Assessment of Physical Properties of a Granite Residual Soil as an Isolation Barrier
Postgraduate Student, Department of Civil & Structural Engineering, University Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor, Malaysia email@example.com
Associate Professor, Dept of Civil & Structural Engineering, University Kebangsaan Malaysia, 43600 UKM, Bangi, Selangor, Malaysia firstname.lastname@example.org
This study investigates the physical properties of a granite residual soil for its potential use as an isolation barrier for waste materials in subsurface disposal facilities. The results of this investigation show that the soil has hydraulic conductivity lower than the suggested limit (1x10-7 cm/s) of the various waste regulatory agencies. In addition, it has adequate strength for stability, and exhibits small shrinkage potential upon drying. These properties suggest the potential use of the soil as a soil-based isolation barrier for containment of waste materials.
Keywords: residual soil; soil liner; isolation barrier; hydraulic conductivity; volumetric shrinkage strain.
Waste materials in waste containment facilities (such as landfills) are made isolated from surrounding environment by providing liners or isolation barriers. Isolation barriers control or restrict the migration of pollutant from the landfill into the environment. Commonly used isolation barriers are composed of compacted natural inorganic clays or clayey soils. Compacted clayey soils have the longest history of successful application as isolation barriers in landfills. The low hydraulic conductivity of compacted clayey soils combined with their availability and relatively low cost makes them potential materials to use as liners in landfills for environmental protection.
It is desirable for waste containment systems to achieve its required purposes at minimum cost. Careful consideration should therefore be given to the choice of materials for the construction of isolation barrier. Granite residual soil is widely distributed all over the Peninsular Malaysia. Its traditional geotechnical properties has much been studied in the past (Taha et al. 2000, 2001, 2002). Its potential use as isolation barrier may significantly reduce the cost of construction of the waste containment facilities. However, for use as isolation barrier, a particular soil should have include the following physical properties: (1) hydraulic conductivity of at least 1x10-7 cm/s (Daniel 1993a; Mohamed and Antia 1998), (2) minimum unconfined compressive strength of 200 kPa (Daniel and Wu 1993), and (3) volumetric shrinkage upon drying of less than about 4% (Daniel and Wu 1993; Tay et al. 2001).
This paper aims to study the physical properties of granite residual soil, for potential usage as isolation barriers in landfills. Typical tests that are generally used to investigate clayey soils proposed as isolation barriers such as particle size distribution, Atterberg limits, compaction, hydraulic conductivity, volumetric shrinkage, and compressive strength were conducted on samples of compacted granite residual soil. If on the basis of these tests, the soil proves to have properties desirable for a barrier material, then it should be considered as a potentially suitable material for the isolation of waste materials in landfills.
MATERIAL AND METHODS
The material used for this study was granite residual soil. The soil was obtained from a granite formation in the compound of Hospital University Kebangsaan Malaysia (HUKM) in Cheras, about 8 km south of Kuala Lumpur, the capital city of Malaysia. The soils were excavated with a shovel from the depth of 0.6m below the profile to avoid humus layer and roots, placed in plastic bags, and transported to the geotechnical laboratory of University Kebangsaan Malaysia. Then, the soils were removed from the plastic bags, carefully blended, and stored in a plastic container.
The basic tests such as specific gravity, particle size distribution and Atterberg limits of the soil were performed according to British Standard (BS1377: 1990). The data of these index properties were used to classify the soil in the Unified Soil Classification System (USCS).
The soil was air dried and crushed into small pieces. The crushed soils were then passed through a No. 4 sieve (4.75 mm openings). The sieved soils were wetted spraying tap water (pH= 6.65). Afterward the moistened soils were sealed in plastic bags and stored for at least 3 days to allow moisture equilibration and hydration. This processed soil was used for compaction, hydraulic conductivity, compressive strength and volumetric shrinkage tests. These experiments were conducted in triplicate for each particular soil condition to ensure the reliability of the test results. The average result of the three replicate measurements has been reported in this paper.
