Design Parameters for Abandoned Dumpsite Soil as Liner Material
Adefemi Afeez BELLO
Department of Civil Engineering, Faculty of Engineering,College of Science, Engineering and Technologt, Osun State University, P.M.B 4494, Osogbo, Nigeria.
E-mail: adefemisola@yahoo.com
* Corresponding author: Phone: +2348062481383
Abstract
This paper presents the findings of laboratory tests that were carried out on abandoned dumpsite soil from Orita-Aperin, Ibadan, and Southwestern Nigeria, to determine its potentiality as hydraulic barrier. Three soil samples were collected (labeled AB1 - AB3) and compacted at –2, 0, 2 and 4% of optimum moisture content using four compactive energy levels namely reduced Proctor (RP), Standard Proctor (SP), West African Standard (WAS) and modified Proctor (MP). The design parameters namely: hydraulic conductivity, volumetric shrinkage and unconfined compressive strength showed appreciable variations in their respective values as moulding water contents and compactive efforts were varied. The soil samples studied had hydraulic conductivity that was less than or equal to 1 x 10-7cm/s provided that: the initial dry unit weight is greater than or equal to 16.48 kN/m3; the initial degree of saturation is greater than or equal to 85%; compaction is carried out at a compactive effort greater than or equal to that of the standard Proctor. Hence, the samples can be used as hydraulic barrier in waste containment structure.
Keywords
Hydraulic conductivity; Volumetric shrinkage; Unconfined compressive strength; Waste containment structure.
Introduction
Solid waste disposal problem in our cities has become a key issue in these environmentally conscious times. Coincidentally, there has been a phenomenal increase in the volume and range of solid waste generated daily within the past few years. This is due largely to the increasing rate of population growth, urbanization and industrialization. This scenario has really brought the culmination of knowledge into environmental geotechnics (a diverse discipline relative to several other disciplines concerned with subsurface environmental control and protection). This includes a wide range of soils and materials, such as synthetically manufactured materials (e.g., geosynthetics and geosynthetic clay liners), low-permeability fine-grained soils used in containment barriers, and high-permeability coarse-grained materials (e.g., aquifer materials) typically associated with subsurface pollution of ground water.
Many communities in Nigeria rely on surface and groundwater as a primary source of drinking water of which a variety of threats to its quality exist. Rapid industrial development in developing countries has increased hazardous waste generation several folds. Heavy metals, organic compounds and other toxic effluents continue to be deliberately released into the environment by manufacturing, mining, oil firm etc. Streams and other sources of domestic water consumption especially those in rural areas are now known to have recorded lethal levels of toxicity with attendant risks to human lives [1, 2]. Although there are some efforts to reduce and recover the waste, disposal in landfills is still the most common method for waste destination. Subsurface pollution from these wastes occurs when water that has leached potentially harmful chemical species migrate through these wastes ultimately reaches the environment beneath the waste. The amount and quality of the contaminated water generated by the waste depends primarily on the conditions, the physical and chemical properties of the waste involved.
Natural liners have the following attributes which made them suitable as liners system: they contain significant amounts of clay minerals and have hydraulic conductivities less than or equal to 1 x 10-7 cm/s; the material should have adequate shear strength (a minimum unconfined compressive strength of 200 kN/m2) and be durable to withstand the destructive forces of alternating wet/dry and freeze/thaw cycles [3, 4]; it should also have minimum potential for shrinkage and cracking (natural or compaction induced) upon drying. Maximum allowable value of 4 % volumetric shrinkage of compacted cylinder upon drying has been adopted by some investigators – [5, 6]. Highly plastic clays, clayey soils of low plasticity, sand-bentonite mixtures, sand-kaolinite mixtures, bentonites, pozzolanic fly ash or pozzolanic fly ash-sand mixtures, bagasse ash mixture, blast furnace slag, geosynthetic clay liners, geomembrane liners, foundry green sands, laterite-bentonite mixture, laterite-bagasse ash mixture, etc [7 - 9] are examples of materials used as liners and covers in waste containment structures.
Thus, the main purpose of soil liners and covers is to serve as hydraulic barriers to impede flow of fluids and contaminants across them. They are therefore designed to have (1) low hydraulic conductivity, (2) minimal desiccation-induced volumetric shrinkage, and (3) adequate shear (or in general mechanical) strength to ensure their structural integrity. Laboratory evaluation of soils to be used as hydraulic barriers will usually involve hydraulic conductivity, volumetric shrinkage, unconfined compressive strength as well as indirect tensile strength tests on compacted soil specimens. As a result of this, this research investigates the design parameters of abandoned dumpsite soil from Orita-Aperin, Ibadan, and Southwestern Nigeria to determine its potentiality for hydraulic barriers.
