potential of soil and groundwater contamination due to mine subsidence under a landfill

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Potential of Soil and Groundwater Contamination Dueto Mine Subsidence Under a LandfillSanjeev Kumar aa Department of Civil Engineering, Southern Illinois University-Carbondale, Carbondale, IL62901Published online: 24 Jun 2010.

To cite this article: Sanjeev Kumar (1999) Potential of Soil and Groundwater Contamination Due to Mine Subsidence Under aLandfill, Journal of Soil Contamination, 8:4, 441-453

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441

Journal of Soil Contamination, 8(4):441–453 (1999)

Potential of Soil and GroundwaterContamination Due to

Mine Subsidence Under a Landfill

Sanjeev Kumar

Department of Civil Engineering,Southern Illinois University–Carbondale,Carbondale, IL 62901

KEY WORDS: contamination, hydraulic conductivity, landfill, liner, subsidence.

Results of a study to evaluate the effects ofmine subsidence on the integrity of a clayliner and potential for soil and groundwatercontamination below a previously minedlandfill are presented. The results showthat for the existing site conditions, surfacesubsidence features are expected to besimilar to subsidence troughs, and the sitehas minimal potential of being contami-nated due to deep-sited subsidence in themine. To further reduce potential for soiland groundwater contamination at the site,it is recommended that the minimum thick-ness of the compacted clay liner be 4 ftinstead of the 2 to 3 ft generally used.Discussion is presented indicating that fur-ther study is required to develop an ad-equate design procedure to determine theeffects of nonuniform settlement of founda-tion soils or refuse on the hydraulic con-ductivity of landfill clay barriers.

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INTRODUCTION

UNICIPAL waste landfills are currently the standard method for ultimatelydisposing of solid wastes. One of the key elements to the successful

operation of a landfill is the adequate performance of hydraulic barriers surround-ing the waste material. One such barrier is a Compacted Clay Liner (CCL), whichis the bottommost component of a landfill and is placed on foundation soil. Thecompacted clay liner in a landfill provides a relatively impervious layer of soil toprevent migration of leachate to an aquifer below the landfill. In accordance withregulatory standards, hydraulic conductivity of the CCL should be 10–7 cm/s orless. Hydraulic conductivity greater than 10–7 cm/s would indicate less time isrequired for leachate to cross the barriers. An increase in the hydraulic conductivityof CCL can jeopardize the intended performance of landfill and is likely to resultin soil and groundwater contamination. Remediation of the CCL and soil below alandfill is very difficult and cost prohibitive.

With the rapid increase in demand of landfills and limited space available,landfills are constructed on sites that require additional design and environmentalimpact studies. One type of site that falls under this category is that which haspreviously been mined for coal, clay, or other resource. If a landfill is existing orproposed on or close to an underground mine, propagation of subsidence in deep-sited mine can result in a nonuniform settlement of soils below the landfill.Nonuniform settlement of the foundation soil results in tensile, flexural, or bend-ing, and shear stresses in the CCL (Jessberger, 1995). Soils, unlike other construc-tion materials, are weak in tension and bending. As a result, tensile and bendingstresses in CCL are likely to develop cracks in the CCL. Cracking of the liner maygenerate preferential flow paths through the liner (i.e., increase in its hydraulicconductivity) and decrease the leachate breakthrough times (Cheng et al., 1994).An increase in hydraulic conductivity of the liner can lead to soil and groundwatercontamination. Figure 1 illustrates a possible deformed shape of CCL and potentiallocations of development of cracks due to non-uniform settlement of foundationsoils.

This article presents results of a study to determine the effects of deep-sitedmine subsidence on the integrity of the clay liner, and potential of soil andgroundwater contamination below a landfill proposed at a site that had previouslybeen mined to recover coal. The discussion and results presented show that for thetype of surface depressions expected due to mine subsidence, the proposed landfillis likely to perform adequately. The subject site is located in the Central U.S.Details about the exact site location are not presented at the request of the siteowner. The project budget did not allow performing advanced laboratory or fieldtests on compacted clay liners subjected to bending stresses. Therefore, informa-tion available in literature and conventional laboratory tests were used to developconclusions presented herein.

