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53 Seiten, Note: 9.36
1.1 Background and motivation
1.2 Objective of the work
1.3 Book Overview
2 Literature Review
2.2 Identification of expansive soil
2.3 Structure of clay mineral
2.4 Structure of compacted bentonite
2.5 Forces and charges in clay system
2.6 Particle association in Clay
2.7 Swelling Mechanism in Clay
2.8 Swelling pressure
2.8.1 Swelling pressure testing in Laboratory
2.8.2 Swelling pressure measurement devices
2.9 Compressibility behavior of bentonite
2.10 Effect of higher temperature on swelling and compressibility
3 Material and Methods
3.1 Properties of Soil studied
3.1.1 XRD Analysis
3.1.2 SEM Analysis
3.2 Modified oedometer
3.3 Swelling Pressure Test
3.3.1 Experimental setup
3.3.2 Constant volume swelling pressure test
3.4 Consolidation Test
3.4.1 Experimental setup
3.4.2 Methodology to Measure Compressibility
4 Result and Discussion
4.1 Swelling Pressure Test Result
4.2 Compressibility Test Result
5 Conclusions and Scope for Further Study
5.2 Scope for Further Study
An expansive soil can be identified by the potential of the soil to swell independently of field conditions such as water content and surcharge pressure. Because of swelling behavior properties, these types of soil produces problems like settlement and foundation crack. Many times these problems become unfavorable. The plasticity index of soil is the main indicator whether the soil is expansive in nature or not. Excess shrink or swelling behavior of these soils make the ground slope unstable and cause the unfavorable problem to the building foundation. In this work, the temperature effect on the swelling and compressibility of the clay were investigated. Most designs of geological repositories constructed at a greater depth for nuclear wastes based on the concept of multi-barrier. The concept of multi-barrier includes the natural geological barrier, engineered barriers made from compacted bentonite and the metal canisters. The natural geological barrier is nothing but host rock. Compacted bentonite-based materials are relevant materials for the purpose of the barrier and backfilling material in the waste repository, because of their high swelling, high radionuclide reduction capacity and low value of permeability (Pusch, 1979; Yong et al., 1986; Villar and Lloret, 2008; Komine and Watanabe, 2010). Stability of high-level radioactive waste disposal repository mainly depends on the swelling pressure value of compacted bentonites (Tripathy et al., 2014).
Compacted bentonites preferred as a barrier and backfilling material in the waste disposal repositories. The reason is to (i) hold the nuclear waste canisters structurally in place and (ii) increase the water tightness and at the same time it transfer the generated heat by the waste to the host rock (Tripathy et al., 2004). At the time of saturation and under the confined boundary conditions the bentonites in compacted form produces swelling pressures. The section of bentonite that is in contact with waste canisters is subjected to elevated temperature upon receiving heat from the radioactive waste, whereas the other section of the compacted bentonite in contact with saturated host rock receive groundwater from the saturated host rock. Once in operation, the host rock is expected to serve as the source of hydration for the bentonites which is in compacted condition and also it act as a confinement against the volume change because of hydration process. The stress convergence of the host rock, as well as the relationship between pressure and void ratio of the bentonite in saturated and compacted form, is important (Tripathy et al., 2004). On the other hand, a bentonite buffer near the canister will initially dry and shrink due to receiving heat induced by the canister. Because of the complexity of the repository, interests on the long term behavior of the bentonite buffers have increasingly gained recognition (Thomas et al., 2003; Villar and Lloret, 2004; Pusch and Yong, 2005).
The change in the moisture content of the soil due to temperature change is the crucial issues in geoenvironmental engineering (Villar and Lloret, 2004; Delage and Romero, 2008). Most problems associated with the behavior of clay liners and backfill materials are related to the change in volume because of variations in the moisture content (Delage and Romero, 2008). The volume change behavior of fine-grained soil during drying and wetting influenced by the physical and chemical forces (Bolt 1956; Mitchell, 1993; Tripathy and Schanz, 2007; Schanz and Tripathy, 2009). Not only physicochemical force, the microstructural arrangement of the clay particles and frictional forces are also important in controlling the behavior of soils (Mitchell, 1993; Saiyouri, et al., 2000; Delage et al., 2006; Pusch and Yong, 2005). Therefore, understanding the behavior of compacted bentonites under thermal, hydraulic and mechanical loading necessary for the long-term safety assessment of the waste disposal repositories. Also at this time, it is very important to study the variation of moisture content, temperature, and degree of saturation in compacted bentonite.
