Frozen ground may be used to provide ground support, groundwater control, or structural underpinning during construction.
Ground freezing may be used in any soil or rock formation, regardless of structure, grain size, or permeability. The mechanical properties of frozen ground are more dependent on time and temperature than on geology.
Groundwater must exist, supplied either by a high water table or artificially. The frozen water (ice) becomes the bonding agent, fusing particles of soil or blocks of rock to increase their combined strength and make them impervious to water seepage.
Typical applications include vertical shafts, deep excavations, tunnels, groundwater control, structural underpinning, and hazardous waste containment.
A preliminary review of the suitability of ground freezing for a particular project requires information about the geometry of the excavation or frozen barrier, soil and groundwater conditions at the site, and proximity of adjacent utilities and structures.
A typical ground freezing system for a shaft or tunnel consists of a series of freeze pipes installed along the perimeter of the proposed excavation, extending into the subsurface strata. To freeze an area, freeze pipes are installed in a grid pattern and extend into the subsurface strata.
Calcium chloride (brine) is used as the cooling medium and is chilled by one or more electrically powered mobile refrigeration units. The cold brine (at -30 to -25 °C) is pumped from the refrigeration unit through a distribution manifold to each freeze pipe. The manifold has supply and return lines. Larger ground freeze systems often require a reverse return manifold line. Chilled brine flows down a pipe inserted within each freeze pipe and then flows back to the surface in the annulus created by the downpipe and the freeze pipe. As the warmer brine returns from the freeze pipes, it flows into the return manifold, which permits flow back to the refrigeration plant. As the refrigerated brine is circulated through individual freeze pipes, frozen cylinders form. After approximately six to eight weeks, the cylinders merge together, forming a massive frozen earth wall.
Soil conditions
Borings that extend well below the planned excavation depth must be thoroughly reviewed. These borings must provide samples for classifying the soil and undisturbed soil samples for frozen and unfrozen strength laboratory tests. Soil type, density, and water content are needed to estimate soil properties. The soil properties are used as input variables into a thermal analysis model, which is a time-dependent heat transfer finite element computer program. The model evaluates the required freezing time as related to freeze pipe spacing, coolant temperature, and coolant flow rates. Model results also assist in determining the required heat load, which is incorporated in the design of the refrigeration and coolant distribution systems.
Groundwater flow
Lateral groundwater flow through a proposed site adds heat, which may cause problems relative to forming a continuous frozen wall. If the water velocity is too large — greater than 1 to 2 meters/day — the freeze columns will not merge, leaving openings in the frozen wall. If the groundwater flow is greater than 1.5 but less than 3 meters/day, either reduced freeze pipe spacing or a second row of freeze pipes is a feasible solution. The formation's permeability or gradient must be reduced if the groundwater flow exceeds 3 meters/day. This can be accomplished by grouting before or during the freeze pipe installation.
Groundwater quality
The presence of contaminated water in the ground to be frozen can lead to several problems, including lower freezing temperatures, reduced ice content, and lower strength.
Shape of the frozen earth structure and freeze pipe spacing
In a plan view, the shape of the ground freezing system is determined in terms of the required excavation limits and the available space on-site. Due to frozen soil's relatively high compressive and low tensile strengths, curved arch walls, particularly circular ones, provide the strongest frozen structure. An ellipse may be effectively employed for rectangular structures if the length-to-width ratio does not exceed approximately 2. Complex structures may involve combinations of curved or straight walls and some form of bracing. In a cross-sectional view, the shape of the frozen soil zone is dependent primarily on the frozen thermal conductivity of the soil layers. Silts and clays have lower thermal conductivity values and will, therefore, have relatively thinner frozen zones that form around the freeze pipes. Conversely, more granular layers with higher thermal conductivity values will have thicker frozen zones.
Site preparation
The site must be graded to ensure that surface water is collected and drained away from the planned frozen structure.
Protection of utilities
In urban areas, the potential impact of the frozen soil on nearby utilities must be considered. Most utilities are located close to the ground surface. Protection of the utility may include exposing the line/pipe and insulating it with sprayed polyurethane foam or installing heat cables adjacent to the utility.
