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0 INTRODUCTION
The use of microorganism in other fields of studies by researchers to produce better result is increasing in recent time. Geotechnical and geo-environmental engineering have been exploring biogeochemical which processes involve geochemical reactions that are moderated by surface microorganisms which is the commonly investigated processes that contains mineral precipitation, gas generation, bio-film formation and bio polymer generation Delong, et al, (2013).
These processes usually utilize surface microbial activities to create the environment favorable for geochemical reaction between organic matter, minerals, gasses, and pore fluid that make up the soil, Delong, et al, (2013). In geotechnical engineering the general precipitation appear to gain more momentum. The commonly used bio-mineralization process in the precipitation of inorganic solute is microbial incited calcium carbonate precipitate (MICP) technique.
This soil improvement technique which utilizes the multifaceted understanding of chemistry and civil engineering at the confluence of microbiology has recently emerged as a research field and is well documented in literature (Mitchel and Santamarina, 2005, Ivanov and Chu, 2008, Dejong et al.
, 2010; Dejong et al; 2013; Umar et al., 2016). The technique that is commonly refers to as microbial incited calcium carbonate precipitate (MICP) employs the use of soil microbes to precipitate calcium carbonate in a given matrix of soil. The bio-product of this process is capable of cementing and clogging the soil particle to gather thereby enhancing strength as well as reducing permeability properties of the soil.
The techniques can be used for old and new structures and is previously used in stabilization of slope, sub grade reinforcement and soil liquefaction (Dejong et al., 2006, Achal et al., 2009, Wei et al., 2012, Cheng et al., 2013).
According to Fritzgez 2005, since bacteria are indigenous, there is low potential for feature environmental hazards. The bacteria usually use in MICP soil improvement technique are urease positive. Bacteria that are urease effective include clusteridium, desulfotomaculum, Sporosarcina, genera Bacillus, spoloactobacilus, (Kucharski et al., 2006). Pathogenicity of bacteria species to be employed in the MICP process should ensure safety to the environment and humans alike. According to Umar et al., (2016), hydrolysis of urea and calcium carbonate formation is as shown in equation (1).
Ca(NH2)2 + 2H2O + Ca2+¬¬¬¬¬¬ 2NH4+ + CaCO2 (1)
Bio-mediated method has been applied in different areas in soil improvement which include liquefaction prevention (Alvarado, 2009), reduction in hydraulic conductivity as well as upgrade in shear strength (Whifin et al., 2007; Ivanov, 2008; Dhami et al., 2013).
Environmental protection consciousness especially the surface water over the years cannot be over emphasized especially in the developed economy of the world. The awareness is stemmed from common cases recorded in the past which were later traced to indiscriminate dumping of waste in the environment. The adoption of properly designed landfills for disposal of waste is therefore the best option of waste management. The construction of liner system in the landfill repository has evolved over the years with new practices such as the addition of synthetic lining material, engineered clays and designing of more specialized gas and leachet collection systems (Jayasekera and Mohajerani, 2001). The sole aim of these new practices is to provide an efficient waste containment facility that will minimized or prevent the transport of wastes contaminants into adjoining environment (Jayasekera and Mohajerani, 2001).
The increase in world population and industrialization has put the generation of waste on the increase. The management of this waste is of paramount important if the serenity of the environment is prioritized especially the surface ground water. Waste is the unwanted materials that are thrown away after primary use. These materials usually constitute environmental hazard and nuisance in the surrounding. In Nigeria waste management has been a source of concern especially in the popular open dumping of refuge in urban and rural settlements despite regulatory environmental protections laws. To effectively manage the disposal of huge waste generated in the country, engineered land filling system is the ultimate solution.
The factor controlling the operation and performance of landfill is the hydraulic conductivity. The general requirement by most regulatory agencies world over is that the clay liners have hydraulic conductivity of less or equal to 1x 10-7cm/s. Also of equal importance is the diffusion mechanism of liner material, since diffusion is also considered and important transport mechanism especially if the seepage velocity nears or equal to zero (Shaekelford and Daniel, 1991).
Lateritic soil is highly weathered material that is predominant in the tropic due to high amount of rainfall. It is reach in oxide and sesquoxides. The study is aimed at evaluating the use of microbiological incited calcium carbonate precipitate (MICP) to evaluate engineering properties of lateritic soil for use in the lining system in a waste containment repository.
1.1 Statement of the Problem
There are lots of concerns over the use of natural clay material for lining system in waste containment facility. This led to consideration of alternative materials such as geo-membrane, geo-synthetic or composite materials as barrier system in a waste containment application. Also, the uses of these alternative materials which are highly expensive led to the continued research in to the adoption of cheaper alternative with less effect on the environment with viable alternative. Furthermore, the pollution of subsurface with the open dumping system called for the adoption of engineered landfill system to mitigate the environmental health issues associated with these practices.
