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Movement of liquid contaminants in partially saturated frozen granular soils
Cold Regions Science and Technology 25 Ž1997. 111–117
Movement of liquid contaminants in partially saturated frozen granular soils David C. Wiggert, Orlando B. Andersland, Simon H. Davies Department of CiÕil and EnÕironmental Engineering, Michigan State UniÕersity, East Lansing, MI 48824, USA Received 1 November 1995; accepted 7 September 1996
Abstract Understanding the migration of nonaqueous phase liquids in frozen subsurface soils is becoming increasingly important in permafrost regions and in temperate zones where frozen subsurface barriers have been proposed to confine contaminants. Tests were performed on 32 specimens of gravelly sands from the Hanford, Washington reservation to determine the relationship between degree of ice saturation and intrinsic permeability. Decane, a representative nonaqueous phase liquid, was employed as the permeant, and was infiltrated through the frozen specimens at y108C. In addition to pure water, a NaCl brine and a mixture of water and decane were utilized as the pore liquids. For all specimens the intrinsic permeability correlated linearly with the ice saturation, varying from approximately 2.7 = 10y7 cm2 at 0% saturation to negligible values at nearly 100% saturation. The different pore liquids did not affect the correlation significantly. Bentonite was added to some of the specimens prior to freezing, reducing the intrinsic permeability to negligible levels. Keywords: Decane; Ice saturation; Intrinsic permeability; Nonaqueous phase liquid; Frozen granular soil; Migration of contaminants
1. Introduction In recent years there has been an increased interest in understanding the fate and movement of contaminants in the frozen subsurface environments. Contamination of nonaqueous phase liquids ŽNAPLs. such as gasoline or oil is an ongoing concern in regions where permafrost exists, and in more temperate regions where there are soils that undergo annual freeze–thaw cycles. Additionally, subsurface barriers consisting of frozen soil have been proposed to temporarily confine contaminated soil during site remediation, and to act as backup containment during removal of hazardous waste from underground storage containers ŽVick, 1994; Andersland et al.,
1995, 1996a,b.. Placement of these barriers is based on established ground freezing technology that is practiced in the construction industry as a means to temporarily stabilize soil and control the movement of ground water ŽAndersland and Ladanyi, 1994.. After a site has been remediated, these barriers can be thawed and the freeze pipes removed, returning the soil to nearly its original state with minimal impact on the environment. When granular soils are frozen to serve as a barrier for containment of liquid contaminants, or a naturally occurring frozen soil is exposed to a NAPL contaminant, a fundamental question that arises is: How impervious is the frozen soil to a liquid contaminant? Water will move into cold soil until the
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D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
heat balance becomes negative. At this point the water is converted to ice, blocking further movement of any water. Liquid contaminants with low freezing points Ž- 08C. will move through partially icesaturated soils as shown by the experimental data summarized in this paper. An additional problem arises when the liquid contaminants are highly miscible with water and introduce a freezing point depression. These liquids cause melting Žerosion. of the ice matrix for temperatures below 08C. This latter problem has been addressed in the paper by Andersland et al. Ž1996b..
2. Background A NAPL is a liquid that has very low solubility in water Žon the order of 0.01 mgrL., and hence does not depress the freezing point of water to cause melting of the ice matrix 1. The problem of forming a continuous, frozen barrier impervious to NAPLs in a granular, low water content soil at approximately y108C has been reported by Andersland et al. Ž1996b.. The site in question was the Hanford, Washington reservation ŽVick, 1994.. Andersland et al. Ž1996b. also dealt with ice erosion within the barrier when liquids with a freezing point depression come in contact with the ice matrix. In another publication, Andersland et al. Ž1996a. reported measurements of hydraulic conductivity in the frozen granular Hanford soil that contained water in the form of ice varying in content from zero to approximately 100% saturation. The permeant employed in the study was decane, a nonaqueous-phase liquid whose viscosity is nearly the same as water at just above 08C. In addition to those reported by Andersland et al. Ž1996a., conductivity tests were performed in Hanford soils ŽAndersland et al., 1995. where bentonite was added to increase the water content of the granular soil prior to freezing and where the pore water contained dissolved NaCl that depressed the freezing point. In order to characterize the movement of a NAPL
1
The effects of solutes on freezing are discussed by Andersland and Ladanyi Ž1994..
