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Mineral Liners
7.2 MINERAL LINERS Marco Favaretti and Raffaello Cossu
INTRODUCTION Mineral liners in landfills are a fundamental component of the physical barrier systems (Chapter 7.1). They should be purpose-designed and constructed to withstand the effect of clayey soils, compaction procedures, degree of densification, water content, and hydraulic conductivity. This chapter describes the property of mineral liners with regard to landfill applications and presents a series of fundamental experimental methods to be performed in the laboratory and the field to ensure the correct design of mineral liners and to verify efficiency following emplacement. Performance of clay liner in attenuating dispersed leachate emissions is finally discussed.
GEOTECHNICAL PROPERTIES OF THE MINERAL LINERS Mineral landfill liners are in general subject to regulations imposing a geometric limit, establishing a minimum value for thickness, and a geotechnical limit setting the maximum value for hydraulic conductivity k (see Chapter 7.1). The first limit is represented by a simple measure of length and therefore in situ control does not constitute a problem. The difficulty in determining coefficient k is associated with the precision, uncertainty and repeatability of its experimental measure, and contrasts with the prescriptive character of the regulation, fixing a specific kmax that cannot be overcome. Even a slight deviation from the kmax implies, from a purely legal point of view, nonacceptability of the used soil or the barrier. It is easy to grasp the paradox of a standard that defines with absolute precision kmax, without illustrating the experimental methods of determination, with possible and consequential technical and legal issues. The mineral liner should be constructed using mediumelow plasticity clayey soils (Plasticity Index, PI ¼ 10% O 40%), with a percentage of passing through ASTM 200 sieve (opening 0.075 mm) greater than 20%. The minimum value of the PI is due to the fact that below 10% it would be highly unlikely to achieve the required low hydraulic conductivity, whereas the maximum value of PI is related to the need to have a workable and compactable soil, with limited shrinkage and swelling properties. Any clods of clayey soil must not exceed 25 O 50 mm. Finally, a greater than 10% presence of gravel granulometric fractions should be avoided.
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The following topics will be treated in sequence: • compaction procedures in the laboratory: interactions between water content w, saturation degree Sr, hydraulic conductivity k, and dry density rd; • main experimental procedures for the determination of hydraulic conductivity in the laboratory and in situ; • in situ control of hydraulic conductivity and density of the mineral liners; • short notes on transport of contaminants through mineral liners and on compatibility between clay liners and leachate.
LABORATORY COMPACTION TESTS ON CLAY SOILS Compaction relates to the mechanical stabilization or densification of a soil, aimed at increasing its density and shear strength or decreasing hydraulic conductivity. Compaction may also be applied to influence the behavior of a clayey soil with regard to shrinkage and swelling. The properties of a compacted soil are usually studied in the laboratory by carrying out a standard (ASTM D698) or modified (ASTM D1557) version of the so-called Proctor tests. A soil sample, prepared at a specific initial water content, is inserted in successive layers inside a cylindrical steel mold and a steel hammer vertically dropped several times onto the soil (Fig. 7.2.1).
Figure 7.2.1 Schematic representation of Standard and Modified Proctor test apparatus.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.1 Main properties of the proctor tests apparatus Compaction Test
Compaction Mold Size
Hammer Size
Number of Layers
Number of Drops per Layer
Height of Drop (mm)
Compaction Energy (kJ/m3)
f (mm)
H (mm)
f (mm)
Weight (N)
Proctor standard
101.6
117
50.8
24.5
3
25
305
593
Proctor modified
101.6
117
50.8
44.5
5
25
457
2693
The size of the hammer, its height of drop, the number of drops for each soil layer, and the number of layers are standardized and reported in Table 7.2.1 according to the different Proctor procedures. The experimental results obtained are generally plotted on a “water content w versus dry density rd“ plane, which has a downward concavity and a maximum coordinate point defined through “optimum water content wopt”and “maximum dry density rd,max” (Fig. 7.2.2). The curve werd is uniquely defined for a specific compaction energy, as well as ideal water content wopt, corresponding to the maximum degree of densification rd,max of the specimen. Dry density rd is equal to the ratio between the mass of the soil particles and constant volume of the Proctor mold. An increase in rd signifies that a greater amount of solid particles is contained within the mold.
Figure 7.2.2 Typical compaction curve w versus rd obtained by a Proctor test.
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Figure 7.2.3 Variability of solid, liquid, and gas phases during a Proctor test.
On the side of the curve referred to as dry of optimum (w < wopt), an increase in water content lubricates the particles, promoting a mutual sliding and interlocking, and giving rise to an increase in the degree of final densification rd. On the wet side of optimum (w > wopt), rd decreases as the water, present in increasing quantities, not only fills the air voids (as on the dry of optimum side) but also replaces part of the volume occupied by the solid particles (Fig. 7.2.3). The two curves shown in Fig. 7.2.4 refer to the same soil, compacted to the two different compaction energies and characteristics
Figure 7.2.4 Typical plot w versus rd of Standard and Modified Proctor test for the same soil.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
of the standard and modified Proctor test procedures. The compaction energy used in the modified test is approximately 4.5-fold greater than that applied in the standard test. For this reason, the standard test is generally reserved for soils used in the construction of river embankments, earth dams, mineral liners of controlled landfills, etc., whereas the modified test is aimed at soils used in the foundations of main roads and industrial pavements. It is highly important that the most suitable Proctor test is selected because the compaction energy used in the laboratory should approximately reproduce that used in situ. Fig. 7.2.4 shows additional theoretical (dashed) curves that represent the trend werd for constant degrees of saturation Sr (increasing to the right side), obtained through the following equation: rd ¼
rw $Sr rs $Sr ¼ rw G$w þ Sr w þ $Sr rs
(1)
with rs and rw density of solid particles and water, respectively, and G specific gravity of soil equal to rs/rw. Both Proctor curves feature low and highly variable saturation levels on the dry side of optimum as the water content increases, whereas on the wet side of optimum Sr varies in a much narrower range. Curve “werd” moves to the left and upward of the diagram with increasing compaction energy. This implies that optimum water content decreases, whereas the maximum dry density increases as energy increases (Fig. 7.2.4). Likewise, in determining hydraulic conductivity k on a large number of specimens, prepared with the same soil and the same compaction energy, we obtain a specific curve “wek.” Using N different compaction energies, we obtain as many different “wek” curves (Fig. 7.2.5). For all compaction energies, as the water content of the specimen increases, coefficient k drastically decreases on the dry of optimum side, even by 1 O 2 orders of magnitude, whereas it increases very slowly on the wet of optimum side. Having established a critical limit of hydraulic conductivity kcrit, which for municipal solid waste (MSW) landfills is generally equal to 109 m/s for the base barrier and 108 m/s for the capping barrier, it is not rare to obtain an in situ hydraulic conductivity kfield greater than kcrit. This may be due simply to the fact of having prepared the soil to an incorrect in situ water content wfield, much less than wopt. On the dry of optimum side variability of coefficient k is, as stated above, very high. For this reason, it would be feasible to prepare the clayey material to be used in construction of the mineral liner, with a water content equal to wopt þ (2% O 3%); this approach would cause the coordinates (w; rd) to fall into the wet of optimum side, less subject to changes of coefficient k. By adopting an in situ water content wfield greater than the corresponding optimum water content, the barrier would be less sensitive to swelling (Lambe, 1958a,b); conversely, shrinkage would be slightly greater to that manifested on the dry of optimum side of the compaction curve (Seed and Chan, 1959). The above, however, are only valid if the field compaction and Standard Proctor energies applied are comparable. In this case, the laboratory compaction curve werd indeed represents the actual behavior of the mineral liner.
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Figure 7.2.5 Plots w versus rd and w versus k obtained using three different compaction energies.
What occurs, however, if the coordinate point (wfield; rd, field) does not lie along the reference Standard Proctor curve? In this case, the compaction energies used in the laboratory and in situ are not comparable. Indeed, we could hypothesize that a much higher or lower energy has been applied in compacting the clayey barrier than the Standard Proctor energy. In Fig. 7.2.6 the Standard Proctor curve is indicated with a continuous line, concavity downward, and peak of coordinates (wopt; rd, max), which represents the ideal combination of water content and dry density to obtain a maximum degree of densification of the barrier. The figure shows that by varying the compaction energy, the curves move upward to the left or downward to the right (for compaction energies higher and lower, respectively, than Standard Proctor energy). The optimum water content wopt of the Standard Proctor curve corresponds to two points in the higher (point A) and lower (point B) energy curves. Point A lies on the dry of optimum side of the higher energy curve, whereas point B is located on the wet of optimum side of the lower energy curve. By being situated to the right or left of their optimum water content, important effects may be produced on structure, compressibility, permeability, shrinkage, and swelling. It is therefore clear that
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Figure 7.2.6 Plots w versus rd when compaction energy used during the construction of the mineral
liner differs (þ or -) from Proctor standard energy.
the control procedure currently adopted on the majority of construction sites is not always the most appropriate or reliable. Accordingly, preliminary large-scale field tests should be carried out to define a detailed and correct compacting procedure (roller type, thickness of the layer to be compacted, number of roller passes per layer, initial water content of the soil, etc.) as similar as possible to the Standard Proctor compaction energy or to a percentage of the same. For example, on the capping of an MSW landfill, it is very difficult to achieve a degree of compaction comparable to that of the Standard Proctor test, due to the compressibility of the underlying mass. Finally, a short note focuses on the installation and compaction of the mineral liner along the side banks of the landfill. If the slope does not exceed the value of 1H:2.5V (Fig. 7.2.7A), the installation can be completed on the banks in the same way as the lower and horizontal base. If the slope is higher, the layers should be implemented as indicated in Fig. 7.2.7B (USEPA, 1989).
