Evaluation of the GeotechnicalProperties of Lwera Sand – Bentonite Mixture as a linermaterial in Solid Waste Landfills

Mr. Kaahwa Archillies Austin*, Mr. Tumwesigye James

Kaahwa Archillies graduated in 2022 from Kyambogo University with a B.Eng. in Environmental Engineering. He believes in preserving the environment and its natural resources as a key factor for sustainable infrastructure development.

Corresponding Author: archillies1994@gmail.com

1.0 INTRODUCTION

This paper reports findings from laboratory tests carried out on four Lwera sand–bentonite mixtures. The main objective of this research was to find out the mix proportion that can be used as a liner material in landfills. Unlike other liners on the market that cannot contain leachate generated from e-waste and radioactive wastes, bentonite can contain such leachate. However, bentonite alone can’t be used for reasons discussed in the subsequent chapters.

Human activities create waste, but it is how these wastes are handled, stored, collected, and disposed of that poses a risk to the environment and public health. In places with intense human activities such as urban centers, appropriate and safe solid waste management (SWM) is of great importance in providing healthy living conditions for residents.

To ensure the safe handling of waste, landfilling is the most preferred solid waste disposal method globally. A landfill can be considered a dumpsite for solid waste disposal. It is the cheapest and simplest method and requires adequate land for the landfill site. Once in the landfill, solid waste starts to decompose and generate leachate. Most of the waste will decompose naturally in landfill, and leachate and gas will be generated throughout the decomposition process (Scott et al. 2005). Hence, the operation of landfills may produce several environmental problems including the generation of leachate, which is disastrous to the groundwater and soil quality.

In Kampala, the capital city of Uganda, the Kampala Capital City Authority (KCCA) is mandated by the Local Government Act of 1997 to provide solid waste management services to all five divisions of the City (Noble Banadda, 2012). This is done by collecting and disposing of solid waste from the five divisions to Kiteezi Landfill, 12 km from the city center.  The 35-acre site at Mpererwe is the only formal landfill and was commissioned in 1996. This has helped reduce the amount and content of hazardous substances and adverse impacts on human health and the environment from waste.

However, people living near the landfill site have complained that this landfill has made their place uninhabitable and that their land has lost value. These conflicts stem from bad odor and leachate (which pollutes water resources) as a result of indiscriminate waste disposal. (Mwiganga M & Kansiime F., 2005).

1.2 Problem Statement

Kiteezi is currently the only licensed waste disposal facility and there are no other licensed facilities to do so (Kampala Capital City Authority, 2017). The number of years of service, the increasing population, and the amount of waste generated has constrained its performance. The landfill has been overwhelmed by the volume and type of waste material being dumped there. The environmental degradation caused by this inadequate and indiscriminate disposal of waste, which is normally mixed with human and animal excreta, electronics among others, results in; flooding (where the waste is dumped in drains), contamination of the surface and groundwater through leachate, soil contamination through direct waste contact or leachate, and uncontrolled release of methane by anaerobic decomposition of waste.

Therefore, with the inefficiency of the current landfill liner, there is a need to construct new landfills with liners that can contain leachate from indiscriminate waste disposal.

The main objective of the research was to Evaluate the geotechnical properties of Lwera sand–bentonite mixture as a liner material in solid waste landfills. To remain on track with the research problem and main objective, the study was guided by the following specific objectives;

1. To identify the research materials.
2. To determine the engineering properties of varying sand-bentonite mixtures.
3. To carry out a cost-benefit analysis of the appropriate mixture identified.

1.3 Justification of the Study

Therefore, using an appropriate and environmentally friendly liner material would be of great importance in protecting the ground below. This study was carried out at Teclab Limited, a geosciences firm majoring in geotechnical surveys/engineering. Bentonite and sand mixtures of mix ratios 0%, 5%, 10%, and 20% were subjected to tests such as classification tests, compaction, consolidation, and undrained shear, and the results analyzed

2.0  METHODOLOGY

This chapter covers the procedures and tools that were used in the study. Several tests as described later in the chapter were done on the samples.

A bentonite content of 0, 5%, 10%, and 20% (Fan, 2014) was adopted. For mixtures containing 5%, 10%, and 20%, sand, and bentonite were homogeneously mixed to obtain a single sample. Subsequent tests thereafter followed from this step and laboratory tests as summarized below were conducted. These tests were conducted with reference to British Standard procedures for testing soils for civil engineering purposes in the procedure shown in Figure 1.