in a porous, partially ice saturated frozen soil, we begin with Darcy’s law in the one-dimensional form rg Õsk i Ž 1. m in which Õ s specific discharge, i s hydraulic gradient, r s density of the NAPL permeant, g s gravitational constant, m s viscosity, and k s intrinsic or specific permeability. Implicit in the use of Eq. Ž1. is the assumption that the NAPL is the only liquid present in the porous matrix. By virtue of the low temperature Žy108C. of the granular soils, close to 100% of the pore water contained in the medium prior to freezing will have been transformed to ice. In a given frozen soil, for a liquid NAPL contaminant whose density r 1 , viscosity m 1 , and hydraulic conductivity K 1 are known, the intrinsic permeability can be estimated in the manner m1 K 1 ks Ž 2. r1 g The hydraulic conductivity K 2 of a second contaminant in the same frozen soil matrix can then be estimated by knowing the intrinsic permeability and the contaminant’s viscosity m 2 and density r 2 : r2 g K2 sk Ž 3. m2 The intrinsic permeability is an empirical constant that accounts for the permeable characteristics of the porous frozen soil. For non-frozen granular soils, k has been correlated with the square of the effective grain diameter of the soil ŽDavis and DeWiest, 1966.. In a frozen soil, it is postulated that in addition to the square of the effective grain size, the intrinsic permeability is also dependent upon the ice saturation Sice , which is the fraction of the unfrozen pore volume that is occupied by ice. For uncontaminated granular soils, the unfrozen water content is close to zero at soil temperatures below 08C, so that nearly all of the moisture is in the form of ice. In order to achieve approximately 100% ice saturation Ž Sice s 1. in the soil, it has been shown that 91.7% water saturation in the unfrozen soil is required 2 ŽAndersland et al., 2 Here it is assumed that there is no change in bulk volume on thawing and that a 9% volume change occurs on conversion of liquid to ice.
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
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1995.. For partially saturated soils capillary forces tend to hold the water in the vicinity of contact points between particles of the granular matrix. When frozen, those water particles are solidified and continuous voids remain, with the result that Sice - 1.
3. Measurement of intrinsic permeability A falling-head permeameter was employed to measure the intrinsic permeability of the frozen soil specimens. In order to maintain the required constant temperature control throughout each test, the specimen and permeant were placed in a coolant bath, Fig. 1. The height of the permeameter was 200 mm, its diameter was 100 mm, and the diameter of the standpipe was 31.8 mm. A typical specimen height was 100 mm. Since the changes in liquid level were very small, no liquid was drained from the permeameter during a test. The permeability was computed from the relation ks
m aL r gA Ž t 2 y t 1 .
ln
h1
ž / h2
Ž 4.
in which a s area of the standpipe, A s the area of the specimen, L s length of specimen, and h1 and h 2 are the initial and final heads in the standpipe that occur over the time interval t 2 y t 1. The apparatus provided accurate measurements of k, except when the medium was nearly 100% ice saturated. Since the focus of the study was to evaluate values of permeability that would result in significant leakage through
Fig. 1. Schematic of hydraulic conductivity apparatus ŽAndersland et al., 1995..
Fig. 2. Particle size distribution curves for the five samples of Hanford soil ŽAndersland et al., 1995..
a frozen barrier, no refinements were attempted to accurately determine k for ice saturations close to 100%. Soil samples from the Hanford site ŽFig. 2. were analyzed and classified as gravelly sand, SW and SP in the Unified Soil Classification system. For the permeability tests, samples were reconstituted to give particle size distributions approximating field samples. Prior to freezing, moisture was added to the samples to a desired degree of saturation using upward seepage of water into the dry soil in the test cell followed by controlled drainage. Intermediate degrees of saturation in samples below the field saturation level were attained by omitting some of
Fig. 3. Viscosity of decane compared to several other liquids ŽAndersland et al., 1995..
114
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
the finer soil grains andror oven drying, if necessary. Since the granular soil contained relatively large voids, prior to freezing close to 100% of the water was held at the solid particle contact points by capillary forces. On the macro scale it would be difficult to show any variation in water content, in a 100-mm-high specimen, hence it was reasonable to assume a uniform distribution of water throughout the specimen. The permeant employed for the tests was decane. Its freezing point is y298C, and its viscosity at y108C is similar to the viscosity of water at nearly 08C, Fig. 3. At 208C the solubility of decane in water is 0.009 mgrL, and its density is approximately 730 kgrm3.
