INFILTRATION TESTS ON FRACTURED COMPACTED CLAY

By M. C. McBrayer; Student Member, ASCE, M. Mauldon/ Associate Member, ASCE,E. C. Drumm/ Member, ASCE, and G. V. Wilson4

ABSTRACT: Desiccation and freeze-thaw of compacted clay barriers may result in cracks that serve as pref­erential flow paths. A series of infiltration tests on compacted kaolin samples was conducted to explore theimportance of preferential flow paths during infiltration, and their effect on the infiltration rate. Clod size at thetime of compaction was found to have a strong influence on both the rate and depth of infiltration. We suggestthat flow and infiltration through fractured clay may be described in terms of two stages: an initial dynamicstage in which the infiltration rate is initially high but decreases rapidly due to the clay swelling and closingfractures, and a steady-state stage usually characterized by ksa" during which the infiltration rate is relativelyconstant. Our study has shown that cracks do not fully heal upon hydration and readily reopen during subsequentdehydration. Infiltration rates during the dynamic stage of infiltration, while cracks are closing, are orders ofmagnitude higher than the steady-state rate used to estimate ksa" for barrier evaluation.

TABLE 2. Summary of Infiltration Tests

OBJECTIVES

and consequently the infiltration rate decreases (Freeze andCherry 1979). Soil swelling and closing of pore spaces canalso contribute to this decrease and may ultimately close pref­erential flow paths, forcing water to flow through the matrix.During the dynamic stages of infiltration, however, preferentialflow paths can dominate flow despite making up only a smallportion of the total soil volume (Beven and Germann 1982).

Recognizing that steady-state Darcy experiments fail to cap­ture the influence of preferential flow before cracks close, wecarried out a series of transient infiltration experiments(McBrayer 1995) with the following objectives: (1) Investigatethe transient hydraulic response of clay fractures during infil­tration tests; and (2) investigate the extent of preferential flowpaths in compacted clay.

0.0-0.1%0.3-3.5%92.0-97.0%3075451731.6%1.37 g/cm'

Measured value(3)

Soil properties(2)

>45 mm>5 mm<2mmPlastic LimitLiquid LimitPlasticity IndexShrinkage LimitOptimum water contentMaximum dry density

TABLE 1. Kaolinite Properties

Index test(1 )

"Phifer (1993).

Particle size"

Atterberg Limits·

Standard Proctor"

Initial Initial dry Fracture DegreeSpecimen water density mecha- of infiltra-

designation content (g/cm3) nism Clod size tion

(1 ) (2) (3) (4) (5) (6)

AD-l 35.9% 1.36 uncracked mixed sizes lowAD-2 34.9% 1.34 uncracked pass No.4 sieve mediumAD-3 34.9% 1.35 uncracked retained on No. 4 medium

sieveD-4 34.9% 1.28 desiccation pass No.4 sieve highD-5 34.9% 1.36 desiccation retained on No. 4 high

sieveFT-3 36.9% 1.34 freeze/thaw mixed sizes mediumFT-4 36.9% 1.33 freeze/thaw mixed sizes mediumFT-5 36.9% 1.34 freeze/thaw mixed sizes mediumFT-6 36.9% 1.35 freeze/thaw mixed sizes highFT-7 36.9% 1.35 freeze/thaw mixed sizes medium

INTRODUCTION

Compacted clay barriers are used in solid waste landfills toisolate waste from infiltrating water and to minimize the flowof leachate from the waste. The effectiveness of these barriersdepends on their ability to limit the movement of water andcontaminants. Saturated hydraulic conductivity is the param­eter usually used to judge the quality and integrity of com­pacted clay liners. Hydraulic conductivity of porous media istypically a maximum at saturation so that the saturated hy­draulic conductivity is generally believed to be a conservativeparameter for barrier evaluation. Cracks resulting from pro­cesses such as desiccation or freeze-thaw, however, may leadto preferential flow paths (Kleppe and Olson 1985; Bowdersand McClelland 1994). In fractured clays, the formation andsubsequent opening and closing of fractures are dynamic pro­cesses with cracks that tend to open during unsaturated con­ditions and to close following hydration. During the initialstage of infiltration when the cracks are open, infiltration andflow may be governed by flow through cracks. Water contentchanges as small as 3% have been shown to produce a four­fold increase in hydraulic conductivity, although the conduc­tivity of the clay matrix was found to decrease (Phifer et al.1995). Thus saturated hydraulic conductivity of the intact claymatrix after crack closure underestimates the role of prefer­ential flow through fractures.

