Ground Freezing Cool Technology Keeps Contaminants on Ice By Adam Chen, PE, Joe Chittet, PE, and inserted into vertical bore holes drilled along Joan Gonzalez the edge of area to be shored or contained. If you’ve ever had to try to dig a hole in the As the cooling agent, typically chilled brine

ground after an extended period of sub-freezing (CaCl2), is circulated through the pipes, weather, you know how tough frozen is. It’s heat is extracted from the soil into the brine nearly twice as strong as concrete, and nothing solution, causing the ground to freeze around passes through it, not water, chemicals or the pipes. The warm brine is returned to the hazardous materials. refrigeration plant where it is again cooled. Frozen earth forms around the freeze pipes in Those attributes are the basis for the latest the shape of vertical, elliptical cylinders. As the applications of ground freezing, a cool cylinders gradually enlarge, they intersect to technology for construction shoring, control form a continuous wall. With heat extraction of groundwater and isolation of subsurface continued at a rate greater than the heat contamination. Wherever soil is loose, soft replenishment, the thickness of the frozen or wet, ground freezing can create a safe, wall will expand with time. simple, reliable and surprisingly cost-effective barrier, either as a temporary application or a permanent solution. Frozen ground is nearly twice as

Time-Tested Technique strong as concrete and is essentially The history of artificial ground freezing and impermeable. its engineering applications stretches back almost 150 years and across an ocean. Invented by the German scientist F.H. Poetsch in 1863, Once the frozen wall has achieved the design the technique was reported to have first been thickness, the freeze plant may be operated at a used in the coal mining valleys of South Wales. reduced rate to maintain the condition during In the United States, the first ground freezing excavation. Monitoring of conditions during application, to a depth of 30 meters (100 feet), formation and maintenance is accomplished by occurred in 1888 for the Chapin Mine Co. in Iron temperature sensors installed at various levels Mountain, Mo. Today, artificial ground freezing in monitor pipes located strategically along the is used extensively for groundwater control frozen wall. and excavation support in the underground construction industry and, recently, for Following excavation, refrigeration is environmental remedial applications. discontinued, allowing the ground to return to its normal state. Basic Concepts Still Valid The concept behind 21st century ground While the principle behind the ground freezing freezing is the same as that developed by process may appear simple, proper execution Poetsch. When in-situ pore water in the of the work, particularly at depth, is complex. subsurface is frozen, it acts as a bonding Successful ground freezing operations require understanding of refrigeration and soil thermal Adam Chen, PE, is a agent, fusing particles of soil or rock to create project manager in the a frozen soil mass with remarkably improved analysis. In addition, understanding of the Burns & McDonnell Environmental compressive strength and impermeability. strength and behavior of frozen earth is vital. Group in Chicago with more While advanced refrigeration technology has The alignment of the freeze pipes is critical to than 20 years of experience. refined modern efforts, the core method of acceptable performance of the ground freezing He specializes in hazardous achieving the freeze still dates back to Poetsch. system. The design, with respect to strength waste remedial design. Small-diameter, closed-end freeze pipes are and time of formation, is directly related to the

Burns & McDonnell 5 TECHBriefs 2010 No. 3 spacing between pipes and temperature of the management and control have become more chilled brine solution. If the installed pipes are problematic because of increased regulatory permitted to deviate too much from the design, requirements in many states. Often it is unexpected windows or zones of less-than- not possible to remove and dispose of large design thickness can occur. volumes of groundwater, even for temporary construction purposes. Construction sites have Freezing Wall Applications become more congested and complicated. Ground freezing is ideal for a multitude of Often, contractors must work around existing construction and environmental remedial infrastructure such as , cables, sewer applications. It is effective in almost all soil lines and other utilities, prohibiting the use of conditions, including heaving , cobbles, conventional shoring with sheet piles. , and . It provides excellent A frozen soil barrier, on the other hand, flexibility for sites with buried utilities, as the often can be installed without disturbing freeze pipes can be installed around or beneath underground infrastructure or requiring existing underground facilities. expensive relocation of utilities.

Since soil freezing equipment is small, portable Another factor driving the move toward ground and uncomplicated to install, soil freezing can freezing is the presence of contaminated be easily adapted for difficult or remote sites. groundwater and soil at many construction Power outages during construction are not a sites. Treating and hauling off hazardous Joe Chittet, PE, is a problem either, because frozen ground stays materials or contaminated groundwater is a senior civil engineer In the frozen for days or weeks without additional pricey proposition. But a frozen barrier can be Burns & McDonnell Environmental chilling, even in the summer. And at between installed on a temporary or permanent basis, Group in Chicago. He has more $15 and $25 per square foot, the price of a safely isolating a work area from external than 10 years of experience in remediation services. frozen soil wall is much more attractive than contaminants or preventing the migration many other options, such as sheeting piling of contaminants to new locations. with a grout sealing, to provide a shore system and prevent groundwater seeping into the Summary excavation area. The modern ground-freeze wall is a reliable and effective technology. Its use is worthy of Several factors contribute to the current consideration, especially for site conditions enthusiasm for ground freezing. Groundwater in which the excavation area is in the middle of active utility lines and/or next to existing buildings. Implementing a ground freezing wall Ground freezing to provide both to provide both earth shoring and groundwater cutoff during construction and soil excavation, earth shoring and groundwater cutoff including removing the contaminated soil, during soil excavation is a viable and is also a cost-effective option compared to traditional approaches such as sheet pile and cost-effective option compared to the Joan Gonzalez is an . associate civil engineer in traditional approach. the Burns & McDonnell Environmental Group in Chicago. She has more than 20 years of experience, and her technical specialties include familiarity with the Illinois Site Remediation Program.

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TECHBriefs 2010 No. 3 6 Burns & McDonnell