Figure 1: Illustration of the Methodology

3.0  RESULTS AND DISCUSSION

After conducting laboratory tests, results and observations are discussed and interpreted in this section.

3.1 Determination of particle size distribution and classification of research material

The major aim for carrying out this was to know and understand how particles were distributed in each of the five data collection points. Sieve analysis as well as atterberg consistency tests were carried out on all the samples. At the end of these tests, samples were classified according to an internationally recognized Unified Soil Classification System.

Neat Sand

Sand produced a fines modulus of 2.43 and the curve produced showed that this representative sand sample was within the grading envelope as shown in Figure 2

Figure 2: A Particle Size Distribution (PSD) chart for Lwera Sand

Neat Bentonite

Bentonite had the highest grading modulus of 0.91 among the soils that were sieved using BS 1377: Part 2: 1990 standard. Bentonite produced a typical grading curve of clay soils. The curve showed that bentonite has a lot of fine particles and thus poor particle distribution, something that was easily observed from the 54.5g of fine particles that passed the 75µm sieve.

Figure 3: A Particle Size Distribution (PSD) chart for bentonite

5% Bentonite

Because of the addition of sand in bentonite, the grading modulus decreased from 0.91 to 0.62. The decrease is attributed to the addition of more coarse sand particles. The curve for this blend showed that particles were well distributed with 0.2g of the mixture passing the 75µm sieve.

Figure 4: A Particle Size Distribution chart for 5% bentonite content

10% Bentonite

Because of having more bentonite particles compared to the 5% mixture, the grading modulus at 10% was at 0.09 with 1.6g of the mixture passing the 75µm sieve. The curve showed that the particles in the blend were well distributed.

Figure 5: A Particle Size Distribution chart for 10% bentonite content

20% Bentonite

On thorough mixing of the mixture, this blend was not distinguishable from plain bentonite. This blend had a grading modulus of 0.13 higher than at 10% blend and this observation is attributed to the blend having more poorly distributed bentonite particles. The blend also had 5.3g of the test sample passing the 75µm sieve and the distribution curve showed that the particles were well distributed.

Figure 6: A Particle Size Distribution chart for 20% bentonite content

Observation

With an exception of sand that was graded using BS 882:1992, the 5% bentonite blend had well-distributed particles since it had fewer particles pass the 75µm sieve through curves of the 10% and 20% also show well-distributed particles compared to plain bentonite. The percentage of fine, medium, and coarse gradation for the mixtures used in this study is summarized in the table below.

Table 1: Percentage of fine, medium, and coarse gradation for the mixtures used

Figure 7: A relationship of coarse, medium, and fine particles of the mixtures.

From the above relationship; 1 represents sand, 2- bentonite, 3- 5% bentonite, 4- 10% bentonite, and 5 represents 20% bentonite

Atterberg Limits

The Atterberg consistency test was a property aimed at seeing how the mixtures manifested themselves by their cohesive resistance to flow. These properties included;

Shrinkage Limit 

Sand proved to be unfriendly to work with for the determination of the shrinkage limit. This is attributed to it not being plastic and therefore the crystalline particles do not undergo any change in shape with the addition of water. On the other hand, neat bentonite had the highest linear shrinkage. This is attributed to the highly cohesive particles of bentonite being able to absorb a lot of water molecules and lose the same at a high rate. 5% bentonite content had the least linear shrinkage because the sample contained a lot of sand particles that don’t shrink. Linear Shrinkage was seen to increase at 10% and 20% bentonite contents from 3.54% at 5% bentonite content to 12.96% and 21.54% respectively because of the introduction of more bentonite particles.

Plastic Limit

Sand is crystalline in nature and therefore it is practically impossible to find its plastic limit. From the results, it was seen that neat bentonite had the highest average plasticity of 57.68 and a plasticity index of 251.4. This implies that bentonite is a highly cohesive soil and difficult to work with compared to other soils. The decrease in plasticity and plasticity index at 5% and 10% bentonite content can be attributed to the presence of more non-cohesive crystalline sand particles and an increase of 20% caused by the addition of more cohesive bentonite particles.

