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Miscellaneous methods
Miscellaneous methods 8.1
8
Thermal stabilization
The thermal stabilization of soil refers to heating or freezing to improve the ground conditions. Although, this method is very costly compared with many other stabilization techniques, sometimes it might be the only option left to the project personnel for improvement of soil at a particular site. Site conditions, time-constraints, and the importance of the project or the structure are some of the factors that may compel engineers to adopt this technique. A brief explanation of ground heating and freezing techniques is presented in the following section. The effectiveness of these methods depends upon several factors, for example, the thermal properties of the in situ soil, the level of saturation of the ground, the amount of subsurface groundwater, the response of the ground to being frozen and its postthaw behaviour, and the impact of freezing or heating on public utilities and the nearby structures.
8.1.1 Heating Heating of soil to about 400°C brings about many changes to the engineering properties of soils. For example, the compressibility of clay soils as well as their swelling capacity is significantly reduced by thermal treatment. Thermal treatment is also used to prevent shear failure in clay soils and to stabilize clay slopes. However, this method is very expensive and nowadays it is used for limited applications, such as in landfills where energy is inherently available. The use of geothermal piles is a fairly new technique that uses geothermal energy and involves the supply of power and energy to the building whilst also working as a structural member. Vitrification is also one type of thermal treatment, where soil is melted at a very high temperature (about 1500–2000°C) with the help of electrodes followed by cooling. The melted soil transfers its heat into the surrounding soil and forms a glass-like product. The formation of the glass-like substance is due to the breakdown of the inorganic material present in the soil into silica and aluminium oxides. Sometimes, glass-forming additives are used to accelerate the process. The vitrification product is chemically stable and leach-resistant, and hence this process is generally used for the treatment of hazardous and radioactive contaminated ground. A simple diagram of vitrification process is presented in Fig. 8.1. In situ vitrification is a better option for removing pollutants from contaminant grounds since it does not involve excavation, transportation, or burial. However, this method has some limitations and disadvantages. First, vitrification is not suggested for soil with a high water content because of the high cost involved in the required evaporation. Also, if the rate of subsurface water flow is high, contaminant diffusion may take place reducing the effective of this method. Second, there are many Geotechnical Investigations and Improvement of Ground Conditions. https://doi.org/10.1016/B978-0-12-817048-9.00008-1 © 2019 Elsevier Inc. All rights reserved.
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Geotechnical Investigations and Improvement of Ground Conditions
Power supply
Electrode
Off-gas collection and treatment
Vapour cap
Contaminated ground Vitrified ground
Melting zone
Fig. 8.1 Vitrification.
controlling factors (e.g. arrangement of electrodes, soil properties including its mineralogical composition, variations in soil composition, groundwater profile, waste characteristics, presence of subsurface combustible material, etc.), all of which makes the vitrification process very complicated. Another major issue associated with vitrification is the damage that can be caused to the nearby structures if proper care is not taken during its implementation.
8.1.2 Ground freezing Ground freezing is applicable to all types of soils. The basic mechanism of ground freezing for soil stabilization is the conversion of the soil pore water into ice, which acts as a cementing agent, bonding together the soil particles or the blocks of rocks. The soil thus become impervious and its strength is also increased. There are many applications for this technique in civil engineering as follows: l
l
l
It generates a stable ground condition, which facilitates the excavation activities on site. It can freezes loose and wet sand to prevent liquefaction during an earthquake. It can create an in situ barrier to prevent soil and groundwater contamination.
There are basically two types of freezing techniques: (1) liquid nitrogen, and (2) brine. The liquid nitrogen from plant is transported to the site in a special tank where it is kept at a temperature of 196°C and at a pressure of about 0.2–0.3 MPa. It is then circulated in the ground, as presented in Fig. 8.2. Once it is injected into the freeze pipes, it vaporizes as nitrogen gas at extremely cold temperatures, from 82°C to 88°C, and it directly evaporates into the atmosphere as it rapidly freezes the ground. Liquid brine (calcium chloride) is also used as a cooling agent that circulates through the freezing pipes. Chilled brine is pumped down to the bottom of the freeze pipe through an open-ended inner pipe and flows up through the annulus. As the
Miscellaneous methods
79
Fig. 8.2 Ground freezing by using liquid nitrogen. Nitrogen gas outlet
Liquid nitrogen storage tank
Drop tube Freeze pipe
Frozen ground
circulation continues the warm energy is extracted from the ground and converts soil pore water into ice. A schematic diagram of this method is presented in Fig. 8.3. Depending upon the size and shape of the frozen ground and the spacing of the installed freeze pipes, this method may take several weeks to months to create frozen ground. Freezing is more effective when the ground is fully saturated and the rate of groundwater flow is low and nonturbulent. However, it should be noted that more soil creep may take place if the clay content is higher. Again, there might be postthaw loss of shear strength of the soils, depending upon the rate of freezing, surcharge loading, and drainage characteristics of the treated site. It is very important to determine the effective freezing point of the in situ soil/water systems to be frozen, as well as the stress deformation behaviour of frozen ground.