HYDRAULIC CONDUCTIVITY OF THE MINERAL LINERS Regulations relating to the design and construction of MSW landfills generally establish a hydraulic conductivity for mineral liners no greater than a critical value. However, these regulations are only prescriptive with no experimental procedure being defined to determine hydraulic conductivity in situ and
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Figure 7.2.7 Layout of the mineral liner with parallel (A) and horizontal (B) lifts. From USEPA (1989).
in the laboratory. Therefore, a series of issues remain to be resolved and should be taken into account during the design stage: • it is preferable to perform an in situ permeability test on the actual barrier or a laboratory permeability test on an undisturbed or reconstituted portion of the barrier? • what should be the minimum duration adopted for the tests? • what should be the optimal size and shape of the specimen? • what should be the nature of the permeating fluid (distilled or tap water or real leachate)? • what magnitude should be for the test gradient? • is the critical hydraulic conductivity kcrit referred to a condition of complete saturation? • what methods should be applied to guarantee complete saturation of the sample? Laboratory Permeability Tests Hydraulic conductivity k of a clayey soil can be determined in laboratory by means of various procedures using different experimental equipment. Experimental procedures include constant-head and falling-head permeability tests; inlet and outlet sections of the specimen are maintained with a constant or variable hydraulic head, respectively. The expressions used to determine coefficient k in the two cases are as follows: Constant-head test: k ¼
Q $L A$Dt$Dh
(2)
Variable-head test: a$L h1 $ln k ¼ A$Dt h2
(3)
where: Q: total discharge volume, m3, in time Dt ¼ t2 t1, expressed in seconds; L: height of specimen; A: cross-sectional area of specimen; Dh: difference of the constant hydraulic head between inlet
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.8 Laboratory permeameters: (A) compaction mold; (B) oedometric cell; (C) triaxial cell.
Modified from Daniel (1994). and outlet sections of the specimen; a: cross-sectional area of the graduated standpipe used in variable-head test; h1, h2: hydraulic heads corresponding to times t1 and t2 (t2 > t1) in a variablehead test. Laboratory permeameters can be distinguished into rigid-wall (RWP) and flexible-wall (FWP) permeameters. Typical RWP are the Proctor mold (constant-head test) and the oedometric cell (variablehead test) (Fig. 7.2.8A and B), whereas triaxial cell is an FWP whereby the test is performed at constant-head (Fig. 7.2.8C). Tests conducted using RWP are less expensive and easier to perform. If actual leachate is used as a permeating fluid, the material constituting the permeameter must be resistant to chemical aggression. The main disadvantage of RWP versus FWP is the potential sidewall leakage with consequent overestimation of coefficient k. When choosing the laboratory test to be used, the advantages and disadvantages associated with the use of an RWP or FWP (Table 7.2.2) should be specifically taken into account, together with the potential effects produced on the hydraulic conductivity by numerous factors indicated in Table 7.2.3. Hydraulic conductivity k may be indirectly determined from the results of an oedometric test, considering a specific effective vertical stress s0 v and the corresponding values of the coefficient of consolidation cv and oedometric modulus M: k ¼
gw $cv M
(4)
where gw is the unit weight of water.
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Table 7.2.2 Main advantages and disadvantages of RW and FW permeameters Permeameter
Advantages
Disadvantages
Compaction Mold (RWP)
• simple and cheap test
• specimen may be partially saturated • stress level of specimen may be not controlled • possible sidewall leakage
Oedometric Cell (RWP)
• simple and cheap test • vertical stress and strain of specimen may be controlled
• difficulty on specimen trimming • limited thickness of specimen • possible sidewall leakage
Triaxial Cell (FWP)
• stress and strain of specimen may be controlled • water volume may be controlled • sidewall leakage is limited
• complex and expensive test • difficulty on specimen trimming
Table 7.2.3 Factors influencing performance of the main laboratory permeameters Factor
Error
Experimental high hydraulic gradient applied for a more rapid seepage through the specimen
• not representative of the in situ condition • migration of the finer grains
Migration of the finer grains due to used high hydraulic gradient
• possible underestimation of k
Degree of saturation influenced by pressurized water coming from an air/water pressure generator
• possible decrease of k due to air effects in soil pores
Specimen size and Height/Diameter ratio
• k decreases as well as H/D ratio increases
Test duration generally too short
• possible overestimation of k
In Situ Permeability Tests The hydraulic conductivity of a mineral liner may be determined by means of one of the following in situ tests: • Boutwell permeameter test (ASTM 6391); • Sealed Double-Ring Infiltrometer test (SDRI) (ASTM D5093); • Guelph permeameter (GP) (ASTM D5126).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Boutwell Permeameter Test Each test requires a horizontal portion of a mineral liner with an area of approximately 16 m2 and a thickness of at least 1 m. A cylinder casing should be installed in a slightly wider diameter predrilled hole. The volume between the hole wall and the side surface of the casing should be hydraulically sealed to prevent lateral water leakage. The rate of water flow into soil through the bottom of the casing is measured in one or two stages, normally using a falling-head procedure. If the soil is considered anisotropic, with different hydraulic conductivity along vertical and horizontal directions, a falling-head test may be carried out in two stages (Fig. 7.2.9A). During Stage 1 the permeant fluid can filter only across the bottom section of the borehole; when the borehole is extended below the bottom of the casing, the permeant fluid can filter both vertically and horizontally (Stage 2). The borehole is extended for Stage 2 after Stage 1 is over (Fig. 7.2.9B). A limiting hydraulic conductivity is computed from the falling-head data in both stages (K1 and K2). Stages 1 and 2 continue until the limiting conductivity for each stage is relatively constant. Phase 1 Date, time, water level, and temperature in the graduated standpipe and in the Temperature Effect Gauge (TEG) should be noted throughout the test. TEG is identical to Boutwell permeameter, with a closed lower part, used to purify the experimental data from the effects of thermal variations. These effects are more relevant with smaller diameter standpipes and soils with a k 5 1010 m/s. Phase 1 is completed when a steady-flow condition is achieved.
Figure 7.2.9 Boutwell permeameter test: (A) Schematic section during Stage 1 and Stage 2, (B) Seepage during Stage 1 and Stage 2.
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Phase 2 Following emptying of the casing, the hole should be deepened (keeping the casing in its original position) up to one- or twofold the casing diameter. During excavation, lateral filtering side of the excavation should remain undisturbed and not smoothed. At the end of these operations, the test will develop as in the previous phase. Methods to calculate actual vertical and horizontal hydraulic conductivities (kv and kh) from K1 and K2, determined during the Stages 1 and 2, are described as follows. STAGE 1 p$d2 Z1 $ln K1 ¼ R T $ 11$D$ðt2 t1 Þ Z2
(5)
RT ¼ 2.2902 (0.9842 T)/T0.1702 with temperature T in C; d ¼ internal diameter of standpipe (cm); D ¼ internal diameter of casing (cm); Z1 ¼ (Zc þ Ro þ R) at time t1; Z2 ¼ (Zc þ Ro þ Rec) at time t2; c ¼ change in TEG scale reading between times t1 and t2; an increase in the height of water in the TEG standpipe is positive; t1 ¼ time at beginning of increment(s); t2 ¼ time at end of increment(s). STAGE 2 A Z1 (6) K2 ¼ RT $ $ln B Z2 with: 0
1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L L A A ¼ d2 $ln@ þ 1 þ D D B ¼ 8$L$ðt2 t1 Þ$ 1 0:56$e1:57$L=D
(7)
(8)
and L ¼ length of Stage 2 extension below bottom of casing (cm), D ¼ internal diameter of Stage 2 extension. Anisotropy of the mineral liner can be evaluated by a coefficient of anisotropy m defined as follows: pffiffiffiffiffiffiffiffiffiffiffiffi m ¼ k h =k v (9) k v $m ¼
kh m
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Once K1, K2, L, and D have been established, coefficient m can be determined with the following expression: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L L ln þ 1þ D D K2 ¼ m$ (11) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 K1 L L ln m$ þ 1 þ m$ D D And then kv and kH can be found by: K1 ¼ k v $m ¼
kh m
(12)
Sealed Double-Ring Infiltrometer Test This test method provides a direct measurement of infiltration rate I (amount of permeant fluid filtering across a soil sample (m3) per unit area (m2) per unit time (s), expressed in m/s). Hydraulic conductivity k cannot be directly measured unless the hydraulic boundary conditions, particularly hydraulic gradient, are known. A double-ring infiltrometer with a sealed or covered inner ring consists of an open outer and a sealed inner ring (Fig. 7.2.10). The rings are embedded and sealed in trenches excavated in the soil. Both rings are filled with water until the inner ring is submerged. The rate of flow is measured by connecting a flexible bag filled with a known weight of water to a port on the inner ring. As water infiltrates into the ground from the inner ring, an equal amount of water flows into the inner ring from the flexible bag. After a known interval of time, the flexible bag is removed and weighed. The weight loss, converted into a volume, is equal to the
Figure 7.2.10 Section of Sealed Double-Ring Infiltrometer.