The soil as compacted with three energies (modified, standard, and reduced) as recommended by Daniel and Benson (1990). The modified and standard Proctor tests were done according to BS methods (BS1377: Part 4:1990:3.3 and BS1377: Part 4:1990:3.5). Reduced compaction was identical to standard Proctor compaction except that only 15 blows of the hammer per lift were applied rather than the usual 27 blows. This range of energy was selected in an effort to bracket the range of reasonable compactive energy normally encountered in the field (Daniel and Wu 1993).
The hydraulic conductivity was measured using rigid wall permeameter under falling head condition as recommended by Head (1994). Processed soils were compacted with three energies (modified, standard, and reduced) at different water content within the permeameter moulds. The permeant liquid was deaired tap water, and hydraulic gradient was 15. Permeation was conducted on the samples until steady conditions were achieved.
The volumetric shrinkage upon drying was measured by extruding compacted cylindrical specimens from the compaction moulds and allowing the cylindrical specimens to dry on a laboratory table in an air-conditioned room (Daniel and Wu 1993). Every day the diameter and the height of the samples were recorded with a digital caliper accurate to 0.01mm. At each reading a minimum of three height and three diameter measurements for each height at 120° intervals were recorded. The average diameter and height were used to compute volume, and the measurements were continued until the volume ceased to change further.
The unconfined compression test was performed in accordance with the ASTM D2166 procedure. The test as performed on cylindrical specimens having a diameter and length of 50 mm and 100 mm respectively, which were trimmed from the larger compacted cylinders. The samples were tested in triaxial compression test machine without applying cell pressure. The rate of strain was 0.83% per minute (Mohamed et al. 2002).
EXPERIMENTAL RESULTS AND DISCUSSIONS
Specific gravity of the granite residual soil is about 2.63. The particle size analysis shows that the soil contains 45% clay (<0.002 mm), 66% fines (<0.075 mm), 35% sand (0.063 to 2 mm). Moreover, the results of Atterberg limits reveal the liquid limit (LL) = 68%, the plastic limit (PL) = 35% and the plasticity index (PI = LL – PL) = 33%. On the basic of these data, the granite residual soil is classified as CH (Inorganic clay with high plasticity) according to the USCS. Inorganic clay with high plasticity (CH) is typical material for landfill liner (Oweis and Khera 1998).
The soil has similar properties to cohesive soils, and therefore is likely to have desirable characteristics to minimize hydraulic conductivity. The hydraulic conductivity value of the liner material is used as the principal indicator of its containment potential. Hydraulic conductivity behavior of soil liners is greatly influenced by the particle size distribution because the relative proportions of large and small particle sizes affect the size of voids conducting flow. Liner soil should have at least 30% fines (Daniel 1993b; Benson et al. 1994) and 15% clay (Benson et al. 1994) to achieve hydraulic conductivity = 1x10-7 cm/s. Thus, the granite residual soil can be used for liner to achieve a hydraulic conductivity = 1x10-7 cm/s, as it possesses suitable amount of clay and fine fractions. Moreover, the soil contains adequate amount of sand, which may offer notable protection from volumetric shrinkage and impart adequate strength as well.
Liquid limit is an important index property since it is correlated with various engineering properties. Soils with high liquid limit generally have low hydraulic conductivity. Benson et al. (1994) recommended that the liquid limit of the liner material be at least 20%. However, soils with very high liquid limit have poor volume stability and high shrink-swell potentials. Most of the specifications for soil liners proposed by various researchers or waste regulatory agencies do not generally prescribe any limit (maximum value) for their liquid limit. As long as it does not create any working problem, soils with high liquid limit generally preferred because of their low hydraulic conductivity. Thus, the granite residual soil with liquid limit about 68% appears to be promising for use as liner.
