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Earthen Materials in Surface Barriers Craig H. Benson, University of Wisconsin-Madison; and Milind V. Khire, GeoSyntec Consultants, Boca Raton, Florida INTRODUCTION The rising cost of remediating contaminated sites has made {ong-term isolation a viable alternative to the removal and treatment of contaminated soils and wastes. Isolation generally consists of installing subsurface barriers (vertical cutoff walls and/or horizontal liners placed in situ) and caps. In some cases, where lateral and vertical migration of contaminants can be limited by minimizing percolation, isolation may consist of only installing a cap. The focus of this paper is on caps constructed with earthen materials. Caps may also be constructed with geosynthetic materials (geomembranes, geonets, etch. However, for large remediation projects, the costs of constructing a cap with geosynthetics may be enormous relative to the cost of a cap constructed with earthen materials. Furthermore, in some regions such as the semi-arid and arid parts of the United States, caps constructed with earthen materials may perform equally as well as caps constructed with geosynthetics. An effective cap design requires that the cap limit percolation into the underlying soils. For earthen caps, design consists of selecting an arrangement of earthen materials that can be used to divert water away from the cap or store the water until it can be removed later by evaporation and/or transpiration. The materials and their arrangement must be selected in such a way that the cap will minimize percolation and continue to function throughout a long design life. WATER BALANCE OF EARTHEN CAPS Earthen caps generally exploit the unique characteristics of unsaturated flow, the storage capacity of fine-grained soils, and the natural capacity of plants to remove water entering the cap during wet periods. These factors are linked by the water balance, which accounts for movement of water into, within, and out of a cap. In algebraic form, the water balance can be described by the following equation: Pr= P -Of- S - E - T -Lf where P is precipitation in the form of snow, rain, or ice; Of iS overland flow; S is soil water storage; E is evaporation; T is transpiration by vegetation; Lf is lateral drainage; and Pr is percolation from the base (Figure 1). In some cases, evaporation and transpiration are combined as evapotranspiratin (Et) Design of an earthen cap to minimize percolation entails manipulating soil water storage capacity, lateral drainage, transpiration, and evaporation such that an acceptable value for percolation occurs in a worst-case design year (e.g., year when rainfall or snowfall is abnormally high and temperature and solar radiation are abnormally low). In particular, the cap is designed such that it has adequate capacity to store or divert water that infiltrates during late fall and winter and sufficient vegetation and evaporative potential to remove the stored water during spring, D-79

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D-80 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT summer, and early fall. Three types of earthen caps that are designed on this pnuciple are (1) traditional resistive banders, (2) capillary baIIiers, and (3) monolayer soil barriers. In some cases, these baniers are used In combination. T E Overland Flow Precipitation E = Evaporation ~ ~T = Transpiration T T \ ~ E T I, Lateral Drainage - .. . _ ~ . .. .. ~ _ ~- - _ __ ~ . . ~ .. _ __ ~ _ _ ~_ . ~ ~ ~: . . 7~ ~ v ~- . . ._ _ _ ... , _ it_ i i - ~.-~ ~_ , ~ at, ~ t , ~ ~ ~ . ~ w ~L ~,-,T,- ,, i, .............. in_- _ Waste or . . . . . . . _ . : : :.:': ': :~: 1 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~. ~ . , . ~ . ............ ~.,~- _ Ila l la =~ .;, ,.~,., ,~,,, ~, ~, , ,~ FIGURE 1 Water Balance of a Cap. RESISTIVE BARRIlERS Principle Traditional resistive barriers are the most commonly used earthen caps today, even though their cost, performance, and durability can be poorer than other types of earthen caps. A traditional resistive cap consists of a battier layer of compacted fine-gra~ned soil and a vegetated surface layer (Figure 21. In some instances, a drainage layer is added between the compacted barrier layer and the vegetated surface layer. Also, a "rooting zone" or "weathering layer" may be added beneath the vegetated layer to protect the barrier layer from root intrusion, desiccation, and frost action.

