**3. Role of clay in geotechnical engineering**

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

88 Current Topics in the Utilization of Clay in Industrial and Medical Applications

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 1925, 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 1932, 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,

the voids was reduced.

**Figure 4.** Clay particle orientations.

**Figure 5.** Casagrande's fabric model (1932) [4].

Studies on soil behavior that do not consider the physico-chemical and microstructural properties of clay soils may be missing important information regarding the soil's physical and mechanical properties. This is because most physical and mechanical behaviors can be explained by the soil's physico-chemical and microstructural properties. 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.

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

The Importance of Clay in Geotechnical Engineering http://dx.doi.org/10.5772/intechopen.75817 91

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].

The effect of swelling-shrinkage on fine-grained soils is often seen as a problem in geotechnical engineering applications. Shrinkage behavior in clay soils is effective in reducing the strength in a slope and a foundation's bearing capacity. 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].

Jones and Holtz [20] estimated that shrinking and swelling soils cause approximately \$2.3 billion in damage annually to small buildings and road surfaces in the United States. This amount of damage is twice the amount of damage incurred from floods, earthquakes and hurricanes. Krohn and Slosson [21] estimated that swelling soils cause approximately \$7 billion in damage each year. According to Holts and Hart [22] 60% of 250,000 newly constructed

earthen dams and earth-retaining structures that are subject to seepage forces [19].

*3.1.2. Swelling-shrinkage behavior of clay*

**Soil type k (cm/s)** Clean gravel 100–1.0 Coarse sand 1.0–0.01 Fine sand 0.01–0.001 Silty clay 0.001–0.00001 Clay <0.000001

**Table 1.** Hydraulic conductivity of soils [19].

#### **3.1. Physical and mechanical behavior of clay**

In geotechnical engineering, it is important to identify a clay type, as the type directly affects the important properties of clay, such as Atterberg's limits, hydraulic conductivity, swellingshrinkage, settlement (compression) and shear resistance. Atterberg's limits, known as consistency limits, define the relationship between ground particles and water and the state of the soil relative to varying water contents. 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. 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. These limits are used to classify a fine-grained soil, according to the Unified Soil Classification system, AASHTO system or TS1500 (Turkey).

#### *3.1.1. Hydraulic conductivity properties of clay*

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

**Figure 7.** Water content-volume relationship of soils.

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].
