Keywords
1. Introduction
The initial days of geotechnical engineering as a specialised discipline within civil engineering were strongly pivoted around physics of multiphasic porous media with consideration of several empiricism that the practicing engineers noted while dealing with several soil investigation projects. Over time, the discipline extended to include chemical considerations to encompass geoenvironmental engineering challenges. It was not until fairly recently that the importance of the biological considerations in geotechnics was recognised. The resulting discipline of biogeotechnics describes coupled physico-chemo-biological processes in multiphasic porous media and engineering applications thereof.
Millions of microbes present in soil surface and subsurface play a major role in the occurrence of a number of geochemical reactions. These geochemical reactions alter the physical and chemical nature of soil and impact the engineering behaviour of soil significantly. Since the last decade, increasing attention from the geotechnical community has been rendered to augmenting and simulating some natural biogeochemical processes to address a range of geoenvironmental challenges. In this regard, major studies have been conducted on the areas of biomineralisation, i.e. microbially induced carbonate precipitation (MICP) and application of biopolymers in different geoengineering projects. The basic mechanisms, pathways, influencing factors, and applications of MICP process have been studied and reviewed extensively in the past (Dhami et al., 2013; Achal and Mukherjee, 2015; Jain et al., 2021; Jiang et al., 2022). Similarly, the basic concept of biopolymer interaction with soil and its application have been demonstrated in Fatehi et al. (2021). However, most of these studies focus mostly on describing biomediation of soil properties in controlled laboratory environments. Table 1 provides a summary of some of the critical reviews available in the literature that discusses biomediation along with the room for improvement that the present article provides.
Table 1. Some key review articles in the field of biogeotechnics and potential room for improvement.
| Review article | Focus | Room for improvement |
|---|---|---|
| Mitchell and Santamarina (2005) | This study introduces, identifies and illustrates microbiological processes and potential effects on soils and rocks, stimulates interest in seeking an improved understanding of their importance and potential for advancing the state of knowledge and practice in geotechnical engineering | Lack of case studies on MICP and biopolymer application, the importance of monitoring techniques, bioremediation, and its importance in solving the geoenvironmental problems |
| Ivanov and Chu (2008) | Two notable applications, bioclogging and biocementation, have been explored | Lack of review of all the biological processes such as bioremediation, and industrial waste valorisation by biomodification |
| Dejong et al. (2013) | This paper assesses the progress, opportunities, and challenges in the biomediated geochemical processes, which comprise of geochemical reactions regulated by subsurface microbiology. The study mainly focused on mineral precipitation, gas generation, biofilm formation, and biopolymer generation |
These studies mostly focus on the fundamentals of MICP, its application, factors affecting MICP, modelling and monitoring of MICP. Though all the microbial processes for carbonate biomineral precipitation are discussed, soil microbes responsible for other biological processes and associated challenges need to be reviewed |
| Dhami et al. (2013) | In the present review, the detailed mechanism of production of calcium carbonate (CaCO3) biominerals by ureolytic bacteria has been discussed along with the role of bacteria and the sectors where these biominerals are being used | |
| Achal and Mukherjee (2015) | This paper reviews current progress and potential of microbial precipitation of CaCO3. Prior research on the modes of application of the technology and consequent gains in strength and durability of construction materials has been summarised | |
| Achal and Mukherjee (2015) | This paper elaborates nature's way of construction based on biomineralisation and discusses the progress of different biological pathways for sustainable construction. A variety of applications of biomineralisation based technology in the construction have been reported. The paper briefly documents the future directions of the technology | |
| Umar et al. (2016) | This paper presents a review of the soil microorganisms responsible for the bicarbonate mineral formation process and the factors that affect their metabolic activities and geometric compatibility with the soil particle sizes. Two mechanisms of biomineralisation, i.e. biologically controlled and biologically induced mineralisation, were also discussed | |
| Jain et al. (2021) | This review article explains all the metabolic pathways and their mechanism involved in the MICP process in detail along with the benefits of using denitrification over other pathways during MICP implementation. The potential application of denitrification in building materials pertaining to soil reinforcement, bioconcrete, restoration of heritage structures and mitigating the soil pollution has been reviewed by addressing the finding and limitation of MICP treatment | |
| Tang et al. (2020) | This paper reviews various influential factors for MICP process, including bacterial species, concentration of bacteria, temperature, pH, composition and concentration of cementation solution, grouting strategies, and soil properties. | |