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APPENDIX~PAPERS PRESENTED \\1 Vegetated Surface Layer -ad -2a'-2-'a ~_ Rooting Zone .,~;5 ~ a Fuji jet ~ __) ~ r F ~ FIGURE 2 Cap with Traditional Resistive Barrier. D-81 Lateral Drain Barrier Layer Adequate performance of a traditional resistive battler is predicated on the barrier layer having low saturated hydraulic conductivity. If the battier layer has low saturated hydraulic conductivity, percolation from the base of the cap is controlled by the saturated hydraulic conductivity of the barrier layer and the gradient imposed by water pending on the barrier layer. Furthermore, when the saturated hydraulic conductivity is low, water is more likely to be diverted as overland flow or lateral flow in a drainage layer, and thus pending on top of the balTier layer is minimized. Consequently, the maximum rate of percolation is equal to or slightly larger than the saturated hydraulic conductivity of the battier layer because the hydraulic gradient is close to unity. Potential Problems Resistive baITiers are prone to failure because fine-gra~ned barrier layers are easily damaged by weathering and distortion. As a result, their saturated hydraulic conductivity increases significantly and the performance of the cap degrades. For example, Benson et al. (1995) have shown that barrier layers exposed to frost become severely cracked, and their saturated hydraulic conductivity can increase dramatically. In some cases, the saturated hydraulic conductivity can increase as much as four orders of magnitude (Figure 3~. Desiccation can also cause extensive cracking of the battier layer, which results In preferential pathways for flow. A large desiccation crack is shown in Figure 4. This crack formed in a resistive-barrier landed! cap within eight years of construction. Similar cracks have been reported by Montgomery and Parsons (19901. Desiccation also causes networks of small cracks in the matrix between the large cracks. These small cracks result In the saturated hydraulic conductivity of the matrix increasing by one to two orders of magnitude (Benson, 1994~. Finally, distortion of the balkier layer due to differential settlements also results in cracking, which will degrade the performance of a cap (lessberger and Stone, 19911.

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D-82 BARRIER TECH~OLOGIESFORENVIRONMENTALMANAGEMENT Hydraulic Conductivity (cm/see) 10-8 10-7 10-6 10-5 10-4 O I ' I I I '1111 1 ' ' " "11 ' ' ' " "'1 ' ' ' " "'1 ' ' ' " "'I 20 - ~ 40 - s it_ Q 8 60 80 100 - - 1 - - - - - ~ Park~iew o Block-Before Block-After 10-3 FIGURE 3 Hydraulic Conductivity Before and After Freeze-Thaw (adapted from Benson et al. 1995). FIGURE 4 Desiccation Crack in Cap Designed as a Resistive Barrier

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APPENDIX DIAPERS PRESENTED D-83 Case History Khire, Benson, and Bosscher (1994) show how desiccation cracks can impact the performance of a resistive battier. A test section was constructed in East Wenatchee, Washington (annual rainfall = 23 cm) to simulate a resistive barrier. The resistive battier consists of 600 mm of compacted silty clay and 150 mm of vegetated silty topsoil. The test section is instrumented extensively to obtain detailed information about the water balance (Benson et al., 1994~. Cumulative percolation from the test section is shown In Figure 5. The data through Summer 1994 show that a small quantity of percolation is generated each year from late fall through early spring. This percolation is the result of winter rainfalls and snow melts, which saturate the barrier. In 1995, however, the quantity of percolation increased dramatically in the spring, even though the amount of precipitation In Winter 1995 was essentially the same as In Winter 1994 and much less than in Winter 1993. Examination of the battier layer showed that vertical desiccation cracks had formed dIlIing drying the previous summer. These cracks apparently became conduits for preferential flow during winter. a_ a_ 4 o 3 _ A_ Ct o C' a) ILL a) - ~ - ~ 1 1 1 1 1 1 1 1 1 1 1 1 2 Percolation Without ~ Cracks \ O 1 1 ) 1 1 1 1 1 1 1 1 1 Fa W 92 93 ~ J Percolation With | Cracks \ ~ l Sp Su 93 93 Fa W Sp 93 94 94 Su Fa W Sp 94 94 95 95 FIGURE 5 Cumulative Percolation Tom Resistive Barrier with Preferential Flow.

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D-84 BARRIER TECHNOLOGIESFORENVIRONMENTALMANAGEMENT CAPILLARY BARRIERS Principle In their simplest form, capillary barriers employ a fine-gra~ned layer over a coarse-grained layer (Figure 6a). Flow across the interface of these layers is restricted under nonsaturated conditions because the unsaturated hydraulic conductivity of the coarse-grained layer is much Tower than the unsaturated hydraulic conductivity of the fine-grained layer (Figure fib) (Hillel, 19801. Also, suctions in the fine-grained layer normally exceed suctions in the coarse-grained layer, which results in an upward hydraulic gradient (Khire, ~ 9951. Thus, the fine-gra~ned soil can store or divert water that infiltrates the cap, and yet flow into the coarse-grained layer is restricted. More elaborate designs employing multiple layers having contrasting grain size are also possible. These caps employ the capillary barrier principle to divert infiltrating water via lateral flow while ensuring that deep percolation does not occur (Nyhan et al., 1993; Yeh et al., 19944. pi pi P 1~ 1 Fine-Grained Soil coarse-Grained Soil (a) - i, In o ,Coarse-Grained Soil ,Fine-Grained Soil Matric Suction (lo) (b) FIGURE 6 Schematic of Capillary Barrier (a) and Hydraulic Conductivity Functions (b).