8.2
Bioengineering treatment
It is reported (DeJong et al., 2010) that about 40,000 soil improvement projects are performed per year at a total cost exceeding US$6 billion/year worldwide. This requires a huge quantity of materials and energy for the treatment of soils and implementation of the various soil-improvement techniques. Hence, there is a need to
80
Geotechnical Investigations and Improvement of Ground Conditions
Centrifugal pump Freezing plant Brine circulation
Cooling tower
Drop tube Freeze pipe
Frozen ground
Fig. 8.3 Ground freezing by using brine solution.
develop new techniques and to expand any existing methods that are energy efficient and environmentally friendly. With this in mind some of the nontraditional techniques are presented herewith. Some of these methods, like plantation and vegetation growth, have been used to prevent soil erosion and to stabilize soil slopes since ancient times. In present day, however, it is done in more scientifically.
8.2.1 Plantation and vegetation growth Plantation and vegetation growth is the most ecofriendly way of soil stabilization as compared to other forms of ground improvement. This method can be used for the stabilization of cut or fill slopes, or for the construction of earth-retaining structures in environmentally sensitive areas. Growing vegetation generates more friction at the soil–stem interface and imparts an artificial cohesion value to the soil through the growing roots. Moreover, evapo-transpiration helps to dissipate pore water pressure and thus help to increase the stability of earth slopes. The stability of slopes is also increased because woody vegetation acts as a hydraulic drain. This breaks the erosive force of running water and thus prevents the erosion of soil particles along the sloping grounds. To improve the effectiveness of this method: l
l
l
suitable plants should be selected as per the site and climatic conditions preliminary studies should be carried out to understand the effects of depth and root concentration in different soils. In this regard, the lifespan and growth rate of the vegetation must be determined the vegetation needs to be planted correctly and maintained.
Miscellaneous methods
81
Depending upon the types of plants and the site and climatic conditions, it may sometimes take time for the vegetation to establish itself in order to impart the required strength to the soil. In such cases, the temporary use of geosynthetic materials, wooden stakes, and soil nails, etc., may be used to protect the structure against initial failure. Moreover, different structural elements, like wire mesh and gabions, may be used in combination with the vegetation to get better results. Natural structural elements can be used to protect mountains and valleys against erosion include brush layers, brush mats, fascines, i.e. a cylindrical bundle of wooden sticks, and wattle fences, which are made by weaving flexible green woods between upright posts. Moreover, the effectiveness of bioengineering structures can be increased by using a combination of two or more of these materials along with suitable installation techniques.
8.2.2 Phytoremediation Phytoremediation is one of the bioengineering treatments in which plants are used to clean pollutants from the contaminated soil. The absorptive capabilities of plants and their ability to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues help them to extract the toxic heavy metals and organic pollutants from contaminated site. Plants work with soil organisms and transform the contaminants into harmless compounds. Phytoremediation can be used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydrocarbons, and landfill leachates. Phytoremediation is cost-effective and environmentally friendly. However, the selection of plants and their growth rates in different sites and environmental conditions are the key to the success of this method. Moreover, this method is limited to shallow sites with low levels of contamination.
8.2.3 Microbial geotechnology The basic mechanisms of soil stabilization using microbial geotechnology are (1) bioclogging, and (2) biocementation. In bioclogging the pore spaces of a soil mass are blocked with microbial biomass (i.e. their bodies and byproducts) such as calcite (CaCO3), etc. Biocementation refers to the bonding of soil grains by the binding agents produced due to microbial reactions. Microbial geotechnology can be used for a large number of civil-engineering applications, such as the enhancement of soil-bearing capacity, reduction of drain channel erosion and seepage control, liquefaction mitigation, prevention of piping in dams and dykes, prevention of contaminant migration, sealing of cracks in concrete to prevent leakage in certain structures, treatment of pavement surface, etc. The significant increase in some of the primary soil properties, like permeability, stiffness, compressibility, shear strength, and volumetric behaviour, provides numerous possibilities for various applications of this method. However, the effectiveness of this method depends upon several biological factors (e.g. selection of microorganism, microbial concentration, activity state, activity potential, biomass, nutrient
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Geotechnical Investigations and Improvement of Ground Conditions
concentrations, etc.), chemical factors, environmental conditions (e.g. pH, chemical concentrations, conductivity, salinity, oxidation–reduction potential, concentration of nutrients and water content, etc.), and geotechnical factors (e.g. pore size and geometric compatibility for progress of microbial process). The long-term behaviour of biotreated soils and the cost efficiency for the large-scale application of this method are yet to be explored in detail. For the successful application of microbial geotechnology the integration of microbiology, ecology, geochemistry, and geotechnical engineering knowledge is required.
