Unsaturated Hydraulic Conductivity of Compacted Abandoned Dumpsite Soils

Transcription

1 Annals of Science, Engineering and Technology 2012 Vol. 2, No 1. Pp ISSN: C ff(x,y)dxdy U N I O S UN College of Science, Engineering and Technology, Osun State University, Osogbo, Nigeria. Unsaturated Hydraulic Conductivity of Compacted Abandoned Dumpsite Soils Bello, Afeez Adefemi Department of Civil Engineering, Faculty of Engineering, College of Science, Engineering and Technology Osun State University, PMB 4494, Osogbo adefemisola@yahoo.com ABSTRACT Unsaturated hydraulic conductivities were measured in the Laboratory for compacted Abandoned dumpsite soil collected from Orita-Aperin, Ibadan, South-western Nigeria. The samples were compacted using four compactive efforts (reduced Proctor, standard Proctor, West African Standard and modified Proctor) at -2%, 0% and +2% wet of optimum moisture content. It is generally observed that Brooks-Corey (BC) model tends to over predict the volumetric water content (θ) particularly at low suction values, while van Genuchten (VG) model tends to under predict it and these values are closer to laboratory measured values. Generally, the measured and predicted Soil Water Characteristics Curve (SWCC) did not show great variations from each other, showing fairly close agreements between measured and predicted θ values. Specimens compacted with higher energies gave lower unsaturated hydraulic conductivity values. It was also noticed that the unsaturated hydraulic conductivity reduced with higher moulding water content using both the VG and BC models. The unsaturated hydraulic conductivity is below 1 x 10-7 cm/s for all the compactive efforts at low 10 kpa matric suction with the exception of reduced Proctor effort. Keywords: Compactive effort, Brooks-Corey model, van Genuchten model, Matric suction, Unsaturated hydraulic conductivity. INTRODUCTION Measurement and Prediction of unsaturated hydraulic conductivity are important for simulating unsaturated water migration through soil for engineering problems, such as in the design of earth dams or tailings impoundments, resource development, and waste management practice (Chiu and Shackelford, 1998; Fredlund et al, 2000). In waste management practice, the primary purpose of a final cover system for a waste disposal facility is to prevent the generation of leachate by minimizing the amount of precipitation percolating through the waste during inactive (postclosure) period. Water balance models are commonly used to simulate the hydrology of final covers placed on waste containment facilities because soils used are generally unsaturated (Khire et al., 1997). For earthen covers, the unsaturated hydraulic conductivity function for the barrier is of particular importance. Khire et al. (1997) have shown that using the proper unsaturated hydraulic conductivity function is essential for accurately simulating the hydrology of earthen landfill covers. Knowledge of the hydraulic conductivity as a function of water content, K (θ), or pressure head, K (h), is essential for many problems involving water flow and mass transport in unsaturated soils (Arya et al; 1999; 1

