In this study, the characteristics of clay are discussed and its importance in geotechnical engineering practices is noted.
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This chapter is composed of five main sections. In the first section, the importance of clay in geotechnical engineering is presented.
In Section 2, clay is defined and its properties are discussed. Section 3 presents the use of clay in geotechnical engineering practices. In Section 4, previous, related studies are summarized. Finally, in Section 5, the subject of clay is summarized and conclusions from this chapter are provided. Clay minerals are called secondary silicates, because they are formed from the weathering of primary rock-forming minerals. Clay minerals consist of two basic structures. First, silica oxygen is formed through the bonding of silicon ions to the oxygen atoms on all four sides tetrahedron. Second, an octagon forms with aluminum and magnesium ions coordinated on eight-sides with oxygen and hydroxyl ions octahedron.
All clay minerals are formed from octahedral and tetrahedral sheets with certain types of cations, which are in various forms and connected to each other in a certain system. Changes in the structures of the octahedral and tetrahedral sheets result in the formation of different clay minerals [ 4 ]. More common clay mineral groups include kaolinite, illite and smectite montmorillonite.
Kaolinite consists of silica and alumina plates, and these plates are connected very strongly, because kaolin clay is very stable Figure 2a. Illite has layers made from two silica plates and one alumina plate Figure 2b.
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However, illite contains potassium ions between each layer; this characteristic makes the structure of the clay stronger than smectite. Smectite has layers made from two silica plates and one alumina plate. Because there is a very weak bond between the layers, large quantities of water can easily enter the structure Figure 2c.
This event causes the swelling of such clay [ 5 ]. Certain features of the clay affect the structure of the soil, which determines its properties such as strength, hydraulic conduction, settlement and swelling. These features include isomorph substitution and surface anion and cation exchange capacity. This event is called isomorphic substitution if the octahedral or tetrahedral sites are replaced by a different atom normally found elsewhere.
The specific surface area is the property of solids, which is defined as the total surface area of a material per unit of mass. With the separation of hydroxyl ions from the clay surface, which results in crystal deficiency at the crystal head, anions subsequently attach to the surface and organic molecule content causes an electrical load imbalance.
Water is a dipolar molecule, namely, it has one positive and one negative charge. The surface of the clay crystal is electrostatically held to the water molecule.
In addition, water is held to the clay crystal by hydrogen bonding. Also, negatively charged clay surfaces attract cations in the water. Thus, water influences clay minerals. For example, the water content changes consistency limits and this affects the ground plasticity. Ultimately the change in clay plasticity directly affects the mechanical behavior of the soil. Studies generally accept clays as fully saturated in geotechnical engineering. Therefore, the behavior of clays is affected by the individual clay particle arrangements and pore water content.
The surfaces of clays are negatively charged, and so they tend to adsorb the positively charged cations in pore water. In this way, the cations on the surface of a clay particle that are entering the water spread into the liquid. This spreading is called the double layer. Briefly, the cations are distributed around the negatively charged surface of the clay particles, with the greatest density near the surface and decreased density with increasing distance from the surface. The cations form a positively charged layer and the double layer is created with a negatively charged surface of the clay particles.
The double layer affects the arrangements of the clay particles, and hence, the physical and mechanical properties of the soil are also affected [ 6 ]. The interaction of these forces controls the engineering behavior of soils to a great extent. At the same time, this interaction leads to the formation of different compositions and settlements in the soil planes, which are defined as structures in clay soils [ 4 ].
Environmental temperature, precipitation, groundwater level and pH and salinity all play roles in clay properties, as well as in the conversion of rock into clay. Clay derived from the same rock can be different under different environmental conditions. Around the clay that is faced with a liquid, there are distance-varying push-pull curves.
If there is a force lifting the two clay minerals, the particles clump together. This is called flocculation. If the net force is thrust, the particles are separated from each other; this is called dispersion. Particle orientations of soils vary between flocculated and dispersed Figure 4. Forces between the particles are important for clay, because the behavior of clay depends on the geological history and structure. This difference in the orientation of fine-grained soils affects the engineering behavior of the soil.
The geological process in the formation of soils in nature constitutes the arrangement of soils. For this reason, geotechnical engineering studies are interested in the physical and mechanical behavior of soils-bearing structures, as well as the strength between the structure, texture and performance of soils. There are many studies on the soil-orientation effects on the soil properties such as strength, hydraulic conductivity and the swelling-shrinkage relative to each particle [ 7 — 12 ].