| Jiang et al. (2020) | Critically reviews the fundamental microbial, chemical, and flow processes involved in MICP process | |
| Sharma et al. (2020) | Review in the application of MICP/biocementation in the mitigation of soil liquefaction | |
| Harran et al. (2023) | In this work, a comprehensive review is provided, ranging from MICP's core fundamentals to recent breakthroughs and milestones, specifically targeting three core axes, namely, the achieved mechanical performance, modeling approaches, and upscaling considerations | |
| Ramdas et al. (2021) | This review provides (1) an overview of soil stabilisation techniques; (2) the primary challenges that lay ahead for future research in bio-based stabilisation products application in the road sector; and (3) innovations to address the challenges of using modernised techniques in the road construction industry (i.e. weak subgrade and the required maintenance thereof, as well as the development of potential bio-based additives (enzymatic and polymeric) for unpaved road construction application) | These two studies focused on only biopolymer soil interaction and the application of biopolymer in soil stabilisation. However, the challenges faced for upscaling and the level of maturity of these techniques for field application could be described in further detail |
| Fatehi et al. (2021) | This paper aims to provide a review on the environmental assessment of using biopolymers as binders in soil improvement, and biopolymer-treated soil characteristics, as well as the most important factors affecting the behaviour of the treated soil | |
| Yu et al. (2023) | The study highlights the heavy metal remediation through bio-solidification i.e. immobilising heavy metal ions using two types of bacteria: urease-producing bacteria (UPB) and phosphatase-producing bacteria (PPB) | Several other bioremediation processes have not been reviewed |
To that end, in the first part of the review paper titled “Biological perspectives in geotechnics: theoretical developments” (Jain et al., 2023), we focused on summarising the theoretical developments in the discipline and presented the state of the art. We also advocated strongly in favour of consideration of the “fourth phase” (i.e. the living organisms in soils and impact of their lifecycles on soil properties) while studying effective stress, strength, stiffness, volume change and conduction phenomena in Geotechnical Engineering.
Building on these concepts, the present paper explores several practical applications of biological pathways on engineering the properties of soils. In particular, we summarise and highlight the advancements in usage of biological pathways in decontaminating soils, strengthening soils through biopolymers and biomineralisation along with biomediated pH control for soils. For each of the applications, current state of practice and the challenges faced have also been described. Next, we discuss how the results of biomediation in engineering application can be spatially and temporally monitored using minimally invasive electromagnetic (EM) methods. This constitutes an important step in quantifying the relative success of the applied technique in practice. Finally, maturity of each of the approaches described is assessed through technology readiness levels (TRLs).
Fig. 1 demonstrates the organisation of sections across the first and the present article and highlights the sections that have been covered under the purview of application and monitoring of biomediated approaches in geotechnics.
Fig. 12. Biomediated soil remediation
Various anthropogenic activities lead to generation of heavy metals and radionuclides that contaminate soil (Jain and Arnepalli, 2019). In addition, scarcity of natural aggregates and sand compels us to utilise industrial solid wastes as an alternative to natural soil for construction and infrastructure projects. This necessitates decontamination and remediation of soils and solid wastes to make them suitable for usage while minimising the environmental risks.
Various types of soil remediation techniques that are utilised in the field include: (1) thermal treatment; (2) electrokinetic treatment; (3) soil encapsulation or capping; (4) chemical blending; (5) air sparging; (6) soil washing; and (7) bioremediation.
Many of the above-mentioned physical and physico-chemical remediation techniques have several drawbacks; while, some of the methods are energy and capital intensive, many rely on large quantities of chemical reagents (Abdel-Sabour, 2007; Lacalle et al., 2020). Moreover, the generation of secondary toxic pollutants accelerates soil contamination (Sharma et al., 2018). To that end, bioremediation is a natural soil remediation technique that uses various biological mechanisms and pathways to neutralise toxic chemicals and wastes in soils. These mechanisms generally do not have any negative impact on the environment. Therefore, bioremediation techniques are amongst the cost-effective and ecofriendly methods that can be applied to a wide selection of soil and water contaminants. In most of the bioremediation processes, the contaminants change to harmless gaseous or liquid phases, which provides a permanent solution for the contaminant degradation. Bioremediation processes can be applied to a wide range of contaminants starting from leachates from the municipal solid waste to oil spills. They offer a sustainable alternative for soil decontamination that saves time, labour and cost.
