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Artificially Frozen Ground as a Subsurface Barrier Technology Steven A. Grant and Iskandar K. Iskandar, Cold Regions Research and Engineering Laboratory, U.S. Army, Harlover, New Hampshire INTRODUCTION Frozen water-saturated ground has great mechanical strength and very low permeability. These properties are exploited by construction ground freezing, a mature civil-eng~neering practice that has long been used in North America, Europe and Asia, usually to restrict ground-water seepage or~stabilize the walls during excavations. Construction ground freezing is often competitive economically with other hairier methods; its principle disadvantage is the long time it can take to form a frozen wall. Recently, artificial ground freezing has been used to form barriers at contaminated sites. The principle advantages of artificial ground freezing for this application, which we designate here as "environmental growing freezing." are thought to be its ability to form a, - impermeabie banders across transitions from soil to bedrock, the minimal amount of solid material hrnu~ht to or taken from the site duIina battier construction or decommissioning, and the great a flexibility in designing or adjusting the ba:Tier's size and shape (]iskandar and Jenkins, 19651. Some believe that, as a natural process, soil freezing is likely to find public and regulatory acceptance, although the technique has been implemented at too few contaminated sites to judge this belief. PHYSICS, CEIEMISTRY AND MECHANICS OF FROZEN GROUND Frozen ground is composed of four phases: gas (often as entrapped bubbles), liquid aqueous solution, solid ice, and solid mineral matnx. A schematic of this assemblage is presented in Figure 1. The amount of liquid aqueous solution remaining at a particular temperature is determined by two physical-chemical phenomena: interracial forces caused by the distortion of the ice-solution interface as it is forced to adjust itself to the geometry of the ground's mineral matrix; and colligative properties of the water as a solvent, that is, the freezing-po~nt depression of the pore solution. Of the two, interracial forces are the more important In determining the amount of liquid aqueous solution persisting at subzero temperatures in ground. As would be expected, the larger the specific surface area of the minerals, the larger the fraction of water remaining unfrozen at subzero temperatures. Figure 2 presents the calculated volumetric liquid-water content of a water-saturated Royal sandy loam as a fimction of temperature. Due to freezing-point depression, the presences of solutes in the pore solutions will cause the amount of liquid present at a given subzero temperature to be increased. This can become an important consideration when ground saturated with seawater is being frozen. The amount of liquid water remaining in frozen ground is not solely of academic interest. In many cold regions, it is this liquid fraction that wicks water Tom deeper in the soil profile to the ground's freezing Font, causing Cost heaves--a serious engineering problem. Liquid-water contents of frozen ground affect the ground's strength, its permeability, and the diffusion rates of solutes in D-153
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D-154 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT it. All of these could be factors in determining the suitability of environmental ground freezing at a given site. Gas Unfrozen Bubbles Water / / ~ Amp I N~ \ ~ t Vg a. Natural Soil Vi V Vs t , vw . ~ b. Soil Separated into Phases FIGURE 1 Schematic drawings showing the phase distribution in Dozen ground. ~ 0.3 Cal - ~ . o 0.2 Cl' 0' 0.1 o Wg Ww Ws Wj -25 -20 -15 -10 -5 0 W Temperature (C) FIGURE 2 Calculated relationship between temperature and liquid water in a frozen Royal sandy loam.
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APPENDIX DIAPERS PRESENTED D-155 Mechanical Properties The strength of frozen ground is due to the strengths and volumetric fractions of its constituents: ice, mineral grains, and liquid water. By most measures, frozen ground can be a very strong material. The measured unconfined compressive strength of frozen Ottawa sand at -50°C was 40 MPa (Sayles, 19881. The corresponding strengths of competent granite without macroscopic flaws are between 150 and 250 MPa. The resistances of frozen ground to load and deformation are affected by its mineral composition, water content, temperature, previous loading, and rate of loading. The effect on the proportion of water is presented in Figure 3. As a general rule, the resistance of frozen ground to load and deformation increases with decreasing temperature but decreases with fume. These two tendencies are shown In Figure 4, which is reproduced from Sayles (19881. Permeability The permeability of frozen ground can be estimated based on the ground's degree of saturation by liquid pore solutions. Figure 5 presents the calculated permeabilities of a Royal sandy loam as a function of temperature. ARTIFICIALLY FROZEN GROUND AS AN ENGINEERING TECHNOLOGY Many, including practicing civil engineers, are unaware of construction ground freezing, yet it is hardy a secret. Perhaps the most famous structure to be affected by construction ground freezing is the tower in Pisa, Italy, the foundation of which is being stabilized by the technique while a more permanent solution to its inclination is being constructed. Perhaps the largest construction project to use construction ground freezing was the Tokyo Metro. A total vo! Me of 38,000 ma of material was excavated (lessberger, 19801. In 1883, the technique was patented by F.H. Poetsch in Germany as a mine-shaft construction method. While improvements have been made, the