Ingles [ 7 ] examined soil fabric during consolidation. Due to an increase in the degree of particle orientation, the total volume of the voids was reduced. Flocculation increases depending on the electrolyte concentration, ion valence, temperature, decreasing dielectric constant, hydrated ion diameter, pH value and surface absorbed ions. Soil engineering properties depend on the size, shape, a high amount of surface area and a negative surface charge of clay particles. In , Terzaghi proposed the concept of clay arrangement. He said that clay minerals stick to each other at the points of contact, with forces sufficiently strong to construct a honeycomb structure.
In , it was shown by Casagrande that this honeycomb shape is a special structure in clay-containing soils and this structure can vary depending on many characteristics of the environment [ 4 ]. Figure 5 shows further compression with progress of soil sedimentation. Later, other researchers also proposed fabric models [ 13 — 17 ].
Collins and McGown [ 17 ] defined the elementary particle arrangement, particle assemblages and pore spaces in the fabric model. Researchers provided an elementary particle arrangement, a single clay, silt or sand, which is shown in Figure 6a and b ; the group effect of clay plates is shown in Figure 6c , and the interaction between silt and sand is shown in Figure 6d. Particle assemblages contain one or more elementary particle arrangements or small particle clusters.
Pore spaces are defined with spacing between elementary particle arrangements and particle assemblages. Bennet and Hulbert [ 18 ] suggested that the fabric of soils is mostly determined by the physical arrangement of particles, which is gained at the time of sediment deposition by the physical—chemical conditions of the depositional environment. The fabrics of soils describe clusters, clusters are formed by other clusters and space between the clusters and structure of the soils describes the fabric, mineral content and decontamination forces.
Also, the fabrics of soils can sometimes be visualized under a microscope. Arrangement of clay particles . In general, clay is an unwanted material because it creates significant engineering problems. Unlike other minerals of the same size, clay forms mud when mixed with water. Clay has plasticity and can be shaped into dough, and when cooked it turns into a solid with great strength increments. Clay generally shows a volume increase when wet, and when it is dried, its volume decreases, which creates many cracks.
With increasing moisture content, clay changes from solid state, to semisolid state, to plastic state and to liquid state, which is given in Figure 7. In Figure 7 , the clay-water mixture shows a total volume reduction, which is equivalent to the volume of water lost around the liquid and plastic limits, as the clay transitions from liquid to dry, and if the decrease in water content continues, no reduction in volume is observed.
This limit value is called the shrinkage limit. Therefore, the shrinkage limit is the moisture content at which the soil volume will not reduce further if the moisture content is reduced. The plastic limit is the moisture content at which the soil changes from a semisolid to a plastic flexible state.
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The liquid limit is the moisture content at which the soil changes from a plastic to a viscous fluid state [ 19 ]. In geotechnical engineering, the liquid and plastic limits are commonly used. Water is a problem in geotechnical engineering, such as water in voids in the ground mass, flowing in pores, or in the pressure or stress that water creates in the pores.
Clay plays an important role in the emergence of water problems, especially on fine soils, and these problems include permeability, shear resistance, setting and swelling problems. In addition, capillarity, freezing and infiltration can be additional issues. Structures built on clay and slope stability are particularly problematic when affected by water. Dams and dikes also cause the destruction of structures without leakage and piping [ 4 ]. Therefore, it is necessary to estimate the quantity of underground seepage under various hydraulic conditions to investigate problems that involve pumping water for underground construction and for stability analyses of earthen dams and earth-retaining structures that are subject to seepage forces [ 19 ].
The hydraulic conductivity coefficient commonly used in geotechnical engineering is also used for permeability. Hydraulic conductivity is a property that expresses how water flows in the soil. Soils are permeable due to the existence of interconnected voids, through which water can flow from the points of high energy to the points of low energy [ 4 ].
Fluid viscosity, pore-size distribution, grain-size distribution, void ratio, roughness of particles and the degree of soil saturation affect the hydraulic conductivity of soils.
Clay soil has electrical ions, so the hydraulic conductivity of clays affects the ionic concentration and thickness of water layers held to the clay particles. Table 1 provides the typical values for soils. The hydraulic conductivity value of soils determines the constant head test for coarse soils and the falling head test for fine-grained soils [ 19 ]. Hydraulic conductivity of soils [ 19 ]. The effect of swelling-shrinkage on fine-grained soils is often seen as a problem in geotechnical engineering applications.
Shrinkage is usually visible from evaporation in dry climates, reduction of groundwater and sudden arid periods. Swelling can be seen due to rising water. These volume changes are harmful to heavy construction and road coverings. Swelling occurs when the inflation pressure is greater than the pressure from the covering or structure. The material damage from the swelling-shrinkage of soils is more likely to occur in the United States due to greater water pressure, floods, typhoons and earthquakes [ 4 ].