2 Leong and Rahardjo, 1997). Values of soil hydraulic properties are necessary to quantify water movement and transportation in unsaturated soils (Warrick, 1993). Water coefficient of permeability or the effective water conductive porosity in an unsaturated soil is a function of pore-water pressure or water content (Fredlund, 1996; Leong and Rahardjo, 1997). The water content of a given soil is in turn a function of the stress state of the soil; there is then a relationship between the hydraulic conductivity and the Soil water characteristics curve (SWCC) (Fredlund, 1996). The determination of unsaturated hydraulic conductivity of soils is difficult due to its variability and time dependency, thus it is predicted mostly using models that are based on the saturated hydraulic conductivity and parameters of the SWCC. The numerous permeability or hydraulic conductivity functions available for unsaturated soils can be categorized into three groups (Leong and Rahardjo, 1997): empirical, macroscopic and statistical models. The assumptions, theoretical background and performance of each category of function (these categories were suggested by Mualem (1986) as an indication of the degree of theoretical sophistication, with the statistical models being the most rigorous) are presented by Leong and Rahardjo (1997). Empirical equations arise from the need for an equation to describe the variations of permeability with matric suction or volumetric water content w, that is, (Leong and Rahardjo, 1997): k r = ( ) or f( w ) (1) where k r is the relative hydraulic conductivity defined as the hydraulic conductivity of the soil, k [=f( m )], relative to the saturated hydraulic conductivity, k sat, of the soil, or Relative hydraulic conductivity function (K r ) is given by (2) where K = Unsaturated hydraulic conductivity of the soil (which is a function of ψ) K sat = Saturated hydraulic conductivity of the soil (from the falling head permeameter). Predicted values are based on Brooks Corey relative hydraulic conductivity and van Genuchten relative conductivity functions (Brooks and Corey, 1964; van Genuchten 1980). Brooks - Corey relative hydraulic conductivity function is given as. = 1 (3) van Genuchten s relative hydraulic function is given as: (4) where α, m and n are parameters of the SWCC described above Variability in soil properties significantly affects the hydraulic conductivity. In most saturated soils, it can vary up to 12 orders of magnitude, and depends on the nature of soil and compaction energy level (Fredlund et al., 2000). The measurement of hydraulic properties of unsaturated soils is cumbersome and time consuming due to its variability in the field (Kasim et al., 1999). According to Fredlund et al. (2000), numerous researchers have so far investigated the effective flow and transport properties for geologic formations under saturated conditions but very few have examined the effective properties for unsaturated media. Thus, in this study, unsaturated hydraulic conductivity were predicted for three compacted Abandoned dumpsite soil from Orita-Aperin, Ibadan, Southwestern Nigeria based on van Genuchten and Brooks Corey relative hydraulic conductivity functions. MATERIALS AND METHODS 2

3 Soils The soil used in this study is a reddish brown tropical soil from Ibadan, (latitude 7 o 27 and longitude 4 o 59 ) Southwestern Nigeria using the method of disturbed sampling. The soil samples were obtained at depths of m and designated as MP1, MP2 and MP3. The soils are classified as A-7-6 according to the Association of American States Highway and Transportation Officials Classification System and are also classified as lean clay with sand (CL), according to the Unified Soil Classification System (BSI, 1990; Head, 1992; Nigeria General Specification, 1997). The specific gravity of the soil range from while its ph range from The percentage passing BS No.200 sieve range from % (Bello and Osinubi, 2011; Bello, 2012). Compaction The 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 Head (1992) as well as Albrecht and Benson (2001). The specimens were moulded at water content in the range % 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 Daniel and Benson (1990) as well as Benson and Trast (1995) 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) are in accordance with BS 1377 (1990). The West African Standard (WAS) compaction is outlined in the Nigerian General Specification (1997). Preparation and testing of specimen Sample specimens were prepared with four compactive efforts (reduced Proctor, standard Proctor, West African Standard and modified Proctor) at -2, 0 and +2% from the dry to the wet side of the line of OMC. 2.5kg of each specimen was moistened with tap water, mixed thoroughly and compacted in BS moulds and later cored into stainless steel rings with inside diameters of 50mm and heights of 50mm with the aid of a mallet. Each of the 36 specimens was covered with caps at both ends before saturation. The samples were subjected to full saturation by capillary action for a period of 3 weeks. Volumetric pressure plate extractor The Soil water Characteristic curve (SWCC) is measured in the laboratory using volumetric pressure plate extractor, which works on the principle of axis-translation, u w is the pore water pressure). A pressure plate extractor consists of two main components: a porous plate air-entry pressure higher than the maximum matric suction to be applied during the test and a sealed pressure cell (Fredlund and Rahardjo, 1993; Tinjum et al.; 1997; Wang and Benson, 2004). The porous plate is made of either ceramic or polymeric membranes. In the drying test, the soil starts at a saturated condition and the matric suction is gradually increased leading to a reduction in the water content in the soil specimen. The airentry ceramic disk and the soil are first saturated. After saturation, excess water is removed from the cell. The soil specimen is placed on the high air-entry ceramic disk inside the retaining cylinder. The cell cover is then mounted and tightened into place and airpressure is applied to the soil specimen in series of increments to achieve different matric suction (ψ). Each increment in air-pressure cause water to be expelled from the specimen until an equilibrium state is reached for the ψ that has been established. Additional increments in outflow are applied only and increment is measured (gravimetrically or volumetrically) to define the water content corresponding to each suction (Benson and Gibb, 1997; Kasim et. al., 1999; Miller et al., 2002; Wang and Benson, 2004). Application of pressure Pressure was applied in three batches as the pressure plate equipment has capacity for only 16 specimens. The entire pressure application lasted about 2 weeks, while the entire process from specimen preparation, saturation and pressure application lasted about 3 months. Pressure was applied using regulated compressed air from a compressor. The soils were subjected to pressures of 10, 30, 100, 500, 1000 and 1500 kpa, respectively. On completion of the test, the equipment was disassembled, the soil specimen removed and placed in an oven to determine its final gravimetric water content. All computations 3