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FIGURE 1

Possible deformed shape of clay barriers and potential locations of cracking.

SUBSURFACE CONDITIONS

Several borings were drilled at the site to evaluate subsurface conditions, includingthickness of the soil overburden and the mined coal seam. Based on the drillingdata, bedrock existed at depths between 60 and 120 ft (elevations between El 757and 765 ft) from the ground surface. The soil overburden consisted of glacial drift,till, and loess deposits. The glacial soils were comprised mainly of sandy to siltyclay with some thin layers of silt and sand. The soil overburden is underlain byPennsylvanian bedrock, which consisted of fissile shale, clay shale, coal, andunder-clay with thinner beds of limestone and sandstone.

The top of the coal seam, which has been previously mined, was encounteredat depths between 115 and 165 ft (elevations between El 708 and 718 ft) from theground surface. The thickness of the extracted out coal seam ranged between 3 and5.5 ft. Based on available mine maps, the mining occurred in the late 1800s andearly 1900s, and the mining was most likely by the room and pillar method. Atypical crosssection showing subsurface conditions at the site is presented inFigure 2.

SURFACE SUBSIDENCE FEATURES

Subsidence or collapse of room and pillar mining generally results in two types ofsurface features, sinkholes, and subsidence troughs. Sinkholes typically develop inareas where mining rooms are large and the soil overburden is shallow. The

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development of a sinkhole is initiated by collapse of the mine roof, which createsa void above the mine into which overlying weaker rock can fall. Propagation ofcollapse of the weaker rock and soil into the void can result in a sinkhole at thesurface. Subsidence troughs are circular or elongated depression features generallycovering larger areas than sinkholes. Subsidence troughs generally occur due toinstability of mine pillars or punching of pillars into the floor or ceiling of the mine.

Sinkholes are common surface subsidence feature associated with room andpillar mining where the thickness of overburden soils is less than 50 ft (Whittakerand Reddish, 1989). Presence of a unit of good rock can stop the propagation ofthe collapse chimney to the surface (Whittaker and Reddish, 1989). Mine subsid-ence at a site where thicker soil overburden and a layer of good quality rock arepresent generally results in surface subsidence features similar to troughs. Thick-ness of the overburden soils at the site was observed to be more than 50 ft. Asshown in Figure 2, layers of good limestone and moderately hard shale existed

FIGURE 2

Typical cross section showing subsurface profile at the site.

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above the mine. Therefore, it was concluded that occurrence of sinkholes at the sitedue to mine subsidence is unlikely and the surface subsidence features, if any, willbe similar to subsidence troughs. (Additional discussion on this topic is presentedin section titled Subsidence Prediction.)

Bruhn et al. (1968) evaluated the time interval between the mining and subsid-ence observed at the surface above a coal mine. Their results show that 90% of thesurface subsidence features developed within 55 years of mining. Mining at thesubject site occurred over 110 years ago, yet no surface subsidence features havebeen observed at the site. Based on the rock samples extracted from the borings,the shale encountered above the mine (where mine was detected) was observed tobe intact. Therefore, it was concluded that the mines below the subject site arerelatively stable.

SUBSIDENCE PREDICTION

When mineral is extracted from vein, the overburden and surface can collapse orsubside, forming depressions at the ground surface. The extent of surface subsid-ence depends on many factors, including the extent of mining and the thickness ofmineral vein. Some of the components of subsidence that can affect surfacestructures include: vertical displacement, horizontal displacement, slope of thesubsidence trough, strain in the horizontal direction, and vertical curvature.