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APPENDIXI}PAPERS PRESENTED D-85 Potential Problems Several problems can be encountered with capillary baITiers. A particularly problematic condition is when the storage capacity of the fine-gra~ned layer is overwheirned in regions where significant snow cover accumulates. When the storage capacity is exceeded, rapid breakthrough across the interface between the fine and coarse-grained layers occurs. Percolation during this condition can be significant (Hakonson et al., 1994; Khire, 19951. Defects such as cracks and macropores In the fine-grained layer can also affect the performance of a capillary barrier. [f defects exist, water can immediately enter the underlying coarse-grained layer and percolation is generated. Finally, the performance of capillary barriers is sensitive to heterogeneities In the soil layers. Variations In texture or hydraulic properties can result in premature breakthrough across the interface or finger flow in the coarse-grained layer (Khire, 1995~. Consequently, percolation can be greater than expected. Case Histories Several studies have shown that capillary barriers can be effective in limiting percolation, even In more humid regions. For example, in a study of five test sections located at a landfill near Hamburg, Germany (average annual precipitation is 800 mm), Melchoir et al. (1994) report that capillary barriers, when used in conjunction with clay resistive barriers, can reduce percolation to virtually zero. Nyhan et al. (1993) also report that capillary barriers can be superior to traditional resistive barriers. In their study, test sections were constructed at Los Alamos National Laboratory. The test sections consisted of the following cap designs: top soil underlain by crushed luff, an EPA (Environmental Protection Agency)- recommended resistive battier cap (topsoil layer, sand drainage layer, and a 600-mm-thick soil layer having saturated hydraulic conductivity less than 1 x loo m/s), a loam capillary barrier (topsoil layer, fine sand wicking layer, grave! layer, and coarse sand layers, and a ciay-Ioam capillary baITier (cIay loam layer, fine sand wicking layer, grave} layer, and coarse sand layer). The test sections were monitored for 15 months. Analysis of the data showed that all of the test sections produced percolation, and that the EPA-recommended resistive battier and the loam capillary balTier produced the largest quantity of percolation (~.5 and 7.4 percent of precipitation, respectively). The clay-loam capillary balTier produced the least amount of percolation (0.7 percent of precipitation). Hakonson et al. (1994) field tested four designs being considered for a cap at Hill Air Force Base in Utah (precipitation is 510 mm/yr). The first cap consisted of sandy loam overlying a gravel drainage layer, which effectively was a simple, two-layer capillary battier. The second cap was a resistive battier and consisted of sandy loam topsoil overlying a sand drainage layer and a layer of clay loam amended with bentonite. The remaining two caps were capillary barriers having sandy loam topsoil over washed gravel. These two capillary battier test sections were seeded with different vegetation. Hakonson et al. (1994) report that percolation from the resistive battier was the least (0.006 percent of precipitation), whereas percolation from the simple, two-layer capillary barrier was greatest (24 percent of precipitation). Both capillary barriers produced percolation that was approximately 15 percent of the precipitation. Percolation Tom the capillary barriers was large because snow accumulated on the test sections, which overwhelmed their capacity for storage and . diversion.