The plasticity index is the most important criteria for selection of soil for liner construction. It is the key property in achieving low hydraulic conductivity. Literatures suggest that the plasticity index must be more than 7% (Daniel 1993b; Benson et al. 1994; Rowe et al. 1995). However, very high plasticity soil becomes sticky when wet and then becomes difficult to work with in the field. Also high plasticity soil forms hard lumps when they are dry and are difficult to break down during compaction. The hard lumps, if not properly compacted, form zones of higher hydraulic conductivity. Moreover, high plasticity soil tends to be more susceptible to desiccation cracking. For plasticity index value greater than 35, excessive shrinkage can be expected (Daniel 1991). Thus, the granite residual soil has suitable plasticity property (PI is about 33%) to minimize hydraulic conductivity and shrinkage susceptibility as well.
The activity (PI/%clay fraction) of granite residual soil is about 0.74. Thus, according to Skempton’s classification it is inactive clay. Inactive clayey soils are the most desirable materials for compacted soil liners (Rowe et al. 1995). In order to achieve a hydraulic conductivity = 1x10-7 cm/s for soil liners, soils with an activity of 0.3 or greater may be specified (Benson et al. 1994; Rowe et al. 1995). Activity is an index of the surface activity of the clay fraction. Soils with higher activity are likely to consist of smaller particles having larger specific surface area and thicker electrical double layers. Therefore, hydraulic conductivity should decrease with increasing activity. However, soils with high activity are more readily affected by pollutant if they used in containment structures (Oweis and Khera 1998).
Thus, the comparison between the index properties of granite residual soil and the index properties as recommended by various researchers for a good liner material shows that granite residual soil have suitable properties to use as liner material.
In the construction of soil liners, compaction is done to achieve a soil layer of improved engineering properties. Compaction of soils results a homogeneous mass that is free of large, continuous inter-clods voids; increases their density and strength, and reduced their hydraulic conductivity. Hydraulic conductivity is the key design parameter when evaluating the acceptability of a liner material. Low hydraulic conductivity is achieved when the soil is compacted close to its maximum dry density. Thus, compaction test is performed to determine the maximum dry density and corresponding optimum water content for a soil under a specific compactive effort.
The compaction curves for the granite residual soil are shown in Figure 1. The compaction curves clearly illustrate that the dry density is the function of compaction water content and compactive effort. For each compactive effort, at the dry side of optimum water content the dry density increases with the increasing water content. This is due to the development of large water film around the particles, which tend to lubricate the particles and make them easier to be moved about and reoriented into a denser configuration (Holtz and Kovacs 1981). Whereas, at the wet side of optimum water content water starts to replace soil particles in the compaction mold and since the unit weight of water is much less than the unit weight of soil the dry density decreases with the increasing water content.
Figure 1. Compaction curves
The curves (Figure 1) are single peaked and parabolic in shape, which is typical of most clayey soils. Since the liquid limit of the soil is in between 30% and 70%, the yielding of single peaked curves (figure 1) are therefore generally expected (Lee and Suedkamp 1972). The most important feature on the compaction curve is its peak, which represents the maximum dry density and corresponding optimum water content for a given compactive effort. The maximum dry density and the optimum water content obtained from this test are given in Table 1. An increase in compactive effort increases the maximum dry density but decreases the optimum water content. Because higher compactive effort yields a more parallel orientation to the clay particles, which gives a more dispersed structure; the particles become closer and a higher unit weight of compaction results (Das 1998).