The main objectives of the work are as follows
1. Development of modified oedometer for conducting swelling pressure and consolidation test at elevated temperature.
2. Determination of swelling pressure of compacted bentonite (dry density = 1.6 Mg/m3) at temperatures of 25, 40, 50, 70, and 900 C.
3. To conduct compressibility test of bentonite and local soil at temperatures of 25, 50, 70 and 900 C.
The whole book divided into five chapters.
Chapter 1 mainly focusses on the background and motivation of the earlier work, primary objectives of the work, and overview of the book.
Chapter 2 presents a detailed literature review of the studies undertaken. The chapter covers introduction, structural unit of clay minerals, structure of compacted bentonite, forces, and charges in the clay system, particle association of clay i.e. orientation of fabric presented in detailed. The chapter also presents a review of swelling mechanism in clay, swelling pressure, and it’s testing in the laboratory, swelling pressure testing devices available and the experimental setup to carry out thermal and thermo-hydraulic tests briefly reviewed. Compressibility behavior of bentonite and effect of higher temperature on swelling and compressibility of clay soils also discussed.
Chapter 3 mainly presents the test to conduct the physical properties of the clays used in this study and the methods adopted for determining the properties of the clays. The physical properties determined include specific gravity, particle size distribution, minerals composition using XRD and SEM analysis, natural moisture content, consistency limits, linear shrinkage, differential free swell and determination of specific surface areas. The chapter also presents the schematic diagram of the developed modified oedometer its calibration chart and the experimental setup for swelling pressure and consolidation test, also cover the procedure adopted for preparing saturated slurried specimens and compacted specimens of the clay.
Chapter 4 outlines the results and discussions coming from swelling pressure test and consolidation test at various temperature. Also, cover water content and degree of saturation variation with temperature for both the bentonite and local soil.
Chapter 5 covers the main conclusions drawn based on the findings of this study and the scope of further study.
At the time of construction and long-term process of a repository, compacted bentonite can work as a barrier, protect the waste canister and restrict the transfer of radionuclide coming from the waste after the possible failure of the canister (Wersin et al., 2007). Bentonite is a type of clay mineral mainly composed of montmorillonite in a smectite group. Montmorillonite is a swelling mineral. Other minerals which also found in bentonite are non-swelling minerals such as quartz, feldspars, micas, carbonate and sand. Nath and Dalal (2004) has done an experiment on physical properties of soil and found that plasticity index of the soil mainly increases, because of increase in the value of the liquid limit of the soil. Clayey soils pose many problems to Geotechnical Engineering structures due to their large scale volume changes and poor shear strength. Moreover, clayey soils have more ability to swell than other soils.
Soils with high swell potential are called expansive soils. These soil types change in size as a result of variation in moisture content. Normally, expansive soils increase in size and swell when they absorb water and reduce in size and shrink when they become dry. Volume change in soil leads to distortions in the form of settlement due to contraction as a result of dryness or in the form of expansion due to swelling as a result of the absorption of water and increased humidity. The damage caused by expansive soils is very significant because of their high volume change which comes from swell-shrink behavior. At high plasticity index, the probability of shrinkage and swelling increases. There is a various application where these expansive soils have to be engineered in such a way so that it can achieve the desired behavior as a barrier for landfill liner (Alawaji, 1990) and back-filling materials for nuclear waste disposal system (Yong et al., 1986).
The volume change behavior of compacted bentonite under different temperature was studied by some of the researchers such as Romero (1999), Romero et al. (2001) and Sultan et al. (2002). The study shows that temperature produces opposite effects to the soil such as swelling or retraction, softening or stiffening, a decrease of the elastic domain or over consolidation i.e. it can be reversible or irreversible. Effect of temperature cause a contraction in the volume of normally consolidated soil, whereas expansion in volume of over-consolidated soils (Baldi et al., 1988 and Romero et al., 2001). The maximum swelling pressure of the sample achieved when the degree of saturation equal or more than 95% for the dry densities (1.29-1.95 g/cm3) at a temperature ranging from 20 to 80 °C (Shirazi et al., 2008).