8.3
Geopolymerization
Cement and lime are some of the common additives used for soil stabilization. Alhough cement is an effective stabilizer for sandy soil, it is costly and it is difficult and expensive to stabilize soil containing extensive clay minerals using cement alone. In addition, cement and lime are becoming increasingly unpopular due to the associated emission of greenhouse gases like carbon dioxide (CO2) and nitrous oxide (N2O) during their production process. The production of 1 ton of cement emits about same amount of CO2 into the atmosphere and consumes a large amount of energy. Keeping in mind the above problems, a low-energy intensive alternative to OPC was searched for. Thus low-cost industrial wastes like ground granulated blast furnace slag, fly ash, phosphogypsum, waste marble dust, cement kiln dust, and lime stone dust have gained popularity as additives for stabilization, all of which would otherwise have ended up either in landfills or lagoons. The use of industrial waste has significantly reduced emissions of greenhouse gases and has provided an effective method of waste disposal. Considering the amount of materials used and the strength requirements, a new generation of binders synthesized from silica- and alumina-rich precursor material has attracted the attention of investigators in recent years. The synthesis of this type of binder requires an alkaline environment and it is known as a geopolymer. It is an inorganic aluminosilicate material formed by polycondensation of tetrahedral silica (SiO4) and alumina (Al2O4) by sharing all the oxygen items forming a polymer. It is generally represented by an empirical formula Mn { (SiO2) z–AlO2}nwH2O, where the symbol “M” represents alkali metals, subscript “n” represents degree of polymerization, the symbol “–” is the presence of bond and subscript “z” is Si/Al ratio. It exhibits performance comparable to Portland cement and is known to have acid resistance, rapid gain in strength, low alkali reactivity, the ability to immobilize toxic waste, excellent binding properties, and the ability to reduce emissions of greenhouse gases. In addition, geopolymer can be synthesized at low temperature of 25–90°C. The geopolymerization process involves the dissolution of the solid aluminosilicates present in the precursor in a strong alkaline medium, followed by the formation of oligomers of silica and alumina, and polycondensation to form an inorganic polymeric structure. The change in the soil matrix that ultimately leads to strength gain due to geopolymerization can be seen in Fig. 8.4.
Miscellaneous methods
83
Fig. 8.4 SEM micrograph of NaOH treated soil samples.
8.4
Nano particles additives and carbon nanotubes
Due to higher ratio of surface area to volume and hence a higher rate of chemicomineralogical activity, nanoparticles significantly influence various properties of soils. Because of this many researchers have studied the soil behaviour after the addition of different nanoparticles like nanoalumina, nanolime, nanoclay, nanosoil, etc. Certain index and engineering properties of soils have been observed to be improved by the addition of the nanoparticles, although these techniques are still in the experimental stages. In recent years the use of carbon nanotubes (CNT), especially the plastic wastegenerated CNT for soil stabilization, has attracted the attention of many researchers. A typical SEM image of CNT synthesized from a high-density polyethylene (HDPE) is presented in Fig. 8.5A. The advantages of using CNTs are as follows: l
l
Its production does not release any greenhouse gases into the atmosphere and hence is ecofriendly. Moreover, the use of waste plastic for CNT production reduces its effects on the environment. It helps to reduce air and soil pollution by minimizing the use of cement, lime, chemicals, etc.
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Geotechnical Investigations and Improvement of Ground Conditions
Fig. 8.5 (A) SEM image of CNTs synthesized using autoclave, (B) simple mix of 0.2 g CNT in 0.5 L of distilled water, and (C) mixing with 2 mL of carboxylate-based surfactant with the help of a magnetic stirrer and sonicator. (A) Courtesy Dr. RP Vijay Kumar, VNIT Nagpur. l
l
l
l
CNT is a durable material. It is less brittle (CNT has about 12% strain at failure). Its increased tensile strength (tensile strength of CNT is about 150 times that of steel) forms a stronger and stiffer soil skeleton. It decreases the hydraulic conductivity of soil (CNT should be tested for its application in landfills covers and other such applications.
The major limitation of this method for soil stabilization is cost efficiency. More research is required to bring down its cost for field application in the construction industry. In addition, due to its low weight, tiny structure, immiscibility with water, and tendency towards clog formation during mixing, handling and mixing, CNT can be very difficult to use in soil. In addition, the long-term behaviour of soil–CNT mix under different site conditions is yet to be explored.
