SOIL STRUCTURE and CLAY MINERALS
Soil Structure Structure of a soil may be defined as the mode of arrangement of soil grains relative to each other and the forces acting between them to hold them in their positions. Soil structure includes the mineralogical composition, the electrical properties of the particle surface, the physical characteristics, ionic composition of the pore water, the interactions among the solid particles, pore water and the adsorption complex. Structural composition of soil influences many engineering properties such as permeability, compressibility and shear strength.
Soil structure headings discussed in detail under two 1. Soil particle structure, and 2. Soil mass structure 1. Soil particle structure deals with structure individual atoms and minerals, of 2. Soil mass structure discusses the pattern arrangement of soil particles in a mass of soil of THAPAR UNIVERSITY, PATIALA 3
Specific Surface of Clay Minerals For fine-grained soils, surface bonding is predominant rather than gravitational force (for coarse-grained soils), Specific surface is a parameter which is often used to decide the importance of surface bonding forces relative to forces due to gravity, Ratio of the surface area of a material to its mass or volume For a cube of side D, Specific surface = D 2 /D 3 ; hence, specific surface α 1/D As D, specific surface For same void ratio, water contents are far more for fine grained soils than for coarse grained soils, Water required to just wet the surface of smaller particles in a given volume is more than that for larger particles. For same void ratio, water contents are more for fine grained soils than coarse grained soils Shape is an important property from physical point of view. Sphere has the smallest surface area per unit volume whereas a plate exhibits the maximum. Clay particles are mostly plate shaped active portions of a soil. THAPAR UNIVERSITY, PATIALA 4
Clay More than 50% of soil sample finer than 2μ Most important grain property of fine-grained soil materials is the mineralogical composition. Possess plasticity and cohesion Presence of water plays a decisive role in engineering behaviour THAPAR UNIVERSITY, PATIALA 5
Combine atoms to form molecule Atomic Bonds Primary Valence Bond Secondary Valence Bond Relative strength: 40-400 van der Walls forces Hydrogen Bond Relative strength: 1-10 Relative strength: 10-20 THAPAR UNIVERSITY, PATIALA 6
Primary Valence Bond or Ionic Bond Due to chemical combination of atoms of two molecules, i.e., combine atoms into molecule Two atoms join by adding or losing some electrons to it s outer shell Atoms which have lost or gained electrons in this manner are called as ions Na +, Al 3+, Si 4+, Cl -, O 2- are the resultant ions Example is Al 2 O 3 excess electrons exchanged THAPAR UNIVERSITY, PATIALA 7
van der Waals force Attractive forces between two parallel clay mineral particles separated by water. Depends on crystal structure of the minerals and on the distance separating the particles van der Walls force Orientation effect Induction effect Dispersion effect 77% contribution 4% contribution 19% contribution THAPAR UNIVERSITY, PATIALA 8
Hydrogen bond Primary valence bond between H and O Weaker than primary valence bond, but much stronger than van der Walls bond Not easily broken under stress applied in soil system THAPAR UNIVERSITY, PATIALA 9
Different Clay Minerals Hydrous aluminium silicates and other metallic ions (Magnesium or iron replacing wholly or in part for aluminium, in some minerals) Particles are small in size, flaky shaped Considerable surface area Can only be viewed with an electron microscope Three main groups Kaolin group Kaolinite Halloysite Montmorillonite group Illite group THAPAR UNIVERSITY, PATIALA 10
Name of Mineral Structural formula Kaolinite (OH) 8.Al 4 Si 4 O 10 Halloysite (OH) 4.Al 4 Si 4 O 10.4H 2 O Montmorillonite (OH) 4.Al 4 Si 8 O 20.nH 2 O Illite (OH) 4.K y (Si 8 - y Al y )(Al 4.Fe 2.Mg 4.Mg 6 )O 20 THAPAR UNIVERSITY, PATIALA 11
Fundamental building blocks Clay minerals are essentially crystalline in nature. Two fundamental building blocks a) Tetrahedral sheet or silica sheet b) Octahedral sheet Symbolic representation THAPAR UNIVERSITY, PATIALA 12
Tips pointing in same direction. Oxygen at base of all units in same plane * Multiple units constitute a sheet a) Silica sheet a) Gibbsite (Aluminium) sheet c) Silica-Gibbsite sheet THAPAR UNIVERSITY, PATIALA 13
