).

Table 1.  Summary of calcium chloride brine and liquid nitrogen properties for AGF

In colder climates, thermosyphons may be used to achieve the temperatures necessary to freeze the soil.  Thermosyphons implement convection of a working fluid to remove heat from the ground and transfer it to the air at ground surface.  The ambient air temperature must be lower than the ground temperature for this process to work, thus it is typically used in cold regions.  The working fluid of the thermosyphon is buried in the ground, where the contained fluid absorbs heat, evaporates, and rises to the top of the syphon.  There, it is cooled by the surrounding air causing it to condense and return to the bottom of the thermosyphon. This process is shown in Figure 6 below.  The process is energy efficient, however it requires air temperatures that are below freezing in order to effectively be used in an AGF process.  If further freezing is required, powered thermosyphons may be used to decrease ground temperatures once they have reached ambient air temperature (Wagner and Yarmak 2013). 

Figure 7.  Passive thermosyphon diagram (Wagner and Yarmak 2012)

Freeze pipes can be made from a variety of materials.  A typical setup may include 5 inch diameter steel outer pipes and 3 inch diameter plastic (e.g. polyethylene) inner pipes (Klein 2012).  Freeze pipes must be able to stand upright and withstand the lateral earth pressures associated with the site.  As a historical rule of thumb, freeze pipes must be capable of withstanding 13 kPa per meter of shaft burial depth (Klein 2012).  Freeze pipe integrity must be monitored to prevent damage to the pipes due to soil heave.

One of the most important aspects of an AGF project is monitoring the soil conditions during freezing and thawing.  Typically, a hole is drilled near the frozen wall, where temperature gauges are installed to monitor soil temperature.  This is of vital importance to the final product (frozen cutoff wall, frozen soil mass, etc).  Additionally, ground heave and settlement due to both freezing and thawing of the soil after project completion are monitored.  If excavation behind a frozen wall is to take place, deflectometers, extensometers, and inclinometers may be used to measure wall deflections.  To determine if any windows of unfrozen soil exist in the frozen soil mass, ultrasonic measurements may be taken.  Finally, project specific measurements such as heave pressures and deformations on existing structures due to heave (tunneling, foundations, special project considerations) are performed if necessary (Jessberger, 1980).

Using computer systems, much of the AGF process is automated.

Automatic data acquisition is used for temperature and deflection measurements.  Additionally, computer systems regulate the flow of coolant into the freeze pipes to more accurately control the temperature of the ground.

Design Methods and Considerations

Perhaps the most important step in ensuring a successful implementation of AGF is site characterization, as is true for all geotechnical engineering projects.  Soil type and groundwater must be accurately characterized to ensure the frozen soil meets the design specifications.  Specifically, for AGF projects, soils should always be sampled and tested for thermal properties.  Groundwater is also tested for freezing temperature and velocity.  A high groundwater velocity (> 2 m/day) creates problems during freezing of the soil, and may yield discontinuities.  Smaller pipe spacing, multiple rows, or using LN2 may be used to counter high groundwater velocity (FHWA 2013, Klein 2012).

Xanthakos et al. 1994 recommends a freeze pipe spacing to diameter ratio of less than or equal to 13 be used for 120 mm diameter pipes or smaller.  Groundwater salinity must also be considered.  High salinity sites will see freezing temperature degradation and lower frozen strengths.  As salinity increases, frost heave, thaw settlement, and heaving force will decrease (Hu et al. 2010).  Some of these changes are beneficial, however the net result is a less conservative design if salinity is not properly accounted for.  In addition, salinity may not be homogenous in the pore water.  Areas of higher concentrations may form pockets of unfrozen water or films of unfrozen water around particles (Hu et al. 2010).

Further consideration outside of soil and groundwater properties include ambient air temperature, project timescale and risk, and expected soil heave and settlement.  If the ambient air temperature is cold enough, thermosyphons may be a more energy efficient solution.  In the case of an emergency situation requiring immediate ground freezing, such as a containment of contaminated soil or construction scheduling issues, liquid nitrogen may be used as the coolant instead of chilled brine.  Finally, designs must also be sensitive to the expected soil heave during freezing and settlement during thawing.  The phase change from water to ice can cause volume increases up to 9%, which translate to soil heave during freezing (Lackner et al, 2005).  