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D-86 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT Khire et al. (1994) describe the water balance of a capillary battier test section constructed adjacent to the previously discussed resistive baITier test section In East Wenatchee, Washington. The capillary barrier has a sand layer overlain by a vegetated silty surface layer. The capillary barrier has been effective, limiting percolation to 0.6 percent of precipitation. This is substantially less percolation than has been produced by the resistive battier test section (4.4 percent of precipitation). Khire et al. (! 994) also report that the capillary battier can be ineffective when snow accumulates on the cap and the subsequent melt overwhelms the storage and diversion capacity of the fine-gra~ned layer. MONOLAYER BARRIlERS Principle Monolayer barriers are caps consisting of a thick layer of fine-grained soil. Monolayer baITiers exploit two characteristics of f~e-grained soil: (1) their large soil water storage capacity when unsaturated, and (2) their low saturated hydraulic conductivity relative to coarse-grained soils. Low saturated hydraulic conductivity limits infiltration through the surface during rainfall or snow melt. High soil water storage capacity provides the capability to store water that does infiltrate until it can be removed by evapotransp~ration. The bander must be sufficiently thick to ensure that changes In water content do not occur near its base, i.e., all changes In soil water storage occur In the upper portion of the baIlier (Figure 71. Otherwise, percolation will occur. Monolayer barriers are constructed from silty sands, silts, and clayey silts, and are cost-effective when large quantities of fine-grained soil requiring little processing are available on site. ~'w_ Fine-Grained Soil ~ ~ ~ ,~ ~ ~ , ~ ~ ~ . ~ ~ ~ Jr ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ \\ // // ~~ Increase (due to Precipitation) or J Decrease (due to Evapotranspiration) of Soil-Water Content in Upper Layers Little or No Water Movement ~ . ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ A ~ at- A A A ~ ~ ~ ~ ~ ~ A ~ ~ ~ ~ do- A ~ r. ~ ~ in' A. A '. A ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ f is' ~ do- ~ A- ~ ~ do' ~ ~ do' ~ A ~ A do' do- ~ lAI^^tm ~ ~ ~ ~ ~ f ~ ~ ~ Z ' ~ ~ ~ ~ ~ ~ ~ I. v Y "O t~ ,. a,. ,,,. , do' ~ ~ A' ~ ~ f ~ ~ f do- do- f ~ ~ ~ do' A do' do- ~ ~ '. ~ ~ ~ ~ A- A f ~ ~ f ~ ~ ~ ~ '- f ~ f r ~ ~ '- '' f f ~ ~ ~ f ~ ~ ~ ~ ~ ~ f f f , FIGURE 7 Principle of Monolayer Cap.

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APPENDIX~PAPERS PRESENTED D-87 Potential Problems The potential problems associated with a monolayer barrier are similar to those for a capillary battier. Exceeding the storage capacity during snow melt, however, is probably not significant if a monolayer battier is thick. Monolayer barriers may have problems shedding moisture via evapotranspiration if deep percolation occurs after intense infiltration events. Case Histories Morr~son-Knudson (1993, 1994) describe the performance of a monolayer barrier proposed as a cap over contaminated soil at Rocky Mountain Arsenal in Commerce City, Colorado. The battier, which is constructed over trenches containing hazardous constituents, consists of a vegetated layer of topsoil underlain by a compacted layer of fine-grained soil (Figure 81. Analysis of monitoring data has shown that most changes in water content occur In the upper 0.6 m of the cap. However, gradual changes in water content have been observed in the deepest probes (1.2 m). Unfortunately, no percolation lys~meters were installed beneath the cap. Thus, the true performance of the cap cannot be evaluated. \\\ r;~;~;~;~;~;~;~;~;~;~;~ ~ ~ -I ~~ '.. ~ ~ . \\\ r ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ r "~. Fir in, ~ r I ODSOII ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ (Uniform Thickness) '~.~.~.~.~.~.~,~.~.~.~. . 1220 mm ~ r ~ '~, Venetated Layer ~. _ _ 150 mrn _ L ~ (Variable Thickness, ~ ~ = ~'7 Waster FIGURE ~Monolayer Cap Considered for Rocky Mountain Arsenal. Biotic Barrier: / Poured Concrete; Fractured In-Place Following Cure