Table 1. Maximum dry density and corresponding optimum water content
|Compactive efforts||Optimum water content, wopt (%)||Max. dry density, gd (kN/m3)|
The relationship between hydraulic conductivity, water content and compactive effort is shown in Figure 2. The hydraulic conductivity decreases with the increasing compactive effort. Because increasing compactive effort decreases the frequency of large pores and can eliminate the large pore mode (Acar and Oliveri 1989). These changes in pore size yield lower hydraulic conductivity. The hydraulic conductivity also changes with the change of compaction water content. Soils compacted at dry of optimum water content tend to have relatively high hydraulic conductivity whereas soils compacted at wet of optimum water content tend to have lower hydraulic conductivity. Increasing water content generally results in an increased ability to break down clay aggregates and to eliminate inter-aggregate pores (Mitchell et al. 1965; Benson and Daniel 1990; Garcia-Bengochea et al. 1979). In addition, the clay particles are more uniformly dispersed and the macropores become constricted and tortuous (Barden 1974). Moreover, increasing water content result in reorientation of clay particles and reduction in the size of interparticle pores (Lambe 1954; Acar and Oliveri 1989). The hydraulic conductivity is the key parameter affecting the performance of most soil liners and covers (Daniel 1987, 1990), thus great attention is generally focused on ensuring that low hydraulic conductivity is achieved. Therefore, it is usually preferred to compact the soil wet of optimum.
Figure 2. Hydraulic conductivity versus compaction water content
Soil liners should have a hydraulic conductivity of at least 1x10-7 cm/s. Figure 2 shows that all the three different compaction efforts cause hydraulic conductivity less than 1x10-7 cm/s. The minimum hydraulic conductivity and corresponding water content at various compactive efforts is presented in Table 2. In the case of each compactive effort the minimum hydraulic conductivity is obtained at water content of slightly (0.5 to 1.7%) wet of optimum water content. Generally the lowest hydraulic conductivity of clayey soil is achieved when the soil is compacted at water content slightly higher than the optimum water content (Mitchell et al. 1965; U.S. Environmental Protection Agency 1989; Daniel and Benson 1990).
Table 2. Minimum hydraulic conductivity and corresponding water content at various compactive efforts
|Compactive efforts||Minimum hydraulic conductivity (cm/s)||Water content (%) at minimum hydraulic conductivity||Optimum water content (%)|
Compacted soil liners are subject to frequent desiccation due to evaporative water losses. Desiccation leads to the development of shrinkage cracks. Cracks provide pathways for moisture migration into the landfill cell, which increases the generation of waste leachate, and ultimately increases the potential for soil and groundwater contamination. Thus, the soil liner significantly losses its effectiveness as an impermeable barrier. Literature suggested that cracking do not likely to occur in soil liners when compacted cylinders of the same soil undergo less than about 4% volumetric shrinkage strain upon drying (Daniel and Wu 1993; Tay et al. 2001).
Figure 3. Volumetric shrinkage strain versus compaction water content
In this study compacted cylindrical specimens were used to determine shrinkage potential of the soil. In the field, the soil shrinks under the overburden pressure. Soil shrinks simply due to water loss, which is independent of the pressure if water and soil particles are considered incompressible. Much information is not available on the relationship between overburden pressure and volumetric shrinkage of compacted soil. In a recent study Briaud et al. (2003) reported that vertical pressure does not influence the volumetric shrinkage. However, in this study shrinkage tests were performed with no overburden pressure applied. The specimens were allowed to dry at approximately 27oC (the mean temperature for the Peninsular Malaysia) temperature to simulate the slow rate of drying that occurs in the field. The Cylindrical specimens began to shrink into smaller cylinders. Volume change occurs as the water surrounding the individual soil particles of the specimens is removed, the soil particles move closer together. The drying tests were conducted for a period of approximately 1 month to get the possible maximum volume change, although volume change did not occurred past the two weeks of drying. During drying the sides of the specimens were open to the atmosphere, which does not replicate the field condition. Drying from the top surface only requires much longer times, and was not practical. Nevertheless, although the laboratory drying conditions did not replicate the field conditions precisely, the relative effects of soil type on volumetric shrinkage are supposed to be preserved. The result of volumetric shrinkage test is presented in Figure 3. Test results indicate that shrinkage strains are influenced by compaction conditions. Shrinkage increases with increasing compaction water content, but the relationship between compactive effort and shrinkage strain is less clear. At low compaction water contents, shrinkage decreases with increasing compactive effort. No clear trend is apparent at higher water contents. Similar results have been reported by other researchers (Klepe and Olson 1985; Daniel and Wu 1993). In this study, each of the three different compactive efforts show little volume change behavior of less than 4%, which is typical maximum permissible limit for compacted soil liners. Thus, if drying takes place, the compacted soil will undergo minimal shrinkage and desiccation cracking.