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amount of water that has infiltrated into the ground. An infiltration rate is then determined from this volume of water, the area of the inner ring, and the interval of time. This process is repeated and a plot of infiltration rate versus time is constructed. The test is continued until the infiltration rate becomes steady. The purpose of the sealed inner ring is to provide a means to measure the actual amount of flow and to eliminate evaporation losses. The purpose of the outer ring is to promote one-dimensional, vertical flow beneath the inner ring. The use of large diameter rings and large depths of embedments helps to ensure that flow is essentially one dimensional. Changes in water temperature can introduce significant error in the volume change measurements. Temperature changes will cause water to flow in or out of the inner ring due to expansion or contraction of the inner ring and the water contained within the inner ring. The problem of temperature changes can be minimized by insulating the rings, by allowing enough flow to occur so that the amount of flow resulting from a temperature change is not significant compared to that due to infiltration, or by connecting and disconnecting the bag from the inner ring when the water in the inner ring is at the same temperature. If the soil being tested will later be subjected to increased overburden stress, then the infiltration rate can be expected to decrease as the overburden stress increases. Laboratory hydraulic conductivity tests are recommended for studies of the influence of level of stress on the hydraulic properties of the soil. The rings shall be made of metal, plastic, or fiberglass. The shape of the rings can be circular or square. The minimum width or diameter of the inner ring is 610 mm and a minimum distance of 610 mm is maintained between the inner and outer rings. Two clear flexible bags have a capacity of 1 O 3 L. Representative samples of the soil to be tested shall be taken before and after the test to determine its moisture content, density, and specific gravity. The thickness of the layer being tested shall be determined as well as the approximate hydraulic conductivity of the layer beneath it. Estimation of hydraulic conductivity k may be performed using the following expression: k ¼
I V 1 V Lf ¼ $ ¼ $ i A$Dt i A$Dt ðH þ Lf Þ
(13)
where: I ¼ infiltration rate; i ¼ hydraulic gradient; A ¼ cross-sectional area of inner ring; Dt ¼ test duration; V ¼ amount of water filtered during Dt; H ¼ depth of ponded water; Lf ¼ depth of wetting front. Evaluation of the water front thickness may be difficult. For this reason two different procedures have been developed to determine coefficient k. The apparent gradient method (Fig. 7.2.11A) is more conservative in view of the tendency to underestimate hydraulic gradient and overestimate hydraulic conductivity. The test pad is considered fully saturated over its entire thickness. This assumption for compacted clay liners with k < 109 m/s is conservative. The hydraulic gradient i is equal to: i ¼ ðH þ DÞ=D
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(14)
Figure 7.2.11 Different methods for permeability measurements with sealed double-ring infiltrometer: (A) apparent gradient method, (B) wetting front method.
The wetting front method (Fig. 7.2.11B) assumes only a partial saturation of the test pad. The water pressure at the wetting front is conservatively assumed at atmospheric pressure. Tensiometers are used to monitor the depth of wetting of the soil over time, and the variation of water content with depth is determined at the end of the test. The wetting front method is the method that is usually recommended. The hydraulic gradient i is equal to: i ¼ ðH þ D0 Þ=D0
(15)
Guelph Borehole Permeability Test The GP (Fig. 7.2.12) is an instrument intended for use in rapidly determining hydraulic conductivity of a mineral liner. The GP is easily movable and suitable for use by a single technician. Testing times range from 300 to 1200 depending on soil nature. According to the reference standard (ASTM D5126) soil thickness of the barrier undergoing testing ranges from 0.15 to 0.75 m below ground level. A shallow cylindrical hole is excavated in the barrier and GP is inserted into the hole. The GP test employs Mariotte’s bottle principle and is carried out measuring steady-state rate of water recharge into unsaturated soil from the test hole. The field hydraulic conductivity kfs is estimated by means of a constant-head test procedure. kfs is referred to the field-saturated bulb (Fig. 7.2.13) of soil surrounding the test hole. Firstly, a constant hydraulic head in the hole is fixed and held at the same level as the lower part of the air tube, located at the center of the GP. When the water level in the hole starts to drop below the air inlet tip, air bubbles exit the tip and rise into the tank air space. Vacuum is then partially relieved and the tank provides water to the hole. Size of opening and geometry of the air inlet tip are suitable to control air bubble size to avoid fluctuations of well water level. When a water level is maintained constant in the hole, a bulb of saturated soil quickly develops all around, assuming shape according to soil nature, well radius, and imposed hydraulic head. When a steady-state water flow has been reached, field-saturated conductivity kfs can be determined. A certain amount of air is usually entrapped in soil voids during the infiltration process, influencing the obtained values of k, which may be lower than those under conditions of complete saturation.
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Figure 7.2.12 Guelph permeameter.
Figure 7.2.13 Bulb of saturated soil (Eijkelkamp, 2011).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.4 Main advantages and disadvantages of field permeameters Permeameter
Advantages
Disadvantages
cheap test equipment easy installation kv and kH can be evaluated test can be performed on slopes
• test involves a limited volume of barrier • partial saturation of soil cannot be properly taken into account • testing times are long for soils having kv z 1010 m/s
Boutwell
• • • •
Sealed double-ring infiltrometer
• moderate test equipment cost • kv can be evaluated • suitable for soils with kv z 1010 m/s • test involves a large volume of barrier • minimal lateral seepage from inner ring
• testing times are long for soils having kv z 1010 m/s • only kv can be evaluated • wetting front suction head must be estimated • test cannot be performed on slopes
Guelph
• moderate test equipment cost • testing times are very short • uniformity of compaction can be controlled
• only an average k can be estimated • test involves a very small volume of barrier • great influence of possible discontinuities in the barrier • more suitable for soils having kv < 108 m/s
From Daniel (1989).
Differences Among Testing Techniques Boutwell permeameter and SDRI may be applied indifferently for field determination of k-coefficient. The main advantages and disadvantages deriving from use of the two devices are summarized in Table 7.2.4. GP allows fast but relatively inaccurate tests to be performed. Its use with mineral liners in MSW landfills should therefore be limited to controlling homogeneity. In other words, by obtaining a series of fairly close k-coefficients, it may be confirmed that the barrier was constructed using homogeneous and uniformly compacted soil.
QUALITY CONTROL DURING CONSTRUCTION Field control of compaction suitability is carried out by referring to the laboratory werd curve and determining in situ water content wfield and dry density rd of the mineral liner. Thus, the so-called degree of compaction Dc is obtained equal to:
Dc ¼ rd;field rd;max;lab 100ð%Þ (16)
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Figure 7.2.14 Sand cone test.
which should be not less than 90% O 95% to confirm suitability of the mineral liner. Field control tests may be destructive or nondestructive. In the first case, a limited amount of barrier should be excavated, while in the second measurements are made without any removal of soil. The most popular destructive test is the Sand Cone Test (ASTM D-1556-1556M). A hole of approximately 150 mm in diameter and depth is excavated at the top of the barrier. The excavated soil is stored in a plastic bag and its natural water content determined in the laboratory. The unknown volume of the excavated hole is filled with calibrated sand (dry sand with known dry density) introduced through a cone pouring device (Fig. 7.2.14). The dry mass of the excavated soil divided by the volume of calibrated sand required to fill the hole allows the dry density of the barrier to be determined. This may be compared with the Standard Proctor dry density. The nondestructive tests include the so-called Nuclear Methods that allow the rapid and relatively reliable determination of both in situ density and water content (ASTM D6938). The instrument uses a radiative isotope (Cesium 137), which generally emits g rays intercepted by a meter present at the lower base of the nucleodensimeter. Dense soil absorbs more radiation than loose soils (Fig. 7.2.15). The instrument is therefore able to estimate both the density of the soil and, after a few minutes, its natural moisture. TRANSPORT OF CONTAMINANTS THROUGH CLAY LINERS Transport of contaminants in soils may occur through three different processes: • Advection: contaminant is transported by water flow due to a hydraulic gradient; • Mechanical Dispersion: it consists of a mixing process occurring into an advection flow, due to different velocities among different points; and
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.15 Nuclear density apparatus. From www.utest.com.tr.
• Molecular Diffusion: it is due to concentration gradients of ionic or molecular constituents that may occur also without any advection process. The main transport process interesting clay liners is usually diffusion. Diffusion Mass of solute F, expressed per unit area and per unit time [ML2T1], solute concentration C [ML3], diffusion gradient vC/vz, and diffusion coefficient D [L2T1] may be correlated, in onedimensional process along the direction z, through the Fick’s first law as follows: vC F ¼ D$ vz
(17)
Negative sign means that diffusion occurred toward concentration is lower (direction z). In Table 7.2.5 a set of molecular diffusion coefficients Dm of contaminants in water are reported. These coefficients are strongly depending on temperature, decreasing about 50% at 5 C. The effective molecular diffusion De in saturated soils is lower than the molecular diffusion Dm of contaminants in water. This is mainly due to the tortuosity t (1, generally ranging between 0.01 O 0.5) of the paths available for diffusion. The following expression can be written: D ¼ De ¼ t$Dm
(18)
Rate of diffusion of the solute in a porous material can be expressed by means of Fick’s second law, combining Fick’s first law and the continuity equation: vC v2 C ¼ D$ 2 vt vz
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Table 7.2.5 Tipycal values of molecular diffusion coefficient Dm for some contami-
nants in water Contaminant
Dm (m2/s)
Naþ, Kþ, Mg2þ, Ca2þ
1E-9 O 2E-9
Cl-, HCO-3, SO24
1E-9 O 2E-9
Chlobenzene
8.4 E-10
Ethylbenzene
7.5 E-10
Methyl alcohol
1.5 E-9
Boundary conditions at the start of the process must be defined. In Fig. 7.2.16 a clay liner, typical of the base of an MSW landfill is drawn. At t ¼ 0 the solute concentration at the top of the clay liner is C0 (>0), whereas it is null at its bottom. In steady condition the solute concentration C0 may be considered constant with time. The solution of Eq. (20) can be found through the following expression: z pffiffiffiffiffiffiffiffi (20) Cðz; tÞ ¼ C0 $erfc 2$ D$t
Attenuation Attenuation is a process that may occur when a leachate filtrates through a mineral barrier placed at the bottom of an MSW landfill. Attenuation produces a decrease of the concentration of the leachate constituents and may develop by means of one or more among the following mechanisms: • Adsorption: molecules adhere to the surface of the clay particles causing a decrease in the total dissolved solid; • Biological uptake: microorganisms break down or adsorb leachate constituents attenuating leachate; • Cation- and Anion Exchange: exchange of ions of one type by ions of another type without disturbing the mineral structure (isomorphous substitution); • Filtration: leachate constituents are physically trapped in the random pore structure;
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.16 Clay liner system in which molecular diffusion is the main transport mechanism.