Liquid Limit

Neat bentonite with cohesive particles produced the highest liquidity index of 309.1. A decrease in liquidity index was observed at 5% and 10% bentonite contents due to the presence of more sand particles that loosen up the more cohesive bentonite particles. 20% bentonite content had a high liquidity index compared to 38.9 and 73.2 of 5% and 10% content because it had more cohesive bentonite particles.

Summary of Classification of the Materials that were used.

The results obtained above described the physical characteristics of Sieve analysis and showed how particles were distributed in my mixtures. Atterberg consistency tests illustrated how my mixtures responded to the change of state.

Table 2: A summary of USCS classification of the samples

Symbols; CH denotes clay with high plasticity, S-C clayey sand, SC very clayey sand, and CS sandy clay, with the symbol on the right representing the most dominant soil in the mixture.

3.2 Determination of Dry Densities and Optimum Moisture Content

Compaction can be defined as a process whereby the solid particles in soil are packed more closely together, usually by mechanical means to increase its dry density. Values of maximum dry density (MDD) and optimum moisture content (OMC) are useful in determining the degree to which soils are compacted under field conditions. Below is a summary of the dry densities that were obtained after compacting the five data collection points by the procedures described in the previous chapter.

Table 3: A summary of dry densities and optimum moisture contents for the mixtures

The maximum dry density and optimum moisture for each soil were determined by appointing the peak of the compaction curves. Bentonite had the lowest MDD and highest moisture content because it had closely packed soil particles hence fewer air voids therefore a high density. Below is a graph showing the relationship between bentonite content, moisture content, and dry density for the mixtures.

Figure 8: A relationship of bentonite content, moisture content, and dry densities for the mixtures

From the above relationship, numbers on the x-axis represent bentonite content. 1 corresponds to values of Lwera sand, 2-values of bentonite, 3 –values for 5% bentonite content, 4 –values for 10% bentonite, and 5 represents 20% bentonite.

The above relationship reveals that MDD increased with an increasing amount of bentonite added. This increase in MDD with the addition of more bentonite is because the added bentonite filled the air voids within the sand particles and that led to an increase in the amount of compacted soil compared with the case without bentonite or with less bentonite.

3.3 Determination of compressibility and permeability of the mixtures

When soil is under an applied load, there is a tendency for soil particles to be pushed together. Initially, water in the soil carries this load until the soil is completely drained but with time soil particles are continually forced closer thereby producing the volume change termed as settlement. A one-dimensional consolidation test attempts to estimate in an accelerated manner both the rate and total amount of settlement of a soil layer under an applied load.

Samples were prepared and tested with reference to BS 1377: Part 5: 1990. Parameters that were derived from this test were the compressible index, the coefficient of consolidation, the degree of consolidation, and lastly the permeability constant k. Below is a summary of the results that were obtained.

Table 4: A summary of results obtained from a one-dimensional consolidation test

Samples were subjected to increased loading forces of 25kPa, 50kPa, 100kPa, 200kPa, 400kPa, and 800kPa, and from the above table, it can be seen that for any type of mixture, the void ratio decreases with an increase in bentonite content. However, 10% content had a significantly higher initial void ratio e0 and voids ratio e and this can be attributed to how particles are distributed in this mixture.

The coefficient of the volume of compressibility from the above data is seen to increase from bentonite to 5% to 10% and at 20%.

From the above results, it can be observed that for any mixture there is a significant improvement in the permeability compared to sand alone. These results reveal that the permeability decreases with increased loading force, and for any type of mixture, the permeability decreases with the increase of bentonite content.

Figure 9: A relationship between permeability k and the loading forces

Observation

It was observed that the highest permeability was achieved by sand (2.01*10-8 m/s) and the lowest permeability by bentonite (3.88*10-9m/s). The results of the coefficient of consolidation are high; ranging from 32.04 m2/year to 30.46 m2/year. Ground stabilization must be done before application in the field to overcome this challenge. 

3.4 Determination of strength characteristics of the mixtures

This was the last test and it was aimed at observing how these mixtures responded to shearing forces. From this test, three aspects were studied namely; shear stress, axial strain, and mode of failure of the mixtures. Two specimens were tested at each data collection point at cell pressures of 30kPa and 60kPa. Below is a summary of the results.