8.5
Use of waste materials
Millions of tonnes of solid waste are produced every year. This includes inorganic solid wastes from industries and organic wastes from agricultural and the municipal sector. The inorganic solid wastes from industries mainly consist of coal combustion residues (CCR), aluminium, copper, iron and zinc tailings, red mud from alumina refinery, etc. The major agricultural wastes consist of bagasse, rice husk, jute fibre, coir fibre and coir pith, paddy straw, and groundnut shell. Various researchers and field engineers have used these waste materials in combination with earth and suitable binders for different construction applications, including soil stabilization. One of these applications is the use of recycled plastic (mainly polyethylene) waste in road construction. For example, a road constructed by mixing shredded waste plastic into road tar in the Rourkela city, Odisha is presented in Fig. 8.6. The good thing is that the
Miscellaneous methods
85
Fig. 8.6 A road made of plastic waste in Rourkela city. From The Samaja, June 25, 2018.
use of plastic waste in bitumen significantly reduces some of the common problems related to the flexibility of a pavement like rutting and cracking, etc. Moreover, it improves the stability, strength, and fatigue life of roads.
8.6
Weight reduction method
The benefits of reducing the weight of fill materials are as follows: l
l
l
Settlement of embankments standing over soft compressible ground can be greatly reduced by the use of lightweight fill materials for construction. Slope stability can be enhanced, since the increase in slope weight is one of the major causes of slope failure. The lightweight fill materials also help to increase the earthquake resistance of the structures.
There are various lightweight materials (both natural as well as manmade) that can be used in construction. Some of the examples are EPS (expanded polystyrene) geofoams, crumb rubber tire, sawed lumber waste, foam glass aggregates, clam shells, LECA (lightweight expanded clay aggregate), expanded shell clay and slates (i.e. ESCS, which is a ceramic material produced by expanding and vitrifying select shales, clays, and slates in a rotary kiln), cellular concrete, pumice (a highly porous igneous rock), industrial wastes like slag, etc. However, the lightweight fill materials need to be properly tested for structural strength, permeability, durability, water absorption value, thermal properties, and density, as applicable to different construction applications. The lightweight materials can be placed over the native soil using many methods, i.e. by spreading followed by compaction, pumping in a flowable form, or by stacking in blocks with proper arrangement.
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Geotechnical Investigations and Improvement of Ground Conditions
Reference DeJong, J.T., Mortensen, B.M., Martinez, B.C., Nelson, D.C., 2010. Bio-mediated soil improvement. Ecol. Eng. 36 (2), 197–210.
Further reading Alsharef, J.M.A., Taha, M.R., Firoozi, A.A., Govindasamy, P., 2016. Potential of using nanocarbons to stabilize weak soils. Appl. Environ. Soil Sci. 5060531. 9 p. https://doi. org/10.1155/2016/5060531 Chang, I., Im, J., Cho, G.C., 2016. Introduction of microbial biopolymers in soil treatment for future environmentally-friendly and sustainable geotechnical engineering. Sustainability 8 (251), 1–23. Chang, D.K., Lacy, H.S., 2008. Artificial ground freezing in geotechnical engineering. In: 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11–16, 2008. Khairul, M.U., Kenny, A.K., Chiet, T.P., 2016. Biological process of soil improvement in civil engineering: a review. J. Rock Mech. Geotech. Eng. 8 (5), 767–774. Kim, D., Kyungho, P., Kim, D., 2014. Effects of ground conditions on microbial cementation in soils. Materials 7 (1), 143–156. Majeed, Z.H., Taha, M.R., 2013. A review of stabilization of soils by using nanomaterials. Aust. J. Basic Appl. Sci. 7 (2), 576–581. Michael, F., Jr, H., 2002. Nanoscience and technology: the next revolution in the earth sciences. Earth Planet. Sci. Lett. 203, 593–605. National Council for Cement and Building Materials, 2008. Report on Demonstration Project for Aggregate-Free Pavement Technology UsingFujibeton for Rural Road Construction, New Delhi. Ng, W.S., Lee, M.L., Khun, T.C., Ling, H.S., 2013. Improvements in engineering properties of soils through microbial-induced calcite precipitation. KSCE J. Civil Eng. 17 (4), 718–728. Patel, A., 2012. Mountain erosion and mitigation-global state of the art. J. Environ. Earth Sci. Form. Environ. Geol. 66 (6), 1631–1639. Zhang, M., Guo, H., El-Korchi, T., Zhang, G., Tao, M., 2013. Experimental feasibility study of geopolymer as the next-generation soil stabilizer. Constr. Build. Mater. 47, 1468–1478. https://doi.org/10.1016/j.conbuildmat.2013.06.017.