• Precipitation: chemical process involving a phase change in which dissolved chemical species are crystallized and deposited from a solution because their total concentration exceeds their solubility limit. The consequences of these attenuation processes are the variation over time of both the permeability of the mineral barrier and the composition of the leachate. It is very difficult to quantify the effects of these processes in the design stage, given their duration and dependence on the specific nature of the leachate. Laboratory tests are not long enough to activate all the long-term interactions that could develop between clayey minerals and leachate. Finally, it should be emphasized that the diffusion rate of a nonattenuated contaminant, through the mineral barrier, can be very high and therefore create unexpected pollution problems.
COMPATIBILITY BETWEEN CLAY LINERS AND LEACHATE The most common clay minerals in soils belong to three groups, kaolinite, illite, and smectite (montmorillonite). In the kaolinite crystals the unit layers are held together by hydrogen bonding. In montmorillonite the interlayers bond is due to cations exchanged for those contained in hydrating solution. Water molecules can also enter between the layers. Therefore, the van der Waals interlayer bonds are generally weak. The exchange complex of soils normally consists of a mixture of the cations Ca2þ, Mg2þ, Kþ, and Naþ, in which Ca2þ comprises the larger part, and Kþ and Naþ only the smaller. The ability of soils to exchange cations with those contained in the liquid solution has important effects on soil behavior. Exchangeable cations are located on particle surfaces and are attracted by their negative charges. The resulting capacity for exchangeable cations (C.E.C.) varies according to the type of
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clay; and it can be about 100 in kaolinite, 300 in illite, and 1000 in montmorillonite (C.E.C. expressed as milliequivalents per kg of clay). Variation occurs because of the effects of crystal size, isomorphous substitution, and pH. Oxides and organic matter also have a cation exchange capacity. The exchange of one cation for another is governed by Coulomb’s law. Cations of a higher valence (higher charge) are attracted more strongly to the negatively charged clay than those of a lower valence. Hence Ca2þ generally replaces Naþ in any reaction. However, if the concentration of lower valence ions (i.e., Naþ) is relevant and much greater than that of higher valence (i.e., Ca2þ), the ion exchange phenomenon can be in an opposite direction. There is also a tendency for larger cations to enter more readily in exchange than smaller ones of the same valency. This is attributed to a greater hydration of the smaller ions that, with a larger hydrated radius, are thus more distant from the negative charges on the clay. When adsorption of water has progressed far enough for swelling, clay has then enough water for a diffuse layer of exchangeable cations to extend out from the negatively charged surface of the mineral sheets. Phenomenon is governed by the opposing effects of electrostatic attraction toward the surface and diffusion away from it in the direction of decreasing concentration. The clay surface, with its localized negative charges, and the swarm of cations of opposite charge constitute a diffuse electrical double layer. If an outer solution bathes the clay, this will affect the distribution of the cations. Also because of the charge on the clay, the concentration of anions near its surface will be lower than in the outer solution (negative adsorption of anions). The effects of liquids may be evaluated with the GouyeChapman theory, which states that the thickness (t) of the diffuse double layer varies as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D$K$T (21) t ¼ 8$p$n0 $ε2 $v where D is the dielectric constant, K is the Boltzmann constant (1.38 106 erg/K), T is the temperature, n0 is the electrolyte concentration, ε is the unit electronic charge (16 1020 C), and v is the cation valence. When the thickness of the diffused double layer decreases, the structure of the clayey soil tends to be flocculated with an increase in permeability. The process is inverse when the thickness t increases. For example, solutions containing monovalent cations tend to produce lower k than solutions with polyvalent cations. Pore fluid composition influences not only the double-layer thickness but also the soil structure (which can alternatively become flocculated or dispersed) and the geotechnical properties of soil (Table 7.2.6). The complexity of the above-mentioned phenomena can produce large fluctuations of the geotechnical parameters of clayey soils.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.6 Effect of pore-fluid parameters on Diffuse Double-Layer thickness
and soil structure Parameter
Parameter related to the pore fluid
DDL Thickness
Soil Structure
Electrolyte concentration
þ
þ
F D
Ion valence
þ
þ
F D
Dielectric constant
þ
þ
D F
Temperature
þ
þ
D F
pH
þ
þ
D F
(þ) increase, (-) decrease. (F) flocculated, (D) dispersed.
CONCLUSIONS Mineral barriers at the base of an MSW landfill must be carefully designed. The material composing them must be carefully selected and put in place using compaction procedures initially defined by test field. Clay liners can be crossed by pollutants in several ways. When hydraulic conductivity is very low, possible release of pollutants will be mainly by molecular diffusion. Attenuation processes can slow the transport of several contaminants. Leachate can attack and compromise the efficiency of the barriers, but only concentrated chemicals are of serious concern. Based on transit time calculations, clay liners can be very effective barriers and minimize release of pollutants from landfills.
References Alberta Environment, March 1985. Design and Construction of Liners for Municipal Wastewater Stabilization Ponds. prepared by Komex Consultants. Ltd.. Alberta Environment Publication, Edmonton, Alberta, Canada. ASTM Test Method D698: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort. ASTM Test Method D1557: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort. ASTM Test Method D1556/D1556M: Standard Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method. ASTM Test Method 6938: Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth). ASTM Test Method D6391: Standard Test Method for Field Measurement of Hydraulic Conductivity Using Borehole Infiltration.
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ASTM Test Method D5093: Standard Test Method for Field Measurement of Infiltration rate Using Double-Ring Infiltrometer with Sealed-Inner Ring. ASTM Test Method D5126: Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone. Daniel, D.E., September 1989. In situ hydraulic conductivity tests for compacted clay. Journal of Geotechnical Engineering 115 (9), 1205e1226. Daniel, D.E., 1994. State-of-the-art: laboratory hydraulic conductivity tests for saturated soils. In: Daniel, D.E., Trautwein, S.J. (Eds.), Hydraulic Conductivity and Waste Contaminant Transport in Soil, ASTM STP 1142. ASTM, Philadelphia, PA, pp. 30e78. Eijkelkamp Soil & Water, November 2011. Guelph Permeameter e Operating Instructions. Evans, J.C., Fang, H.Y., Kugelman, I.J., 1985. Organic fluid effects on the permeability of soil-bentonite slurry walls. In: Proc. National Conf. On Hazardous Wastes and Environmental Emergencies, Cincinnati, OH, USA, pp. 267e271. Lambe, T.W., 1958a. The structure of compacted clay. Journal of the Soil Mechanics and Foundation Division 84 (SM2), 16541e1654-34. ASCE. Lambe, T.W., 1958b. The engineering behaviour of compacted clay. Journal of the Soil Mechanics and Foundation Division 84 (SM2), 1655-1e1655-35. ASCE. Seed, H.B., Chan, C.K., 1959. Structure and strength characteristics of compacted clays. Journal of the Soil Mechanics and Foundation Division 85 (SM5), 87e128. ASCE. Sharma, H.D., Lewis, S.P., 1994. Waste Containment Systems, Waste Stabilization and Landfills. John Wiley & Sons Inc., USA. ISBN:0-471-575336-4. U.S. Environmental Protection Agency (USEPA), 1989. Requirements for Hazardous Waste Landfill Design, Construction and Closure. Seminar Publication, EPA/625-89/022, USEPA, Cincinnati, OH, 127 pp.
Further Reading ASTM Test Method D2167: Standard Test Method for Density and Unit Weight of Soil in Place by the Rubber Balloon Method. ASTM Test Method D6780/D6780M: Standard Test Method for Water Content and Density of Soil In situ by Time Domain Reflectometry (TDR). Daniel, D.E., Anderson, D.C., Boynton, S.S., 1985. Fixed-walls versus flexible-wall permeameter. In: Hydraulic Barriers in Soil and Rock, ASTM STP 874. ASTM, Philadelphia, pp. 107e126. ISBN:0-08030-04117-0. Favaretti, M., Moraci, N., 1995. Effects of leachate on the behaviour of sand-clay mixtures. In: 5th International Landfill Symposium “Sardinia 95”, S. Margherita di Pula, Cagliari, Italy. Favaretti, M., Moraci, N., 1997. Geotechnical behaviour of sand-sodium bentonite mixtures. In: 6th International Landfill Symposium “Sardinia 97”, S. Margherita di Pula, Cagliari, Italy. Holtz, R.D., Kovacs, W.D., Sheahan, T.C., 2011. An Introduction to Geotechnical Engineering, second ed. Pearson, USA. ISBN:0-13-701132-6. Qian, X., Koerner, R.M., Gray, D.H., 2002. Geotechnical Aspects of Landfill Design and Construction. Prentice Hall, USA. ISBN:3-13-012506-7.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
INTRODUCTION Mineral liners in landfills are a fundamental component of the physical barrier systems (Chapter 7.1). They should be purpose-designed and constructed to withstand the effect of clayey soils, compaction procedures, degree of densification, water content, and hydraulic conductivity. This chapter describes the property of mineral liners with regard to landfill applications and presents a series of fundamental experimental methods to be performed in the laboratory and the field to ensure the correct design of mineral liners and to verify efficiency following emplacement. Performance of clay liner in attenuating dispersed leachate emissions is finally discussed.
