Soil Structure and Soil Fabric

The physical arrangement of particles and particle groups in different patterns is known as soil fabric. Soil structure includes soil fabric as well as the intra- and inter-particle forces of attraction and repulsion.

Soil Fabric:

The term soil fabric refers to the geometrical arrangement of individual particles in a soil, including the geometrical distribution of pore spaces. Soil fabric may be used to describe the particle arrangement in both cohesive soils such as clays and granular soils such as silts, sands, and gravels. However, the term packing is also used to describe the geometrical arrangement of particles in pure granular soils where clays are not present.

In general, clay particles occur in multiple groups called fabric units, rather than as individual discrete units. A comprehensive description of soil fabric therefore involves the arrangement of individual particles in each fabric unit and also the arrangement of various fabric units and pore spaces or voids.

Fabric units in clays that are visible to naked eye are known as peds. Fabric units that can be distinguished using an optical microscope are called clusters. Several clusters together form peds. Individual clay particles can be distin­guished using a TEM or SEM, and these fabric units are called domains or factoids or packets. In soils containing montmorillonite mineral, it may not be possible to distinguish individual particles using SEM or TEM.

Soil fabric for clays is different from that of sands or other coarse-grained particles because of their shape. As evident from the mineral structure, clay particles are formed by combinations of silica and alumina sheets.

Thus, the individual clay particles are in the shape of a sheet or plate with thickness considerably less than the other dimensions. This is in contrast to the sand particles that are more or less equi-dimensional in all directions, though they are not truly spherical.

Attractive and Repulsive Forces in Clays:

By considering the attractive and repulsive forces existing at the inter-particle and particle group level along with the soil fabric, soil structure helps to understand the engineering behavior of clays under structural loads.

Clay particles are not electrically neutral but carry some charge due to:

1. Broken bonds at edges leading to positive charge at the particle edges as in kaolinite.

2. Isomorphous substitution resulting in negative charge on particle surface. The magnitude of this charge depends on cation exchange capacity and specific surface, both of which are high in montmorillonite, medium in illite, and the least in kaolinite.

Attractive and repulsive forces of electrical nature therefore exist in clays.

Attractive forces between particles occur due to:

1. Attraction between positive edge and negative surface when there is edge-to-face particle association.

2. Short range London-van der Waals forces when clay particles are at atomic distances apart.

3. Presence of cementing or bonding materials such as iron oxide, aluminum oxide, carbonates, etc.

4. Organic matter present in the soil.

Repulsive forces occur in clays mainly due to repulsion between diffuse double layers of adjacent clay particles. The magnitude of the repulsive forces, relative to the gravity forces due to weight of the clay particles, increases with increase in the thickness of diffuse double layers, decrease in particle thickness and size.

Depending on the predomi­nance of attractive or repulsive forces in clays, the following two principal types of soil structure may occur in clays:

1. Flocculent structure.

2. Dispersed structure.

The concept of diffuse double layer is explained before considering the discussion on soil structure

Diffuse Double Layer in Clays:

The negatively charged particle surface attracts positively charged ions (cations) present in the pore water. The cati­ons, attracted to the particle surface, are not a fixed part of the clay particle, but are “exchangeable” by other cations of higher valence or size, whenever there is a change in the soil chemistry due to natural causes or environmental pollution.

The number of cations on a particle surface depends on the ion valence and the CEC of the mineral. The lower the valence of the cations, the more is the number of cations on the particle surface. The higher the CEC of the soil, the more is the number of exchangeable cations on the particle surface.

When the soil is in dry state, the exchangeable cations cling or adhere to the particle surface. When the water content of soil increases due to rainfall or the flow of groundwater, water acts as a shield and it penetrates the space between the particle surface and the cations, decreasing the attractive force between the two.

The cations thus spread over a distance from the particle surface. As the influence of the negative charge of the particles is maximum near the surface and decreases with the increase in distance, the concentration of cations is more at the surface and decreases with the increase in distance from the particle surface.

Further, the cations, because of their positive charge, attract negatively charged ions (anions) in the pore water. The concentration of anions is minimum near the particles surface because of repulsion from the negatively charged surface, and it increases with the increase in distance from the particle surface.

The pore water, in the vicinity of the particle surface, is not free, but it is under the influence of attractive force between the cations and the negatively charged particle surface and anions and also the repulsive force between the particle surface and the anions. This layer of water that is under the influence of electrical forces of attraction and repulsion and in which the cations and anions diffuse, up to some distance from the particle surface, is known as diffuse double layer.