Important properties of clay soil such as low permeability, less porous structure, and plasticity to act as a barrier lost due to the effect of temperature, which may develop from the waste. These buffer material in compacted condition played an important role to prevent the waste at its initial stage. At this stage radioactivity and the temperatures levels due to the production of the heat of the stored waste very high. This time, available information related to the effect of temperature on swelling characteristics of compacted clay soil is limited. Therefore, it is s a challenge to scientists and researchers working in the field to understand the effect of temperature on swelling characteristics of compacted clay thoroughly.
An expansive soil can be identified by the potential of the soil to swell independently of field conditions such as water content and surcharge pressure. The potential for swell depends on many factors: 1) amount and type of clay mineral; 2) soil structure, such as particle arrangement, bonding, and fissures; and 3) nature of the pore fluid and exchangeable cations. According to Chen (1975), expansive soils can be identified by three methods. The first method is mineralogical identification, the second one is indirect measurements, such as the index property, including Atterberg limits tests, linear shrinkage tests, free swell tests, and activity method and the third one is a direct measurement in the laboratory. Federal Housing Administration (1973), classified the expansive soils based on their plasticity index and liquid limit value as shown in Table 2.1. The Department of Army (1983) also classified the potential of swell according to their Atterberg limits as shown in Table 2.2.
Table 2.1 Classification of expansive soils (Federal Housing Administration, 1973)
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Table 2.2 Classification of swelling soils (Department of Army, 1983)
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Altmeyer (1955), suggested a classification method to determine the swell potential of soil according to their shrinkage limits and linear shrinkage value as shown in Table 2.3.
Table 2.3 Soil expansivity prediction by shrinkage limits and linear shrinkage (Altmeyer, 1955)
The mineral composition of the expansive soil includes quartz, and smectites as major components and feldspar, calcite, chlorite minerals as minor components. Primary minerals (e.g. quartz and feldspar) having dimensions larger than the secondary clay minerals (e.g. illite, kaolinite, and montmorillonite). Because of the smaller in size, secondary minerals having a very high specific surface area (m2/g) than primary minerals. Clay minerals are hydrated aluminum silicates available in the crystalline form of relatively complicated structure. These minerals divided into three groups according to their crystalline arrangement, and it found that all the three groups have roughly similar Engineering properties. Although mineral structure complicated, mineralogical investigation of different clay mineral demonstrated that they made of two basic building blocks silica tetrahedral and alumina octahedral.
The silica tetrahedral consists of a central silicon atom surrounded by four oxygen atom arranged as the apex of the equilateral triangle. A number of tetrahedral combines to form a sheet of silicon and called as silica sheet. The alumina octahedral consists of a central alumina atom surrounded by six hydroxyl ion. A number of octahedral combines to form a sheet of alumina and called as gibbsite sheet. These sheets stacked together and formed a crystal. In the case when it is air dry, bonding between these layers possible by van der Waals and cation exchange process. As above mentioned types of bonding are weak, and they broke very easily when any polar liquid enter between the layers (Mitchell, 1993). The particular arrangement and chemical compositions of silica tetrahedral sheets and alumina octahedral sheets determines the type of clay mineral and its general characteristics. The structural arrangement of clay mineral families shown in Figure 2.1.
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Figure 2.1 Clay minerals structural arrangement: (a) kaolinite, (b) illite and (c) montmorillonite (Mitchell, 1993).
Sheet or plate-like structure in the shape of folded, twisted or rod. It consists of alternating sheets of single silica tetrahedral and single alumina octahedral produces 1:1 clay layer of kaolinite mineral. In between the layers both the sheets strongly bonded by hydrogen bonding (Yong and Warkentin, 1975). Because of the presence of hydrogen bond a strong bond, kaolinite crystal does not hydrate and therefore, a very stable mineral. It has very fewer chances of volume change when contact with water or moisture. This mineral contains no interlayer water since both the sheets fit together very closely. The specific surface area (SSA) of the mineral is about 15m2/g.
It consists of two silica tetrahedral sheets with a central alumina octahedral sheet. Both the alumina and silica sheets share their oxygen atoms with each other, which result in the development of four oxygen thick layer. Potassium ions take the position between adjacent oxygen base planes. The potassium ions bind the two sheets together more firmly than that of montmorillonite with the result that the lattice is less susceptible to cleavage. Illite, therefore, does not swell so much in the presence of water as that of montmorillonite though it expands more than kaolinite. In this type of mineral the negative charge to balance the potassium ions comes from the aluminum substitution within the octahedral layer. These mineral show properties between both the mineral kaolinite and montmorillonite. It has fewer chances of volume change when in contact with moisture or water. The specific surface area of the mineral is about 90m2/g.