8
Thermal stabilization
The thermal stabilization of soil refers to heating or freezing to improve the ground conditions. Although, this method is very costly compared with many other stabilization techniques, sometimes it might be the only option left to the project personnel for improvement of soil at a particular site. Site conditions, time-constraints, and the importance of the project or the structure are some of the factors that may compel engineers to adopt this technique. A brief explanation of ground heating and freezing techniques is presented in the following section. The effectiveness of these methods depends upon several factors, for example, the thermal properties of the in situ soil, the level of saturation of the ground, the amount of subsurface groundwater, the response of the ground to being frozen and its postthaw behaviour, and the impact of freezing or heating on public utilities and the nearby structures.
8.1.1 Heating Heating of soil to about 400°C brings about many changes to the engineering properties of soils. For example, the compressibility of clay soils as well as their swelling capacity is significantly reduced by thermal treatment. Thermal treatment is also used to prevent shear failure in clay soils and to stabilize clay slopes. However, this method is very expensive and nowadays it is used for limited applications, such as in landfills where energy is inherently available. The use of geothermal piles is a fairly new technique that uses geothermal energy and involves the supply of power and energy to the building whilst also working as a structural member. Vitrification is also one type of thermal treatment, where soil is melted at a very high temperature (about 1500–2000°C) with the help of electrodes followed by cooling. The melted soil transfers its heat into the surrounding soil and forms a glass-like product. The formation of the glass-like substance is due to the breakdown of the inorganic material present in the soil into silica and aluminium oxides. Sometimes, glass-forming additives are used to accelerate the process. The vitrification product is chemically stable and leach-resistant, and hence this process is generally used for the treatment of hazardous and radioactive contaminated ground. A simple diagram of vitrification process is presented in Fig. 8.1. In situ vitrification is a better option for removing pollutants from contaminant grounds since it does not involve excavation, transportation, or burial. However, this method has some limitations and disadvantages. First, vitrification is not suggested for soil with a high water content because of the high cost involved in the required evaporation. Also, if the rate of subsurface water flow is high, contaminant diffusion may take place reducing the effective of this method. Second, there are many Geotechnical Investigations and Improvement of Ground Conditions. https://doi.org/10.1016/B978-0-12-817048-9.00008-1 © 2019 Elsevier Inc. All rights reserved.
78
Geotechnical Investigations and Improvement of Ground Conditions
Power supply
Electrode
Off-gas collection and treatment
Vapour cap
Contaminated ground Vitrified ground
Melting zone
Fig. 8.1 Vitrification.
controlling factors (e.g. arrangement of electrodes, soil properties including its mineralogical composition, variations in soil composition, groundwater profile, waste characteristics, presence of subsurface combustible material, etc.), all of which makes the vitrification process very complicated. Another major issue associated with vitrification is the damage that can be caused to the nearby structures if proper care is not taken during its implementation.
8.1.2 Ground freezing Ground freezing is applicable to all types of soils. The basic mechanism of ground freezing for soil stabilization is the conversion of the soil pore water into ice, which acts as a cementing agent, bonding together the soil particles or the blocks of rocks. The soil thus become impervious and its strength is also increased. There are many applications for this technique in civil engineering as follows: l
l
l
It generates a stable ground condition, which facilitates the excavation activities on site. It can freezes loose and wet sand to prevent liquefaction during an earthquake. It can create an in situ barrier to prevent soil and groundwater contamination.
There are basically two types of freezing techniques: (1) liquid nitrogen, and (2) brine. The liquid nitrogen from plant is transported to the site in a special tank where it is kept at a temperature of 196°C and at a pressure of about 0.2–0.3 MPa. It is then circulated in the ground, as presented in Fig. 8.2. Once it is injected into the freeze pipes, it vaporizes as nitrogen gas at extremely cold temperatures, from 82°C to 88°C, and it directly evaporates into the atmosphere as it rapidly freezes the ground. Liquid brine (calcium chloride) is also used as a cooling agent that circulates through the freezing pipes. Chilled brine is pumped down to the bottom of the freeze pipe through an open-ended inner pipe and flows up through the annulus. As the
Miscellaneous methods
79
Fig. 8.2 Ground freezing by using liquid nitrogen. Nitrogen gas outlet
Liquid nitrogen storage tank
Drop tube Freeze pipe
Frozen ground
circulation continues the warm energy is extracted from the ground and converts soil pore water into ice. A schematic diagram of this method is presented in Fig. 8.3. Depending upon the size and shape of the frozen ground and the spacing of the installed freeze pipes, this method may take several weeks to months to create frozen ground. Freezing is more effective when the ground is fully saturated and the rate of groundwater flow is low and nonturbulent. However, it should be noted that more soil creep may take place if the clay content is higher. Again, there might be postthaw loss of shear strength of the soils, depending upon the rate of freezing, surcharge loading, and drainage characteristics of the treated site. It is very important to determine the effective freezing point of the in situ soil/water systems to be frozen, as well as the stress deformation behaviour of frozen ground.