Lateritic soil is characterized by huge amount of sesquioxides of iron (Fe2O3) as well as that of aluminum (Al2O3) with regard to other chemical components (Amadi, 2008). These soils are coated with sesquioxides gel inherent in the fine fractions of the soil. Thus, activity on the surface of clay minerals is reduced which in turn reduces the water absorption capacity of the lateritic soil. This also results to cementation of adjoining particles leading to aggregation of individual grains forming coarser particles. The above factors generally lower plasticity and by extension soil ability to establish low hydraulic conductivity (Townsend et al., 1971; Evans, 1993 Osunibi and Amadi, 2003; Osinubi and Nweiwu, 2005; Amadi 2008). The decision exposed the limitations of lateritic soils if used in its natural state as the contaminant barrier in a waste containment repository which is of primary concern to a geo-environmental engineer. To achieve low permeability of lateritic soil it is important to modify its engineering properties. The use of Bacillus brevis treated lateritic soil to take care of the highlighted deficiencies to better the hydraulic performances using the bio-logging and bio-cementation of the bacteria species.
Justification for the study
Microbial incited calcium carbonate precipitation (MICP) is recently used in geo-technical engineering in enhancing soil properties such as upgrade in shear strength as well as reduction in soil permeability (Wei et al., 2012). Although the research area is still in infancy a lot of studies pertaining the field have been carried out some of which include Mitchell and Santamarina (2005), Lian et al., (2006), Okwadha and Li (2010), Dejong et al., (2010), Wei et al., (2012) Gat et al., (2014), Osinubi et al., (2019a, b, c).
In order to develop sustainable and cost effective barrier system in the waste containment facility peculiar to the tropical region, the geo-environmental assessment of tropical soil such as lateritic soil treated with microbes has to be under taken. This is particularly so since most barrier technologies that are presently in use ware developed base on conditions in the temperate regions, therefore application of such technology in this part of the world may not necessarily be doubled (Amadi 2008).
The increase in construction cost associated with the use of geo-synthetic and geo-membrane materials also necessitate the choice of effective alternative but cost friendly materials for barrier system in a waste containment facility (Amadi, 2008).
Research Hypothesis
It is required that;
The compacted microbial induce calcite treated soil will have low hydraulic conductivity (k) (k?1×10-7cm/s )
The unconfined comprehensive strength of the compacted microbial induced calcite precipitate treated with soil will not be less than 200 KN/M2.
The Volumetric Shrinkage strength of microbial induced calcite precipitin treated compacted soil will not be greater than 4% after drying.
Aim and Objectives
The research is aimed at evaluating the effect of bacillus brevis on some engineering properties of soil for use in waste containment barrier system, with the following specific objectives;
To evaluate the effect of microbial calcite precipitate on compaction properties lateritic soil using energy levels of RBLS, BSL,WAS and BSH.
To investigate the effect of treatment on hydraulic conductivity, k, volumetric shrinkage strain and unconfined compressive strength of compacted lateritic soil.
Delineation of acceptable zones for bacillus brevis treated latrite soils.
Scanning electron microscopy (SEM) and X-ray florescence (XRF) analysis of microbial induced calcite precipitate treated laterite soils.
To determine the hydraulic properties (soil water characteristic curve) of the compacted natural and MICP treated late rite soil.
Diffusion, batch equilibrium and reliability assessment of calcite precipitate treated lateritic soil based on hydraulic conductivity using first order reliability method (FORM).
Scope of Work
The study is limited to microbial induced calcite precipitation treatment of lateritic soil for application in municipal solid waste containment repository. BS 1377 (1990) and BS 1994 (1990) for both natural and treated soil, respectively will be adopted for all geotechnical procedures throughout the research.
2.0 REVIEW OF RELATED LITERATURE
The application of biological methods in remedying deficiency in soil is gaining wide acceptability in the geotechnical community. The method has proved viable and potent for the purpose of enhancing soil properties and sustenance of the natural habitat (Ivanov and Chu, 2008; Umar et al., 2016). The research outcomes underlined the huge potential and wider applicability of the technique in diverse fields including geo-environmental engineering. Bio-mediation offers novelty in geotechnical engineering that can be taped to improve geotechnical properties of soil (Alvarado, 2009). The method offer the use of biological treatment as demonstrated in many utilizations, such as decreasing the permeability and enhancing shear strength of soil (Whiffin el al., 2007; Harkes et al., 2010; van Paassesn, 2011), it has also been used to enhance concrete strength and durability and remediation of building cracks (Qian el al., 2010; Dhami et al., 2013; Achal et al., 2013), employed cementing of sand column (Achal et al., 2009). Bio-mediation as method of soil enhancement generally refers to biogeochemical reaction within the soil matrix resulting in the formation of calcium carbonate precipitation which modify engineering characteristics of such soil (Dejong et al., 2010).
The technique uses microbes in the soil in a process simply referred to as microbially induced calcite precipitation (MICP), which precipitates calcite in the given soil. The produced calcite cements the soil grains resulting in the formation of clogs in the soil. This is responsible for increased strength as well as reduction in hydraulic conductivity of the soil matrix. MICP as soil enhancement approach offers an alternative approach for soil remediation which has been utilized a lot of civil engineering problems such as stabilization of soil slopes, sub grade reinforcement and liquefiable soil deposits (Dejong et al., 2006; Qian el al., 2010; Cheng et al., 2013).