GEOTECHNICAL PROPERTIES OF THE MINERAL LINERS Mineral landfill liners are in general subject to regulations imposing a geometric limit, establishing a minimum value for thickness, and a geotechnical limit setting the maximum value for hydraulic conductivity k (see Chapter 7.1). The first limit is represented by a simple measure of length and therefore in situ control does not constitute a problem. The difficulty in determining coefficient k is associated with the precision, uncertainty and repeatability of its experimental measure, and contrasts with the prescriptive character of the regulation, fixing a specific kmax that cannot be overcome. Even a slight deviation from the kmax implies, from a purely legal point of view, nonacceptability of the used soil or the barrier. It is easy to grasp the paradox of a standard that defines with absolute precision kmax, without illustrating the experimental methods of determination, with possible and consequential technical and legal issues. The mineral liner should be constructed using mediumelow plasticity clayey soils (Plasticity Index, PI ¼ 10% O 40%), with a percentage of passing through ASTM 200 sieve (opening 0.075 mm) greater than 20%. The minimum value of the PI is due to the fact that below 10% it would be highly unlikely to achieve the required low hydraulic conductivity, whereas the maximum value of PI is related to the need to have a workable and compactable soil, with limited shrinkage and swelling properties. Any clods of clayey soil must not exceed 25 O 50 mm. Finally, a greater than 10% presence of gravel granulometric fractions should be avoided.
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The following topics will be treated in sequence: • compaction procedures in the laboratory: interactions between water content w, saturation degree Sr, hydraulic conductivity k, and dry density rd; • main experimental procedures for the determination of hydraulic conductivity in the laboratory and in situ; • in situ control of hydraulic conductivity and density of the mineral liners; • short notes on transport of contaminants through mineral liners and on compatibility between clay liners and leachate.
LABORATORY COMPACTION TESTS ON CLAY SOILS Compaction relates to the mechanical stabilization or densification of a soil, aimed at increasing its density and shear strength or decreasing hydraulic conductivity. Compaction may also be applied to influence the behavior of a clayey soil with regard to shrinkage and swelling. The properties of a compacted soil are usually studied in the laboratory by carrying out a standard (ASTM D698) or modified (ASTM D1557) version of the so-called Proctor tests. A soil sample, prepared at a specific initial water content, is inserted in successive layers inside a cylindrical steel mold and a steel hammer vertically dropped several times onto the soil (Fig. 7.2.1).
Figure 7.2.1 Schematic representation of Standard and Modified Proctor test apparatus.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.1 Main properties of the proctor tests apparatus Compaction Test
Compaction Mold Size
Hammer Size
Number of Layers
Number of Drops per Layer
Height of Drop (mm)
Compaction Energy (kJ/m3)
f (mm)
H (mm)
f (mm)
Weight (N)
Proctor standard
101.6
117
50.8
24.5
3
25
305
593
Proctor modified
101.6
117
50.8
44.5
5
25
457
2693
The size of the hammer, its height of drop, the number of drops for each soil layer, and the number of layers are standardized and reported in Table 7.2.1 according to the different Proctor procedures. The experimental results obtained are generally plotted on a “water content w versus dry density rd“ plane, which has a downward concavity and a maximum coordinate point defined through “optimum water content wopt”and “maximum dry density rd,max” (Fig. 7.2.2). The curve werd is uniquely defined for a specific compaction energy, as well as ideal water content wopt, corresponding to the maximum degree of densification rd,max of the specimen. Dry density rd is equal to the ratio between the mass of the soil particles and constant volume of the Proctor mold. An increase in rd signifies that a greater amount of solid particles is contained within the mold.
Figure 7.2.2 Typical compaction curve w versus rd obtained by a Proctor test.
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Figure 7.2.3 Variability of solid, liquid, and gas phases during a Proctor test.
On the side of the curve referred to as dry of optimum (w < wopt), an increase in water content lubricates the particles, promoting a mutual sliding and interlocking, and giving rise to an increase in the degree of final densification rd. On the wet side of optimum (w > wopt), rd decreases as the water, present in increasing quantities, not only fills the air voids (as on the dry of optimum side) but also replaces part of the volume occupied by the solid particles (Fig. 7.2.3). The two curves shown in Fig. 7.2.4 refer to the same soil, compacted to the two different compaction energies and characteristics
Figure 7.2.4 Typical plot w versus rd of Standard and Modified Proctor test for the same soil.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
of the standard and modified Proctor test procedures. The compaction energy used in the modified test is approximately 4.5-fold greater than that applied in the standard test. For this reason, the standard test is generally reserved for soils used in the construction of river embankments, earth dams, mineral liners of controlled landfills, etc., whereas the modified test is aimed at soils used in the foundations of main roads and industrial pavements. It is highly important that the most suitable Proctor test is selected because the compaction energy used in the laboratory should approximately reproduce that used in situ. Fig. 7.2.4 shows additional theoretical (dashed) curves that represent the trend werd for constant degrees of saturation Sr (increasing to the right side), obtained through the following equation: rd ¼
rw $Sr rs $Sr ¼ rw G$w þ Sr w þ $Sr rs
(1)
with rs and rw density of solid particles and water, respectively, and G specific gravity of soil equal to rs/rw. Both Proctor curves feature low and highly variable saturation levels on the dry side of optimum as the water content increases, whereas on the wet side of optimum Sr varies in a much narrower range. Curve “werd” moves to the left and upward of the diagram with increasing compaction energy. This implies that optimum water content decreases, whereas the maximum dry density increases as energy increases (Fig. 7.2.4). Likewise, in determining hydraulic conductivity k on a large number of specimens, prepared with the same soil and the same compaction energy, we obtain a specific curve “wek.” Using N different compaction energies, we obtain as many different “wek” curves (Fig. 7.2.5). For all compaction energies, as the water content of the specimen increases, coefficient k drastically decreases on the dry of optimum side, even by 1 O 2 orders of magnitude, whereas it increases very slowly on the wet of optimum side. Having established a critical limit of hydraulic conductivity kcrit, which for municipal solid waste (MSW) landfills is generally equal to 109 m/s for the base barrier and 108 m/s for the capping barrier, it is not rare to obtain an in situ hydraulic conductivity kfield greater than kcrit. This may be due simply to the fact of having prepared the soil to an incorrect in situ water content wfield, much less than wopt. On the dry of optimum side variability of coefficient k is, as stated above, very high. For this reason, it would be feasible to prepare the clayey material to be used in construction of the mineral liner, with a water content equal to wopt þ (2% O 3%); this approach would cause the coordinates (w; rd) to fall into the wet of optimum side, less subject to changes of coefficient k. By adopting an in situ water content wfield greater than the corresponding optimum water content, the barrier would be less sensitive to swelling (Lambe, 1958a,b); conversely, shrinkage would be slightly greater to that manifested on the dry of optimum side of the compaction curve (Seed and Chan, 1959). The above, however, are only valid if the field compaction and Standard Proctor energies applied are comparable. In this case, the laboratory compaction curve werd indeed represents the actual behavior of the mineral liner.
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Figure 7.2.5 Plots w versus rd and w versus k obtained using three different compaction energies.
What occurs, however, if the coordinate point (wfield; rd, field) does not lie along the reference Standard Proctor curve? In this case, the compaction energies used in the laboratory and in situ are not comparable. Indeed, we could hypothesize that a much higher or lower energy has been applied in compacting the clayey barrier than the Standard Proctor energy. In Fig. 7.2.6 the Standard Proctor curve is indicated with a continuous line, concavity downward, and peak of coordinates (wopt; rd, max), which represents the ideal combination of water content and dry density to obtain a maximum degree of densification of the barrier. The figure shows that by varying the compaction energy, the curves move upward to the left or downward to the right (for compaction energies higher and lower, respectively, than Standard Proctor energy). The optimum water content wopt of the Standard Proctor curve corresponds to two points in the higher (point A) and lower (point B) energy curves. Point A lies on the dry of optimum side of the higher energy curve, whereas point B is located on the wet of optimum side of the lower energy curve. By being situated to the right or left of their optimum water content, important effects may be produced on structure, compressibility, permeability, shrinkage, and swelling. It is therefore clear that
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.6 Plots w versus rd when compaction energy used during the construction of the mineral
liner differs (þ or -) from Proctor standard energy.
the control procedure currently adopted on the majority of construction sites is not always the most appropriate or reliable. Accordingly, preliminary large-scale field tests should be carried out to define a detailed and correct compacting procedure (roller type, thickness of the layer to be compacted, number of roller passes per layer, initial water content of the soil, etc.) as similar as possible to the Standard Proctor compaction energy or to a percentage of the same. For example, on the capping of an MSW landfill, it is very difficult to achieve a degree of compaction comparable to that of the Standard Proctor test, due to the compressibility of the underlying mass. Finally, a short note focuses on the installation and compaction of the mineral liner along the side banks of the landfill. If the slope does not exceed the value of 1H:2.5V (Fig. 7.2.7A), the installation can be completed on the banks in the same way as the lower and horizontal base. If the slope is higher, the layers should be implemented as indicated in Fig. 7.2.7B (USEPA, 1989).