8.2
Bioengineering treatment
It is reported (DeJong et al., 2010) that about 40,000 soil improvement projects are performed per year at a total cost exceeding US$6 billion/year worldwide. This requires a huge quantity of materials and energy for the treatment of soils and implementation of the various soil-improvement techniques. Hence, there is a need to
80
Geotechnical Investigations and Improvement of Ground Conditions
Centrifugal pump Freezing plant Brine circulation
Cooling tower
Drop tube Freeze pipe
Frozen ground
Fig. 8.3 Ground freezing by using brine solution.
develop new techniques and to expand any existing methods that are energy efficient and environmentally friendly. With this in mind some of the nontraditional techniques are presented herewith. Some of these methods, like plantation and vegetation growth, have been used to prevent soil erosion and to stabilize soil slopes since ancient times. In present day, however, it is done in more scientifically.
8.2.1 Plantation and vegetation growth Plantation and vegetation growth is the most ecofriendly way of soil stabilization as compared to other forms of ground improvement. This method can be used for the stabilization of cut or fill slopes, or for the construction of earth-retaining structures in environmentally sensitive areas. Growing vegetation generates more friction at the soil–stem interface and imparts an artificial cohesion value to the soil through the growing roots. Moreover, evapo-transpiration helps to dissipate pore water pressure and thus help to increase the stability of earth slopes. The stability of slopes is also increased because woody vegetation acts as a hydraulic drain. This breaks the erosive force of running water and thus prevents the erosion of soil particles along the sloping grounds. To improve the effectiveness of this method: l
l
l
suitable plants should be selected as per the site and climatic conditions preliminary studies should be carried out to understand the effects of depth and root concentration in different soils. In this regard, the lifespan and growth rate of the vegetation must be determined the vegetation needs to be planted correctly and maintained.
Miscellaneous methods
81
Depending upon the types of plants and the site and climatic conditions, it may sometimes take time for the vegetation to establish itself in order to impart the required strength to the soil. In such cases, the temporary use of geosynthetic materials, wooden stakes, and soil nails, etc., may be used to protect the structure against initial failure. Moreover, different structural elements, like wire mesh and gabions, may be used in combination with the vegetation to get better results. Natural structural elements can be used to protect mountains and valleys against erosion include brush layers, brush mats, fascines, i.e. a cylindrical bundle of wooden sticks, and wattle fences, which are made by weaving flexible green woods between upright posts. Moreover, the effectiveness of bioengineering structures can be increased by using a combination of two or more of these materials along with suitable installation techniques.
8.2.2 Phytoremediation Phytoremediation is one of the bioengineering treatments in which plants are used to clean pollutants from the contaminated soil. The absorptive capabilities of plants and their ability to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues help them to extract the toxic heavy metals and organic pollutants from contaminated site. Plants work with soil organisms and transform the contaminants into harmless compounds. Phytoremediation can be used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydrocarbons, and landfill leachates. Phytoremediation is cost-effective and environmentally friendly. However, the selection of plants and their growth rates in different sites and environmental conditions are the key to the success of this method. Moreover, this method is limited to shallow sites with low levels of contamination.
8.2.3 Microbial geotechnology The basic mechanisms of soil stabilization using microbial geotechnology are (1) bioclogging, and (2) biocementation. In bioclogging the pore spaces of a soil mass are blocked with microbial biomass (i.e. their bodies and byproducts) such as calcite (CaCO3), etc. Biocementation refers to the bonding of soil grains by the binding agents produced due to microbial reactions. Microbial geotechnology can be used for a large number of civil-engineering applications, such as the enhancement of soil-bearing capacity, reduction of drain channel erosion and seepage control, liquefaction mitigation, prevention of piping in dams and dykes, prevention of contaminant migration, sealing of cracks in concrete to prevent leakage in certain structures, treatment of pavement surface, etc. The significant increase in some of the primary soil properties, like permeability, stiffness, compressibility, shear strength, and volumetric behaviour, provides numerous possibilities for various applications of this method. However, the effectiveness of this method depends upon several biological factors (e.g. selection of microorganism, microbial concentration, activity state, activity potential, biomass, nutrient
82
Geotechnical Investigations and Improvement of Ground Conditions
concentrations, etc.), chemical factors, environmental conditions (e.g. pH, chemical concentrations, conductivity, salinity, oxidation–reduction potential, concentration of nutrients and water content, etc.), and geotechnical factors (e.g. pore size and geometric compatibility for progress of microbial process). The long-term behaviour of biotreated soils and the cost efficiency for the large-scale application of this method are yet to be explored in detail. For the successful application of microbial geotechnology the integration of microbiology, ecology, geochemistry, and geotechnical engineering knowledge is required.