Materials and Method
Sampling of soils
The soil samples used in this study work are a natural material that is yellowish brown soil from a borrow pit at Orita-Aperin abandoned dumpsite, Ibadan, (latitude 7°30’ and longitude 4°56’) Oyo state, Nigeria using the method of disturbed sampling. The soil samples were obtained at depths of 1.80 – 3.90m and designated as AB1, AB2 and AB3. The soil samples were collected in large-to-medium-sized bags and thereafter transported to the Soil Mechanics Research Laboratory of the Department of Civil Engineering, Ahmadu Bello University (ABU), Zaria. Each soil sample was spread and allowed to air-dry under laboratory conditions.
Compaction
The sample specimens tested were prepared by mixing the relevant quantity of dry soil samples previously crushed to pass through BS No.4 sieve with 4.76 mm aperture as outlined by [10] as well as [11]. The specimens were moulded at water content in the range 5.25 - 25.5% and four different compactive efforts similar to those that might be achieved in the field. The compaction methods used included the reduced Proctor (RP) effort described by [10] which is equivalent to the Reduced British Standard Light (RBSL). The standard Proctor (SP) or British Standard Light (BSL) and modified Proctor (MP) or British Standard Heavy (BSH) is in accordance with [12]. The West African Standard (WAS) compaction is outlined in the [13].
Five to seven batches of soil each weighing 2.5 kg was placed in a tray and mixed with tap water. The reduced and standard Proctor compactions utilized 3 layers applying 15 and 27 blows each of a 2.5kg rammer falling from a height of 300mm using 1000cm3 moulds respectively. The modified Proctor compactive effort involved the use of the same mould with a 4.5 kg rammer falling from a height of 450 mm applying 27 blows each and compacting in 5 layers. For the West African Standard compactive effort which is the conventional energy level commonly used in the region [14, 15] consist of energy level derived from a 4.5 kg rammer falling through 450 mm height onto five layers using 10 blows each.
Design parameters
Materials for landfill liners/covers are usually investigated for a number of parameters which are considered to be relevant to their proper functioning under service condition. The design parameters investigated in this study include hydraulic conductivity; desiccation-induced volumetric shrinkage and unconfined compressive strength of the materials i.e. soil. Other parameters that must be considered in clay liner design include indirect (splitting or Brazilian) tensile strength, bearing capacity, trafficability, internal and interface shear strengths as well as compressibility [2]. However, in this investigation, only the three afore-mentioned design parameters have been considered.
Hydraulic conductivity
Air-dried soils specimens for hydraulic conductivity tests were first mechanically crushed to smaller sizes enough to pass through 4.76 mm aperture (BS No. 4 sieve size). The test specimens were mixed with ABU, Zaria tap water to the desired moulding water contents. The hydraulic conductivity tests were carried out in rigid-wall compaction mould permeameters using the falling head method similar to the method reported by [4]. Samples compacted at optimum (0), two (2) and four (4) percents dry (-) and wet (+) of the optimum using the four compactive efforts were soaked for at least 48 hours to achieve full saturation in accordance with Head [10]. Compacted specimens together with compaction moulds were first placed in immersion tanks and water was gradually introduced such that the top of the compacted soil was covered with about 3-5 cm of water. The placement of the compacted specimen and mould in the immersion tank was to ensure that there would not be any drying of the sample specimen from the lower open end of the mould. A relatively short sample was connected to a stand pipe, which provided both the head of water and the means of measuring the quality of water flowing through it. After saturation, test specimens were connected to a permanent liquid (distilled water). Readings were taken intermittently and the changes in water height under 8 hour’s intervals were measured. Permeation was terminated when the hydraulic conductivity values were generally within 10% of the average values or when steady state condition was reached for the more permeable specimens. The geometric mean of the last three readings were computed and reported as the hydraulic conductivity of each of the thirty (30) samples
Strength
Unconfined compressive strength (UCS) tests were conducted on soil specimens previously mixed with ABU Zaria, tap water and compacted at moulding water contents in the range 5.25 - 25.5% using four compactive efforts. Compacted specimens were sealed in plastic lugs and allowed to stand for at least 24 hours before trimming (for UCS test specimens) and testing. Each strength test involved the use of sixty (60) specimens. At least two trimmed specimens (38 mm diameter by 76 mm high) per moulding water content per compactive effort were used in the UCS testing.