The design parameters determined from site characterization are often modeled using finite element method (FEM) computer programs, such as Ansys.  This may be necessary for more complex scenarios and subsurface conditions.  Additional programs such as GeoStudio’s SEEP/W and AIR/W are able to model convective surface boundary conditions if required (Geo-Slope).  TEMP/W is used in GeoStudio to model thermal changes in the ground.

Due to its impermeable properties, frozen soil makes an excellent groundwater cutoff material.  Ground freezing has been used to create a water-tight seal around excavations in and around salt mines.  It also has the ability to tie in with bedrock and other subsurface features (Schmall and Braun 2006).  This creates an impermeable barrier that may extend down into fractured bedrock.  It should be noted that the hydraulic conductivity of rock may increase after thawing due to further opening of fractures during freezing.

Freezing Time

Artificial ground freezing can be a time intensive process.  Chilled brine is better suited for projects with longer time frames on the order of weeks to months.  The brine is cycled through the piping system during the freezing phase until the ground is fully frozen.  After the soil has been sufficiently frozen, the temperature is kept constant during the maintenance phase.  Liquid nitrogen can be used for rapid ground freezing as its temperature is much lower and freezing can be achieved in a matter of days.  It is often used for emergency situations in which rapid stabilization or containment is necessary (van Dijk and Bouwmeester-van der Bos 2001). Freezing time is a function of several factors, chief among them being pipe spacing and temperature.  A column of freezing spreads radially around each pipe.  The soil is considered fully frozen when the freezing columns have overlapped and all space between them has been frozen.  Larger spacing correlates with longer freezing time (Johansson 2009).

Figure 8.  AGF with brine, required freezing time as a function of pipe spacing (After Jessberger and Vyalov 1978)

Figure 8 illustrates this relationship in both sandy and clayey soils. Clay soils will generally require a longer freezing time than sandy soils for the same pipe spacing. Greater moisture contents will require longer freezing times because a greater amount of water must be frozen. As expected, a lower brine temperature will decrease the required freezing time.

Cost

The cost of a typical AGF project can vary greatly depending on the energy requirements, size of the freezing area, site specific difficulties, coolant (liquid nitrogen is much more expensive than brine), and timescale.  Ground freezing becomes cost efficient relative to other methods when the specific benefits of AGF lend themselves to a project (Schmall and Braun 2006).  AGF may become the desired method due to difficult ground conditions (e.g. weak layers, non-engineered fills) or when a suite of improvement techniques would otherwise be needed (van Dijk and Bouwmeester-van der Bos 2001).

Internally, the cost effectiveness of increasing the size of a ground freezing project has been analyzed by the Army Corps of Engineers on a frozen ground waste containment operation at Ft. Detrick, MD.  The geometry of the frozen area increased, and consequently additional materials costs, energy requirements, and total capital costs were estimated.  Approximately 6.3 times the length of freeze pipe of the initial design was required to achieve approximately 6.7 times the energy requirement of the original design.  The capital costs incurred were estimated at 6.4 times the costs of the original design (Grant, 2001).  The Grant study suggests an approximately linear relationship between freeze pipe length, energy requirement, and cost increase on some projects.  It is important to recognize that this may not reflect the energy to cost relationship of an AGF project.  Each project will be different in design and requirements.   

Currently, ground freezing is becoming an increasingly competitive method on a cost basis for even basic geotechnical applications.  Typically, a frozen soil wall can cost anywhere in the range of $30-$60 per square foot of frozen soil (Daniel Mageau, personal communication, April 14, 2014).

The following sections describe typical advantages and disadvantages of utilizing AGF.

Advantages

Ground freezing is an extremely versatile method for temporary ground improvement or cutoff.

It is applicable to the entire range of soils, provided that the soil is near saturation or completely saturated.  If water contents are not acceptable, water can be added, provided the water will not drain out of the soil quickly (Schmall and Braun 2006).  In addition to being applicable to the entire range of soils, it is also applicable to difficult ground conditions including large boulders and cobbles, or debris-rich non-engineered fills.  A good example of the applicability of ground freezing is demonstrated in the Boston Central Artery/Tunnel (CA/T) project, discussed later in this report in the Modern Applications to Civil Engineering section.

Additionally, ground freezing can create cutoff walls or frozen soil masses in a variety of geometries (shown in Figure 8), simply by modifying the placement and spacing of the freeze pipes.  This is especially important during tunneling applications, where freeze pipes are installed horizontally and at various angles to create stable frozen ground for tunnel support and excavation.

Figure 9. Examples of frozen barrier configurations (wall, enclosure, solid block) (Wagner and Yarmak 2012)

Furthermore, ground freezing will likely be cost effective when the site conditions are such that stability and/or containment must be achieved with multiple methods, which combined have the same effect as applying ground freezing alone.  Again, the Boston CA/T project was one such project that determined ground freezing was the most cost effective solution using a value engineering process.  

Disadvantages

Ground freezing is a highly energy intensive process, requiring refrigeration of massive quantities of soil over extended periods of time, which is very expensive.  Costs only increase if liquid nitrogen is required for quicker soil freezing.  

Additionally, implementing ground freezing requires plenty of monitoring: brine temperatures, soil temperatures, deflections of adjacent or nearby structures, heaving and settlement at the ground surface, groundwater salinity, pressures within freeze pipes (leak detection), frozen wall thickness, and the location and dimensions of possible windows within the frozen wall, among other site specific measurements.  

Possible failures of an AGF project can occur as a result of inadequate monitoring or installation.  The spacing of the freeze pipes may be such that the frozen wall barrier is not complete, leaving windows of unfrozen soil, or such that the frozen wall thickness is not controlled, and grows too large, placing unnecessary stresses on nearby structures and soil.  Additionally, improper securing of freeze pipes may cause leakage of the brine.  

Furthermore, there is the inherent disadvantage of the volume expansion of water during freezing, leading to soil heave and thaw settlement, which may damage adjacent or nearby structures if not monitored and accounted for with regular structure maintenance.  Soil heave and settlement may also damage AGF equipment, most commonly the freeze pipes, causing leakage and requiring maintenance. In cases where significant settlements could damage overlying or embedded structures, care is taken not to freeze ground beneath the structures as to avoid settlements upon thawing.

The following sections provide brief summaries of projects which have successfully implemented ground freezing, including the specific considerations and obstacles which were overcome by implementing AGF.  

Boston Central Artery/Tunnel (CA/T)

The Boston CA/T project is perhaps the most well-known application of massive soil freezing to date.  The project involved the construction of  underground expressway tunnels to replace an aging elevated highway system.  Three tunnels were to be constructed using tunnel jacking.

The subsurface profile consisted of miscellaneous fill materials (boulders, cobbles, concrete and steel fragments, wood, brick, granite blocks, among others) for 6-8 meters, overlying 3-5 meters of organic silts, clays, and peat, overlying 1.5 meters of dense silty sand, overlying 5 meters of marine clays.  To stabilize the tunnel face, the initial design called for a combination of ground improvement techniques to handle the extreme soil variability, including chemical grouting, de-watering, horizontal jet grouting, and soil nailing.

Figure 10. Vertical section of tunnel during operation (Dijk and Bouwmeester-van den Bos 2001)

A value engineering study showed that AGF could provide the stability required through each soil layer at the site at a lower cost than implementing four different ground improvement methods, therefore it was chosen to provide stability of the tunnel face for excavation and support of the tunnel jacking system.

The Boston CA/T project required multiple site-specific design considerations.  The tunnel sections were constructed beneath a railway (shown in Figure 9), therefore the freeze pipes were insulated at the top to keep the ground surface thawed for railway operations and maintenance.  Ground temperature and heave were monitored throughout the project and any damages to the railway system were corrected with routine maintenance.  Additionally, the freeze pipes were terminated approximately 1 meter above the tunnel invert, to prevent the tunnel from experiencing thaw settlements after construction.  Finally, heat pipes were installed at the edges of the frozen cutoff walls to prevent heave pressure from the expansion of the frozen wall from burdening the tunnel jacking system during excavations.