8.3
Geopolymerization
Cement and lime are some of the common additives used for soil stabilization. Alhough cement is an effective stabilizer for sandy soil, it is costly and it is difficult and expensive to stabilize soil containing extensive clay minerals using cement alone. In addition, cement and lime are becoming increasingly unpopular due to the associated emission of greenhouse gases like carbon dioxide (CO2) and nitrous oxide (N2O) during their production process. The production of 1 ton of cement emits about same amount of CO2 into the atmosphere and consumes a large amount of energy. Keeping in mind the above problems, a low-energy intensive alternative to OPC was searched for. Thus low-cost industrial wastes like ground granulated blast furnace slag, fly ash, phosphogypsum, waste marble dust, cement kiln dust, and lime stone dust have gained popularity as additives for stabilization, all of which would otherwise have ended up either in landfills or lagoons. The use of industrial waste has significantly reduced emissions of greenhouse gases and has provided an effective method of waste disposal. Considering the amount of materials used and the strength requirements, a new generation of binders synthesized from silica- and alumina-rich precursor material has attracted the attention of investigators in recent years. The synthesis of this type of binder requires an alkaline environment and it is known as a geopolymer. It is an inorganic aluminosilicate material formed by polycondensation of tetrahedral silica (SiO4) and alumina (Al2O4) by sharing all the oxygen items forming a polymer. It is generally represented by an empirical formula Mn { (SiO2) z–AlO2}nwH2O, where the symbol “M” represents alkali metals, subscript “n” represents degree of polymerization, the symbol “–” is the presence of bond and subscript “z” is Si/Al ratio. It exhibits performance comparable to Portland cement and is known to have acid resistance, rapid gain in strength, low alkali reactivity, the ability to immobilize toxic waste, excellent binding properties, and the ability to reduce emissions of greenhouse gases. In addition, geopolymer can be synthesized at low temperature of 25–90°C. The geopolymerization process involves the dissolution of the solid aluminosilicates present in the precursor in a strong alkaline medium, followed by the formation of oligomers of silica and alumina, and polycondensation to form an inorganic polymeric structure. The change in the soil matrix that ultimately leads to strength gain due to geopolymerization can be seen in Fig. 8.4.
Miscellaneous methods
83
Fig. 8.4 SEM micrograph of NaOH treated soil samples.
8.4
Nano particles additives and carbon nanotubes
Due to higher ratio of surface area to volume and hence a higher rate of chemicomineralogical activity, nanoparticles significantly influence various properties of soils. Because of this many researchers have studied the soil behaviour after the addition of different nanoparticles like nanoalumina, nanolime, nanoclay, nanosoil, etc. Certain index and engineering properties of soils have been observed to be improved by the addition of the nanoparticles, although these techniques are still in the experimental stages. In recent years the use of carbon nanotubes (CNT), especially the plastic wastegenerated CNT for soil stabilization, has attracted the attention of many researchers. A typical SEM image of CNT synthesized from a high-density polyethylene (HDPE) is presented in Fig. 8.5A. The advantages of using CNTs are as follows: l
l
Its production does not release any greenhouse gases into the atmosphere and hence is ecofriendly. Moreover, the use of waste plastic for CNT production reduces its effects on the environment. It helps to reduce air and soil pollution by minimizing the use of cement, lime, chemicals, etc.
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Geotechnical Investigations and Improvement of Ground Conditions
Fig. 8.5 (A) SEM image of CNTs synthesized using autoclave, (B) simple mix of 0.2 g CNT in 0.5 L of distilled water, and (C) mixing with 2 mL of carboxylate-based surfactant with the help of a magnetic stirrer and sonicator. (A) Courtesy Dr. RP Vijay Kumar, VNIT Nagpur. l
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CNT is a durable material. It is less brittle (CNT has about 12% strain at failure). Its increased tensile strength (tensile strength of CNT is about 150 times that of steel) forms a stronger and stiffer soil skeleton. It decreases the hydraulic conductivity of soil (CNT should be tested for its application in landfills covers and other such applications.
The major limitation of this method for soil stabilization is cost efficiency. More research is required to bring down its cost for field application in the construction industry. In addition, due to its low weight, tiny structure, immiscibility with water, and tendency towards clog formation during mixing, handling and mixing, CNT can be very difficult to use in soil. In addition, the long-term behaviour of soil–CNT mix under different site conditions is yet to be explored.