techniques used today are essentially those of Poetsch. Energy is removed from the ground by circulating cold liquids through pipes drilled into the ground. The two refrigeration approaches are: using a mechanical refrigeration plant through which a heat-transfer fluid is circulated to freeze pipes, or pumping expendable cryogenic liquids through the freeze pipes. Freeze pipes, which consist of two nested pipes for supply and return of the heat-transfer fluid (usually CaCI2 or MgCI2 brines) or cryogenic liquid (typically liquid nitrogen), are drilled into the ground at regular intervals, typically ~ m. In principle, any soil can be frozen. Shuster (1980) noted that successfi~l implementation of ground freezing may be limited as follows: 1. The degree of saturation of the ground by water must be high enough to bind the sol! grains when Dozen. Generally, fully water-saturated ground is preferred. 2. The concentrations of solutes in pore-water solution should not be too high. This is a consideration, for example, when freezing marine sediments. The melting point of sea ice is lower and its strength less than that of fresh-water ice. (There have been instances of catastrophic failures
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D-156 I, ~6 LO - it, ~ 4 lo o ~ 2 ~ 4 o BARRIER TECHNOLOGIES FOR ENI'IRONMENTAL MANAGEMENT ; en' ~ ·\ O am_ · GO~;FNOUR ANI) ANDERSlAND, 1968 - ° BAKER, 1979 0 20 40 60 80 100 TOTAL MO I STORE CONTENT, % FIGURE 3 Effect of total moisture content on unconfined compressive strength of a frozen fine sand at -12°C and at a strain rate of 2.2 x 10-6 s~i (Dom Andersland and Ladanyi, 19941. 10 - CL A. 8 Strength from test data. '3 -- Predicted strength from Vialovs eat: ~u''~ ~,n`~/B' . .. (1500hr) . ._ (4000 hr.) 1 . . . ~-0.6 1 ~(2000 hr.) 0 100 200 300 400 500 I, Time (mint) FIGURE4 Calculated relationship between temperature and hydraulic conductivity of a frozen Royal sandy loam.
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APPENDIX~PAPERS PRESENTED 10-3 10-4 10-5 10-6 no 10-7 .E C' ~ 10-9 ._ ._ co E a) 10-8 1o-1o 0-11 0-12 1o-l3 10-14 10-15 10-16 10-17 -25 -20 D-157 -15 -10 Temperature (C) -5 0 FIGURES Calculated relationship between temperature and hydraulic conductivity of a frozen Royal sandy loam. Of frozen walls formed in salt-water saturated ground [E.~. Chamberlain, personal communication, 199411. 3. Ground-water seepage does not bring to the forming barrier more energy than can be removed by the refrigeration plant. Calculations have shown that flows should not be greater than 2 m/day. The ground has comparatively high heat capacity and low thermal conductivity (Sullivan and Stefanov, 19901. Once formed, a frozen-ground barrier effectively eliminates energy loss due to convection and can be maintained indefinitely with Tow power consumption. There is a small, but stable market for construction ground-freezing services; four firms in the United States provide the service. ARTIFICIALLY FROZEN GROUND AS A WASTE-CONTAINMENT TECHNOLOGY We are aware of two contain" inated sites at which artificial ground freezing is being used for waste containment. The first is a National Priority List (NPL) site in the eastern United States situated next to a river. Frozen ground walls are being formed in conjunction with sheet-pile walls to limit the migration of contaminants off-site to the river. This site will be excavated after the walls
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D-158 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT are formed. The second site is a petroleum-contaminated site on permafrost in Alaska. Here a frozen-ground bander has been formed to prevent snowmelt-runoff driven horizontal flows of dissolved petroleum products as the active layer above the permafrost thaws. Ground freezing has been proposed, though not as yet adopted, at two DOE reservations: Oak Ridge and Hanford. A successes! field trial was conducted at Oak Ridge in 1994. The soils of the Solid Waste Storage Areas within the Oak Ridge Reservation (ORR) are fairly shallow (3-9 feet), weakly developed, and overlay weathered rock (saprolite). Environmental ground freezing is seen as having two potential advantages at ORR. First, the barrier can be formed continuously across the transition Tom soil to the underlying saprolite. Second, when compared with other methods, less earth will be excavated In Installing a ~ozen-ground baITier. Environmental growing freezing has also been proposed at the Hanford Reservation, although an unrealistically Tong time may be needed to develop a reliable technique to saturate and cool an arid soil simultaneously, without water loss. Results of bench-scale experiments, however, are encouraging (AndersTand et al., 1994; Dash et al., 19941. SCREENING MATRIX Development Status Construction ground freezing is routine, but environmental ground freezing is in its infancy. The firms that participated in the environmental ground freezing projects and demonstrations have been able to meet the design criteria without difficulty. A principle difficulty appears to be that there is no ~n-situ sensing technology for continual assessment of the integrity of the barrier. As with other barrier technologies, sensing leaks Tom beneath the barrier are problematic. Ava;RabiJity As noted above, a limited number of firms provide this service, which is as much an art as a construction practice. Were the demand for environmental ground freezing to increase markedly, experienced practitioners could be finely booked. This appears to be a hypothetical, not imminent, problem. Implementability At sites that meet the criteria for successful Implementations of construction ground freezing, one would expect that environmental ground freezing could be implemented straightforwardly.