Table 5: A summary of results obtained from a triaxial test

Results from Table 5 indicate that sand alone experienced the highest average shear stress of 98kPa with 5% bentonite content experiencing the least shear stress of 46.5kPa. Axial strain is observed to increase with increasing bentonite content from 9.30% at 5% content to 16.33% at 20% bentonite content.

Conditions at failure

Sand, bentonite, 5% bentonite, and 10% bentonite produced an angle of shear when subjected to cell pressures of 30kPa and 60kPa. 20% bentonite content tended to bulge.

Images 1, 2, 3, 4, and 5 are a summary of the modes of failure of samples when subjected to forces.

4.0 CONCLUSION

The main purpose of this research was to evaluate the geotechnical properties of Lwera sand-bentonite mixture as a liner material in landfills. Laboratory results obtained showed that sand-bentonite mixtures can be applied as a liner material in landfills. From these results, the following conclusions were made;

i.              Bentonite alone cannot be used as a liner material even though it possesses the unique properties of containing leachate. This is because it has a permeability (3.88*10-9) less than the standard (10-6 – 10-8) m/s and shears when subjected to forces.

ii.            Likewise 5% bentonite content and 10% bentonite content cannot be used as a liner material because they shear when subjected to shearing forces despite having a permeability in the range of (10-6 – 10-8) m/s.

iii.           20% bentonite content can be applied as a liner in landfills because it fulfilled all the tests it was subjected to.

A cost-benefit analysis conducted showed that the application of sand-bentonite mixture as a liner material in landfills is feasible.

5.0 RECOMMENDATIONS

From the above results, ground stabilization will be required to counteract the high coefficient of consolidation obtained (32.46m2/year). The 20% sand-bentonite mixture will be efficient in performance when applied with 15-30cm.

However, this area needs further research. From the obtained results and my evaluation, I would wish to make the following recommendations;

i.              Research can be conducted using sand from other areas.

ii.            Results for consolidation were obtained for 4 loading days, research can be conducted for results beyond 4 loading days.

iii.           My permeability constant k was obtained using a – dimensional constant head permeability test. Research can as well be conducted using other permeability tests such as Rowe cell constant head test, falling head test, and standard compaction permeameter. 

REFERENCES

BSi. (1990). Methods of testing for soils for civil engineering purposes. In British Standards Institution, Part 7: Shear Strength tests (pp. 23-27). London: Controls Testing Equipment Limited.

BSi. (1990). Methods of testing for soils for civil engineering purposes. In British Standards Institution, Compaction- related tests (pp. 1-22). London: Controls Testing Equipments Limited.

BSi. (1990). Methods of testing for soils for civil engineering purposes. In British Standards Institution, Part 2: Classification tests (pp. 1-36). London: Controls Testing Equipments Limited.

Fan, R. D. (2014). Compressibility and hydraulic conductivity of sand/clay-bentonite backfills. Geoenvironmental engineering, 21-30.

IFC. (2013). Kampala Solid Waste Management PPP Project. Pre-bid Investors Conference. Kampala, Uganda: KCCA.

Kampala Capital City Authority. (2017). Project Teaser- Kampala Waste Treatment and Disposal. Kampala: IFC.

Mwiganga M, & Kansiime F. (2005). The impact of Mpererwe Landfill in Kampala-Uganda, on the surrounding Environment. Physics and Chemistry of the earth, 30, 744-750.

National Environment Management Authority. (2000). State of Environment Report. Kampala, Uganda: National Environment Management Authority.

Noble Banadda, D. T. (2012). Qualitative Determination of Methane Gas at Selected Sites in Kampala. Kampala: Bentham Open. doi:10.2174/1874829501205010050

Office of Auditor General. (2010). Value for money audit report on solid waste management in Kampala. Kampala, Uganda: Office of Auditor General (OAG), Republic of Uganda.

Shirazi, S. K. (2010). Permeability and swelling characteristics of bentonite. International Journal of Physical Sciences, 1647-1659.

World Bank. (2015, September 16). Promoting Green Urban Development in African Cities. Urban Environmental Profile,1.

Zurbrugg, C. (2003). Urban solid waste management in low-income countries of Asia how to cope with the garbage crisis. Presented for Scientific Committee on Problems of Environment (SCOPE) Urban Solid Waste Management Review Session, Durban, South Africa, November 2002.