4 Volumetric water content (cm3/cm3) made were based on the original as-compacted soil volumes. prediction of soil water characteristics curves Brooks - Corey and van Genuchten equations were used in conjunction with the SWCC to estimate the unsaturated hydraulic conductivity of the samples. The predicted fitting parameters generated from measured laboratory results were used to predict the SWCC. The predicted values were in turn compared with the SWCC obtained from measured soil water retention data. The models were used to generate values of relative hydraulic conductivity for the 36 samples. RESULTS AND DISCUSSION prediction of soil water characteristics curves Brooks - Corey and van Genuchten equations shown in equations (3) and (4), respectively, were used in conjunction with the SWCC to estimate the unsaturated hydraulic conductivity of the samples. The predicted fitting parameters generated from measured laboratory results were used to predict the SWCC. The predicted values were in turn compared with the SWCC obtained from measured soil water retention data. The models were used to generate values of relative hydraulic conductivity for the 36 samples. As the initial water content increased, it became more difficult to de-saturate the specimen due to the dominance of the micro scale pores with minimal void ratio. The values of the residual water content varied in the range , while the saturated water content varied in the range for both models. From the plots, it is generally observed that Brooks-Corey model tends to over predict the volumetric water content (θ) particularly at low suction values, while van Genuchten model tends to under predict it and these values are closer to laboratory measured values. Generally, the measured and predicted SWCC did not show great variations from each other, showing fairly close agreements between measured and predicted θ values. Similar results were obtained by Tinjum et al. (1997); Chiu and Shackelford (1998) and Nwaiwu (2004). Figs. 1 to 9 showed the SWCCs for the 36 samples using standard Proctor effort Measured Brooks-Corey predicted van Genuchten predicted

5 Volumetric water content (cm3/cm3) Volumetric water content (cm3/cm3) Fig. 1: Soil water characteristic curves for AB1 on the dry side of optimum moisture content (standard Proctor compaction) Measured Brooks-Corey predicted van Genuchten predicted Fig. 2: Soil water characteristic curves for AB1 at optimum moisture content (standard Proctor compaction) Measured Brooks-Corey predicted van Genuchten predicted Fig. 3: Soil water characteristic curves for AB1 on the wet side of optimum moisture content (standard Proctor compaction) 5

6 Volumetric water content (cm3/cm3) Volumetric water content (cm3/cm3) Measured Brooks-Corey predicted van Genuchten predicted Fig. 4: Soil water characteristic curves for AB2 on the dry side of optimum moisture content (standard Proctor compaction) Measured Brooks-Corey predicted van Genuchten predicted Fig. 5: Soil water characteristic curves for AB2 at optimum moisture content (standard Proctor compaction) 6

7 Volumetric water content (cm3/cm3) Volumetric water content (cm3/cm3) Measured Brooks-Corey predicted van Genuchten predicted Fig. 6: Soil water characteristic curves for AB2 on the wet side of optimum moisture content (standard Proctor compaction) Measured Brooks-Corey predicted van Genuchten predicted Fig. 7: Soil water characteristic curves for AB3 on the dry side of optimum moisture content (standard Proctor compaction) 7