HYDRAULIC CONDUCTIVITY OF THE MINERAL LINERS Regulations relating to the design and construction of MSW landfills generally establish a hydraulic conductivity for mineral liners no greater than a critical value. However, these regulations are only prescriptive with no experimental procedure being defined to determine hydraulic conductivity in situ and
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Figure 7.2.7 Layout of the mineral liner with parallel (A) and horizontal (B) lifts. From USEPA (1989).
in the laboratory. Therefore, a series of issues remain to be resolved and should be taken into account during the design stage: • it is preferable to perform an in situ permeability test on the actual barrier or a laboratory permeability test on an undisturbed or reconstituted portion of the barrier? • what should be the minimum duration adopted for the tests? • what should be the optimal size and shape of the specimen? • what should be the nature of the permeating fluid (distilled or tap water or real leachate)? • what magnitude should be for the test gradient? • is the critical hydraulic conductivity kcrit referred to a condition of complete saturation? • what methods should be applied to guarantee complete saturation of the sample? Laboratory Permeability Tests Hydraulic conductivity k of a clayey soil can be determined in laboratory by means of various procedures using different experimental equipment. Experimental procedures include constant-head and falling-head permeability tests; inlet and outlet sections of the specimen are maintained with a constant or variable hydraulic head, respectively. The expressions used to determine coefficient k in the two cases are as follows: Constant-head test: k ¼
Q $L A$Dt$Dh
(2)
Variable-head test: a$L h1 $ln k ¼ A$Dt h2
(3)
where: Q: total discharge volume, m3, in time Dt ¼ t2 t1, expressed in seconds; L: height of specimen; A: cross-sectional area of specimen; Dh: difference of the constant hydraulic head between inlet
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.8 Laboratory permeameters: (A) compaction mold; (B) oedometric cell; (C) triaxial cell.
Modified from Daniel (1994). and outlet sections of the specimen; a: cross-sectional area of the graduated standpipe used in variable-head test; h1, h2: hydraulic heads corresponding to times t1 and t2 (t2 > t1) in a variablehead test. Laboratory permeameters can be distinguished into rigid-wall (RWP) and flexible-wall (FWP) permeameters. Typical RWP are the Proctor mold (constant-head test) and the oedometric cell (variablehead test) (Fig. 7.2.8A and B), whereas triaxial cell is an FWP whereby the test is performed at constant-head (Fig. 7.2.8C). Tests conducted using RWP are less expensive and easier to perform. If actual leachate is used as a permeating fluid, the material constituting the permeameter must be resistant to chemical aggression. The main disadvantage of RWP versus FWP is the potential sidewall leakage with consequent overestimation of coefficient k. When choosing the laboratory test to be used, the advantages and disadvantages associated with the use of an RWP or FWP (Table 7.2.2) should be specifically taken into account, together with the potential effects produced on the hydraulic conductivity by numerous factors indicated in Table 7.2.3. Hydraulic conductivity k may be indirectly determined from the results of an oedometric test, considering a specific effective vertical stress s0 v and the corresponding values of the coefficient of consolidation cv and oedometric modulus M: k ¼
gw $cv M
(4)
where gw is the unit weight of water.
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Table 7.2.2 Main advantages and disadvantages of RW and FW permeameters Permeameter
Advantages
Disadvantages
Compaction Mold (RWP)
• simple and cheap test
• specimen may be partially saturated • stress level of specimen may be not controlled • possible sidewall leakage
Oedometric Cell (RWP)
• simple and cheap test • vertical stress and strain of specimen may be controlled
• difficulty on specimen trimming • limited thickness of specimen • possible sidewall leakage
Triaxial Cell (FWP)
• stress and strain of specimen may be controlled • water volume may be controlled • sidewall leakage is limited
• complex and expensive test • difficulty on specimen trimming
Table 7.2.3 Factors influencing performance of the main laboratory permeameters Factor
Error
Experimental high hydraulic gradient applied for a more rapid seepage through the specimen
• not representative of the in situ condition • migration of the finer grains
Migration of the finer grains due to used high hydraulic gradient
• possible underestimation of k
Degree of saturation influenced by pressurized water coming from an air/water pressure generator
• possible decrease of k due to air effects in soil pores
Specimen size and Height/Diameter ratio
• k decreases as well as H/D ratio increases
Test duration generally too short
• possible overestimation of k
In Situ Permeability Tests The hydraulic conductivity of a mineral liner may be determined by means of one of the following in situ tests: • Boutwell permeameter test (ASTM 6391); • Sealed Double-Ring Infiltrometer test (SDRI) (ASTM D5093); • Guelph permeameter (GP) (ASTM D5126).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Boutwell Permeameter Test Each test requires a horizontal portion of a mineral liner with an area of approximately 16 m2 and a thickness of at least 1 m. A cylinder casing should be installed in a slightly wider diameter predrilled hole. The volume between the hole wall and the side surface of the casing should be hydraulically sealed to prevent lateral water leakage. The rate of water flow into soil through the bottom of the casing is measured in one or two stages, normally using a falling-head procedure. If the soil is considered anisotropic, with different hydraulic conductivity along vertical and horizontal directions, a falling-head test may be carried out in two stages (Fig. 7.2.9A). During Stage 1 the permeant fluid can filter only across the bottom section of the borehole; when the borehole is extended below the bottom of the casing, the permeant fluid can filter both vertically and horizontally (Stage 2). The borehole is extended for Stage 2 after Stage 1 is over (Fig. 7.2.9B). A limiting hydraulic conductivity is computed from the falling-head data in both stages (K1 and K2). Stages 1 and 2 continue until the limiting conductivity for each stage is relatively constant. Phase 1 Date, time, water level, and temperature in the graduated standpipe and in the Temperature Effect Gauge (TEG) should be noted throughout the test. TEG is identical to Boutwell permeameter, with a closed lower part, used to purify the experimental data from the effects of thermal variations. These effects are more relevant with smaller diameter standpipes and soils with a k 5 1010 m/s. Phase 1 is completed when a steady-flow condition is achieved.
Figure 7.2.9 Boutwell permeameter test: (A) Schematic section during Stage 1 and Stage 2, (B) Seepage during Stage 1 and Stage 2.
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Phase 2 Following emptying of the casing, the hole should be deepened (keeping the casing in its original position) up to one- or twofold the casing diameter. During excavation, lateral filtering side of the excavation should remain undisturbed and not smoothed. At the end of these operations, the test will develop as in the previous phase. Methods to calculate actual vertical and horizontal hydraulic conductivities (kv and kh) from K1 and K2, determined during the Stages 1 and 2, are described as follows. STAGE 1 p$d2 Z1 $ln K1 ¼ R T $ 11$D$ðt2 t1 Þ Z2
(5)
RT ¼ 2.2902 (0.9842 T)/T0.1702 with temperature T in C; d ¼ internal diameter of standpipe (cm); D ¼ internal diameter of casing (cm); Z1 ¼ (Zc þ Ro þ R) at time t1; Z2 ¼ (Zc þ Ro þ Rec) at time t2; c ¼ change in TEG scale reading between times t1 and t2; an increase in the height of water in the TEG standpipe is positive; t1 ¼ time at beginning of increment(s); t2 ¼ time at end of increment(s). STAGE 2 A Z1 (6) K2 ¼ RT $ $ln B Z2 with: 0
1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L L A A ¼ d2 $ln@ þ 1 þ D D B ¼ 8$L$ðt2 t1 Þ$ 1 0:56$e1:57$L=D
(7)
(8)
and L ¼ length of Stage 2 extension below bottom of casing (cm), D ¼ internal diameter of Stage 2 extension. Anisotropy of the mineral liner can be evaluated by a coefficient of anisotropy m defined as follows: pffiffiffiffiffiffiffiffiffiffiffiffi m ¼ k h =k v (9) k v $m ¼
kh m
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
(10)
Once K1, K2, L, and D have been established, coefficient m can be determined with the following expression: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 L L ln þ 1þ D D K2 ¼ m$ (11) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 K1 L L ln m$ þ 1 þ m$ D D And then kv and kH can be found by: K1 ¼ k v $m ¼
kh m
(12)
Sealed Double-Ring Infiltrometer Test This test method provides a direct measurement of infiltration rate I (amount of permeant fluid filtering across a soil sample (m3) per unit area (m2) per unit time (s), expressed in m/s). Hydraulic conductivity k cannot be directly measured unless the hydraulic boundary conditions, particularly hydraulic gradient, are known. A double-ring infiltrometer with a sealed or covered inner ring consists of an open outer and a sealed inner ring (Fig. 7.2.10). The rings are embedded and sealed in trenches excavated in the soil. Both rings are filled with water until the inner ring is submerged. The rate of flow is measured by connecting a flexible bag filled with a known weight of water to a port on the inner ring. As water infiltrates into the ground from the inner ring, an equal amount of water flows into the inner ring from the flexible bag. After a known interval of time, the flexible bag is removed and weighed. The weight loss, converted into a volume, is equal to the
Figure 7.2.10 Section of Sealed Double-Ring Infiltrometer.