Bentonite, a smectite group of clay, contains a large quantity of mineral montmorillonite. This clay expands in volume when comes in contact with moisture. According to the modern concept, it is composed of units made up of two silica tetrahedral with a central alumina octahedral (Mitchell and Soga, 2005). The structural unit forms a weak bond can break easily. All the tip of tetrahedral points in the same direction and towards the center of the unit. The tetrahedral and octahedral combine so that tip of tetrahedral of each silica sheet and one of the hydroxyl layer of octahedral form a common layer. There is absolutely no bonding between two montmorillonite due to the presence of oxygen base layer. Montmorillonite and illite have similar composition. The outstanding feature of the mineral is that water and other, polar molecule can enter in the layer, and expand in the c-direction depending upon the polar molecules in amount present. Hence, it is of having very high CEC and specific surface area than the other two minerals i.e. illite and kaolinite. The specific surface of the mineral is about 800 m2/g.
As we know clay minerals consists of a lot of particles. Multiple pores exist inside the bentonite structure (Pusch, 1982; Delage et al., 2006). The micropores defined as the pores lying inside the aggregates (intra-aggregate pores) (Delage et al., 2006). The macropores are the pores in between the aggregates (inter-aggregate pores). The pores between the particles are sometimes called as the interparticle pores or mesopores (Delage et al., 2006). The change in the microstructure of compacted bentonites is responsible for the change in hydraulic conductivity, swelling pressure and cation or anion diffusion capacities (Pusch, 2001). Bradbury and Baeyens (2003), subdivided porosity into three different pore namely interlayer pores (0.2 - 1 nm), micropores (less than 5 nm) and macropores (5 - 20 nm). The schematic diagram of the microstructure of powdered air-dry bentonite, obtained with the help of Scanning Electron Microscopy (Pusch, 1982) shown in Figure 2.2. The variety of possible soil fabrics and interparticle forces lead to the variety of pore structures (Mitchell, 1993). The unit layers, particles, and aggregates form various types of pores in clay soil. In compacted soils the clay particles combined, and hence there is a chance of an increase in the formation of the aggregate microstructure (Delage et al., 2006).
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Figure 2.2 Schematic diagram of the microstructure of powdered air dry bentonite (Pusch, 1982).
As we know clay particles, they carry a charge on its surface which is negative in nature. The charge developed because of both the isomorphous substitution and a breakage of the structure at its edges. When it is in a dry condition, the charge on the surface is balanced by some of the exchangeable cations like Na+, Ca2+, Mg2+, and K+ neighboring the clay particle. When the water added to the soil, the cations mentioned above, and some of the anions, they float around the clay particles, and the process is called Diffused double layer (DDL) which is the result of clay water electrolyte interaction. On the negatively charged surface of the clay, cations held strongly. These cations termed as adsorbed cations. The adsorbed cations try to diffuse away from the surface of clay, and it tries to equalize the concentration throughout pore water. The diffusion tendency of adsorbed cations and electrostatic attraction together would result in cation distribution adjacent to each clay particle in suspension. Figure 2.3 shows such ions distribution neighboring to a single clay particle. Close to the surface, there is a high concentration of ions which decrease outwards. Thus, there are double layers of ions compressed layer and diffuse layer and hence the name double layer.
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Figure 2.3 Ion distribution neighboring to a clay surface using diffuse double layer concept (Mitchell and Soga, 2005).
In a clay soil, a basic idea to understand how the soil fabrics formed and how they changed with time and loading throughout the history of that clay is very important. van Olphen (1977) gives the classification as 1) face to face (FF), 2) edge to face (EF), and 3) edge to edge (EE). The second and third classification produce flocculation of viscous gel while the first classification produces less viscous oriented aggregate. Dispersion is used to describe the disassociation of flocculated particles. Here flocculated and aggregated terms used to assemble the multi-particle, whereas, deflocculated and dispersed used to single particle or particle group acting independently.