Freeze pipe installation
Ordinary ¼-inch thick steel with an inside diameter of 3 inches is commonly used for freeze pipes. All subsurface connections on a freeze pipe should be welded; threaded couplings are not recommended. After installation, each pipe is filled with water and pressure tested from the surface. To test the strength of the welds, each pipe must maintain a given pressure for a specific period.
Pipe survey
Following installation, a verticality survey is conducted on each freeze pipe to verify its actual placement. A variety of downhole inclinometer instruments may be used. Additional freeze pipes will be installed if a significant pipe deviation has occurred. More than any other controllable variable, the relative freeze pipe spacing controls the time needed to complete satisfactory freezing.
Distribution manifold
For a circulating coolant system, the distribution manifold is typically made out of 6 or 8-inch diameter HDPE or steel pipe. The manifold pipes are connected to supply the refrigerated coolant from the refrigeration units to each individual freeze pipe. Valves are installed within the manifold to regulate the brine flow from the full capacity during the initial freezing to a restricted flow during the maintenance and construction phase. The manifold will be designed to deliver approximately 25 gallons per minute through each freeze pipe. Typically, the pipes are grouped in series with three or four pipes per group. The freeze pipe headers include two separate valves to permit the hydraulic balancing necessary to ensure uniform heat transfer.
Drill rigs with rotary or resonant sonic tooling are preferred for freeze pipe installation. In situations with numerous subsurface utilities and limited site access, freeze pipes must often be installed at compound angles. Rigs with the ability to drill compound angles are often required to ensure accurate placement of the freeze pipes.
Ideally, a frozen soil barrier should be tied into an impervious layer (e.g., bedrock) to develop a closed bottom condition. This eliminates the need for significant pumping to control groundwater.
Portable refrigeration units, using ammonia as the primary refrigerant, are typically used for temporary, field use. These units operate with either diesel fuel or electricity and have high thermal efficiencies. Electric units are the most common and contain a 350 hp electric compressor, requiring a 480V, three-phase electrical service. The units also require a water supply for the cooling towers. The refrigeration units are equipped with a full range of electronic instrumentation that can be coupled to off-site communication services. This feature allows personnel to monitor plant operations at any time. Two types of refrigerant systems are available: circulating coolant and expendable liquid refrigerant.
Circulating coolant
This type of refrigeration requires a distribution manifold to circulate a cooling medium to the freeze pipes that extract heat from the soil. Most ground freezing projects employ the circulating coolant refrigeration system. A calcium chloride solution (approximately 28%) is used as the circulating coolant. The refrigeration units, rated to provide 150 tons of refrigeration at -25°C, chill the coolant to temperatures ranging from -25 to -30°C.
Expendable liquid refrigerant
Liquid nitrogen and carbon dioxide are expendable refrigerants used in this system. This type of refrigeration system is used when rapid formation of the wall is important and only small earth volumes are to be frozen. Liquid nitrogen does not require a refrigeration plant(s), so only space for a liquid tanker truck to access the manifold is needed. Liquid nitrogen is pumped from a tanker truck to the manifold, distributing the coolant to each freeze pipe. The liquid nitrogen will flow down the inner feed pipe and be vented to the atmosphere through the freeze pipe's annulus. No return is required on the manifold system. Each freeze pipe will have a cryogenic valve to balance the coolant flow to ensure equal distribution through all pipes. As the liquid nitrogen circulates, cylindrical soil columns freeze around each pipe. The size of the columns increases with time, forming a virtually watertight, impermeable barrier, as the columns merge together.
Ideally, the refrigeration equipment requires an area approximately 75 feet x 50 feet and is located within 100 feet of the work site.
Careful observation of the following parameters is required to ensure proper operation of the system:
Ground temperatures
Ground temperatures are measured in temperature pipes located throughout the ground freezing area. The temperature pipes are identical to the freeze pipes, except they are not connected to the manifold system. Instead, they are filled with calcium chloride brine and left open to the atmosphere, reducing the potential for the formation of ice within the monitoring pipe. Hand-held temperature probes or RTD temperature sensors can be used to collect the data. The RTDs can be connected to an on-site computer, which can transmit temperature data to any off-site location.