The following section presents a critical review of some of the bioremediation techniques, by segregating them into two broad sections, i.e. microbial remediation and phytoremediation.
2.1. Microbial remediation
Microbes are the most primitive forms of life and possess the adaptability to resist and overcome a variety of adverse environmental conditions. The industrial bloom has resulted in many toxic elements being constantly produced and affecting the natural ecosystem. Being the primary respondent to the changes in environmental conditions, microbes maintain the structure and function of the ecosystem by either altering their genetic system or transferring genetic elements. Adoption of any of these mechanisms enables microbes not only to sustain the adverse environmental conditions, but also to remediate the toxic elements to non-toxic or relatively lesser toxic forms (Dash and Das, 2012). Looking at the criticality of treating the persistent environmental problems, most of the recent studies advocate a multidisciplinary approach utilising the potential microbes for enhanced bioremediation such as biostimulation, bioaugmentation, bioaccumulation, biosorption, phytoremediation, and rhizoremediation (Das et al., 2016).
It is not the primary intention of a microbe to remediate a toxic element; however, in the process of its adaptation, the lesser toxic or non-toxic forms of the toxic elements are generated. Therefore, the toxic elements regulate the growth, metabolism, and differentiation of the microbes. Though there are many factors affecting the interaction between the microbes and the toxic elements including redox state of metals, type of organism, interacting environment, metabolic activity, and structural components of the organism, the regulation of biogeochemistry of toxic metals is controlled by the microbes leading to their mobilisation and immobilisation (Gadd, 2010). To that end, some of the most common processes of microbial adaptation to the toxic environment and the subsequent remedial practices have been discussed below.
2.1.1. Biosorption
Microorganisms are ubiquitous and have a high surface area to volume ratio. These characteristics allow them to provide a larger reactive area. Besides, the overall negative charge of a microbial cell allows the metal cations to attach on the exterior of the cell surface. Metal biosorption is accomplished as a rapid physiochemical process by which the toxic metal ions are associated with an immediate cell contact (Volesky, 1990). Biosorption of toxic metals can be carried out by both the cell mass of living and non-living microorganisms. In the living cell mass, metabolic pathways such as production of polysaccharides play a major role in the process of enhancing bioavailability of toxic metals to the microorganism (Iyer et al., 2005). Expolysaccharide (EPS) production in Klebsiella aerogenes has shown its capability to survive in cadmium (Cd) stress condition and subsequent removal of Cd from the growth medium (Scott and Palmer, 1990). Besides the production of EPS, other structural molecules of microbial cell wall such as proteins, lipids, and carbohydrates also play a major role in complex formation with the toxic metals by development of various bonds such as ionic interaction, van der Waals force, and electrostatic interaction.
Different bacteria, i.e. P. aeruginosa, B. thuringiensis, Actinomycete sp., Streptomyces sp. and Bacillus sp., and fungi such as A. niger, P. notatum, Absidia cylindrospora, Chaetomium atrobrunneum, and Coprinellus micaceus isolated from contaminated soil and sludge have the ability to biosorb various heavy metals including copper, chromium, cadmium, nickel, and lead, and remediate the soil environment (Karakagh et al., 2012; Nagashetti et al., 2013; Oves et al., 2013; Albert et al., 2019; Oyewole et al., 2019). The studies showed that the native microbes are feasible alternatives for biosorption and can improve the soil quality. However, their biosorption ability is mostly dependent on the soil pH and heavy metal concentration (Oves et al., 2013).