Unconfined Compressive Strength
The result of unconfined compression test against compaction water content is shown in Figure 4. The strength of compacted soil decreases with the increase of compaction water content. As the amount of water increases the electrolyte concentration is reduced, leading to an increase in diffused double layer. Expansion takes place and the distance between the clay particles as well as the distance between the aluminiosilicate unit layers increases, resulting in a reduction of both the internal friction and cohesion. Other researchers (Seed and Chan 1959; Daniel and Wu 1993; Taha and Kabir 2003) observed the same fact. Compactive effort has also a great influence on soil strength. At low compaction water content, unconfined compressive stress increases with increasing compactive effort. But at higher water content no clear trend is noticed; e.g. at 24% compaction water content, modified Proctor effort results the lowest unconfined compressive stress among the three compactive efforts.
Figure 4. Unconfined compression strength versus compaction water content
An isolation barrier used in waste containment a system is supposed to sustain certain amount of static load exerted by the overlying waste materials. In this regard, the barrier material must have adequate strength for stability. The bearing stress acts on the barrier system depends on the height of landfill and the unit weight of waste. Thus, to date, the minimum required strength of soil used for compacted soil liners is not specified. Daniel and Wu (1993) arbitrarily selected them, to support the maximum bearing stress in a landfill. They mentioned that soil used as barrier material should have minimum unconfined compression strength of 200 kPa. Test result shows (Fig. 3) that the soil possesses higher strength than the recommended minimum strength of 200 kPa for all the three compactive efforts.
Acceptable Water Content and Dry Density
A critical step in designing of a compacted soil liner is determination of the range of acceptable water content and a minimum dry density of the soil. Water content and dry density values can greatly affect a soil’s ability to restrict the transmission of flow. Even small variations in water content and dry density may results a tremendous change to the hydraulic conductivity (Mitchell et al. 1965). From the above discussions presented in the previous sections it is clear that the physical properties (such as hydraulic conductivity, strength, and shrinkage potential) controlling the performance of soil liners are greatly influenced by water content. If the soil is too dry at the time of compaction, suitably low hydraulic conductivity may become unachievable. If the soil is too wet, a variety of problems may ensue, e.g., problems with construction equipment operating on soft, weak soils and potential slope instability caused by low strength of the soil. In addition, very wet soil may crack due to desiccation shrinkage. Thus, it is very important to specify the range of water contents within which the compacted soil will exhibit hydraulic conductivity = 1x10-7 cm/s, volumetric shrinkage = 4% and unconfined compressive strength = 200 kPa. The acceptable range of water content is given in Table 3. Only the modified Proctor compaction test meets the acceptable limits. The overall acceptable water content ranges from 16.5% to 21.1%, which is between 4.2% dry of optimum and 0.4% wet of optimum value determined from modified Proctor compaction test. So the granite residual soil meets all the three main requirements at a wider range of water content. In addition, the soil can be compacted at water content of 4.2% dry compared to optimum water content and still possesses a hydraulic conductivity less than 1x10-7 cm/s. This water content is advantageous in terms of minimizing shrinkage potential.
