Soil Structure and Clay Minerals

Clay minerals are hydrous aluminium silicates formed by chemical weathering. They are built from two basic units, the silica tetrahedron and the alumina (gibbsite) octahedron, stacked in sheets.

Building units

• Silica sheet: tetrahedra with Si at the centre and four O at corners, joined at the base corners.
• Gibbsite sheet: octahedra with Al at the centre and six OH (or O) at corners.

1. Kaolinite (1:1 mineral)

One silica sheet bonded to one gibbsite sheet: a layer about 7.2 Å thick.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   ------ hydrogen bond ------
   Silica   [T T T T T]
   Gibbsite [O O O O O]

• Layers are held by strong hydrogen bonds and some van der Waals force, so they do not separate.
• Little isomorphous substitution; low swelling and shrinkage, low plasticity, low activity ($A \approx 0.4$), specific surface about 15 m²/g.

2. Illite (2:1 mineral)

A gibbsite sheet is sandwiched between two silica sheets (~10 Å thick); potassium ions between the layers hold them together.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   Silica   [T T T T T]
   ---- K+ ions (bond) ----
   Silica   [T T T T T]

• Some Si is replaced by Al (isomorphous substitution). Moderate swelling, activity about 0.9, specific surface about 80 m²/g.

3. Montmorillonite (smectite, 2:1 mineral)

Same layered structure as illite, but layers are held by weak van der Waals forces and water or exchangeable cations can enter between the layers.