Volumetric shrinkage
Sixty (60) sample specimens were compacted using four different compactive efforts at optimum moisture content (OMC), two and four percents dry and wet of the optimum moulding water content. These samples were carefully extruded from the 1000 cm3 moulds and left on a table to dry gradually at laboratory room temperature 25±20C. The weights and dimensions of the specimens were measured regularly for 28 days until when no change in the dimensions or weights were observed.
Design of overall acceptable zones
A procedure for developing the compaction criteria for soil liners and covers has been described by [10] and it involves:
(1) Compacting and permeating a soil over ranges of moulding water contents and compactive efforts as well as definition of an acceptable zone of suitable moulding water contents and corresponding dry unit weights.
(2) Modifying the acceptable zone already delineated on the basis of hydraulic conductivity using appropriate geotechnical parameters such as volumetric shrinkage strain and soil strength.
This procedure which was employed by [11] for clayey sand was used in this study. The following three conditions were established here:
(a) The compacted soil should have a low hydraulic conductivity, with 1 x 10-7cm/s as the maximum allowable value for soil liners and covers [10]
(b) In the absence of any other definitive value, 4% volumetric shrinkage strain of compacted soil cylinders upon drying [10, 11] is assumed to be the maximum allowable for soil specimen. Soils with minimal volumetric shrinkage upon drying are considered to have minimal potential to crack upon drying.
(c) A minimum unconfined compressive strength of 200kN/m2 which is the lowest value for very stiff soils based on a classification by [16] was used in this study. It is considered necessary that compacted lateritic and abandoned dumpsite soils liners and covers should have adequate strength in order to ensure their structural integrity. It has been shown [17, 18] that once the unconfined compressive strength condition is satisfied, the tensile strength condition is also satisfied; only unconfined compressive strength is used in defining the acceptable zones for the soil specimen under consideration.
Results and Discussion
Delineation of acceptable zone
The development and execution of a construction quality assurance plan has been described as the basis on which the successful design and construction of hydraulic barriers is based. According to [10], most engineers rely primarily on field-measured water content and dry unit weight to verify proper compaction of the soil. Two compaction criteria have been identified [11]: the common and the modern criteria. The common compaction control criterion has been identified [12] (i.e., traditional approach to construction quality assurance) and described procedures for establishing quality assurance using the modern criterion. For the purpose of this research, the modern approach was used.
Acceptable zones based on modern criterion
The acceptable zone in this case is usually established first with hydraulic conductivity as the primary design parameter. A certain procedure has been worked on by [11] which were used by [5] to develop the hydraulic conductivity-based acceptable zones as shown in Fig.1a-c, while that of the one based on shear strength is shown in Fig.2a-c. Also, those based on desiccation-induced volumetric shrinkage are shown in Fig.3a-c. The hatched zones indicate portions on the compaction plane showing the limits based on different criteria for the acceptable zones.
Figure 1(a). Acceptable zone based on hydraulic conductivity for AB1
Figure1(b). Acceptable zone based on hydraulic conductivity for AB2
Figure 1(c). Acceptable zone based on hydraulic conductivity for AB3
Figure 2(a). Acceptable zone based on shear strength for AB1
Figure 2(b). Acceptable zone based on shear strength for AB2
Figure 2(c). Acceptable zone based on shear strength for AB3
Figure 3(a). Acceptable zone based on volumetric shrinkage for AB1
Figure 3(b). Acceptable zone based on volumetric shrinkage for AB2
Figure 3(c). Acceptable zone based on volumetric shrinkage for AB3
Design of overall acceptable zones based on modern criterion
The acceptable zones based on desiccation-induced volumetric shrinkage strain and strength conditions were superimposed on the earlier defined acceptable water content/dry unit weight ranges which were based on hydraulic conductivity only. This procedure is in conformity with the suggestions of [10] and agrees with that employed by [5]. The results of the superimposition are shown in Figures 4a-c.
Figure 4(a). Acceptable zone for AB1 based on low hydraulic conductivity, low desiccation induced shrinkage and high unconfined compressive strength
Figure 4(b). Acceptable zone for AB2 based on low hydraulic conductivity, low desiccation induced shrinkage and high unconfined compressive strength
Figure 4(c). Acceptable zone for based on low hydraulic conductivity, low desiccation induced shrinkage and high unconfined compressive strength
Conclusion
The soil samples studied had hydraulic conductivity that was less than or equal to 1 x 10-7cm/s provided that: the initial dry unit weight is greater than or equal to 16.48 kN/m3; the initial degree of saturation is greater than or equal to 85%; compaction is carried out at a compactive effort greater than or equal to that of the standard Proctor. Hence, the samples can be used as hydraulic barrier in waste containment structure.
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