Table 3. Acceptable range of water content
|Compactive efforts||Acceptable range of water content (%) for hydraulic conductivity||Acceptable range of water content (%) for Volumetric shrinkage||Acceptable range of water content (%) for Unconfined compressive strength||Overall acceptable range of water content (%)|
|Modified compaction||16.5 to >26||<16 to 21.1||<16 to 23.3||16.5 to 21.1|
|Standard compaction||25.1 to 31.9||<22 to 23.1||<22 to 29||…|
|Reduced compaction||27.1 to 27.9||<23 to 23.8||<23 to 28.8||…|
Since, an overall acceptable range of water content has been selected, for the construction of isolation barrier the soil must be compacted with adequate compactive energy using the overall acceptable water content to compress large voids and to remold clods of soil into a homogeneous, relatively impermeable mass. The dry density of the soil can be a useful indicator of the effectiveness of compaction. Literatures suggest that the minimum dry density can be determined by drawing an overall acceptable zone on water content-dry density curve (Daniel and Benson 1990; Daniel and Wu 1993). In this paper a simple approach has been followed to draw an overall acceptable zone. The overall acceptable zone is shown in Figure 5 has been drawn based on overall acceptable water content (Table 3) on water content-dry density curve, considering the range of the overall acceptable water content as the left and right boundary of the zone. The upper boundary of the overall acceptable zone is the zero-air-void line and the bottom boundary is the modified Proctor compaction curve (because only the modified Proctor compaction test meets the acceptable limits). Thus, the Figure 5 shows that for the granite residual soil, a range of water content and dry densities are existed that satisfy all the three established criteria (hydraulic conductivity, strength, and shrinkage strain).
Figure 5. Overall acceptable zone and recommended acceptable zone
However, the “initial degree of saturation” has significant influence on hydraulic conductivity. Elsbury et al. (1990) plotted the results of hydraulic conductivity test on laboratory-compacted specimens as a function of initial degree of saturation. The general trend was that hydraulic conductivity decreased with increasing initial degree of saturation. Other researchers also have been presented the similar results (Benson et al. 1994; Benson and Trast 1995; Benson et al. 1999). Benson and Trast (1995) achieved the hydraulic conductivity less than 1x10-7 cm/s, the maximum suggested limit of the various waste regulatory agencies, for all the soils in their study by compacting to an initial saturation in excess of 85%. In another study Benson et al. (1999) reported that there is a high probability of achieving the field hydraulic conductivity = 1x10-7 cm/s when most of the field compaction data point lie on or above the line of optimum water contents. The optimum point normally occurs at saturation near 85% (Benson and Boutwell 1992). Thus, the overall acceptable zone should be adjusted based on “85% saturation line” for meeting the field hydraulic conductivity criterion as shown in Figure 5. The “recommended acceptable zone” defines the minimum dry density and a range of water content that will produce compacted soil with acceptable hydraulic conductivity, strength and shrinkage strain. The “recommended acceptable zone” shown in Figure 5 could be recommended for field control directly.
The following conclusions can be drawn from the investigation of granite residual soil: (1) The granite residual soil is inorganic clay with high plasticity. Generally, this type of soil possesses desirable characteristics to minimize hydraulic conductivity, and frequently used for the construction of compacted soil liners. (2) The index properties (liquid limit, plastic limit, % clay content, % fines, activity etc.) of the soil satisfy the basic requirements as a liner material. (3) It is inactive clayey soil. Thus, the soil would be less affected by waste chemicals and also less susceptible to shrinkage. (4) The soil has hydraulic conductivity of less than 1x10-7 cm/s, when it is compacted with modified Proctor compaction effort at a range of water content between 4.2% dry of optimum and 0.4% wet of optimum water content. (5) Moreover, within the above range of water content the soil compacted with modified Proctor exhibits adequate strength of more than 200 kPa and volumetric shrinkage strain of less than 4%.
Thus, it is concluded that the granite residual soil can be used as a suitable liner material for isolating waste materials in landfills. Its potential use as isolation barrier will enhance the waste management programs in Malaysia.
Although the soil meets all the basic requirements as a good barrier material, it would be hard to work with due to its high plasticity. Therefore, during liner construction great attention should be focused on soil preparation. The soil should properly blend and homogenize to achieve a mixer of relatively small clods with reasonably uniform moisture distribution. Blending the soil on site with a pulverizing mixer would be helpful in reducing clod size and producing more uniform moisture content.
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