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amount of water that has infiltrated into the ground. An infiltration rate is then determined from this volume of water, the area of the inner ring, and the interval of time. This process is repeated and a plot of infiltration rate versus time is constructed. The test is continued until the infiltration rate becomes steady. The purpose of the sealed inner ring is to provide a means to measure the actual amount of flow and to eliminate evaporation losses. The purpose of the outer ring is to promote one-dimensional, vertical flow beneath the inner ring. The use of large diameter rings and large depths of embedments helps to ensure that flow is essentially one dimensional. Changes in water temperature can introduce significant error in the volume change measurements. Temperature changes will cause water to flow in or out of the inner ring due to expansion or contraction of the inner ring and the water contained within the inner ring. The problem of temperature changes can be minimized by insulating the rings, by allowing enough flow to occur so that the amount of flow resulting from a temperature change is not significant compared to that due to infiltration, or by connecting and disconnecting the bag from the inner ring when the water in the inner ring is at the same temperature. If the soil being tested will later be subjected to increased overburden stress, then the infiltration rate can be expected to decrease as the overburden stress increases. Laboratory hydraulic conductivity tests are recommended for studies of the influence of level of stress on the hydraulic properties of the soil. The rings shall be made of metal, plastic, or fiberglass. The shape of the rings can be circular or square. The minimum width or diameter of the inner ring is 610 mm and a minimum distance of 610 mm is maintained between the inner and outer rings. Two clear flexible bags have a capacity of 1 O 3 L. Representative samples of the soil to be tested shall be taken before and after the test to determine its moisture content, density, and specific gravity. The thickness of the layer being tested shall be determined as well as the approximate hydraulic conductivity of the layer beneath it. Estimation of hydraulic conductivity k may be performed using the following expression: k ¼
I V 1 V Lf ¼ $ ¼ $ i A$Dt i A$Dt ðH þ Lf Þ
(13)
where: I ¼ infiltration rate; i ¼ hydraulic gradient; A ¼ cross-sectional area of inner ring; Dt ¼ test duration; V ¼ amount of water filtered during Dt; H ¼ depth of ponded water; Lf ¼ depth of wetting front. Evaluation of the water front thickness may be difficult. For this reason two different procedures have been developed to determine coefficient k. The apparent gradient method (Fig. 7.2.11A) is more conservative in view of the tendency to underestimate hydraulic gradient and overestimate hydraulic conductivity. The test pad is considered fully saturated over its entire thickness. This assumption for compacted clay liners with k < 109 m/s is conservative. The hydraulic gradient i is equal to: i ¼ ðH þ DÞ=D
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
(14)
Figure 7.2.11 Different methods for permeability measurements with sealed double-ring infiltrometer: (A) apparent gradient method, (B) wetting front method.
The wetting front method (Fig. 7.2.11B) assumes only a partial saturation of the test pad. The water pressure at the wetting front is conservatively assumed at atmospheric pressure. Tensiometers are used to monitor the depth of wetting of the soil over time, and the variation of water content with depth is determined at the end of the test. The wetting front method is the method that is usually recommended. The hydraulic gradient i is equal to: i ¼ ðH þ D0 Þ=D0
(15)
Guelph Borehole Permeability Test The GP (Fig. 7.2.12) is an instrument intended for use in rapidly determining hydraulic conductivity of a mineral liner. The GP is easily movable and suitable for use by a single technician. Testing times range from 300 to 1200 depending on soil nature. According to the reference standard (ASTM D5126) soil thickness of the barrier undergoing testing ranges from 0.15 to 0.75 m below ground level. A shallow cylindrical hole is excavated in the barrier and GP is inserted into the hole. The GP test employs Mariotte’s bottle principle and is carried out measuring steady-state rate of water recharge into unsaturated soil from the test hole. The field hydraulic conductivity kfs is estimated by means of a constant-head test procedure. kfs is referred to the field-saturated bulb (Fig. 7.2.13) of soil surrounding the test hole. Firstly, a constant hydraulic head in the hole is fixed and held at the same level as the lower part of the air tube, located at the center of the GP. When the water level in the hole starts to drop below the air inlet tip, air bubbles exit the tip and rise into the tank air space. Vacuum is then partially relieved and the tank provides water to the hole. Size of opening and geometry of the air inlet tip are suitable to control air bubble size to avoid fluctuations of well water level. When a water level is maintained constant in the hole, a bulb of saturated soil quickly develops all around, assuming shape according to soil nature, well radius, and imposed hydraulic head. When a steady-state water flow has been reached, field-saturated conductivity kfs can be determined. A certain amount of air is usually entrapped in soil voids during the infiltration process, influencing the obtained values of k, which may be lower than those under conditions of complete saturation.
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Figure 7.2.12 Guelph permeameter.
Figure 7.2.13 Bulb of saturated soil (Eijkelkamp, 2011).
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.4 Main advantages and disadvantages of field permeameters Permeameter
Advantages
Disadvantages
cheap test equipment easy installation kv and kH can be evaluated test can be performed on slopes
• test involves a limited volume of barrier • partial saturation of soil cannot be properly taken into account • testing times are long for soils having kv z 1010 m/s
Boutwell
• • • •
Sealed double-ring infiltrometer
• moderate test equipment cost • kv can be evaluated • suitable for soils with kv z 1010 m/s • test involves a large volume of barrier • minimal lateral seepage from inner ring
• testing times are long for soils having kv z 1010 m/s • only kv can be evaluated • wetting front suction head must be estimated • test cannot be performed on slopes
Guelph
• moderate test equipment cost • testing times are very short • uniformity of compaction can be controlled
• only an average k can be estimated • test involves a very small volume of barrier • great influence of possible discontinuities in the barrier • more suitable for soils having kv < 108 m/s
From Daniel (1989).
Differences Among Testing Techniques Boutwell permeameter and SDRI may be applied indifferently for field determination of k-coefficient. The main advantages and disadvantages deriving from use of the two devices are summarized in Table 7.2.4. GP allows fast but relatively inaccurate tests to be performed. Its use with mineral liners in MSW landfills should therefore be limited to controlling homogeneity. In other words, by obtaining a series of fairly close k-coefficients, it may be confirmed that the barrier was constructed using homogeneous and uniformly compacted soil.
QUALITY CONTROL DURING CONSTRUCTION Field control of compaction suitability is carried out by referring to the laboratory werd curve and determining in situ water content wfield and dry density rd of the mineral liner. Thus, the so-called degree of compaction Dc is obtained equal to:
Dc ¼ rd;field rd;max;lab 100ð%Þ (16)
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Figure 7.2.14 Sand cone test.
which should be not less than 90% O 95% to confirm suitability of the mineral liner. Field control tests may be destructive or nondestructive. In the first case, a limited amount of barrier should be excavated, while in the second measurements are made without any removal of soil. The most popular destructive test is the Sand Cone Test (ASTM D-1556-1556M). A hole of approximately 150 mm in diameter and depth is excavated at the top of the barrier. The excavated soil is stored in a plastic bag and its natural water content determined in the laboratory. The unknown volume of the excavated hole is filled with calibrated sand (dry sand with known dry density) introduced through a cone pouring device (Fig. 7.2.14). The dry mass of the excavated soil divided by the volume of calibrated sand required to fill the hole allows the dry density of the barrier to be determined. This may be compared with the Standard Proctor dry density. The nondestructive tests include the so-called Nuclear Methods that allow the rapid and relatively reliable determination of both in situ density and water content (ASTM D6938). The instrument uses a radiative isotope (Cesium 137), which generally emits g rays intercepted by a meter present at the lower base of the nucleodensimeter. Dense soil absorbs more radiation than loose soils (Fig. 7.2.15). The instrument is therefore able to estimate both the density of the soil and, after a few minutes, its natural moisture. TRANSPORT OF CONTAMINANTS THROUGH CLAY LINERS Transport of contaminants in soils may occur through three different processes: • Advection: contaminant is transported by water flow due to a hydraulic gradient; • Mechanical Dispersion: it consists of a mixing process occurring into an advection flow, due to different velocities among different points; and
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.15 Nuclear density apparatus. From www.utest.com.tr.
• Molecular Diffusion: it is due to concentration gradients of ionic or molecular constituents that may occur also without any advection process. The main transport process interesting clay liners is usually diffusion. Diffusion Mass of solute F, expressed per unit area and per unit time [ML2T1], solute concentration C [ML3], diffusion gradient vC/vz, and diffusion coefficient D [L2T1] may be correlated, in onedimensional process along the direction z, through the Fick’s first law as follows: vC F ¼ D$ vz
(17)
Negative sign means that diffusion occurred toward concentration is lower (direction z). In Table 7.2.5 a set of molecular diffusion coefficients Dm of contaminants in water are reported. These coefficients are strongly depending on temperature, decreasing about 50% at 5 C. The effective molecular diffusion De in saturated soils is lower than the molecular diffusion Dm of contaminants in water. This is mainly due to the tortuosity t (1, generally ranging between 0.01 O 0.5) of the paths available for diffusion. The following expression can be written: D ¼ De ¼ t$Dm
(18)
Rate of diffusion of the solute in a porous material can be expressed by means of Fick’s second law, combining Fick’s first law and the continuity equation: vC v2 C ¼ D$ 2 vt vz
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Table 7.2.5 Tipycal values of molecular diffusion coefficient Dm for some contami-
nants in water Contaminant
Dm (m2/s)
Naþ, Kþ, Mg2þ, Ca2þ
1E-9 O 2E-9
Cl-, HCO-3, SO24
1E-9 O 2E-9
Chlobenzene
8.4 E-10
Ethylbenzene
7.5 E-10
Methyl alcohol
1.5 E-9
Boundary conditions at the start of the process must be defined. In Fig. 7.2.16 a clay liner, typical of the base of an MSW landfill is drawn. At t ¼ 0 the solute concentration at the top of the clay liner is C0 (>0), whereas it is null at its bottom. In steady condition the solute concentration C0 may be considered constant with time. The solution of Eq. (20) can be found through the following expression: z pffiffiffiffiffiffiffiffi (20) Cðz; tÞ ¼ C0 $erfc 2$ D$t
Attenuation Attenuation is a process that may occur when a leachate filtrates through a mineral barrier placed at the bottom of an MSW landfill. Attenuation produces a decrease of the concentration of the leachate constituents and may develop by means of one or more among the following mechanisms: • Adsorption: molecules adhere to the surface of the clay particles causing a decrease in the total dissolved solid; • Biological uptake: microorganisms break down or adsorb leachate constituents attenuating leachate; • Cation- and Anion Exchange: exchange of ions of one type by ions of another type without disturbing the mineral structure (isomorphous substitution); • Filtration: leachate constituents are physically trapped in the random pore structure;
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Figure 7.2.16 Clay liner system in which molecular diffusion is the main transport mechanism.