   Silica   [T T T T T]
   Gibbsite [O O O O O]
   Silica   [T T T T T]
   ~~~ water + cations ~~~ (weak)
   Silica   [T T T T T]

• Large isomorphous substitution of Mg for Al; very high swelling and shrinkage, activity 1 - 7, specific surface up to 800 m²/g. Bentonite is mainly montmorillonite.

Basic structural units

Clay minerals are made from two basic building blocks:

1. Silica tetrahedron: a silicon atom at the centre surrounded by four oxygen atoms (or hydroxyls).
2. Alumina octahedron: an aluminium (or magnesium) atom at the centre surrounded by six hydroxyls (or oxygens).

Their repeated units form sheets, which are stacked to form clay mineral layers.

Silica sheet

Many silica tetrahedra are joined by sharing the three oxygens at the base, forming a flat sheet with hexagonal pattern; the apexes point in one direction. Formula $(\text{Si}_4\text{O}_{10})^{4-}$ or $\text{Si}_4\text{O}_6(\text{OH})_4$. Thickness about 4.93 Å.

Gibbsite sheet

Octahedral units with aluminium in the centre, joined by shared hydroxyls. Only two out of three possible positions of Al are filled (dioctahedral). Formula $\text{Al}_2(\text{OH})_6$ (Al$^{3+}$). Thickness about 5.05 Å.

Brucite sheet

Similar octahedral sheet with magnesium (Mg$^{2+}$) at the centre, in which all three positions are filled (trioctahedral). Formula $\text{Mg}_3(\text{OH})_6$.

 Tetrahedron      Octahedron
     Si              Al / Mg
    /|\            /| |\
   O O O          OH OH OH
     O            OH OH OH

Soil structure is the arrangement of soil particles and the forces between them. For clays it depends mostly on the electrical forces and the water in the soil.

1. Flocculated structure

• Formed when the net force between particles is attractive (high salt concentration, low pH, high electrolyte).
• Particles join edge-to-face or edge-to-edge in an open "card-house" arrangement.
• Properties: high void ratio, high strength, higher permeability, low density, high compressibility on loading.

2. Dispersed structure

• Formed when the net force is repulsive (low salt concentration).
• Particles lie parallel (face-to-face), oriented in the same direction.
• Properties: low void ratio, higher density, low permeability, lower strength.

 Flocculated          Dispersed
  \ / --  |            ===
   /\ |  /             ===
  --  /\               ===
 (open, random)       (parallel)

Effect of compaction

• Dry side of optimum: flocculated structure because of low water and little repulsion; high strength and permeability, and swelling potential.
• Wet side of optimum: dispersed structure because water allows particles to rotate and align; lower strength, low permeability, more shrinkage.
• Compaction effort also pushes soil towards dispersion, mainly on the wet side.

Other structures: single-grained (sand and gravel), honeycomb (fine sand and silt) and cohesive-matrix structure.

• Soil fabric is the geometric arrangement of soil particles, groups of particles (flocs) and pores in a soil mass, including orientation and shape (how the particles are packed).
• Soil structure is the combination of fabric and the interparticle forces between particles; it includes the fabric plus effects of cementation and bonding. Types include single-grained, honeycomb, flocculated and dispersed structures.

Soil structures based on compaction

• Flocculated structure (dry side of OMC): particles remain in random edge-to-face contact because there is not enough water to separate them; high strength, high permeability, high swelling potential.
• Dispersed structure (wet side of OMC): more water increases repulsion, so particles rotate and align face to face; lower strength, low permeability, more shrinkage and less swelling.
• Compaction with higher energy at the same water content increases the orientation (dispersion).

Double diffuse layer (DDL)

Clay particles carry a negative charge on their surfaces. In water they attract positive ions (cations) and polar water molecules. Close to the particle the cations are held strongly (adsorbed layer), and further away the concentration decreases gradually with distance, forming a diffuse layer of ions. The negative particle surface together with this diffuse cloud of positive ions is called the double diffuse layer. Its thickness decreases as the salt concentration and the valence of the cations increase.

Clay particles carry a net negative charge on their surface due to broken bonds at the edges and isomorphous substitution. When the clay is in water, this charge attracts positive ions (cations) and the dipolar water molecules.

• Near the surface, the cations are held tightly together with water molecules (the adsorbed or fixed layer).
• Further away, the attraction decreases, so the cations are free to move and their concentration falls with distance until it equals the concentration in the free water. This region is the diffuse layer.

The negative charge of the particle surface and the diffuse positive layer together form the double diffuse layer (diffuse double layer).

 - + - + - |
 - + -  +  |   conc. of cations
 - +  +    |   falls with distance
 particle   |-- adsorbed -- diffuse -->

Thickness increases with a lower electrolyte concentration, lower valence of cations, higher dielectric constant and lower temperature. A thicker layer gives more repulsion, a dispersed structure, higher plasticity and more swelling (montmorillonite).

• Specific surface is the total surface area of the soil particles per unit mass (m²/g) or per unit volume. It rises as particle size falls, e.g. kaolinite about 15 m²/g, illite about 80 m²/g, montmorillonite up to 800 m²/g.
• Diffuse double layer is the layer of water and positive ions (cations) around a negatively charged clay particle, in which the ion concentration decreases gradually with distance from the particle surface.

Specific surface is the total surface area of the particles of a soil per unit mass (m²/g) or per unit volume. It increases as particle size decreases and as the shape becomes flat (plate-like).

Typical values: sand about 0.01 m²/g, kaolinite about 15 m²/g, illite about 80 m²/g, montmorillonite up to 800 m²/g.

Effects on fine-grained soil

1. Surface forces dominate: in fine soils the electrical (surface) forces are much bigger than the weight of the particle, so the behaviour is controlled by the clay mineral and water, not by gravity.
2. More adsorbed water: a large surface holds a lot of water in the double layer, giving high water content, high liquid and plastic limits, and high plasticity.
3. Cohesion and plasticity: the soil becomes cohesive and plastic.
4. Swelling and shrinkage: high specific surface soils (montmorillonite) swell strongly on wetting and shrink on drying.
5. Low permeability: the large amount of bound water blocks the voids.
6. High compressibility and slow consolidation.
7. Cation exchange capacity and chemical activity increase.
8. Strength: high cohesion but strongly affected by water content.

Adsorbed water

Adsorbed water is the water held around a clay particle by the electrical attraction of the particle surface. It is very strongly attracted near the surface, with a density higher than ordinary water, and behaves almost like a solid. It is only partly removed by oven drying at 105 C, and its influence decreases away from the surface. It gives clay its plasticity and cohesion.

Formation of double diffuse layer

1. A clay particle has a negative charge on its surface because of isomorphous substitution and broken bonds at the edges.
2. In water, the negative charge attracts cations (Na$^+$, Ca$^{2+}$, K$^+$) and dipolar water molecules. The cations are also hydrated.
3. The cations nearest the surface are held strongly, forming the adsorbed (Stern) layer.
4. Because of thermal energy, the cations further out are free to diffuse. Their concentration falls gradually with distance until it becomes equal to that in the free pore water. This is the diffuse layer.
5. The negative surface charge and the diffuse positive ion cloud together form the double diffuse layer.

 particle  adsorbed   diffuse layer
 -----|+ + +|+  +   +    +      (free)
 - - -|+ + +| +    +      +
 conc of cations decreases ->

Thickness increases with the decrease of salt concentration and cation valence, and with an increase in dielectric constant of water and temperature. Thick layers give more repulsion, dispersion, swelling, and higher plasticity.

Double diffuse layer

Clay particles have a negative surface charge. In water they attract cations and polar water molecules. Close to the surface the cations are held tightly (adsorbed layer), and further out their concentration decreases gradually (the diffuse layer). The particle surface and the diffuse cloud of cations make up the double diffuse layer. A thicker layer produces greater repulsion between particles and more water held in the clay.

 Surface (-)  | + + +  +   +    +
              |  adsorbed | diffuse
              |<-- ion concentration falls -->

Mineral that swells most: montmorillonite

Montmorillonite swells the most, followed by illite, with kaolinite swelling the least. Reasons:

1. Structure: it is a 2:1 mineral; the layers are held together only by weak van der Waals forces between two silica sheets, so water and cations enter easily between the layers.
2. Large isomorphous substitution (Mg for Al), giving a high negative charge and thick double layers.
3. Very high specific surface (up to 800 m²/g, compared with about 15 m²/g for kaolinite) that adsorbs a large amount of water.
4. Kaolinite layers are bonded by strong hydrogen bonds so that water cannot enter; illite layers are held by K$^+$ ions, a strong bond, giving only moderate swelling.

Hence montmorillonite (bentonite, black cotton soil) shows the highest swelling, shrinkage, plasticity (activity 1 - 7).

Effect on geotechnical behaviour

• Montmorillonite: very high swelling and shrinkage, high plasticity, high compressibility, low permeability and low shear strength when wet. It causes foundation, slope, and pavement problems (black cotton soil); bentonite is used as a sealing material and drilling mud.
• Kaolinite: low swelling and shrinkage, low plasticity and compressibility, higher permeability and good stability. It is generally a favourable soil for construction and is stable on wetting and drying.

Isomorphous substitution

Isomorphous substitution is the replacement of a central atom in the silica tetrahedron or alumina octahedron by another atom of nearly the same size but lower valence, without any change in the crystal structure. Examples: Al$^{3+}$ replacing Si$^{4+}$ in a silica sheet, and Mg$^{2+}$ replacing Al$^{3+}$ in a gibbsite sheet. This leaves the layer with a net negative charge, which attracts cations and water (the origin of the double diffuse layer and cation exchange).

1:1 versus 2:1 minerals