2.1.2. Intracellular accumulation
Intracellular accumulation of toxic metals has been well documented as a mode of bioremediation by the microorganisms. The metal transporters regularly transporting the essential physiological cations found in the microbial cell membrane also facilitate the entry of toxic metals inside the microbial cell (Summers and Silver, 1978). After entering the cell, toxic metals are either compartmentalised, get converted into non-toxic forms, or get precipitated in the form of phosphide, sulfide, carbide, or hydroxide. Nickel has been reported to be accumulated in Pseudomonas aeruginosa as phosphide salts (Sar et al., 2001). Mostly the intracellular accumulation of toxic metals is limited to the periplasmic space. In Pseudomonads, the accumulated toxic metals have also been reported to be restricted to the periplasm mostly in the form of metal sulfide (Sinha and Mukherjee, 2009). Timková et al. (2018) have concluded the capability of Actinomycetes/Streptomycetes for biosorption and bioaccumulation of various heavy metals in metal contaminated soils.
2.1.3. Extracellular precipitation
Many microorganisms have been reported to secrete specific and non-specific metal-binding entities to the environment. These compounds act as ameliorate to bind with the toxic metals present in the environment. Some of these entities include phosphates, oxalates, and sulfides that facilitate the binding of toxic metals by extracellular immobilisation. Microbial metabolites such as organic acids, though non-specific, have the potential to form complex with metals and thus affect their mobility and toxicity (Gadd, 1990). Certain organic acids and extracellular polymeric substances have also been reported to bind with the toxic metal. Mostly these extracellular metabolites convert the toxic metals to their respective sulfide salts, thereby reducing their toxicity. Metal sulfides are highly insoluble under neutral and anaerobic conditions, and hence they possess the capability to remediate a toxic metal present in the environment.
Besides, many microorganisms have been reported to produce specific extracellular metal-binding compounds in the presence of low level of toxic metals in the environment. Siderophores are produced by microorganisms in presence of low concentration of Iron in the environment. Siderophores, the iron binding compounds, have the potential to form complex with insoluble Fe (III) in the environment and form soluble compounds that can be utilised by the microorganism using specific transport mechanisms (Neilands, 1981). Many studies have hypothesised the application of siderophores in binding with many other metals such as magnesium, manganese, chromium (III), gallium (III), cadmium, and certain radionuclides (Sinha and Mukherjee, 2009). Hence, the potential of siderophores can be explored further for their application in remediation of many toxic metals using extracellular precipitation approach.
Several recent studies have also been carried out to remediate various heavy metals and radionuclides from soil via precipitation and coprecipitation of the metals into their respective carbonate ions by MICP process (Achal et al., 2012; Kumari et al., 2014).
2.1.4. Volatilisation
Microorganisms possess the capability to detoxify the toxic metals into lesser or non-toxic forms through the process of volatilisation. This process is accomplished by oxidation, methylation, reduction, and demethylation mechanisms. Many microbes such as methanoarchaea, fermentative bacteria, and sulfate reducing bacteria are reported to volatilise toxic metals such as mercury (Hg), arsenic (As), selenium (Se), and lead (Pb) (Meyer et al., 2007). Out of all the toxic metals, Hg volatilisation is well established. The narrow spectrum and broad-spectrum Hg resistance is associated with the presence of mer genes in bacteria which is a part of the mer operon. The inducible enzyme mercuric ion reductase (MerA) has been predominantly present in Pseudomonas, Staphylococcus aureus, Escherichia coli, and Bacillus sp. (Dash and Das, 2015). Inorganic mercury enters the bacterial cell through MerT (via MerP), MerC, or MerF present in the bacterial membrane. After entering the microbial cell, MerA coding for mercuric ion reductase transforms Hg2+ to Hg by releasing NADP+ (Dash and Das, 2012). Similarly, MerB provides resistance towards organic form of mercury, which is always associated with MerA. After entering the cell, MerB coding for organomercurial lyase cleaves C–Hg bond and releases Hg2+ in the cytoplasm. Hg2+ is subsequently converted to Hg by the action of mercuric ion reductase enzyme (Priyadarshanee et al., 2022). In this process, the Hg is liberated to the environment, which is considered to be the least toxic form of mercury, and bioremediation of mercury takes place by mercury resistant bacteria.