Fig. 4. Test cell prepared with soil specimen prior to freezing ŽAndersland et al., 1995..
For the permeability tests, a cell temperature of y108C was chosen, one that is commonly used for testing the limiting mechanical properties of frozen soil ŽAndersland and Ladanyi, 1994.. A test cell containing a soil specimen prior to freezing is illustrated in Fig. 4.
4. Experimental results The results of permeability tests on thirty-two frozen soil specimens prepared with a variety of frozen or nearly-frozen pore fluids are summarized in Table 1. The permeant selected was decane at y108C for all tests except for specimens 28 and 29, which were conducted on unfrozen specimens close to 08C. For comparative purposes water was used as the permeant on specimens 28 and 29. Different pore liquids were chosen to provide a variety of situations that might be expected at a contaminated field site where a frozen containment barrier may be formed. Soil specimens 1–12, 16, 17, and 21–25 contained only water as the pore liquid prior to freezing; specimens 13, 14, 18, 26, and 27 contained a NaCl solution; specimens 30–32 had essentially no water content; and specimens 15, 19, and 20 utilized water plus trapped decane as the pore liquid. For specimens 16–18, and 20–25, only one permeability test was performed; with specimen 26 the test was repeated twice; and for the remaining specimens the number of tests varied from four to eight. In the table, dn, dSice , and dk are the uncertainties associated respectively with porosity, ice saturation, and intrinsic permeability. The first two are best estimates based on measurand uncertainties, while the third is the standard deviation of the repeated measurements of each test. A general trend shown by data presented in Table 1 is that permeability decreases as ice saturation increases. This trend is reasonable when considering that an increase in ice content will decrease the continuous unsaturated void space and consequently the permeability. The relationship between k and Sice is clearly shown in Fig. 5. Note that the trend is not highly dependent upon the makeup of the pore liquid. The effect of saline pore water on the formation of a frozen matrix and the resulting permeability is
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
demonstrated with specimens 13 and 14. For both specimens, pure ice crystals formed when the matrix temperature reached approximately y58C, which resulted in a higher concentration of NaCl in the remaining brine. As the concentration of NaCl in the brine increases, the freezing point decreases. Specimen 14 was observed to not freeze completely after
115
12 hours at y108C, with liquid appearing on its surface. It appeared that freezing occurred from the sides and bottom of the specimen to the center, concentrating saline pore liquid at the core. Specimen 18, saturated at 100% with 7.9% NaCl brine, detached from the wall, thereby preventing a permeability test to be performed.
Table 1 Intrinsic permeability tests Specimen No.
Pore fluid andror additive
Porosity n Ž%. d n s "1
Saturation Sice " 2 Ž%. dSice s "2
Intrinsic permeability Ž k " dk . = 10 7 Žcm2 .
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
water water water water water water water water water water water water 3.5% NaCl 7.9% NaCl water q trapped decane water water 7.9% NaCl c water q trapped decane water q trapped decane water q 7.5% bentonite d water q 7.5% bentonite d water q 7.5% bentonite f water q 10% bentonite d water q 10% bentonite d 3.35% NaCl q 10% bentonite g 7.9% NaCl q 10% bentonite g none Žunfrozen. h none Žunfrozen. h none none none
27 30 30 38 42 35 34 40 34 34 34 34 37 39 34 y 35 34 33 36 43 43 48 41 41 37 37 29 29 40 38 34
73 61 61 49 49 12 34 8 11 78 11 11 39 44 62 b 100 100 100 100 b 100 b 98 98 92 100 100 70 68 0 0 0 0 0
0.55 " 0.02 1.26 " .004 1.19 " 0.04 1.30 " 0.08 1.47 " 0.06 2.72 " 0.20 2.30 " 0.20 2.51 " 0.10 1.93 " 0.20 0.21 " 0.02 1.99 " 0.10 1.91 " 0.10 1.78 " 0.10 1.55 " 0.10 1.09 " 0.10 y y y 0.02 " 0.01 - 0.004 y - 0.004 y y y 1.11 " 0.06 0.84 " 0.02 3.67 " 0.10 2.94 " 0.05 2.72 " 0.08 2.07 " 0.06 2.05 " 0.40
a b c d e f g h
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Permeant was decane for tests 1 through 27, and tests 30 through 32. Saturation includes both water and decane. Specimen detached from wall, measurement could not be made. Test specimen drained for 12 hr while precooling in chest freezer. Porosity based on expanded volume due to swelling of bentonite. Test specimen drained for 24 hr at room temperature prior to freezing. Permeant was decane. Permeant was water at room temperature.
a
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D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
Fig. 5. Intrinsic permeability versus ice saturation. The symbols designate the various pore liquids used.