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D-88 BARRIER TECHNOLOGIES FOR E~IRONMENTALMANAGEMENT GeoLogic Associates (1993) describe a thick soil balTier used as a cap for a landfill! in Southern California. The battier is 2 m thick and is constructed from a clayey silt. To date, water movement has been limited to the upper 0.6 m of soil. The data indicate that the water content of the upper soil layers increases rapidly after rainfall and then gradually decreases as water is removed by evapotranspiration. Unfortunately, percolation is not being monitored, and thus, the actual performance of the cap cannot be determined. SUMMARY Earthen caps can be effective barriers to percolation into underlying waste or contaminated soil provided they are designed for the climate In which they will be placed. Currently, three designs for earthen caps are being used: resistive baITiers, capillary barriers, and monolayer barriers. Resistive barriers rely on the low saturated hydraulic conductivity of a barrier layer to limit percolation. In contrast, capillary barriers and monolayer barriers generally rely on the soil water storage capacity of fine-grained soils to store infiltrating water until it can be removed by evapotranspiration. Capillary barriers also exploit the contrasting unsaturated hydraulic properties of fine and coarse-grained soils. Each of the three designs is prone to its own set of problems. Resistive barriers are especially likely to experience degrading performance because it is difficult to protect a barrier layer having low saturated hydraulic conductivity from deteriorating as a result of weathering or settlement. Capillary barriers are not prone to the same problems as resistive barriers but can fail if the storage and diversion capacity of the fine-gra~ned layer is undersized, particularly In regions where significant snow cover accumulates. In contrast, the storage capacity of a monolayer barrier probably will not be overwhelmed, but a monolayer barrier may have difficulty recovering Tom intense infiltration events. Nevertheless, the problems associated with each of these designs can be avoided by carefi~lly considering the conditions to which the cap will be exposed during its design life. ACKNOWLEDGMENT Financial support for a portion of the work described in this paper has been provided by the National Science Foundation through grant number CMS-9157116. This paper has not been reviewed by NSF and no endorsement should be assumed. REFERENCES Benson, C. 1994. Research developments In clay liner construction. Pp. 81-93 In Proceedings of the 32nd Annual International Solid Waste Exposition, Solid Waste Assoc. of N. America. Benson, C., P. Bosscher, D. Lane, and R. Pliska. 1994. Monitoring System for Hydrologic Evaluation of Landfill Final Covers, Geotechnical Testing J. 17~21:138-149. Benson, C., E. Chamberlain, A. Erickson, and X. Wang, 1995. Assessing Frost Damage in Compacted Clay Liners, Geotechnical Testing I. ~ 8~31:324-333, September.

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APPENDIX PAPERS PRESENTED D-89 GeoLogic Associates. 1993. Evaluation of Unsaturated Fluid Flow, Coastal Sage Scrub Habitat Area, Coyote Canyon Final Cover System, Orange County, California, report prepared by GeoLogic Associates for Orange County Integrated Waste Management Department. Hakonson, T., K. Bostick, G. Trulillo, K. Manies, R. Warren, L. Lane, I. Kent, and W. Wilson. 1994. Hydrologic Evaluation of Four Landfill} Cover Designs at Hill Air Force Base Utah Deptartment of Energy Mixed Waste Landfill Integrated Demonstration, Sandia National Laboratory, LAUR-93-4469. Hillel, D. 1980. Fundamental of Soil Physics. New York: Academic Press. lessberger, H. and K. Stone, K. 1991. Subsidence Effects on Clay Barriers. Geotechnique, 4~2~:185-194. Khire, M. ~995. Field Hydrology and Water Balance Modeling of Earthen Final Covers for Waste Containment. Ph.D. dissertation. University of Wiscons~n-Madison. Khire, M., C. Benson, and P. Bosscher. 1994. Final Cover Hydrologic Evaluation-Phase Ill, Environmental Geotechnics Report 94-4, University of Wisconsin-Madison. Melchoir, S., K. Berger, B. Vielhaber, and G. Miehlich. 1994. Multilayered landfill covers: Field data on the water balance and liner performance. Pp. 411-425 In Proceedings of the 33rd Hanford Symposium on Health and Environment, Pasco, Wash., Nov. 7-11. Montgomery, R. and L. Parsons. 1990. The Omega Hills cover test plot study: Fourth year data summary. In Proceedings of the 22nd Mid-Atiantic Industrial Waste Conference, Drexe] University, Philadelphia, Pa. Mornson-Knudsen Corporation. 1993. Implementation of SoiWegetative Covers for Final Remediation of the Rocky Mountain Arsenal, white paper report prepared by Mornson- Knudsen Corporation, Denver, Colo., for Shell Oil Company. Morr~son-Knudsen Corporation. 1994. Annual Monitoring Report for Other Contamination Sources, Interim Response Action, Shell Section 36 Trenches Containment System, prepared by Mornson-Knudsen Corporation, Denver, Colo., for Shell Oil Company, Denver, Colo. Nyhan, I., G. Langhorst, C. Martin, I. Martinez, and T. Schofield. 1993. Hydrologic studies of multilayered landfill! closure of waste landfi~Is at Los Alamos. ~ Proceedings of the DOE Environmental Remediation Conference "ER 93", Oct. 1993, Augusta, Ga. Stormont, J. 1995. The Effect of Constant Anisotropy on Capillary Barrier Performance. Water Resources Research. 31~3~:783-786. Yeh, T., A. Guzman, R. Srivastava, and P. Gagnard. 1994. Numerical Simulation of the Wicking Effect in Liner Systems. Ground Water. 32~1~:2-1 1. 7 ~