Infiltration is simply the entry of water into the soil surface.For initially unsaturated soil, the gradient in matric head (neg­ative pressure head) controls infiltration during early stages butgradually dissipates with time. The decreasing influence of thematric head gradient as the wetting front deepens results in adecrease in the infiltration rate. As more water infiltrates andthe water content increases, the hydraulic gradient becomesincreasingly dominated by gravity and approaches unity. Thedecrease in hydraulic gradient is not fully offset by the in­crease in hydraulic conductivity towards the saturated value,

'Res. Asst., Dept. of Civ. and Envir. Engrg., Univ. of Tennessee, Knox­ville, TN 37996-2010.

2Asst. Prof., Dept. of Civ. and Envir. Engrg., Univ. of Tennessee,Knoxville, TN.

'Prof., Dept. of Civ. and Envir. Engrg., Univ. of Tennessee, Knoxville,TN.

4Asst. Prof., Dept. of Plant and Soil Sci., Univ. of Tennessee, Knox­ville, TN 37901-1071.

Note. Discussion open until October I, 1997. To extend the closingdate one month, a written request must be filed with the ASCE Managerof Journals. The manuscript for this technical note was submitted forreview and possible publication on June 2, 1995. This technical note ispart of the Journal of Geotechnical and Geoenvironmental Engineer­ing, Vol. 123, No.5, May, 1997. ©ASCE, ISSN 1090-0241/97/0005­0469-0473/$4.00 + $.50 per page. Technical Note No. 10828.

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(a)

I I

II I

....... ./

......... V-- -(b)

FIG. 1. Infiltration Tests: (a) Mariotte Device; (b) Specimen Cross Sections (Slice No.1 in Foreground)

(a)

(d)

(b)

(e)

(c)

(f)

FIG. 2. Surface Cracking of Desiccated Compacted Sample: (a) Sample as Compacted; (b) First Desiccation Cycle; (c) First Dehy­dration Cycle; (d) Second Desiccation Cycle; (e) Second Rehydration Cycle; (f) Third Desiccation Cycle

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FIG. 3. Four Compacted Specimens: (a) Infiltration Rate Ver­sus Time for Uncracked (AD-1), Cracked from Freezing (FT-5and FT-6), and Cracked from Desiccation (0-4); (b) Dye Penetra­tion of Four Compacted Specimens: AD-1; FT-6; FT-5; 0-4

MATERIALS

The kaolinite used in all the tests was acquired from the J.M. Huber Corporation under the trade name Barden AG-!.This material has been used for landfill barriers (Phifer 1991).Pertinent material properties are given in Table 1. Tap water(Amoozegar and Wilson 1996) was used in preparation of theclay specimens and for the permeant in the infiltration tests.A solution of 0.1 % Rhodamine B red dye was used in theinfiltration tests to stain active pathways.

Specimens were subjected to three cycles of desiccation orfreeze/thaw to induce the formation of preferential flow paths.Desiccated specimens were compacted in 15.2-cm diameterPVC molds with a thickness of 5.1 em. Freeze/thaw specimenswere compacted in 10.2 cm diameter polyvinyl chloride (PVC)molds with a thickness of 5.1 cm. Larger diameter specimenswere used for desiccation to reduce boundary effects on crack­ing. The edges of the specimens were confined with a flexible

Crack Formation

METHODS

Preparation of Compacted Clay Specimens

The soil was prepared by slowly adding water to the drykaolin in a mechanical mixer while preventing the formationof large clods. Sufficient water was added to obtain a mixtureat about 3% above standard Proctor optimum water content.The soil was then double bagged and placed in a moistureroom to hydrate for a minimum of 24 h. Specimens were thencompacted using standard Proctor energy. Some specimenswere prepared with clods passing the No. 4 sieve, some wereprepared with clods retained on the No. 4 sieve, and otherswere prepared with mixed clod sizes. A summary of the spec­imens' properties is provided in Table 2.

(b)

I--AD-I __FT-5 -+-FT-6 __ 0-41(a)

Time (5)

Fr·!

AD-I Fr-6

&. .s•

I.E+OO

f I.FrO1.......!l 1.E-02~

j I.Fr03

1.&04

I.FrOS

10 100 1000 ooסס1 ooסס10

... *'"

I I ISlice 1 ~ •,..... I .

.,'... » • ;aq

I I I-~Slice 2 . • 'J!'~.

• $ , •

I-~Slice 3"~' '., !

• •

1-Slice 4 ~d' ''{:,:" .' .ac w.