Examples of volatilisation pathway for remediating arsenic and selenium contaminated soils by isolated fungus and genetically engineered Pseudomonas putida bacterium have also been discussed in the literature (Karlson and Frankenberger, 1988).
2.1.5. Active sequestration
Many studies have confirmed the accumulation of toxic metals in the cytoplasm of indigenous microorganisms using their metabolic activity (Sar et al., 2001). The efficacy of such metal accumulation has been enhanced even further by using genetically modified microorganisms. Most of these approaches attempt to overexpress metal-binding peptides such as polyhistidines and metallothioneins. Many researchers have also tried to clone phytochelatin in microorganisms, which is readily found in the plants and has been reported to be a huge success in the process of active sequestration of toxic metals (Sauge-Merle et al., 2003). In this regard, a genetically modified E. coli has been constructed expressing the vanabin genes from vanadium rich ascidian Ascidia sydneiensis samea, for effective copper accumulation (Ueki et al., 2003). Other approaches include expression of bacterial surface metal-binding proteins like hexahistidyl peptides as bacterial surface proteins to increase metal-binding capability of a microorganism. Samuelson et al. (2000) modified E. coli and Staphylococcus carnosus for enhanced binding of Ni2+ and Cd2+ using this approach. Similarly, the expression of mer operon in Deinococcus geothermalis from E. coli has shown a huge reduction of Hg in the environment by these modified microorganisms (Brim et al., 2003). Besides, Dash and Das (2016) constructed a transgenic Bacillus thuringiensis strain having both mercury accumulation and volatilisation potential to remove ∼100% of toxic mercury from the environment.
2.1.6. Passive sequestration
Microbial biomass has also the potential to passively bind the toxic metals through passive sequestration. This technique provides a cost-effective mechanism for industrial waste water treatment. Though biosorption is facilitated by both the living and non-living microbial biomass, it involves cell surface binding, ion exchange, and microprecipitation (Gadd, 1990). Stoichiometric interaction between cell surface molecules and metal, and inorganic deposition of increased amounts of metal(s) are two major mechanisms employed by the microbial biomass to bind with the toxic metals. However, metal-binding capability of microbial cell differs depending on its affinity with the metals, and many microorganisms do not show any affinity towards metals. Glycoproteins present on the cell surface of Gram-negative bacteria show their preferential binding with Cd2+ in comparison with other metals. Similarly, phosphoryl groups of lipopolysaccharides (LPS) present on the surface of E. coli shows affinity for many metal cations. Mostly, metal deposition occurs at the polar head regions of the membrane and along the peptidoglycan of E. coli. In another study, phosphate residues were found to be the uranium binding sites in Streptomyces longwoodensis (Crist et al., 1981). Though Bacillus sp. has been regarded as the most frequently used microorganism for metal binding, Pseudomonas sp., Zoogloea ramigera, and Streptomyces sp. have also been reported to have huge potential in passive sequestration of toxic metals (Mullen et al., 1989).
Recent studies have shown the feasibility of remediating arsenic, mercury and cadmium contaminated soils via microbial sequestration process with the assistance of heavy metal tolerating microbes (Ghosh et al., 2022; Kaur et al., 2022).
2.1.7. Microbial assisted phytoremediation
Plants coabsorb metals and metalloids along with the absorption of water and nutrients from the soil (Dary et al., 2010). Many tall plants across different families have shown their potential to accumulate a large number of toxic metals in their shoot. Such plants are used either to reduce their bioavailability (phytostabilisation) or to transform them into volatile forms (phytovolatilisation) (Shutcha et al., 2015). Several studies have been carried out to enhance the metal uptake capability of these tall plants by their genetic modification using the established microbial genes. In this regard, microbial assisted phytoremediation plays an important role in increasing the microbial metabolism in the rhizosphere to boost the process of phytoremediation. Though many microbes are present in the soil having the potential of enhancing the translocation of metals to the plants, establishing a robust plant-microbial interaction model is required to solve the purpose. In this regard, plant growth promoting rhizobacteria (PGPR) with the ability to grow in the roots play a useful role in the process of phytoextraction of toxic metals. Addition of Microbacterium liquefaciens, M. arabinogalactanolyticum, and Sphingomonas macrogoltabidus to the roots of Alyssum murale has shown significantly augmented Ni2+ uptake from the soil (Abou-Shanab et al., 2003). Another microorganism Brevibacillus isolated from Zn2+ contaminated environment has also shown its potential to enhance plant growth as well as accumulation of N and P in the plants.