This amount of damage is twice the amount of damage incurred from floods, earthquakes and hurricanes. Swelling pressure depends on the type of clay mineral, soil structure and fabric, cation exchange capacity, pH, cementation and organic matter. Any cohesive soil can involve clay minerals, but montmorillonite or bentonite clay minerals are more active regarding swelling-shrinkage. Swelling is calculated by swelling experiments with chemical and mineralogical analysis, soil indices and some empirical formulas from soil classifications. The shrinkage limit is determined from a laboratory test or approximate calculation recommended by Casagrande.
Properties of clay improve with chemical additives such as cement, lime, lime-fly ash, cement-fly ash, calcium chloride and so on.
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Structures transfer loads to the subsoil through their foundations. The imposed stress from the structure compresses the subsoil. This compression of soil mass leads to a decrease in the volume of the mass, which results in the settlement of the structure, and this should be kept within tolerable limits.
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Therefore, settlement compression should be estimated before construction. The settlement is defined as the compression of a soil layer due to the construction of foundations or other loads. The compression is seen in deformation, relocation of soil particles and expulsion of water or air from void spaces. In general, the soil settlement under load falls into three categories: immediate or elastic settlement, which is caused by the elastic deformation of dry soil or moist and saturated soils without change in the moisture content; primary consolidation settlement, which is the result of a volume change in saturated cohesive soils because of the expulsion of water occupying void spaces; and secondary consolidation settlement is the volume change under a constant effective stress due to the plastic adjustment of soil fabrics [ 19 ].
The consolidation settlement is seen when a structure is built on saturated clay or the water level is permanently lowered. Simultaneously, consolidation settlement is seen under its own weight or the weight of soils that exists above the clay. The consolidation settlement of clay takes a long time, and the reason for this is the low hydraulic conductivity and slow drainage of clay. Settlement of the soil is determined by one-dimensional consolidation odometer and hydraulic consolidation Rowe.
In experiments, the vertical loads and void ratios are recorded. Afterwards, the relationship between the pressure and void ratio is determined from the measured data. These data are also useful in determining the consolidation coefficient. The consolidation coefficient is determined by the root of time method and the log-t method. Figure 8 shows the relationship between the void ratio and stress for a typical odometer test for consolidation. The shear strength of soils is one of the most important aspects of geotechnical engineering.
The strength of the soil provides safety for geotechnical structures. The bearing strength, slope stability and bearing wall of the bases are influenced by the shear strength of the soils. Failure in the soils occurs in the form of shear. If the stresses in the soil exceed the shear strength, failure occurs. The shear failure of the soil depends on the interactions between the soil particles.
These interactions are divided into friction strength and cohesion strength [ 2 ]. When the clay soils are subjected to shear, the volume change in the drainage shear depends on the environmental pressure, as well as the stress history of the soil. In addition, loading on clay soils does not allow water to escape from the pores, and thus, this creates excess water pressure. If the loading does not cause failure, the excess water pressure is dampened, consolidation occurs and volume change is observed. The long process of this volume change in the clays is due to very low hydraulic conductivity.
Determination of the shear strength of the clay is performed by a direct shear test, triaxial compression test, vane test and standard penetration tests [ 4 ]. Figure 9 presents the relationship between the shear stress and normal stress for a typical shear strength test and triaxial compression test. After the failure envelope is drawn, the cohesion c and internal friction angle f are obtained. For the determination of the physico-chemical and microstructural properties of clay soils, X-ray diffractometer XRD and scanning electron microscope SEM are commonly conducted.
In addition, to determine the physico-chemical properties and structure of the soils, a pH test, electrical conductivity, cation exchange capacity, helium pycnometer, mercury intrusion porosimetry MIP , surface area analysis SSA , Brunauer-Emmett-Teller BET method or likewise, zeta potential and wavelength dispersive X-ray fluorescence test and Differential Thermal Analysis DTA are conducted.
The change in pH affects the soil-water relations. Low pH indicates flocculation, and high pH indicates dispersion. Undetected location. NO YES. Selected type: E-Book. Added to Your Shopping Cart. Evaluation Copy Request an Evaluation Copy. This is a dummy description. This book presents an integrated systems approach to the evaluation, analysis, design, and maintenance of civil engineering systems. Addressing recent concerns about the world's aging civil infrastructure and its environmental impact, the author makes the case for why any civil infrastructure should be seen as part of a larger whole.
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