Bentonite clay was added to the Hanford soil to observe its effect on water retention and on the permeability for tests 21–27. The magnitude of Bentonite stated in the table is the percent weight fraction of the dry sand sample. Bentonite has the property of absorbing layers of water between silicate layers, and hence it will significantly expand while increasing water retention in the soil ŽGrim, 1962.. Whereas the unfrozen water content in granular soils is negligible, there is a measurable amount of unfrozen water in bentonite ŽAndersland and Ladanyi, 1994.. However, the amount of bentonite in specimens 21 to 27 was so small the unfrozen water content in the specimens was negligible. Insignificant magnitudes of permeability were measured for the specimens with added bentonite using water as the pore fluid Žtests 21–25.. When saline water was used as the pore fluid Žtests 26 and 27., expansion of the bentonite did not occur, resulting in significantly larger permeabilities. Specimens 15, 19, and 20 contained trapped decane and water as the pore liquid. Prior to freezing,
the decane was trapped in the pores by forcing one pore volume of decane into the water-saturated medium, and then back flushing with two pore volumes of water. Approximately 24% of the pore space was occupied by the trapped decane. The partially saturated specimen Žtest 15. yielded a permeability of approximately 1 = 10y7 cm2 , while the completely water-saturated specimens yielded values at least two orders of magnitude lower. These experiments suggest that a frozen soil at y108C containing trapped NAPL such as decane, and with the remaining pore volume saturated with water, a barrier can be formed that may suppress the migration of an insoluble contaminant.
5. Conclusions Gravelly sands of the type described herein may contain liquid contaminants that are insoluble in water and do not lower the freezing point of the pore water. One possible method for isolating sites with
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
these contaminants is to create a frozen soil containment barrier. It is possible that the soil matrix that is to form the barrier may itself be contaminated, or alternately, a permafrost zone has become contaminated. Tests were performed on 32 specimens to study interaction between the level of ice saturation and intrinsic permeability. A NaCl brine solution and a mixture of water and decane were used as pore liquids in addition to pure water. The permeant Ždecane. was passed through the frozen soil at y108C. For all tests, and for the various pore liquids employed, the permeability was seen to decrease nearly linearly with an increase in saturation. In Fig. 5, the permeability ranged from approximately 2.7 = 10y7 cm2 to negligible values when the soil has an ice saturation close to 100%. The addition of bentonite to the partially saturated soil, when exposed to water and allowed to expand prior to freezing, reduced the permeability to negligible values. Soils containing saline pore water will require lower freezing temperatures to account for the freezing point depression. It is recognized that diffusion of contaminants may take place even in frozen soils with very low permeabilities; however, such mass transport would likely take place at a much smaller rate than the convection studied herein. The study of mass transport of contaminants by diffusion in frozen porous media was beyond the scope of the present study.
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Acknowledgements This study was supported by RUST Geotech, Inc., Grand Junction, CO, and by the Division of Engineering Research, Michigan State University. Laboratory assistance was provided by B. Boddy, S. Czewski, D. Fongers, J. Potter, and the late D. Sikarski, and is greatly appreciated.
References Andersland, O.B. and Ladanyi, B., 1994. An Introduction to Frozen Ground Engineering. Chapman and Hall, New York, NY. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1995. Performance and Formation of Frozen Containment Barriers in Dry Soils, Report to RUST Geotech, Inc., Grand Junction CO, Michigan State University, East Lansing, MI, 129 pp. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1996a. Hydraulic conductivity of frozen granular soils. J. Environ. Eng. Am. Soc. Civ. Eng., 122Ž3.: 212–216. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1996b. Frozen soil barriers: formation and ice erosion. J. Contam. Hydrol., 23: 133–147. Davis, S.N. and DeWiest, R.J.M., 1966. Hydrogeology. Wiley, New York, NY. Grim, R.E., 1962. Applied Clay Mineralogy. McGraw-Hill, New York, NY. Vick, J.D., 1994. Description of the Hanford Cryobarrier Demonstration Project. In: Notes from Cryobarrier Technology Workshop, Richland, WA.