Tetrahedral sheet Four O atoms at the apices of a tetrahedron enclosing Si atom Each of O atom at base is common to two adjacent units. Sharing of charges leaves three negative charges at base per tetrahedral unit. This along with two at apex = 5 negative charges. Four positive charges of silicon means net charge of -1 per unit Octahedral sheet Six OH ions at the tips of an octahedron enclosing and Al or Mg or some other metallic atom OH ioins in two planes with ion common to three octahedral units. Net charge of +1 per unit If atom at centre is Al, resulting sheet is called the Gibbsite sheet If atom at centre is Mg, resulting sheet is called the Brucite sheet THAPAR UNIVERSITY, PATIALA 14
Isomorphous Substitution Substitution of metallic ions of one kind by other metallic ions of lower valence. Difference in the valences leads to a negative charge. Difference in size of the cations produces a distortion of the mineral structure. Decreased resistance of a mineral structure to chemical and mechanical weathering, e.g. feldspar. Example is Illite sheet wherein two tetrahedral sheets are sandwiching an octahedral sheet with potassium ions acting as bond between the sheets. The negative charge to balance the potassium ions comes from the substitution of aluminum for some silicon in the tetrahedral sheets. THAPAR UNIVERSITY, PATIALA 15
Kaolinite Symbolic structure of Kaolinite THAPAR UNIVERSITY, PATIALA 16
Kaolinite is one of the most common clay minerals in sedimentary and residual soils, e.g. quartz (most common mineral in sand siltsized particles. 1:1 clay mineral (1 tetrahedral sheet and 1 octahedral sheet) Resulting layer thickness is ~ 0.7nm or 7 A (1A =10-10 m) A kaolinite crystal is a stacking of 70-100 or more of these basic layers Structural units held together by hydrogen bonds between the OH of octahedral sheet and the O of tetrahedral sheet Very strong and stable. Very little isomorphous substitution. Little tendency in interlayers to allow water and to swell Least active of clay minerals Thickness of 500 A - 1000 A Specific surface is 5 15 m 2 /g THAPAR UNIVERSITY, PATIALA 17
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Photomicrograph of Kaolinite particles THAPAR UNIVERSITY, PATIALA 19
Halloysite (Kaoline group) One of the most common minerals in residual soils, especially those derived from volcanic parent material. More randomly stacked than kaolinite It s 7 A layers are separated by water molecules Is tubular and rod like (unlike flaky clay minerals) Heating at temperatures higher than 60 to 75 C reduces it to kaolinite or water Engineering properties are different than kaolinite Both, kaolinite and halloysite, are used for chinaware and medicinal purposes THAPAR UNIVERSITY, PATIALA 20
Montmorillonite (Smectite) Silica sheet Gibbsite THAPAR UNIVERSITY, PATIALA 21
Dominant clay mineral in some clays and shales and in some residual soils derived from volcanic ash. 2:1 mineral. Two silica sheets and one alumina sheet. Thickness of layer ~ 9.6 A. Interlayer bonding between the tops of silica sheets is mainly due to van der Walls forces (weakest bond strength). Octahedral Al is partially replaced by Mg atoms. This isomorphic substitution results in unit negative charge at location of the substituted atom, which is balanced by exchangeable cations, such as Ca +2 and Na +2 situated at the exterior of the sheets. Specific surface is 800m 2 /g (largest specific surface among major clay minerals). THAPAR UNIVERSITY, PATIALA 22
Large amount of water and other exchangeable ions can easily enter between the layers causing separation Susceptible to volume change (excessive swelling capacity) Highly plastic and has little internal friction. Dangerous for stability of structure supported Soils containing montmorillonite mineral are commonly known as black cotton soil (20% of country s area) Bentonite Montmorillonite clay Used in drilling boreholes and oil wells THAPAR UNIVERSITY, PATIALA 23
Photomicrograph of Montmorillonite particles THAPAR UNIVERSITY, PATIALA 24
Illite Isomorphous substitution THAPAR UNIVERSITY, PATIALA 25
Most common clay mineral in stiff clays and shales as well as in postglacial marine and lacustrine soft clay and silt deposits. 2:1 mineral similar to montmorillonite. Potassium ions occupy positions between the adjacent O base planes. Potassium (K + ) ion bonds the two layer firmly. Don t swell as much in presence of water as montmorillonite. Layer thickness is 10 30nm. Specific surface is 80 100 m 2 /g. THAPAR UNIVERSITY, PATIALA 26