8 Volumetric water content (cm3/cm3) Volumetric water content (cm3/cm3) Measured Brooks-Corey predicted van Genuchten predicted Fig. 8: Soil water characteristic curves for AB3 at optimum moisture content (standard Proctor compaction) Measured Brooks-Corey predicted van Genuchten predicted Fig. 9: Soil water characteristic curves for AB3 on the wet side of optimum moisture content (standard Proctor compaction) on unsaturated hydraulic conductivity for different matric suctions were studied. Unsaturated hydraulic conductivity Unsaturated hydraulic conductivity was analyzed using the van Genuchten and Brooks-Corey models. The effects of compactive effort and moulding water content Effect of compactive effort The effect of compactive effort on the variation of unsaturated hydraulic conductivity with matric suction at optimum compaction 8

9 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) moisture content for all specimens using van Genuchten and Brooks-Corey models are shown in Figs. 10 to 15, respectively. The Brooks-Corey model gave a more consistent trend than van Genuchten model. Generally, specimens compacted with higher energies gave lower unsaturated hydraulic conductivity values. This was so because compaction at higher energies reduced the frequency of large pores and average pore sizes resulted in lower unsaturated hydraulic conductivity (Nwaiwu, 2004). RP - AB1 SP WAS MP Matric Suction (%) Fig. 10: Variation of unsaturated hydraulic conductivity with matric suction for AB1 at optimum moisture content using van Genuchten model. RP - AB2 SP WAS MP Matric Suction (%) Fig. 11: Variation of unsaturated hydraulic conductivity with matric suction for AB2 at optimum moisture content using van Genuchten model. 9

10 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) RP - AB3 SP WAS MP Matric Suction (%) Fig. 12: Variation of unsaturated hydraulic conductivity with matric suction for AB3 at optimum moisture content using van Genuchten model. RP - AB1 SP WAS MP Matric Suction (%) Fig. 13: Variation of unsaturated hydraulic conductivity with matric suction for AB1 at optimum moisture content using Brooks-Corey model. 10

11 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) RP - AB2 SP WAS MP Matric Suction (%) Fig. 14: Variation of unsaturated hydraulic conductivity with matric suction for AB2 at optimum moisture content using Brooks-Corey model. RP - AB3 SP WAS MP Matric Suction (%) Fig. 15: Variation of unsaturated hydraulic conductivity with matric suction for AB3 at optimum moisture content using Brooks-Corey model. Figs. 16 to 27 for different compactive efforts Effect of moulding water content The results of the effect of moulding water contents on unsaturated hydraulic at moulding water content 2% dry of optimum, optimum and 2% wet of optimum water content. At lower matric suctions specimen conductivity at various matric suctions compacted wet of optimum water content determined using van Genuchten (VG) and Brooks- Corey (BC) models are shown in generally had lower hydraulic conductivity. It was also noticed that the unsaturated hydraulic 11

12 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) conductivity reduced with higher moulding water content using both the VG and BC models. The unsaturated hydraulic conductivity is below 1 x 10-7 cm/s for all the compactive efforts at low 10 kpa matric suction with the exception of reduced Proctor effort. This could be attributed to the fact that under unsaturated conditions, the larger pores empty first and fill last (Benson and Daniel, 1994; Hillel, 1982). Larger pores responsible for high hydraulic conductivity dry of optimum gradually became inactive first as the soil desaturated, while samples wet of optimum with smaller pores became inactive last in with Meerdink et al. (1996). D W D W Fig. 16: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water contents for AB1 (Reduced Proctor compaction) D W D W 12

13 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) Fig. 17: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB2 (Reduced Proctor compaction) D W D W Fig. 18: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB3 (Reduced Proctor compaction) D W D W 1.00E-14 Fig. 19: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB1 (Standard Proctor compaction) 13

14 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) D W D W 1.00E E-15 Fig. 20: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB2 (Standard Proctor compaction) D W D W 1.00E-14 Fig. 21: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB3 (Standard Proctor compaction) 14