Microorganisms are known to influence the formation of fine-grained materials and alter the characteristics of coarse-grained materials such as strength and hydraulic conductivity. Also, they aid chemical reactions in a soil matrix, facilitate weathering and alter the mechanical and chemical characteristics of materials after sampling. Therefore, microorganisms and their effects on mechanical characteristics of soil in geo-environmental engineering are not fully discovered (Mitchell and Santamarina, 2005).
Microorganisms are bound in soil more than other microbial habitats. The quantum of nutrients in the soil and also due to liquid usually found in the in the pore spaces. These microbes are not evenly distributed in the soil since some species are more than others. This could be attributed to the fact that the factors responsible for their growth and survival are not evenly distributed in the soil. Microorganisms are known to adapt to varying conditions both physiologically and genetically, which is the reason for their existence for more than 3.5 billion years (Stotzky, 1997).
Approximately, there is 109-1012 of microorganisms for every kilogram of a soil mass near the ground surface. Microorganisms present in soil include archaca, eukarya and bacteria. Important properties of archaca and bacteria include distinct chemical composition, simple cell structure without membrane-enclosed nucleus, and have more than one chromosome. Micoorganisms are usually identified, characterized and classified using the types of cell wall, type of biochemical transformation, shape nutrients, and DNA and RAN sequences (Stotzky, 1997; Woese et al., 1990; Ehrlich, 1998; Chapelle, 2001).
Bacteria are the most abundant soil microbes Mitchell and Santamirana (2005). They sporulate to withstand harsh environmental conditions. Their cell diameter ranges from 0.5 mm to 3 mm and have nearly round shape, spiral or rod like. According to Madigan et al. (2008) bacteria can survive in an environment of very low to high temperatures, low to high acidity and/ or salinity. Most of bacteria cells have a negative surface charge for groundwater pH values between 5 and 7, which is typical for near surface soils Chapelle, (2001).
3.0 MATERIALS AND METHODS
3.1 Materials
Soil: The soil sample to be used for the research shall be borrowed from Abagana in Anambra state, Nigeria. While the bacterium (microorganisms) shall be isolated from the soil to be treated (Bio-Stimulation) by serial dilution. The isolates will be store at a temperature of 40C before its characterization and classification.
Microorganism: The urease positive bacteria to be use in this research will be Bacillus brevis. It is a rod shaped Gram positive bacterium.
Cementation reagent: The cementation reagent to be adopted for the study will be that described by Stocks-Fisher et al. (1999), which contains 3g of Nutrient broth, 20 g of urea, 10 g of NH4CI, 2.12 G NaHCO3 and 2.8 g CaC12 per liter of distilled water.
3.2 Research Methodology
Samples for this research work will be collected by method of bulk disturbed sampling. The soil will be treated with Bacillus brevis (in step concentration of 25 ml 50 ml, and 75 ml) and the cementation fluid. Soil sample will be compacted using Reduced British Standard light (RBSL), British Standard light (BSL), British Standard heavy (BSH) and west African Standard (WAS) respectively.
Test to be carried include:
Compaction
Hydraulic Conductivity
Diffusion
Volumetric Shrinkage
Unconfined Compression
Scanning Election microscopy and X-Ray Fluorescence.
BS 1377 (1990). BS 1924 (1990) and Head (1980), Volumes 1 and 2 shall be used for the test procedures.
Isolation of the Bacterium
The bacterium will be isolated from the soil to be treated (Bio-Stimulation) by serial dilution. The isolates will be store at a temperature of 40c before its characterization and classification.
The Culture medium and growth conditions
The procedures to be used will be in line with that outlined by Stocks-Fisher et al., (1999). Bacillus sphaericus will be use all through the study medium (Tris ±YE). The pilot and stock cultures will be made from the following ingredients in one liter of distilled water; (NH4)2SO4, 10 g; Tris ±Hc1, 130 mM (pH 9.0); and yeast extract, 20 g; to which 1.5% agar will be added to obtain a solid medium for the stock culture. All constituting ingredients will be autoclaved separately and thereafter mixed to avoid precipitation. The experiments leading to the precipitation of CaCO3 will be carried out in liquid medium ( urea ± CaC12) consisting of the following in one liter of distilled water; urea, 20 g; nutrient broth (Bacto),3 g; NaHCO3, 2.12 g (equivalent to 25.2 mM) and NH4C1 10 g. Using 6 N HC1, the pH of the medium will be maintained at 6.0 before autoclaving. After autoclaving 10 ml solution containing 2.80 g CaC12 which is filter sterilized will be added into the medium. The final pH of the medium will be measured. B. species will be grown at 300C under aerobic conditions for pilot and stock cultures. Broth cultures will be incubated in a water bath shaker (Lab-line, Model 3540) operated at 200 rpm. Cell concentration will be determined by viable cell counting on Tris ± YE plates.