Coolant temperatures
Coolant return temperatures are measured at each freeze pipe at the connection to the return manifold. This measurement ensures that each pipe has complete circulation.
Groundwater levels
Groundwater levels are measured in nearby piezometers or monitoring wells. Hand-held water level indicators or electronic transducers, providing continuous measurement, can be used to collect the data. Any significant drop in these levels could indicate potential inflows into the excavated zone.
Coolant flow and pressure
Coolant flow and pressure are constantly monitored within the refrigeration unit and are connected to an alarm system. Any decrease in flow or pressure could indicate a broken line or leak, requiring immediate repair operations.
Refrigeration data
Refrigeration units are equipped with a computer that records all operating information. This data is available to on-site personnel to review system performance and ensure the plant operates within expected parameters.
After the refrigeration units have been turned on and the coolant is circulating through the freeze pipes, temperature data is collected from the temperature pipes and recorded, usually daily. When all pipes show consistent temperatures below 0°C, the ground has frozen, and the excavation can begin.
The exposed frozen wall surface is susceptible to deterioration and possible unstable conditions due to several factors, including 1) thermal load from sun, rain, and moving ambient air, 2) sloughing of partially saturated frozen granular soils due to sublimation of ice, and 3) improper construction methods involving water and soil removal from the excavation. The frozen ground can be protected if the following construction techniques are thoroughly considered and possibly implemented:
Protection of exposed frozen earth
An exposed frozen wall, if left uninsulated, will slough a small amount daily until it deteriorates to an unstable condition. A single layer of reinforced reflective plastic or foam insulation is frequently sufficient to prevent sloughing. Line shafts with concrete are typically recommended at 10-foot intervals as the excavation progresses. Frozen earth can be excavated by jetting water, blasting with explosives, cutting with rotating hardened metal bits, or breaking with pneumatic or hydraulic impact tools. Of these alternatives, blasting and water jetting represent the greatest danger to the frozen earth.
Concrete placement against frozen earth
Concrete can be placed directly against frozen earth when necessary Ð although low temperatures reduce the curing rate. Experience has shown that for concrete placed at 15-18°C, the adjacent frozen soil will thaw to a depth approximately equal to 50-100% of the concrete thickness. With time, the soil will re-freeze. Normally, neither freezing nor the reduced curing rate presents a problem for ordinary concrete in sections thicker than 250 mm. For thinner sections, the heat of hydration and/or rate of set must be increased by using the following (in order of desirability and cost): 1) a lower water-cement ratio, 2) a richer mix design, 3) high early or regulated cement, 4) accelerating additives, 5) aluminous cement, or 6) high concentrations (9-15%) of calcium chloride.
The popularity of ground freezing is continuing to grow. For engineers, artificially frozen ground is very solid and waterproof. It is highly reliable and characterized by high safety standards. Structural analysis models of frozen earth are becoming increasingly common. For the environment, no residues remain below ground, and it protects the groundwater.
Ground freezing is a versatile technology that can be applied in various applications. It will become more prevalent as more construction is performed within geologically unfavorable ground, in urban areas, existing structures, and tight working conditions.
A typical ground freezing system using a circulating coolant can take 6 to 8 weeks for a smaller diameter shaft and 10 to 12 weeks for larger areas. This is the only time for the formation of the frozen Earth structure. Time to mobilize and install the freeze pipes occurs before the freeze formation and is not included in the above time estimates.
An expendable refrigerant system, such as liquid nitrogen, can often form a frozen structure in a few days.
Ground freezing is not an inexpensive construction technique.
It is rare to experience any measurable heave or settlement from ground freezing operations. Theoretical computations are available, but they have not been shown to accurately and consistently predict heave or settlement in any soil type.
The chemicals used for ground freezing (calcium chloride, anhydrous ammonia, and liquid nitrogen) require extreme care. On site personnel must be properly trained to work with these materials and have appropriate safety equipment. In enclosed spaces, the refrigerant ammonia may be substituted with other, less toxic alternatives.