2.2. Phytoremediation
The physicochemical techniques used for reclaiming heavy metal contaminated soils and groundwater are associated with several drawbacks, such as extensive labour, high costs, disruption of indigenous soil microorganisms, and permanent changes in soil physicochemical parameters (McGrath et al., 2001; Barcelo and Poschenrieder, 2003; Sheoran et al., 2011).
Advancements in biological approaches for heavy metal extraction, land reclamation, and slope stabilisation are gaining recognition as greener alternatives to the conventional remediation pathways. The early efficiency of biological methods may be low, but it increases with time with aftercare and monitoring until it is self-sustained. Phytoremediation is one of such biological techniques of remediation that incorporates the use of specific types of plants to address issues of soil and water. The term “phytoremediation” is derived from the Greek prefix phyto (plant) and the Latin root remedium (restore or clean). It is a cost-effective, ecofriendly in situ remediation process (Chehregani and Malayeri, 2007; Vithanage et al., 2012). The use of vegetation is largely dependent on the environmental circumstances surrounding the site, and it becomes difficult for the plants to thrive if the surrounding conditions are adverse. In such cases, it becomes necessary to provide external help in form of some amendments to support the initial growth. Hence, using the suitable plant species tolerant to adverse conditions (including drought, fire and pests) as an engineered solution can help with slope stabilisation, erosion control, dust management, metal extraction and land reclamation (Morgan and Rickson, 1994).
The primary importance in phytoremediation is given to the phytostabilisation and phytoextraction strategies (Fig. 2) for immobilisation and removal of metals from soil (reclamation):
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(1)
Phytoextraction. Phytoextraction (also known as phytoabsorption, phytoaccumulation, or phytosequestration) is the absorption of contaminants from soil or water by plant roots and their transfer and deposition in above-ground biomass (shoots) (Ghosh and Singh, 2005).
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Phytostabiliation. Phytostabilisation is associated with the reduction in the bioavailability and mobility of heavy metals in soils as a result of their stabilisation from off-site transport by plants.
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Phytodegradation. Phytodegradation is the breakdown of organic pollutants by plants using enzymes such as dehalogenase and oxygenase, without the assistance of rhizospheric bacteria.
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Phytovolatilisation. For organic pollutants and some heavy metals, such as Se and Hg, phytovolatilisation, or the absorption of pollutants from soil by plants, conversion to volatile form, and subsequent release into the atmosphere, can be employed. The process does not entirely remove the contamination, and it simply transports contaminants from the soil to the atmosphere, where they can be redeposited.
Fig. 2Phytoremediation is a promising method for land reclamation, but it also has some hurdles (Ramamurthy and Memarian, 2012) that have been listed below:
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Prolonged period (several years) for soil remediation.
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Performance is limited by the plants' low biomass and sluggish development rate.
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Sustainable phytoremediation is mostly determined by climatic and meteorological conditions.
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Metals that have accumulated can be transported into the food chain if biomass is mishandled.
3. Biomediated soil stabilisation
3.1. Vegetation aided slope stabilisation
Slope stabilisation using vegetation describes another usage of plants in geotechnical engineering application. The goal is to reduce erosion in the near term through plants cover and to create a self-sustaining ecosystem in the long run through plant colonisation. Plants can be used to fortify the soil and offer stability against sliding due to the mechanical and hydrological effects of the roots (Feng et al., 2020). Plant biomass has been proven to be efficient in lowering erosion potential and infiltration of precipitation, thereby promoting hydrological reinforcement (Baets et al., 2007; Feng et al., 2020).
Each plant has a unique shape and structure. Above-ground biomass (stems and leaves) and below-ground biomass (roots) both contribute to slope stability. The key elements in bioengineering are the roots, which give strength, anchoring, and absorption from the soil. The stem and leaves assist in intercepting precipitation and stopping droplets from making direct contact with the earth. Stems of the plants are effective in dispersing energy from wind and water runoff, hence avoiding topsoil erosion. Because of their root development patterns, grasses, shrubs, and trees are employed to improve slope stability at different locations of the slope. The root branching patterns of plants are of immense significance in terms of soil bioengineering. Fig. 3 shows different root patterns of various plant groups (Yen, 1987).