Of these subsidence components, strain in the horizontal direction is of thehighest concern with respect to landfill development in this type of environment.Although both vertical and horizontal displacements could occur, landfill compo-nents are generally capable of tolerating large uniform settlements (vertical dis-placement) without compromising liner integrity. Tensile strains in the horizontaldirection resulting from non-uniform settlement of foundation soils are of concernbecause of the very limited tensile strength of soils. These strains can jeopardizethe integrity of landfill liners, thus enhancing potential for contamination of soiland groundwater below a landfill.

Vertical settlements and horizontal strains at the liner elevation that are expecteddue to mine subsidence at the site are presented in Table 1. The subsidence profileis shown on Figure 3. The surface area affected by mining is generally larger thanthe excavated vein area. The angle of inclination between the vertical at the edgeof the mine and the point of zero vertical displacement at the edge of the troughat the surface is termed as the ‘limit angle’ or ‘angle of draw’. This angle is afunction of the vein dip and the geology of the area. The angle of draw for theCentral U.S. range between 12 and 30 degrees. To study the sensitivity of defor-mations in selecting a value for angle of draw, analyses were performed for threedifferent values. From the results presented in Table 1, it was concluded that forthe subsurface conditions observed at the site, selection of the angle of draw hasminimal effect on the vertical settlements and horizontal strains.

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TABLE 1Results of Subsidence Evaluation

Horizontal strainsVertical

Case analyzed settlement (ft) Tension (%) Compression (%)

Angle of draw 23.4 degrees 1.6 1.3 2.1Angle of draw 20.0 degrees 1.5 1.0 1.3Angle of draw 30.0 degrees 1.5 1.0 1.2

FIGURE 3

Subsidence profile expected at the site.

BEHAVIOR OF CCL UNDER BENDING AND TENSILE STRESSES

The behavior of compacted cohesive soils subjected to bending has received littleattention. Literature addressing the effects of subsidence on the integrity of com-pacted clay landfill barriers (caps and liners), and tensile strength of compactedclays is presented below.

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Behavior of Landfill Clay Barriers Under Bending Stresses

Chang et al. (1994), Lozano and Aughenbaugh (1995), and Bredariol et al. (1995)studied the effects of bending stresses on the behavior of compacted clay speci-mens prepared in the laboratory. All these studies assumed complete loss ofsupport below a portion of the landfill cover or liner. These results may beapplicable to the cases where surface subsidence features are expected to be similarto sinkholes. As complete loss of support is unlikely at the subject site, results ofthese studies are not directly applicable to the expected conditions at the site.However, general information about behavior of compacted clays to bendingstresses is used to evaluate the integrity of clay liner at the subject site.

Chang et al. (1994) studied axisymmetric flexural distortion of low plasticityfine-grained materials under gravity loads. Models of fine-grained landfill capresponses to differential subsidence were tested in a special device to determine thechange in hydraulic conductivity of soils under differing levels of flexure. Theirresults are presented in Figure 4. They concluded that flexure can cause a increasein hydraulic conductivity, but not complete loss of resistance to permeation.Lozano and Aughenbaugh (1995) studied the factors that influence the flexuralbehavior of fine-grained soils. Fine-grained soils were compacted into small beamsand tested for flexibility. Their results indicated that the flexibility of the materialis directly related to (primarily) the plasticity index and amount of particle size less

FIGURE 4

Distorted permeability results. Tacony soils (After Cheng et al., 1994.)

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than two microns. Bredariol et al. (1995) studied the relationships between differ-ential settlement of landfill caps and resulting distress that could change hydraulicconductivity. Plots relating CCL distortion to cracking intensity were developedand related to local permeability changes (Figure 5). Their results suggest thatCCL’s having higher tensile strength also have lower permeability for the samelevel of distortion.

FIGURE 5

Distortion vs. composite permeability. (After Bredariol et al., 1995.)

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Jessberger and Stone (1991) conducted centrifuge modeling of clay barrierssubjected to differential deformations. They modeled modes of deformation thatwere relevant to those that might occur as a result of the propagation of deep-seatedsubsidence toward the surface leading to deformation of the landfill base liner.They defined the degree of liner deformation in terms of degree of rotation (θ) thatoccurred at the base of the liner. Jessberger and Stone (1991) reported that in thecase of pure kaolin liners, onset of tension cracks was observed at liner deforma-tion, θ, of 3 to 3.5 degrees when no overburden was present. On the other hand,no cracking was observed with a liner deformation of 11 degrees when overburdenwas present. Similar observations were made for the liner consisting of sand/silicaflour/bentonite mixture. Slight surface cracking was observed at a liner deforma-tion, θ, of 7.5 degrees without overburden and no significant cracking was ob-served even after the liner deformation of 16 degrees when overburden waspresent. Their study concluded that the presence of an overburden suppressed theformation of tension cracks, and no significant reduction in the liner efficiency wasobserved.

Jessberger (1995) summarized the state of practice of using compacted clayliners to contain waste and reported on the work of Scherbeck (1992) presented inETC8 (1993). Scherbeck (1992) developed an assessment approach to check ifdeformation will result in a stable, open crack in the liner.

Based on the work of Scherbeck (1992), the induced tensile strain is comparedto the failure strain as follows:

n ultν

τ

εε

= (1)

where εult = the ultimate (failure) tensile strain; εt = the induced tensile strain in theoutermost fiber; ηv = strain ratio.

If the strain ratio is less than 2, stress conditions should be examined to checkif cracking occurs. Scherbeck (1992) showed that for the clay liners, with effectivefriction angle of 20 degrees, the liners are not likely to show any cracking ifoverburden pressure is more than three times the cohesion of the liner material.Scherbeck (1992) recommended that the depth of crack (ZR) in a liner be estimatedusing Eq. 2. The overburden pressure required to avoid cracking can be computedby setting depth of crack equal to zero.

Z CR = ′ + ′

22 45

2 0γφ σtan – (2)

where ZR= depth of crack; γ = unit-weight of the liner material; C′= effectivecohesion of the liner material; φ′= effective friction angel of the liner material; σ0 =overburden pressure.

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Behavior of Compacted Clays Under Tensile Stresses

The studies conducted by Krishnayya et al. (1974) and Narain and Rawat (1970)are frequently used in discussions regarding tensile behavior of compacted cohe-sive soils. Narain and Rawat (1970) used the Brazilian tensile test to determinetensile strength of compacted soils and its variation with respect to molding watercontent. They found that the tensile strength of compacted soils generally increaseswith an increase in molding water content up to a limit below the optimummoisture content, at which point it decreases. The ratio of unconfined compressivestrength and tensile strength of compacted soils at the optimum moisture contentvaried from 6 to 12. This ratio for soils of low to medium plasticity decreased withincreasing liquid limit and plasticity indices. Based on the results of unconfinedcompression tests, they reported that compressive strains at failure varied from 6.1to 18.0% depending on water content.

Krishnayya et al. (1974) studied the results of indirect (Brazilian) tensile andunconfined compression tests on a compacted, well-graded, low-plasticity soil(Mica till with Liquid Limit and plasticity index of approximately 18 and 3.5,respectively). They examined the effect of water content, compactive effort, rateof loading, and the addition of bentonite to the soil on the tensile characteristics.Their results indicated that the tensile strength of the soil decreased with anincrease in water content. These results are not consistent with the results of Narainand Rawat (1970). The inconsistency is likely to be due to the different types ofsoils used in the two investigations.

Krishnayya et al. (1974) also observed that tensile strain at failure increasedwith water content. Failure tensile strains varied from less than 0.25% at moisturecontents of 5% to more than 3% at moisture contents of 11% (optimum moisturecontent of the soil was reported to be 9.2%). The ratio of compressive failure strainfrom unconfined compression tests to tensile failure strain from indirect tensilestrength test (referred to as strain ratio) varied from approximately 4 at moisturecontent of 4% below the optimum to 2 at moisture content of 2% below theoptimum. The strain ratio was observed to be relatively constant up to moisturecontents of approximately 2% above optimum. It should be recognized that theresults presented above are for pure tension failure in soils. The tensile strains ina CCL at the site are expected to be due to bending of the liner rather than puretension. Based on laboratory tests on compacted clay specimens, Bredariol et al.(1995) reported that the ratio of unconfined compression and flexural strengths wasbetween 5:1 and 10:1.

LABORATORY TESTS ON CCL MATERIAL

Laboratory tests performed on the soil specimen to be used to construct thecompacted clay liner show that the liquid limit and plasticity indices of the

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specimen are 42 and 25, respectively, and more than 65% of the soil particles werefiner than the No. 200 U.S. sieve. The standard Proctor maximum dry density ofapproximately 109 lbs per cubic foot (pcf) and optimum moisture content ofapproximately 18% was estimated from the laboratory tests. The results of anunconfined compression test on a specimen of the soil compacted to 107.2 pcf(98.5% of standard Proctor maximum dry density) at 16.7% water content (1% dryof optimum moisture content) yielded an unconfined compressive strength ofapproximately 2800 lb per square foot (psf). Failure strain was observed to beapproximately 12%. The initial tangent modulus from the unconfined compressiontest was approximately 790 lb per square inch (psi). The hydraulic conductivity teston a sample of this material yielded a hydraulic conductivity of approximately2 × 10–9 cm/s. Based on the laboratory tests performed on the CCL material, it wasconcluded that the soil could adequately be used to construct the CCL.

POTENTIAL FOR SOIL AND GROUNDWATER CONTAMINATION

The soil and groundwater below a landfill have a significant potential of beingcontaminated if the compacted clay liner of the landfill develops cracks. Thecompressive failure strain from an unconfined compression test performed on asoil specimen to be used to construct the CCL was observed to be 12%. Based onthe results presented by Krishnayya et al. (1974) and Bredariol et al. (1995), failuretensile strain for a specimen compacted close to the optimum moisture content wasestimated to be between 1.5 to 4%. This range appears to be consistent with thefailure tensile strains of 2 to 3% observed by Bredariol et al. (1995) for soils havingplasticity indices of approximately 20%, compacted at or near optimum moisturecontent. At the subject site, the maximum potential horizontal strain (tension) wasestimated to be 1.6% (Table 1). According to the criteria suggested by Scherbeck(1992), the ratio of failure strain and expected strain due to subsidence wascomputed to be between 1 and 2.5. Therefore, potential of occurrence of cracks waschecked as discussed below.

From the settlement profile shown in Figure 3, the liner deformation, θ, asdefined by Jessberger and Stone (1991), was calculated to be approximately1 degree. As shown in results presented by Jessberger and Stone (1991), nocracking of the liner was observed at liner deformations of 3 and 3.5 degrees fortwo different types of soils when no overburden pressure was present. Their resultsalso showed that overburden pressure reduced the potential of developing tensioncracks in the liner, and no cracks were observed at liner deformations of 11 and16% for the same soils when overburden stresses equivalent to approximately 8 ftof soil were present. From Equation 2, the height of refuse required to minimizethe potential of liner cracking at the site was estimated to be approximately18 ft.

Based on the information presented above, it was concluded that for the orderof deformations and strains expected in the liner, the site has minimal potential for

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development of cracks in the liner and thus minimal potential for soil and ground-water contamination. The potential is further reduced after approximately 18 ft ofrefuse is placed on the liner before initiation of subsidence.