Movement of liquid contaminants in partially saturated frozen granular soils David C. Wiggert, Orlando B. Andersland, Simon H. Davies Department of CiÕil and EnÕironmental Engineering, Michigan State UniÕersity, East Lansing, MI 48824, USA Received 1 November 1995; accepted 7 September 1996
Abstract Understanding the migration of nonaqueous phase liquids in frozen subsurface soils is becoming increasingly important in permafrost regions and in temperate zones where frozen subsurface barriers have been proposed to confine contaminants. Tests were performed on 32 specimens of gravelly sands from the Hanford, Washington reservation to determine the relationship between degree of ice saturation and intrinsic permeability. Decane, a representative nonaqueous phase liquid, was employed as the permeant, and was infiltrated through the frozen specimens at y108C. In addition to pure water, a NaCl brine and a mixture of water and decane were utilized as the pore liquids. For all specimens the intrinsic permeability correlated linearly with the ice saturation, varying from approximately 2.7 = 10y7 cm2 at 0% saturation to negligible values at nearly 100% saturation. The different pore liquids did not affect the correlation significantly. Bentonite was added to some of the specimens prior to freezing, reducing the intrinsic permeability to negligible levels. Keywords: Decane; Ice saturation; Intrinsic permeability; Nonaqueous phase liquid; Frozen granular soil; Migration of contaminants
1. Introduction In recent years there has been an increased interest in understanding the fate and movement of contaminants in the frozen subsurface environments. Contamination of nonaqueous phase liquids ŽNAPLs. such as gasoline or oil is an ongoing concern in regions where permafrost exists, and in more temperate regions where there are soils that undergo annual freeze–thaw cycles. Additionally, subsurface barriers consisting of frozen soil have been proposed to temporarily confine contaminated soil during site remediation, and to act as backup containment during removal of hazardous waste from underground storage containers ŽVick, 1994; Andersland et al.,
1995, 1996a,b.. Placement of these barriers is based on established ground freezing technology that is practiced in the construction industry as a means to temporarily stabilize soil and control the movement of ground water ŽAndersland and Ladanyi, 1994.. After a site has been remediated, these barriers can be thawed and the freeze pipes removed, returning the soil to nearly its original state with minimal impact on the environment. When granular soils are frozen to serve as a barrier for containment of liquid contaminants, or a naturally occurring frozen soil is exposed to a NAPL contaminant, a fundamental question that arises is: How impervious is the frozen soil to a liquid contaminant? Water will move into cold soil until the
0165-232Xr97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 5 - 2 3 2 X Ž 9 6 . 0 0 0 2 0 - 1
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D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
heat balance becomes negative. At this point the water is converted to ice, blocking further movement of any water. Liquid contaminants with low freezing points Ž- 08C. will move through partially icesaturated soils as shown by the experimental data summarized in this paper. An additional problem arises when the liquid contaminants are highly miscible with water and introduce a freezing point depression. These liquids cause melting Žerosion. of the ice matrix for temperatures below 08C. This latter problem has been addressed in the paper by Andersland et al. Ž1996b..
2. Background A NAPL is a liquid that has very low solubility in water Žon the order of 0.01 mgrL., and hence does not depress the freezing point of water to cause melting of the ice matrix 1. The problem of forming a continuous, frozen barrier impervious to NAPLs in a granular, low water content soil at approximately y108C has been reported by Andersland et al. Ž1996b.. The site in question was the Hanford, Washington reservation ŽVick, 1994.. Andersland et al. Ž1996b. also dealt with ice erosion within the barrier when liquids with a freezing point depression come in contact with the ice matrix. In another publication, Andersland et al. Ž1996a. reported measurements of hydraulic conductivity in the frozen granular Hanford soil that contained water in the form of ice varying in content from zero to approximately 100% saturation. The permeant employed in the study was decane, a nonaqueous-phase liquid whose viscosity is nearly the same as water at just above 08C. In addition to those reported by Andersland et al. Ž1996a., conductivity tests were performed in Hanford soils ŽAndersland et al., 1995. where bentonite was added to increase the water content of the granular soil prior to freezing and where the pore water contained dissolved NaCl that depressed the freezing point. In order to characterize the movement of a NAPL
1
The effects of solutes on freezing are discussed by Andersland and Ladanyi Ž1994..