• Precipitation: chemical process involving a phase change in which dissolved chemical species are crystallized and deposited from a solution because their total concentration exceeds their solubility limit. The consequences of these attenuation processes are the variation over time of both the permeability of the mineral barrier and the composition of the leachate. It is very difficult to quantify the effects of these processes in the design stage, given their duration and dependence on the specific nature of the leachate. Laboratory tests are not long enough to activate all the long-term interactions that could develop between clayey minerals and leachate. Finally, it should be emphasized that the diffusion rate of a nonattenuated contaminant, through the mineral barrier, can be very high and therefore create unexpected pollution problems.
COMPATIBILITY BETWEEN CLAY LINERS AND LEACHATE The most common clay minerals in soils belong to three groups, kaolinite, illite, and smectite (montmorillonite). In the kaolinite crystals the unit layers are held together by hydrogen bonding. In montmorillonite the interlayers bond is due to cations exchanged for those contained in hydrating solution. Water molecules can also enter between the layers. Therefore, the van der Waals interlayer bonds are generally weak. The exchange complex of soils normally consists of a mixture of the cations Ca2þ, Mg2þ, Kþ, and Naþ, in which Ca2þ comprises the larger part, and Kþ and Naþ only the smaller. The ability of soils to exchange cations with those contained in the liquid solution has important effects on soil behavior. Exchangeable cations are located on particle surfaces and are attracted by their negative charges. The resulting capacity for exchangeable cations (C.E.C.) varies according to the type of
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clay; and it can be about 100 in kaolinite, 300 in illite, and 1000 in montmorillonite (C.E.C. expressed as milliequivalents per kg of clay). Variation occurs because of the effects of crystal size, isomorphous substitution, and pH. Oxides and organic matter also have a cation exchange capacity. The exchange of one cation for another is governed by Coulomb’s law. Cations of a higher valence (higher charge) are attracted more strongly to the negatively charged clay than those of a lower valence. Hence Ca2þ generally replaces Naþ in any reaction. However, if the concentration of lower valence ions (i.e., Naþ) is relevant and much greater than that of higher valence (i.e., Ca2þ), the ion exchange phenomenon can be in an opposite direction. There is also a tendency for larger cations to enter more readily in exchange than smaller ones of the same valency. This is attributed to a greater hydration of the smaller ions that, with a larger hydrated radius, are thus more distant from the negative charges on the clay. When adsorption of water has progressed far enough for swelling, clay has then enough water for a diffuse layer of exchangeable cations to extend out from the negatively charged surface of the mineral sheets. Phenomenon is governed by the opposing effects of electrostatic attraction toward the surface and diffusion away from it in the direction of decreasing concentration. The clay surface, with its localized negative charges, and the swarm of cations of opposite charge constitute a diffuse electrical double layer. If an outer solution bathes the clay, this will affect the distribution of the cations. Also because of the charge on the clay, the concentration of anions near its surface will be lower than in the outer solution (negative adsorption of anions). The effects of liquids may be evaluated with the GouyeChapman theory, which states that the thickness (t) of the diffuse double layer varies as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D$K$T (21) t ¼ 8$p$n0 $ε2 $v where D is the dielectric constant, K is the Boltzmann constant (1.38 106 erg/K), T is the temperature, n0 is the electrolyte concentration, ε is the unit electronic charge (16 1020 C), and v is the cation valence. When the thickness of the diffused double layer decreases, the structure of the clayey soil tends to be flocculated with an increase in permeability. The process is inverse when the thickness t increases. For example, solutions containing monovalent cations tend to produce lower k than solutions with polyvalent cations. Pore fluid composition influences not only the double-layer thickness but also the soil structure (which can alternatively become flocculated or dispersed) and the geotechnical properties of soil (Table 7.2.6). The complexity of the above-mentioned phenomena can produce large fluctuations of the geotechnical parameters of clayey soils.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann
Table 7.2.6 Effect of pore-fluid parameters on Diffuse Double-Layer thickness
and soil structure Parameter
Parameter related to the pore fluid
DDL Thickness
Soil Structure
Electrolyte concentration
þ
þ
F D
Ion valence
þ
þ
F D
Dielectric constant
þ
þ
D F
Temperature
þ
þ
D F
pH
þ
þ
D F
(þ) increase, (-) decrease. (F) flocculated, (D) dispersed.
CONCLUSIONS Mineral barriers at the base of an MSW landfill must be carefully designed. The material composing them must be carefully selected and put in place using compaction procedures initially defined by test field. Clay liners can be crossed by pollutants in several ways. When hydraulic conductivity is very low, possible release of pollutants will be mainly by molecular diffusion. Attenuation processes can slow the transport of several contaminants. Leachate can attack and compromise the efficiency of the barriers, but only concentrated chemicals are of serious concern. Based on transit time calculations, clay liners can be very effective barriers and minimize release of pollutants from landfills.
References Alberta Environment, March 1985. Design and Construction of Liners for Municipal Wastewater Stabilization Ponds. prepared by Komex Consultants. Ltd.. Alberta Environment Publication, Edmonton, Alberta, Canada. ASTM Test Method D698: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort. ASTM Test Method D1557: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort. ASTM Test Method D1556/D1556M: Standard Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method. ASTM Test Method 6938: Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth). ASTM Test Method D6391: Standard Test Method for Field Measurement of Hydraulic Conductivity Using Borehole Infiltration.
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ASTM Test Method D5093: Standard Test Method for Field Measurement of Infiltration rate Using Double-Ring Infiltrometer with Sealed-Inner Ring. ASTM Test Method D5126: Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone. Daniel, D.E., September 1989. In situ hydraulic conductivity tests for compacted clay. Journal of Geotechnical Engineering 115 (9), 1205e1226. Daniel, D.E., 1994. State-of-the-art: laboratory hydraulic conductivity tests for saturated soils. In: Daniel, D.E., Trautwein, S.J. (Eds.), Hydraulic Conductivity and Waste Contaminant Transport in Soil, ASTM STP 1142. ASTM, Philadelphia, PA, pp. 30e78. Eijkelkamp Soil & Water, November 2011. Guelph Permeameter e Operating Instructions. Evans, J.C., Fang, H.Y., Kugelman, I.J., 1985. Organic fluid effects on the permeability of soil-bentonite slurry walls. In: Proc. National Conf. On Hazardous Wastes and Environmental Emergencies, Cincinnati, OH, USA, pp. 267e271. Lambe, T.W., 1958a. The structure of compacted clay. Journal of the Soil Mechanics and Foundation Division 84 (SM2), 16541e1654-34. ASCE. Lambe, T.W., 1958b. The engineering behaviour of compacted clay. Journal of the Soil Mechanics and Foundation Division 84 (SM2), 1655-1e1655-35. ASCE. Seed, H.B., Chan, C.K., 1959. Structure and strength characteristics of compacted clays. Journal of the Soil Mechanics and Foundation Division 85 (SM5), 87e128. ASCE. Sharma, H.D., Lewis, S.P., 1994. Waste Containment Systems, Waste Stabilization and Landfills. John Wiley & Sons Inc., USA. ISBN:0-471-575336-4. U.S. Environmental Protection Agency (USEPA), 1989. Requirements for Hazardous Waste Landfill Design, Construction and Closure. Seminar Publication, EPA/625-89/022, USEPA, Cincinnati, OH, 127 pp.
Further Reading ASTM Test Method D2167: Standard Test Method for Density and Unit Weight of Soil in Place by the Rubber Balloon Method. ASTM Test Method D6780/D6780M: Standard Test Method for Water Content and Density of Soil In situ by Time Domain Reflectometry (TDR). Daniel, D.E., Anderson, D.C., Boynton, S.S., 1985. Fixed-walls versus flexible-wall permeameter. In: Hydraulic Barriers in Soil and Rock, ASTM STP 874. ASTM, Philadelphia, pp. 107e126. ISBN:0-08030-04117-0. Favaretti, M., Moraci, N., 1995. Effects of leachate on the behaviour of sand-clay mixtures. In: 5th International Landfill Symposium “Sardinia 95”, S. Margherita di Pula, Cagliari, Italy. Favaretti, M., Moraci, N., 1997. Geotechnical behaviour of sand-sodium bentonite mixtures. In: 6th International Landfill Symposium “Sardinia 97”, S. Margherita di Pula, Cagliari, Italy. Holtz, R.D., Kovacs, W.D., Sheahan, T.C., 2011. An Introduction to Geotechnical Engineering, second ed. Pearson, USA. ISBN:0-13-701132-6. Qian, X., Koerner, R.M., Gray, D.H., 2002. Geotechnical Aspects of Landfill Design and Construction. Prentice Hall, USA. ISBN:3-13-012506-7.
SOLID WASTE LANDFILLING j Concepts, Processes, Technologies j R. Cossu, R. Stegmann