CONCLUSIONS AND RECCOMMENDATIONS

Evaluation of potential of soil and groundwater contamination due to a proposedlandfill at a previously mined site is presented. For the existing subsurface condi-tions, surface subsidence features will be similar to subsidence troughs. Based onthe information presented in literature and the expected deformed shape of thecompacted clay liner, it was concluded that the site has minimal potential of beingcontaminated due to deep-sited subsidence in the mine. The potential will furtherreduce once 18 ft of refuse is placed on the liner. The laboratory tests performedon the liner material showed that the material to be used for constructing the CCLwas adequate. To reduce the potential for soil and groundwater contamination, itwas recommended that the minimum thickness of the CCL be 4 ft instead of 2 to3 ft generally used. It was also recommended that the CCL be compacted atmoisture contents above the optimum moisture content of the soil to provideadditional flexibility to the liner. Based on the review of available literature, it isthe author’s opinion that further experimental study is required to develop aprocedure to better estimate the effect of nonuniform settlement of foundation soilsand refuse on the integrity of the compacted clay barriers.

REFERENCES

Bredariol, A. W., Martin, J. P., Cheng, S., and Tull, C. F. 1995. Flexural cracking of compacted clayin landfill covers. Geoenvironment 2000, Characterization, Containment, Remediation, and Per-formance in Environmental Geotechnics, Proceedings of a Specialty Conference, GeotechnicalEngineering Division and the Environmental Engineering Division of the American Society ofCivil Engineers, Geotechnical Special Publication No. 46, February 24–26. pp. 914–931.

Bruhn, R. W., Magnuson, M. O., and Gray, R. E. 1968. Subsidence over the mined out Pittsburghcoal. ASCE Annual Convention, Pittsburgh, PA. pp. 26–55.

Cheng, S. C., Larralde, J. L., and Martin, J. P. 1994. Hydraulic conductivity of compacted clayey soilsunder distortion or elongation conditions. Hydraulic Conductivity and Waste Contaminant Trans-port in Soil, ASTM STP 1142, David E. Daniel and Stephen J. Trautwein, Eds., American Societyfor Testing and Materials, Philadelphia, 1994. pp. 266–283.

ETC8. 1993. Geotechnics of landfill design and remedial works, (2nd ed.). Ernst & Sohn, Berlin.Jessberger, H. L. 1995. Waste containment with compacted clay liners. Geoenvironment 2000,

Characterization, Containment, Remediation, and Performance in Environmental Geotechnics,Proceedings of a Specialty Conference, Geotechnical Engineering Division and the Environmen-tal Engineering Division of the American Society of Civil Engineers, Geotechnical SpecialPublication No. 46, February 24–26. pp. 463–483.

Jessberger, H. L. and Stone, K. J. L. 1991. Subsidence effects on clay barriers. Geotechnique, 41(2),pp. 185–195.

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Krishnayya, A. V. G., Eisenstein, Z, and Morgenstern, N. R. 1974. Behavior of compacted soil intension. J. Geot. Engrg. Division, ASCE, Vol. 100, No. GT9, September pp. 1051–1061.

Lozano, N. and Aughenbaugh, N. B. 1995. Flexibility of fine-grained soils. Geoenvironment 2000,Characterization, Containment, Remediation, and Performance in Environmental Geotechnics,Proceedings of a Specialty Conference, Geotechnical Engineering Division and the Environmen-tal Engineering Division of the American Society of Civil Engineers, Geotechnical SpecialPublication No. 46, February 24–26. pp. 844–858.

Narain, J. and Prakash, C. R. 1970. Tensile strength of compacted soils. J. Soil Mech. Foundat. Div.,ASCE, Vol. 96, No. SM6, November pp. 2185–2190.

Scherbeck, R. 1992. Geotechnisches verhalten mineralischer deponieabdichtungen bei ungleichformigerverform ungseinwirkung. Schriftenreihe des Instituts fur Grundbau, Ruhr-Universitat Bochum.

Whittaker, B. N. and Reddish, D. J. 1989. Subsidence: Occurrence, Prediction, and Control.Developments in geotechnical engineering, 56, Elsevier.

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potential of soil and groundwater contamination due to mine subsidence under a landfill

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