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APPENDIX~PAPERS PRESENTED Reliability/Maintainability D-159 Based on experience with construction ground freezing, the technology should be reliable and easily maintained when installed professionally at an appropriate site. Secondary Waste Produced Intrinsically, very little secondary waste is produced with environmental ground freezing. Stance the method typically relies on water in the ground, only that material brought to the surface for drilling holes for freeze pipes may constitute a secondary waste. Once refiigeration is stopped, the ground returns largely to its original condition, although frozen ground may consolidate while thawing. Implementation Costs These tend to be high but are competitive with other battier technologies (Sullivan, Lynch, and Tskandar, 19841. Operating Costs Although dependent on the intensity of on-site monitoring, these costs should be modest. Regulatory Acceptance As an organization that supports environmental research, the U.S. Environmental Protection Agency (EPA) has fended some studies and demonstration projects, but it has not indicated how readily it will accept environmental ground freezing. Public Acceptance Environmental ground freezing has been implemented at very few sites. To our knowledge, no studies of public acceptance have been made. it is assumed that environmental ground freezing should not incite public resistance since it is an adaptation of a natural process. Natural Resources Impact Extended periods at subzero temperatures would kill many of the flora and farina remaining In the frozen-ground barrier. As noted above, forming the frozen-ground barrier takes roughly a month; presumably, most of the burrowing animals would vacate the barrier as it cools. Rapid re- habitation from the surrounds after decommissioning of the barrier would be anticipated.
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D-160 Risk-Management Reduction BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT It is expected that the risk-management effects of environmental ground freezing would be on par with other hairier techniques. CONCLUDING REMARKS Environmental ground freezing has long been a promising waste isolation technology but one that was all too rarely fielded (Bovay Northwest, Inc., 19921. Recent evaluations and implementations at contaminated sites may yield a sufficient body of experience so that the technique may be implemented more often. REFERENCES Andersland, O. B., and B. Ladanyi. 1994. An Introduction to Frozen Ground Engineering. New York: Chapman and Hall. Andersland, O. B., S. H. Davies, and D. C. Wiggert. 1994. Performance and Formation of Cryogenic Containment BaIriers in Dry Soils. Report Submitted to RUST Geotech, Inc., Grand Junction, Colo. Bovay Northwest, Tnc. 1992. Interim Subsurface Barriers Technologies. Workshop Report. Prepared for Westinghouse Hanford Company, Bovay Northwest, ~c., Richland, Wash. Dash, J. G., H-Y. Fu, and R. Leger. 1994. Bench Ccale Testing of Hanford and Oak Ridge Soils, Formation and Diffusion Testing of Cryogenic Barriers. Final Report Submitted to Scientific Ecology Group, Inc., Oak Ridge, Tenn. Iskandar, r. K. and T. F. Jendcins. 1985. Potential uses of artificial ground freezing for contaminant immobilization. Pp. 128-137 In Proceedings of International Conference on New Frontiers for Hazardous Waste Management. EPA l 9-8 l 5025. lessberger, H. L. 1980. Review, theory and application of ground freezing in civil engineering. Cold Reg. Sci. Technol. 3:3-27. Sayles, F. H. 1988. State of the art, Mechanical properties of frozen soil, In Frozen Ground SS, vo} 1, R. H. Jones and I. T. Holden, eds. Brookfield, Vermont: Balkema. Shuster, I. A. 1980. Engineering quality assurance for construction ground freezing. Pp. 863-879 in Ground Freezing, Proceedings of the 2nd International Symposium on Ground Freezing, Trondheim, Norway, Norwegian Institute of Technology. Sullivan, Ir., I. M. and L. A. Stefanov. 1990. Comparison of numerical simulations with expenmental data for a prototype artificial ground freezing. Pp. 36-44 in Frozen Soil Impacts on Agricultural, Range, and Forest Lands, K. R. Cooley, ed. Cold Regions Research & Engineering Laboratory Spec. Rep. 90-1. Cold Regions Research & Engineenng Laboratory, Hanover, N.H. Sullivan, Ir., I. M., D. R. Lynch, and I. K. Iskandar. 1984. The economics of ground freezing for management of uncontrolled hazardous waste sites. Pp. 386-392 in Proceedings of the 5th International Conference on Management of Uncontrolled Hazardous Waste Sites. . ~ · . ~
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Representative terms from entire chapter: