The International Information Center for Geotechnical Engineers

# Ground Freezing

4.0 IMPLEMENTATION IN THE FIELD

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.

4.1 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.

4.2 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.

4.3 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 7 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.

4.4 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).

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