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Photomicrograph of Illite particles THAPAR UNIVERSITY, PATIALA 28
Chlorite Clay mineral commonly associated with micas and illite. 1:1 clay mineral. Unit sheet consists of one biotite mica sheet and one brucite sheet. Biotite octahedral sites occupied by magnesium (Mg). Brucite octahedral layer in which Mg atoms are in octahedral coordination with hydroxyls Unit sheet of chlorite is 1.4 nm thick. THAPAR UNIVERSITY, PATIALA 29
Adsorbed water The layers of water which surround the clay crystal are called adsorbed water. How and why water is adsorbed on the surface of clay particle? Water as dipole THAPAR UNIVERSITY, PATIALA 30
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Negatively charged soil surface attracted to a) Water dipole (molecule of water is like a rod with positive and negative charges at opposite ends) b) Cation attraction c) Hydrogen bonding THAPAR UNIVERSITY, PATIALA 32
Diffuse - double layer Attraction of water to the particle surface is strongest near the surface (about 10 A thick adsorbed thin layer of water) and decreases with distance. Beyond this 10 A layer, there is an outer layer which is attracted to a lesser degree and is more mobile. This layer is the diffuse-double layer. Water outside this diffuse double layer is nonoriented normal water (don t affect behaviour of clay particles). THAPAR UNIVERSITY, PATIALA 33
Effect of presence of water on Clay Main force of attraction Clay is very sensitive to presence of water Clay particles interact through the adsorbed water layers. Hence, concentration and size of ions present in water (environmental conditions) influence the host of soil structures. Soil structure is the end product of the interplay of the forces of attraction (i.e., van der Walls forces, hydrogen bonding and other linkages) and repulsion (i.e., similar charges on particle surface) between the particles of a soil mass. THAPAR UNIVERSITY, PATIALA 34
Cation (Base) exchange capacity The ability of a clay particle to adsorb ions on its surface or edges is called it s cation or base exchange capacity. Function of the mineral structure and the size of the particles. Cation exchange capacity (CEC) of clay is defined as the amount of exchangeable ions, expressed in milliequivalents, per 100 g of dry clay or meq/100g. Clay particles carry a net negative charge. In an ideal crystal, the positive and negative charges would be balanced. However, isomorphous substitution and broken continuity of structures result in a net negative charge at the faces of the clay particles. (There are also some positive charges at the edges of these particles.) THAPAR UNIVERSITY, PATIALA 35
Cation To balance the negative charge, the clay particles attract positively charged ions from salts in their pore water. These are referred to as exchangeable ions. Some are more strongly attracted than other. Common soil cations include sodium (Na + ), potassium (K + ), magnesium (Mg 2+ ) and calcium (Ca 2+ ). Cations can make clay particles stick together (flocculate).
The cations can be arranged in a series in terms of their affinity for attraction as follows: Li + < Na + < H + < K + < NH 4+ < Mg +2 < Ca +2 < Al +3 This series indicates that, for example, Al +3 ions can replace Ca +2 ions, and Ca +2 ions can replace Na + ions. THAPAR UNIVERSITY, PATIALA 37
Cation (Base) exchange capacity of Principal Clay Minerals Mineral Cation exchange capacity (meq/100g) Kaolinite 3-8 Illite 20-30 Chlorite 20-30 Hydrated halloysite 40-50 Montmorillonite 80 THAPAR UNIVERSITY, PATIALA 38
Uses of cation exchange for soil stabilisation This principle can be used with advantage in many practical situations. An example is stabilisation of sodium clay soil by using lime. Here Ca +2 replace the Na + by virtue of their superior replacing power and reduces the swelling of sodium montmorillonite Be careful if soil has too much salt concentration Sodium saturated failure THAPAR UNIVERSITY, PATIALA 39
Flocculation and Dispersion of Clay Particles Net force is attractive Reduces with distance between particles (van der Walls) Net force is repulsive Flocculated state Dispersed state
Effect of salt concentration on orientation THAPAR UNIVERSITY, PATIALA 41
Chapter Over THAPAR UNIVERSITY, PATIALA 42