Figure 11.  Railway operation around freeze pipes (FHWA 2013)

The project employed the chilled brine cooling method of AGF successfully, and varied pipe spacing to control the freeze time.  The Ramp D tunnel required a faster freeze time than the other two tunnels, therefore freeze pipe spacing was smaller (2.1 meters compared to 2.4 meters).  Freeze times were on the order of 3-4 months, depending on freeze pipe spacing.  

One system failure occurred during freezing.  The freeze pipes were damaged due to frost heave, and leaks were detected.  These leaks were repaired and all pipes were outfitted with redundant closed-end steel sleeves to prevent future leaks.  

The project was completed successfully without any further delays from failures of the ground freezing system.  The Boston CA/T project is an example of the successful implementation of ground freezing under extremely variable soil conditions over a massive volume of soil (van Dijk and Bouwmeester-van den Bos 2001).

Netherlands Sophiaspoortunnel

The purpose of the project at Sophiaspoortunnel was to construct fourteen cross passages between parallel railroad tunnels.

Figure 12. Cross section of a service shaft and cross passages (Crippa and Manassero 2006)

The subsurface consisted of a 15 m thick clay layer above a 10 m thick layer of loose sand (through which most of the tunnels exist) above another layer of clay.  The groundwater table was at a depth of 25 m.

For each of the fourteen cross passages, freeze pipes were installed horizontally between the tunnels to create a horizontal frozen soil column for excavation and support of the tunnel.  Both chilled brine (10 cross passages) and liquid nitrogen (4 cross passages) were used successfully.  Pipe spacing was on average 1 meter.  For each cross passage, 25 to 29 freeze pipes were installed to perform the freezing, with a design wall thickness of 1.8 to 2.3 m.  The total volume of frozen soil for all cross passages was 4400 cubic meters.

Per the design, the brine method took longer to reach full freezing and closure than the liquid nitrogen method.  Specifically, for the brine method, shell closure was reached in 8-15 days, with the minimum design thickness reached in 34-67 days.  For the liquid nitrogen method, closure was reached in 4-7 days, with the minimum design thickness reached in 9-14 days, much quicker than the brine method (Crippa and Manassero 2006).  

The Netherlands Sophiaspoortunnel study is a good example of the application of both brine and liquid nitrogen methods of AGF, and provides a comparison of timescale for each method as implemented in field conditions.

Artificial ground freezing is a versatile technique for ground improvement and stability.  The applicability of AGF covers most soil types including non-engineered fills, boulders and other large obstructions, and weak fine grained soils.  It has been used for vertical shaft construction for mining, stabilization of debris-rich earth fills, horizontal stabilization for tunneling, vertical and/or lateral contaminant containment, contaminant redirection, groundwater cutoff tied into bedrock, and emergency stabilization using liquid nitrogen.  

AGF creates an impermeable, frozen soil barrier or mass, which has higher strength and stiffness than the unfrozen soil.  This also has the capacity for soil heave and subsequent thaw settlements, which could become problematic for nearby structures.  Proper site characterization is key to anticipate the effects of ground freezing on the soils at a particular site.  Laboratory testing standards are available from both ASTM and JGS.  Classification standards for frozen soils are documented by ASTM.

AGF is implemented in the field using a mobile refrigeration plant, which circulates chilled calcium chloride brine through freeze pipes, removing heat from the soil and freezing the soil’s pore water.  Liquid nitrogen may also be used, however it is allowed to evaporate into the atmosphere rather than being recirculated.  A number of design considerations must be taken into account such as freeze pipe spacing, freezing time, groundwater velocity, saturation, pore water salinity, estimated soil heave, and costs.

Temperature of the soil and coolant, as well as soil heave, settlement, and pressures on existing structures as well as the freeze pipes are important to monitor while implementing an artificial ground freezing program.  Overall, artificial ground freezing has a wide variety of applications, and an history of successful application in the field.  It has become economically competitive with traditional ground stabilization methods and has the ability to be applied to a wide variety of projects.

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