An understanding of the different factors which is responsible for various association shown in Figure 2.4. With the help of a type of association viscosity or gel forming tendencies of clay can be determined, which depends on the clay concentration. Microscopic study of the clay soil by different researchers showed that inside the clay aggregates, the clay fabric is mostly oriented in a face to face arrangement (Saiyouri et al., 2000). Clay minerals consist of several unit layers stacked on top of one another to form a particle. A montmorillonite particle is usually made of several unit layers (Saiyouri et al., 2004). The variety of possible soil fabrics and interparticle forces lead to the variety of pore structures (Mitchell, 1993). Understanding structure and fabric of clays are important for understanding their engineering behavior. Delage et al. (2006) have shown that the behavior of clays at microscopic level plays a significant role on the volume change behavior at the macro level. Tripathy et al., (2010) also found the same result for the physicochemical interactions between clay particles and pore fluid due to an increase in suction and normal stress.
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Figure 2.4 Modes of particles association (a) Dispersed and deflocculated (b) aggregated but deflocculated (c) flocculated but dispersed (van Olphen, 1977)
The swelling behavior of expansive clays exposed to water produced because of to two mechanisms one is crystalline swelling and another one is osmotic swelling (Madsen and Müller-Vonmoos, 1989). It produced when expansive clay mineral is allowed to saturate under controlled volume conditions. Both mechanisms influenced by the breakup of montmorillonite particles and by the demixing of cations. According to Yong (1999), the difference between this two mechanism crystalline and the osmotic swelling is only due to hydration structure of the water.
The first mechanism developed because of the hydration of ions when montmorillonite absorbs water is nothing but crystalline swelling. Madsen and Müller-Vonmoos (1989), found that in the case of unconfined condition montmorillonite volume may double its original volume while in the case of confined condition it can be more than 100 MPa only due to crystalline swelling. When the swelling of the mineral is restricted, the crystalline swelling pressure of very high degree will develop, reaching several thousands of kPa. The second mechanism, which appears when montmorillonite absorbs water i.e. change in volume of the clay mineral beyond crystalline swelling is due to osmotic phenomena. It occurs because of the interactions between diffuse double layers and the Vander Waals attraction. It is also called as double layer swelling.
The pressure required to keep a constant void ratio of the specimens during the hydration process of unsaturated expansive soil is called swelling pressure (Sridharan et al., 1986). When bentonite absorbs water under restrained boundary condition, they exhibit swelling pressures. Insertion of water molecules within the interlayer and interparticle pores causes montmorillonite clay to swell (van Olphen, 1962). However, if volume change is not allowed, the specimen will exert swelling pressure equivalent to the net repulsive force exerted between the clay platelets (Bolt, 1956; Tripathy et al., 2004; Schanz and Tripathy 2009). The magnitude of swelling pressure depends on the specific surface area, available exchangeable cations, temperature, initial dry density, and initial water content (Villar, 1999; Villar, and Lloret, 2004; Tripathy et al., 2004; Tripathy and Schanz, 2007; Schanz and Tripathy, 2009).
Swelling pressure behavior of bentonite in compacted form can explain using the concept of before, during and after water uptake. From the Figure 2.5, it can explain that before water uptake, bentonite made of a mixture of montmorillonite, voids, and other minerals having non-swelling properties. From this step, the voids occupied by air and free water. At the time of water uptake, montmorillonite absorbs water and into interlayers and swells occupying the void in the bentonite. Therefore, montmorillonite increases its volume, and the swelling pressure occurs. After water uptake, no any void available to absorb hence the volume of bentonite cannot increase more and at this point the swelling pressure can measure.
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Figure 2.5 Swelling pressure mechanism on compacted bentonite (Komine and Ogata, 1994).
Measurements of swelling pressure of expansive soils during the hydration process have been conducted in the laboratory by different methods (Sridharan et al., 1986; Komine and Ogata, 1994; Villar 1999; Tripathy et al., 2004). These methods are constant volume (i.e. isochoric condition), swell-underload, and swell-load (Sridharan, et al., 1986). All three gives a different value of swelling pressure. In swell-load, the specimen is inundated and allowed to swell under seating load. Loads added after an equilibrium condition reached by the soil sample. In swell-under load, several identical specimens are allowed to swell or to compress under different vertical loads to reach equilibrium conditions.
The swelling pressure defined as the pressure required to bring the soil specimen to its initial void ratio. In the case of constant volume method (no swell method), the volume of specimen is maintained constant by applying pressure until there is neither swelling nor compression occurred during inundation. That is why this method is also known as strain controlled test. Various methods of swelling pressure measurement shown in Figure 2.6. It observes that swell load test gives the highest value, swell under load test gives intermediate value, and constant volume test gives the least value of swelling pressure (Sridharan et al., 1986).
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