in a porous, partially ice saturated frozen soil, we begin with Darcy’s law in the one-dimensional form rg Õsk i Ž 1. m in which Õ s specific discharge, i s hydraulic gradient, r s density of the NAPL permeant, g s gravitational constant, m s viscosity, and k s intrinsic or specific permeability. Implicit in the use of Eq. Ž1. is the assumption that the NAPL is the only liquid present in the porous matrix. By virtue of the low temperature Žy108C. of the granular soils, close to 100% of the pore water contained in the medium prior to freezing will have been transformed to ice. In a given frozen soil, for a liquid NAPL contaminant whose density r 1 , viscosity m 1 , and hydraulic conductivity K 1 are known, the intrinsic permeability can be estimated in the manner m1 K 1 ks Ž 2. r1 g The hydraulic conductivity K 2 of a second contaminant in the same frozen soil matrix can then be estimated by knowing the intrinsic permeability and the contaminant’s viscosity m 2 and density r 2 : r2 g K2 sk Ž 3. m2 The intrinsic permeability is an empirical constant that accounts for the permeable characteristics of the porous frozen soil. For non-frozen granular soils, k has been correlated with the square of the effective grain diameter of the soil ŽDavis and DeWiest, 1966.. In a frozen soil, it is postulated that in addition to the square of the effective grain size, the intrinsic permeability is also dependent upon the ice saturation Sice , which is the fraction of the unfrozen pore volume that is occupied by ice. For uncontaminated granular soils, the unfrozen water content is close to zero at soil temperatures below 08C, so that nearly all of the moisture is in the form of ice. In order to achieve approximately 100% ice saturation Ž Sice s 1. in the soil, it has been shown that 91.7% water saturation in the unfrozen soil is required 2 ŽAndersland et al., 2 Here it is assumed that there is no change in bulk volume on thawing and that a 9% volume change occurs on conversion of liquid to ice.
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
113
1995.. For partially saturated soils capillary forces tend to hold the water in the vicinity of contact points between particles of the granular matrix. When frozen, those water particles are solidified and continuous voids remain, with the result that Sice - 1.
3. Measurement of intrinsic permeability A falling-head permeameter was employed to measure the intrinsic permeability of the frozen soil specimens. In order to maintain the required constant temperature control throughout each test, the specimen and permeant were placed in a coolant bath, Fig. 1. The height of the permeameter was 200 mm, its diameter was 100 mm, and the diameter of the standpipe was 31.8 mm. A typical specimen height was 100 mm. Since the changes in liquid level were very small, no liquid was drained from the permeameter during a test. The permeability was computed from the relation ks
m aL r gA Ž t 2 y t 1 .
ln
h1
ž / h2
Ž 4.
in which a s area of the standpipe, A s the area of the specimen, L s length of specimen, and h1 and h 2 are the initial and final heads in the standpipe that occur over the time interval t 2 y t 1. The apparatus provided accurate measurements of k, except when the medium was nearly 100% ice saturated. Since the focus of the study was to evaluate values of permeability that would result in significant leakage through
Fig. 1. Schematic of hydraulic conductivity apparatus ŽAndersland et al., 1995..
Fig. 2. Particle size distribution curves for the five samples of Hanford soil ŽAndersland et al., 1995..
a frozen barrier, no refinements were attempted to accurately determine k for ice saturations close to 100%. Soil samples from the Hanford site ŽFig. 2. were analyzed and classified as gravelly sand, SW and SP in the Unified Soil Classification system. For the permeability tests, samples were reconstituted to give particle size distributions approximating field samples. Prior to freezing, moisture was added to the samples to a desired degree of saturation using upward seepage of water into the dry soil in the test cell followed by controlled drainage. Intermediate degrees of saturation in samples below the field saturation level were attained by omitting some of
Fig. 3. Viscosity of decane compared to several other liquids ŽAndersland et al., 1995..
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D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
the finer soil grains andror oven drying, if necessary. Since the granular soil contained relatively large voids, prior to freezing close to 100% of the water was held at the solid particle contact points by capillary forces. On the macro scale it would be difficult to show any variation in water content, in a 100-mm-high specimen, hence it was reasonable to assume a uniform distribution of water throughout the specimen. The permeant employed for the tests was decane. Its freezing point is y298C, and its viscosity at y108C is similar to the viscosity of water at nearly 08C, Fig. 3. At 208C the solubility of decane in water is 0.009 mgrL, and its density is approximately 730 kgrm3.
Fig. 4. Test cell prepared with soil specimen prior to freezing ŽAndersland et al., 1995..