I I~ I I~Slice 5

, &Q f

I

I~ I 1-Slice 6

(a) (b) (c)

FIG. 4. Typical Specimens Exhibiting: (a) Low Penetration; (b) Medium Penetration; (c) High Penetration

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Infiltration Test Procedure

membrane at all times to maintain their integrity, and the topwas exposed to the atmosphere. Prior to cracking, all speci­mens were soaked in a water bath at room temperature for 12h except specimen Ff-7. Each 24-h desiccation cycle consistedof 12 h under a heat lamp at approximately 66°e followed by12 h saturating in a water bath at room temperature (approx­imately 21°C). Each 24-h freeze/thaw cycle consisted of 12 hof three-dimensional freezing at about -7°e followed by 12h of thawing at room temperature.

The 15.2 cm desiccated specimens were cut to a 1O.2-cmdiameter with a band saw before infiltration. A Marioue device[Fig. lea)] was set up to maintain a constant head of 5 cm ofred dye solution on the sample. An electronic scale was usedto measure the permeant lost from the reservoir, which wasassumed to be the permeant infiltrated into the specimens. Therate of infiltration was recorded with time until steady-stateflow conditions were obtained, up to a maximum of 1,500 min.After infiltration, specimens were air-dried, and cut into twelveapproximately equal slices to expose the stains left by the dye[Fig. l(b»). The slices were cut with a band saw with 14 teethper inch, and polished on a vertical belt sander with 120-gritsandpaper (Norton Durite). Images of the slices were recordedwith a video camera to evaluate the stained pathways and toqualitatively evaluate the degree of infiltration.

~._--.

~~

~

~.~

RESULTS AND ANALYSIS

Crack Formation and Apparent Healing

Images of the surface of specimen D-4 during cycles ofdesiccation and rehydration reveal that cracking and shrinkingof the sample occurred during the first desiccation cycle [Fig.2(b)]. Based on visual inspection, these cracks appeared toheal during rehydration as the specimen swelled [Fig. 2(c)];as the sample absorbed more water and increased in volume,the interior cracks and perimeter voids closed. These samecracks, however, reopened on the successive desiccation cycles[Fig. 2(d)], often to greater lengths and wider apertures asshown in Fig. 2. New cracks were also created on subsequentcycles [Fig. 2(f)]. This sequence of cracks opening and closingwas repeated for each desiccation and rehydration cycle.

Fracture Effects on Infiltration

The infiltration rate for specimen D-4 was observed to de­crease roughly two orders of magnitude over the first hourfrom its initial value of 0.85 mrn/s [Fig. 3(a)]. Similar de­creases in infiltration rate were observed for specimens sub­jected to freeze/thaw cycles (Ff-5 and Ff-6). In spite of theapparent healing of cracks shown in Fig. 2, the steady-statevalue was about two orders of magnitude greater than for anuncracked air-dried sample (AD-I). The uncracked specimen(AD-I) showed the lowest infiltration rate as well as the shal­lowest penetration of the dye [Fig. 3(b)], as compared to thethree cracked samples. These cracked specimens exhibitedlarge variability in infiltration rate, presumably due to differ­ences in the fracture network. Sample D-4 contained a fracturethat completely penetrated the sample, and as a result, themeasured infiltration rate was the highest and the dye penetra­tion was the most pronounced. The degree of dye penetration(Table 2) was evaluated qualitatively by examination of theslices of the clay specimens. Typical examples of specimensexhibiting low, medium, and high penetration are shown inFig. 4. The depth of dye penetration was strongly correlatedwith infiltration rate. These results suggest that flow throughfractured clay should be characterized in terms of two stages:an initial transient stage during which fractures are closing,and a final steady-state stage during which the fractures appearto be healed.

oo1000ססoo1סס10

Time (5)

100

1.&05

10

I.E+OO

f 1.&01'-'~ 1.&02~1'1

! 1.E-03fe 1.E-04.s

I--AD-l __ AD-2 -+-AD-3!

(a)

AD-I

• C 4' .e

AD-J

Clod Size Effects on Infiltration

Although it is well known that clod size can affect the hy­draulic conductivity for steady-state flow (Benson and Daniel1990), less is known about the influence of clod size duringinfiltration. Transient infiltration experiments were carried outon three compacted uncracked specimens to investigate therole of clod size during infiltration (Fig. 5). Specimen AD-3was compacted with a large clod size, specimen AD-2 wascompacted with a small clod size and specimen AD-l wascompacted with a mixture of these two sizes. Of the threeuncracked specimens, AD-l (with a mixed clod size) displayedthe lowest infiltration rate and the shallowest dye penetration(Fig. 5), suggesting that preferential pathways in specimenswith mixed clod sizes are less significant than in those with auniform clod size. This is consistent with the well known ef­fects of grain size distribution on void ratio and permeabilityin porous media (Sowers 1979).