15 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) D W D WOPT VG Fig. 22: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB1 (West African Standard compaction) D W D W 1.00E-14 Fig. 23: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB2 (West African Standard compaction) 15

16 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) D W D W 1.00E-14 Fig. 24: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB3 (West African Standard compaction) D W D W 1.00E E-15 Fig. 25: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB1 (Modified Proctor compaction) 16

17 Unsaturated Hydraulic Conductivity (cm/s) Unsaturated Hydraulic Conductivity (cm/s) D W D W 1.00E E-15 Fig. 26: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB2 (Modified Proctor compaction) D W D W 1.00E E-15 Fig. 27: Variation of unsaturated hydraulic conductivity with matric suction at dry, optimum and wet of optimum water content for AB3 (Modified Proctor compaction) 17

18 CONCLUSION The result of unsaturated water flow studies showed that the pore size distribution of a compacted soil has considerable influence on the shape of the SWCCs and on the model fitting parameters. Specimen compacted at optimum water content had the least pore size distribution and those compacted dry of optimum had the highest pore size distribution. Smaller pore sizes were associated with high air-entry suctions. Soil composition and compaction conditions generally affected both the SWCCs and the model parameters. Brooks-Corey (BC) model tends to over predict the volumetric water content (θ) particularly at low suction values, while van Genuchten (VG) model tends to under predict it and these values are closer to laboratory measured values. Generally, the measured and predicted SWCC did not show great variations from each other, showing fairly close agreements between measured and predicted θ values. Specimens compacted with higher energies () gave lower unsaturated hydraulic conductivity values. Differences between unsaturated hydraulic conductivities predicted from Brooks- Corey s model and those predicted from van Genuchten model were shown not to be of great significance. It was also noticed that the unsaturated hydraulic conductivity reduced with higher moulding water content (2% wet of optimum) using both the VG and BC models. The unsaturated hydraulic conductivity is below 1 x 10-7 cm/s for all the compactive efforts at low 10 kpa matric suction with the exception of reduced Proctor effort. The results obtained for unsaturated hydraulic properties of these soils can be used to predict the performance and to accomplish the hydrologic modeling of these soils when used in the cover systems of the municipal solid waste landfills. REFERENCES Albrecht, B.A. and Benson, C.H. (2001). Effect of desiccation on compacted natural clays. Journal of Geotechnical and Geo-environmental Engineering, ASCE, Vol.127, No. 1, pp Arya, L.M., Leij, F.J., Van Genuchten, M Th. and Shouse, P.J., (1999). Scaling parameter to predict the soil water characteristics from particle-size distribution data. Soil Science Society of America Journal, Vol. 63, No. 3, pp Bello, A. A. (2012) Geotechnical Evaluation of Reddish brown Tropical Soil Journal of Geotechnical and Geological Engineering, springer publication. 30(2): Bello, A.A. and Osinubi, K.J. (2011) Attenuative capacity of compacted abandoned dumpsite soil Electronic Journal of Geotech. Engineering. (EJGE), 16, Bundle A, pp Benson, C.H and Daniel, D.E (1994). Minimum thickness of compacted soil liners: I. Stochastic models. Journal of Geotechnical Engineering, ASCE, Vol. 120, No. 1, pp Benson, C. H. and Gibb, M. M. (1997). Measuring unsaturated hydraulic conductivity in the laboratory and Field. Proc. of sessions on Unsaturated Soils at Geo-Logan 97 sponsored by the Geo-Institute, ASCE, July 15 th 19 th 1997, Logan, Utah. Benson, C.H. and Trast, J. (1995). Hydraulic conductivity of thirteen compacted clays. Clay and clay minerals Vol. 43, No.6, p 669. Brooks, R.H. and Corey, A.T. (1964). Hydraulic Properties of porous Media. Colorado State University, Hydrology Paper No. 3, Fort Collins, Colorado BS1 (1990). Methods of testing soils for civil engineering purposes. British Standards Institution, BS 1377, London. Chiu, T.F. and Shackelford, C.D. (1998). Unsaturated hydraulic conductivity of

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