For the permeability tests, a cell temperature of y108C was chosen, one that is commonly used for testing the limiting mechanical properties of frozen soil ŽAndersland and Ladanyi, 1994.. A test cell containing a soil specimen prior to freezing is illustrated in Fig. 4.
4. Experimental results The results of permeability tests on thirty-two frozen soil specimens prepared with a variety of frozen or nearly-frozen pore fluids are summarized in Table 1. The permeant selected was decane at y108C for all tests except for specimens 28 and 29, which were conducted on unfrozen specimens close to 08C. For comparative purposes water was used as the permeant on specimens 28 and 29. Different pore liquids were chosen to provide a variety of situations that might be expected at a contaminated field site where a frozen containment barrier may be formed. Soil specimens 1–12, 16, 17, and 21–25 contained only water as the pore liquid prior to freezing; specimens 13, 14, 18, 26, and 27 contained a NaCl solution; specimens 30–32 had essentially no water content; and specimens 15, 19, and 20 utilized water plus trapped decane as the pore liquid. For specimens 16–18, and 20–25, only one permeability test was performed; with specimen 26 the test was repeated twice; and for the remaining specimens the number of tests varied from four to eight. In the table, dn, dSice , and dk are the uncertainties associated respectively with porosity, ice saturation, and intrinsic permeability. The first two are best estimates based on measurand uncertainties, while the third is the standard deviation of the repeated measurements of each test. A general trend shown by data presented in Table 1 is that permeability decreases as ice saturation increases. This trend is reasonable when considering that an increase in ice content will decrease the continuous unsaturated void space and consequently the permeability. The relationship between k and Sice is clearly shown in Fig. 5. Note that the trend is not highly dependent upon the makeup of the pore liquid. The effect of saline pore water on the formation of a frozen matrix and the resulting permeability is
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
demonstrated with specimens 13 and 14. For both specimens, pure ice crystals formed when the matrix temperature reached approximately y58C, which resulted in a higher concentration of NaCl in the remaining brine. As the concentration of NaCl in the brine increases, the freezing point decreases. Specimen 14 was observed to not freeze completely after
115
12 hours at y108C, with liquid appearing on its surface. It appeared that freezing occurred from the sides and bottom of the specimen to the center, concentrating saline pore liquid at the core. Specimen 18, saturated at 100% with 7.9% NaCl brine, detached from the wall, thereby preventing a permeability test to be performed.
Table 1 Intrinsic permeability tests Specimen No.
Pore fluid andror additive
Porosity n Ž%. d n s "1
Saturation Sice " 2 Ž%. dSice s "2
Intrinsic permeability Ž k " dk . = 10 7 Žcm2 .
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
water water water water water water water water water water water water 3.5% NaCl 7.9% NaCl water q trapped decane water water 7.9% NaCl c water q trapped decane water q trapped decane water q 7.5% bentonite d water q 7.5% bentonite d water q 7.5% bentonite f water q 10% bentonite d water q 10% bentonite d 3.35% NaCl q 10% bentonite g 7.9% NaCl q 10% bentonite g none Žunfrozen. h none Žunfrozen. h none none none
27 30 30 38 42 35 34 40 34 34 34 34 37 39 34 y 35 34 33 36 43 43 48 41 41 37 37 29 29 40 38 34
73 61 61 49 49 12 34 8 11 78 11 11 39 44 62 b 100 100 100 100 b 100 b 98 98 92 100 100 70 68 0 0 0 0 0
0.55 " 0.02 1.26 " .004 1.19 " 0.04 1.30 " 0.08 1.47 " 0.06 2.72 " 0.20 2.30 " 0.20 2.51 " 0.10 1.93 " 0.20 0.21 " 0.02 1.99 " 0.10 1.91 " 0.10 1.78 " 0.10 1.55 " 0.10 1.09 " 0.10 y y y 0.02 " 0.01 - 0.004 y - 0.004 y y y 1.11 " 0.06 0.84 " 0.02 3.67 " 0.10 2.94 " 0.05 2.72 " 0.08 2.07 " 0.06 2.05 " 0.40
a b c d e f g h
e e e e e
Permeant was decane for tests 1 through 27, and tests 30 through 32. Saturation includes both water and decane. Specimen detached from wall, measurement could not be made. Test specimen drained for 12 hr while precooling in chest freezer. Porosity based on expanded volume due to swelling of bentonite. Test specimen drained for 24 hr at room temperature prior to freezing. Permeant was decane. Permeant was water at room temperature.
a
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D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
Fig. 5. Intrinsic permeability versus ice saturation. The symbols designate the various pore liquids used.