The interaction between two particles or fabrics units, which are in proximity to each other, is through the inter­action of their diffuse double layers. The thickness of diffuse double layer significantly influences the soil properties in clays such as swelling, shrinkage, compressibility, as well as the permeability and shear strength.

As per Gouy-Chapman theory, for a given clay, the thickness of diffuse double layer increases with the decrease in electrolyte concentration of pore fluid, valence of exchangeable cations, and temperature. The thickness also increases with the increase in dielectric constant of the pore fluid, size of cation, pH, and anion adsorption. The double layer thickness also increases with the increase in the CEC and the specific surface area of the minerals.

Thus, monovalent cations such as Na⁺ and K⁺ form thicker double layers than the divalent cations such as Ca²⁺and Mg²⁺. Montmorillonite, which has more specific surface and CEC, forms thicker double layers than illite and kaolinite.

In general, the liquid limit and swelling of clays increase with the increase in the thickness of diffuse double layer. The shrinkage limit, compressibility, permeability, and shear strength of the clays decrease with the increase in the thickness of double layer. The relative influence of diffuse double layer forces on soil behavior depends on the size of individual clay particle.

In montmorillonite, the particle size is as small as 10 A and hence the soil behav­ior containing montmorillonite mineral is strongly influenced by double layer forces. The particle size of kaolinite is large and hence the gravity forces due to weight predominate over the surface forces of diffuse double layer. In illite, the behavior is intermediate between that of kaolinite and montmorillonite.

Types of Soil Structures and Fabric:

Soil structure is the arrangement of soil particles (soil fabric) or fabric units as well as the magnitude of electrical forces of attraction and repulsion between the individual particles and between the fabric units. It is the soil struc­ture that determines the soil fabric and the magnitude of properties of clay such as swelling, shrinkage, compressi­bility, permeability, and shear strength.

Following are the types of soil structure in clays:

1. Flocculent structure.

2. Dispersed structure.

These two structures are discussed in the following subsections:

1. Flocculent Structure:

A flocculated or flocculent structure in clays consists of clay particles with edge-to-face association. In this arrange­ment, the edge of one particle is in contact with the face of the other particle. As the edge of a clay particle has a small positive charge and the surface of the particle has a negative charge, there is a strong attractive force between the particles giving a stable structure.

Because of the edge-to-face particle arrangement, soils with flocculated structure have large void ratio and more permeability, that is, more water is able to flow through the voids per unit time.

Because of the attractive force at the particle contacts, soils with flocculated structure possess better resistance to external loads and thus have high shear strength. Such soils offer more resistance to deformation under loads and hence their compressibility is less. Similarly resistance to shrinkage of soil, due to drying, is also more.

Flocculated structure in clays is formed when the thickness of diffuse double layer is less, which occurs in a saline environment due to high ion concentration of the pore water. In this case, the decrease of double layer thick­ness and hence of the repulsive forces promotes edge-to-face particle association.

When clay particles settle in salt water, flocculated structure is formed. Flocculated structure is formed when attractive forces are predominant. Hence, flocculent structure is more prevalent in kaolinite and illite, where double layer forces are insignificant because of low double layer thickness, CEC and specific surface and larger particle size.

2. Dispersed Structure:

Dispersed structure in clays consists of face-to-face association of clay particles or fabric units. Because of the face- to-face association, the repulsive forces between the particles or between the fabric units are more in dispersed structure. Further, dispersed structure is more prevalent in montmorillonite, where double layers are thick and consequent repulsive forces are predominant.

As the repulsive forces are dominant over attractive forces, the shear strength of soils in dispersed structure is very low. As the thickness of diffuse double layer around the particle surface is large, the double layers of adjacent particles or fabric units repel each other and there is a greater particle-to-particle spacing resulting in a high void ratio, which is reflected in terms of large swelling in the presence of water.

However, the effective void space available for fluid flow is rather very small because the water in the double layer is not free but under the influence of the electrical forces of attraction and repulsion. Consequently, clays with dispersed structure have low permeability.

As the double layer repulsive forces are strong, soils with dispersed structure offer more resistance to compres­sion. The rate of compression in such soils is very slow and occurs over prolonged periods ranging from several months to years. As the external pressure from the foundations increases, the repulsive forces gradually yield to them, resulting in large compression.

When soils with dispersed structure are subject to drying, the double layer water evaporates resulting in large shrinkage. The magnitude of shrinkage is large because the particles are oriented to one another.