(b)

FIG. 5. Three Alr-drled Compacted Specimens: (a) InfiltrationRate Versus Time on Mixed Clod Size (AD-1), Small Clod Size <No.4 Sieve (AD-2), and Large Clod Size> No.4 Sieve (AD-3); (b)Dye Penetration of Three Air-dried Specimens: AD-1; AD-2;AD-3

CONCLUSIONS

Preferential flow may be insignificant if clay barriers arekept fully saturated. However, clay barriers may undergo de­hydration and subsequent crack formation, especially whencovered with a flexible membrane liner (FML) or drainage

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layer. This technical note addresses the integrity of clay bar­riers that undergo hydration-dehydration cycles. The mainconclusions drawn from this study are:

1. Desiccation and freeze/thaw cycles induce cracks that ap­pear to heal upon rehydration, but reopen on subsequentcycles of desiccation or freeze/thaw.

2. Infiltration in a cracked clay has an initial transient stageduring which the preferential flow paths playa dominantrole.

3. Although cracks appear to be healed, they still contributeto flow.

4. The degree of cracking, as measured qualitatively fromthe dye stains, is strongly correlated with the rate of in­filtration in the transient stage.

5. Mixed clod sizes lead to lower infiltration rates and lessdye penetration.

The saturated hydraulic conductivity is usually used tojudge the quality and integrity of compacted clay barriers. Dueto clay swelling and closing of fractures, flow calculationsbased on hydraulic conductivity determined for steady-stateconditions may underestimate flow during the dynamic stageof infiltration. By the time traditional steady-state measure­ments of hydraulic conductivity are obtained, the fractures arenot as hydraulically active due to closure. We suggest thatthese pathways are more hydraulically active during the dy­namic stage of infiltration during which fractures are closingand that this stage must be considered in the evaluation of claybarriers.

ACKNOWLEDGMENTS

This research was carried out under the DOE-UTK Joint EducationProgram in Environmental Restoration and Waste Management, G.D.Reed (P.L). This work was supported, in whole or in part, by a DOE

cooperative agreement DE-FC05-920R22056. This support does not con­stitute an endorsement by DOE of the views expressed herein. The writersare grateful for this support.

APPENDIX. REFERENCESAmoozegar, A., and Wilson, G. V. (1996). "Methods for measuring hy­

draulic conductivity and drainable porosity." American Society ofAgronomy (ASA) special monograph on agricultural drainage, W.Skaggs, ed., ASA, Inc., Madison, Wis.

Benson, C. H., and Daniel, D. E. (1990). "Influence of clods on hydraulicconductivity of compacted clay." J. Geotech. Engrg., ASCE, 116(8),1231-1248.

Beven, K., and Germann, P. (1982). "Macropores and water flow insoils." Water Resour. Res. 18(5) 1311-1325.

Bowders, J. J. Jr., and McClelland, S. (1994). "The effects offreeze/thawcycles on the permeability of three compacted soils." Hydraulic con­ductivity and waste containment transpon in soil, ASTM STP 1142, D.E. Daniel and S. J. Trautwein, eds., American Society for Testing andMaterials, Philadelphia.

Freeze, R. A., and Cherry, J. A. (1979). Groundwater. Prentice-Hall, Inc.,Englewood Cliffs, N.J.

Kleppe, J. H., and Olson, R. E. (1985). "Desiccation cracking of soilbarriers." Hydraulic barriers in soil and rock, ASTM STP 874, A. I.Johnson, R. K. Frobel, N. J. Cavalli, and C. B. Pettersson, eds., Amer­ican Society for Testing and Materials, Philadelphia, 263-275.

McBrayer, M. C. (1995). "Clay barriers: laboratory study of hydraulicdefects and preferential flow paths," MS thesis, University of Tennes­see, Knoxville.

Phifer, M. A. (1991). "Closure of a mixed waste landfill-lessonslearned." Proc.• Symp. on Waste Magmt., Department of Energy, Wash­ington D.C., 517-524.

Phifer, M. A. (1993). "Clay barrier post compaction water content effectsand particle orientation," MS thesis, University of Tennessee, Knox­ville.

Phifer, M. A., Boles, D., Drumm, E., and Wilson, G. V. (1995). "Com­parative response of two barrier soils to post compaction water contentvariations." Proc., ASCE Spec. Con! Geoenvironment 2000: Charac­terization. Containment. Remediation. and Performance in Envir. Geo­technics, ASCE, New York, 519-607.

Sowers, G. F. (1979)./ntroductory soil mechanics andfoundations: geo­technical engineering. MacMillan Publishing Co., Inc., New York.

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