Bentonite clay was added to the Hanford soil to observe its effect on water retention and on the permeability for tests 21–27. The magnitude of Bentonite stated in the table is the percent weight fraction of the dry sand sample. Bentonite has the property of absorbing layers of water between silicate layers, and hence it will significantly expand while increasing water retention in the soil ŽGrim, 1962.. Whereas the unfrozen water content in granular soils is negligible, there is a measurable amount of unfrozen water in bentonite ŽAndersland and Ladanyi, 1994.. However, the amount of bentonite in specimens 21 to 27 was so small the unfrozen water content in the specimens was negligible. Insignificant magnitudes of permeability were measured for the specimens with added bentonite using water as the pore fluid Žtests 21–25.. When saline water was used as the pore fluid Žtests 26 and 27., expansion of the bentonite did not occur, resulting in significantly larger permeabilities. Specimens 15, 19, and 20 contained trapped decane and water as the pore liquid. Prior to freezing,
the decane was trapped in the pores by forcing one pore volume of decane into the water-saturated medium, and then back flushing with two pore volumes of water. Approximately 24% of the pore space was occupied by the trapped decane. The partially saturated specimen Žtest 15. yielded a permeability of approximately 1 = 10y7 cm2 , while the completely water-saturated specimens yielded values at least two orders of magnitude lower. These experiments suggest that a frozen soil at y108C containing trapped NAPL such as decane, and with the remaining pore volume saturated with water, a barrier can be formed that may suppress the migration of an insoluble contaminant.
5. Conclusions Gravelly sands of the type described herein may contain liquid contaminants that are insoluble in water and do not lower the freezing point of the pore water. One possible method for isolating sites with
D.C. Wiggert et al.r Cold Regions Science and Technology 25 (1997) 111–117
these contaminants is to create a frozen soil containment barrier. It is possible that the soil matrix that is to form the barrier may itself be contaminated, or alternately, a permafrost zone has become contaminated. Tests were performed on 32 specimens to study interaction between the level of ice saturation and intrinsic permeability. A NaCl brine solution and a mixture of water and decane were used as pore liquids in addition to pure water. The permeant Ždecane. was passed through the frozen soil at y108C. For all tests, and for the various pore liquids employed, the permeability was seen to decrease nearly linearly with an increase in saturation. In Fig. 5, the permeability ranged from approximately 2.7 = 10y7 cm2 to negligible values when the soil has an ice saturation close to 100%. The addition of bentonite to the partially saturated soil, when exposed to water and allowed to expand prior to freezing, reduced the permeability to negligible values. Soils containing saline pore water will require lower freezing temperatures to account for the freezing point depression. It is recognized that diffusion of contaminants may take place even in frozen soils with very low permeabilities; however, such mass transport would likely take place at a much smaller rate than the convection studied herein. The study of mass transport of contaminants by diffusion in frozen porous media was beyond the scope of the present study.
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Acknowledgements This study was supported by RUST Geotech, Inc., Grand Junction, CO, and by the Division of Engineering Research, Michigan State University. Laboratory assistance was provided by B. Boddy, S. Czewski, D. Fongers, J. Potter, and the late D. Sikarski, and is greatly appreciated.
References Andersland, O.B. and Ladanyi, B., 1994. An Introduction to Frozen Ground Engineering. Chapman and Hall, New York, NY. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1995. Performance and Formation of Frozen Containment Barriers in Dry Soils, Report to RUST Geotech, Inc., Grand Junction CO, Michigan State University, East Lansing, MI, 129 pp. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1996a. Hydraulic conductivity of frozen granular soils. J. Environ. Eng. Am. Soc. Civ. Eng., 122Ž3.: 212–216. Andersland, O.B., Davies, S.H. and Wiggert, D.C., 1996b. Frozen soil barriers: formation and ice erosion. J. Contam. Hydrol., 23: 133–147. Davis, S.N. and DeWiest, R.J.M., 1966. Hydrogeology. Wiley, New York, NY. Grim, R.E., 1962. Applied Clay Mineralogy. McGraw-Hill, New York, NY. Vick, J.D., 1994. Description of the Hanford Cryobarrier Demonstration Project. In: Notes from Cryobarrier Technology Workshop, Richland, WA.