Fig. 33.2. Biopolymers in soil stablisation
A variety of laboratory experiments have been conducted to assess the future potential of biopolymers in geotechnical applications. Based on the outcomes from laboratory experiments, field-scale application of biopolymer-treated soils has also been demonstrated.
3.2.1. Laboratory-scale studies
3.2.1.1. Mechanical properties
Several studies have focused on laboratory-scale assessments for shear strength (Chen et al., 2013, 2019; Ayeldeen et al., 2016; Smitha and Sachan, 2016; Cabalar et al., 2017; Latifi et al., 2017; Chang and Cho, 2019; Lee et al., 2020; Chang et al., 2021) and compressive strength (Chang and Cho, 2012; Akbulut and Cabalar, 2014; Chang et al., 2015a, 2017, 2018; Das et al., 2015; Aguilar et al., 2016; Dove et al., 2016; Cabalar et al., 2017) of biopolymer-treated soils.
The strengthening characteristics of biopolymer-treated soils were dependent on three factors: geometric conditions of soils (particle size distribution, and soil type), hydraulic conditions of pore spaces (water content), and biopolymer conditions (biopolymer content, and mixing methods) (Chang et al., 2015a).
Biopolymers enhance the shear resistance of soils by improving the bonding strength between soil particles. In case of non-cohesive soils (e.g. sand), biopolymers enhance the cohesion of soils by coating around the granular surface, while the friction angle remains constant. On the other hand, biopolymers form direct bonds with surface charge of cohesive soils (e.g. clays) producing conglomerates with dilation effects during the shearing. Thus, cohesion and friction angle both increase with biopolymer treatment for cohesive soils. As a result, biopolymers show a better result for cohesive soils than non-cohesive soils.
3.2.1.2. Hydraulic properties
Along with their interaction with soil particles, biopolymers also engineer the hydraulic properties of pore fluids. Biopolymers absorbs pore fluids forming hydrogel filling pore spaces. Therefore, biopolymer treatments achieve the hydraulic conductivity of impermeable layers (i.e. <10−7 cm/s), showing it potentials for hydraulic barrier applications (Bouazza et al., 2009; Kwon and Ajo-Franklin, 2013; Wiszniewski and Cabalar, 2014; Eires et al., 2015; Ayeldeen et al., 2016; Chang et al., 2016a; Cabalar et al., 2017; Sujatha et al., 2021; Fu et al., 2022) (Fig. 6).
Fig. 6Furthermore, biopolymers store pore fluids encouraging plant growth (Larson et al., 2010; Chang et al., 2015b; Tran et al., 2019). Previous laboratory studies demonstrate that the biopolymers function as a plant nutrient and water holding agent (Fig. 7). These findings are encouraging the field applicability of biopolymers for anti-desertification efforts.
Fig. 7In summary, the laboratory verifications of biopolymer-treated soils have shown their field applicability for stabilisation, hydraulic control of geo-structures, and preventing the desertification.
3.2.2. Case studies
3.2.2.1. Slope surface protection: In situ wet-spraying method details and strengthening effect verification
Slope stability, the condition that an inclined slope can withstand its own weight and external forces without displacement, is of significant importance in geotechncial engineering because the destruction or loss of containment from such structures may result in severe physical and economic losses in the surrounding area.
Seo et al. (2021) have conducted field-scale implementation of biopolymer-treated slopes. The wet-spraying system used in this study is composed of a nozzle, air compressor, generator, mixing tank, water tank, and supplier (Fig. 8).
Fig. 8Implementation factors are varied to assess the effect of mixing time, biopolymer phase, and biopolymer contents on the effectiveness of biopolymer spraying onto the slope surfaces. The results revealed the gradual increase in UCS with mixing time due to the formation of uniform biopolymer-soil complex. A mixing time of 20 min was suggested as the adequate duration for in situ biopolymer applications (Fig. 9).