8.5
Use of waste materials
Millions of tonnes of solid waste are produced every year. This includes inorganic solid wastes from industries and organic wastes from agricultural and the municipal sector. The inorganic solid wastes from industries mainly consist of coal combustion residues (CCR), aluminium, copper, iron and zinc tailings, red mud from alumina refinery, etc. The major agricultural wastes consist of bagasse, rice husk, jute fibre, coir fibre and coir pith, paddy straw, and groundnut shell. Various researchers and field engineers have used these waste materials in combination with earth and suitable binders for different construction applications, including soil stabilization. One of these applications is the use of recycled plastic (mainly polyethylene) waste in road construction. For example, a road constructed by mixing shredded waste plastic into road tar in the Rourkela city, Odisha is presented in Fig. 8.6. The good thing is that the
Miscellaneous methods
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Fig. 8.6 A road made of plastic waste in Rourkela city. From The Samaja, June 25, 2018.
use of plastic waste in bitumen significantly reduces some of the common problems related to the flexibility of a pavement like rutting and cracking, etc. Moreover, it improves the stability, strength, and fatigue life of roads.
8.6
Weight reduction method
The benefits of reducing the weight of fill materials are as follows: l
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Settlement of embankments standing over soft compressible ground can be greatly reduced by the use of lightweight fill materials for construction. Slope stability can be enhanced, since the increase in slope weight is one of the major causes of slope failure. The lightweight fill materials also help to increase the earthquake resistance of the structures.
There are various lightweight materials (both natural as well as manmade) that can be used in construction. Some of the examples are EPS (expanded polystyrene) geofoams, crumb rubber tire, sawed lumber waste, foam glass aggregates, clam shells, LECA (lightweight expanded clay aggregate), expanded shell clay and slates (i.e. ESCS, which is a ceramic material produced by expanding and vitrifying select shales, clays, and slates in a rotary kiln), cellular concrete, pumice (a highly porous igneous rock), industrial wastes like slag, etc. However, the lightweight fill materials need to be properly tested for structural strength, permeability, durability, water absorption value, thermal properties, and density, as applicable to different construction applications. The lightweight materials can be placed over the native soil using many methods, i.e. by spreading followed by compaction, pumping in a flowable form, or by stacking in blocks with proper arrangement.
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Reference DeJong, J.T., Mortensen, B.M., Martinez, B.C., Nelson, D.C., 2010. Bio-mediated soil improvement. Ecol. Eng. 36 (2), 197–210.
Further reading Alsharef, J.M.A., Taha, M.R., Firoozi, A.A., Govindasamy, P., 2016. Potential of using nanocarbons to stabilize weak soils. Appl. Environ. Soil Sci. 5060531. 9 p. https://doi. org/10.1155/2016/5060531 Chang, I., Im, J., Cho, G.C., 2016. Introduction of microbial biopolymers in soil treatment for future environmentally-friendly and sustainable geotechnical engineering. Sustainability 8 (251), 1–23. Chang, D.K., Lacy, H.S., 2008. Artificial ground freezing in geotechnical engineering. In: 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11–16, 2008. Khairul, M.U., Kenny, A.K., Chiet, T.P., 2016. Biological process of soil improvement in civil engineering: a review. J. Rock Mech. Geotech. Eng. 8 (5), 767–774. Kim, D., Kyungho, P., Kim, D., 2014. Effects of ground conditions on microbial cementation in soils. Materials 7 (1), 143–156. Majeed, Z.H., Taha, M.R., 2013. A review of stabilization of soils by using nanomaterials. Aust. J. Basic Appl. Sci. 7 (2), 576–581. Michael, F., Jr, H., 2002. Nanoscience and technology: the next revolution in the earth sciences. Earth Planet. Sci. Lett. 203, 593–605. National Council for Cement and Building Materials, 2008. Report on Demonstration Project for Aggregate-Free Pavement Technology UsingFujibeton for Rural Road Construction, New Delhi. Ng, W.S., Lee, M.L., Khun, T.C., Ling, H.S., 2013. Improvements in engineering properties of soils through microbial-induced calcite precipitation. KSCE J. Civil Eng. 17 (4), 718–728. Patel, A., 2012. Mountain erosion and mitigation-global state of the art. J. Environ. Earth Sci. Form. Environ. Geol. 66 (6), 1631–1639. Zhang, M., Guo, H., El-Korchi, T., Zhang, G., Tao, M., 2013. Experimental feasibility study of geopolymer as the next-generation soil stabilizer. Constr. Build. Mater. 47, 1468–1478. https://doi.org/10.1016/j.conbuildmat.2013.06.017.