SOIL STRUCTURE AND FABRIC
The structure of a soil is taken to mean both the geometric arrangement of the particles or mineral grains as well as the interparticle forces which may act between them. Soil fabric refers only to the geometric arrangement of particles (from Holtz and Kovacs, 1981). *Fabric and structure are used interchangeably sometimes.
The interparticle forces (or surface forces) are relatively important for fine-grained soils at low confinement (low state of stress). Although the behavior of a coarse-grained soil can often be related to particle size distribution, the behavior of a fined-grained soil usually depends much more on: geological history and structure than on particle size.
SOIL FABRIC AND STRUCTURE Fabric is the arrangement of particles, particle group and pore spaces in a soil. Structure is the combined effects of fabric, composition and interparticle forces. Microfabric at least an optical microscope is needed. Macrofabric stratification, fissuring, voids and large scale inhomogeneties (by naked eye or a hand lense).
NET ENERGY AND FORCE OF INTERACTION Dispersion or flocculation Fabric of soil Engineering properties. If repulsion dispersion If attraction flocculation
Very small particles provide very large surface area. Negatively charged surface provide very active surface for chemical interaction. (From Bennett and Hulbert, 1986)
DISPERSION AND FLOCCULATION OF CLAY Colloidal clay Clay is a colloid. Colloidal particles have special properties due to their very small size. Firstly, their large surface area in relation to their mass makes them very reactive; in clays, this reactivity is shown as an electrostatic attraction of cations.
Secondly, colloids can exist in water as either: o suspensions (dispersed) or o as gels (flocculated).
The tendency of a colloid to o flocculate or o disperse depends on three things: the nature of the colloidal particles; the total salt concentration; the nature of the adsorbed ions.
The type and amount of different cations in a clay-water-electrolyte system have a major influence on double layer interaction. Flocculation to describe particles that are connected edge to edge or edge to face, Aggregation to describe particles that are connected face to face.
TERMINOLOGY Face (F) Edge (E) Clay Particle Dispersed: No face-to-face association of clay particles Aggregated: Face-to-face association (FF) of several clay particles. Flocculated: Edge-to-Edge (EE) or edge-to-face (EF) association Deflocculated: No association between aggregates or particles. van Olphen, 1991 (from Mitchell, 1993)
Flocculated fabric Dispersed fabric Edge-to-face (EF): positively charged edges and negatively charged surfaces (more common) Edge-to-edge (EE) Aggregated fabric The net interparticle force between surfaces is repulsive Face-to-Face (FF) Shifted Face-to-Face (FF)
CLAY FABRIC edge-to-face contact face-to-face contact Flocculated Aggregated
ENVIRONMENT EFFECT ON CLAY FABRIC Electrochemical environment i.e.: ph, acidity, temperature, cations present in the water during the time of sedimentation influence clay fabric significantly.
Flocculation is the first step in aggregate formation. Examples of flocculated and dispersed organic molecules.
Thickness of the diffuse double layer will depend on: Concentration of soil solution: High concentration of soil solution yields a thin DDL.
Valence of exchange ions: Monovalent ions yield a thick DDL Size of an ion (or hydration radius): Strongly hydrated ions yield a thick DDL. Particles with thick DDL tend to Particles with thin DDL tend to DISPERSE FLOCCULATE
Colloidal particles are either: hydrophilic (water-loving) or hydrophobic (water-hating). Hydrophilic colloids form stable suspensions and do not readily flocculate.
Hydrophobic colloids (such as clay) form unstable suspensions and flocculate easily. The nature of the colloidal clay particle (hydrophobic) means that clay will flocculate if allowed to. This is good for soil structure!
The more concentrated the salts (electrolytes) in the soil solution, the more likely it is that clay will flocculate. This is the 'electrolyte effect'. The salt is not necessarily common salt, sodium chloride. Any soluble salt, such as gypsum, will have this effect.
An 'electrolyte' is any salt. It is not necessarily common salt (sodium chloride). It could be any combination of cation and anion. Salts in soil can come soil minerals. from the weathering of
Weathering releases cations such as sodium, potassium, calcium, iron and magnesium. Anions produced by weathering include: sulphate, chloride, carbonate and phosphate.
Calcium adsorbed onto the clay surface allows the clay to flocculate when the total salt concentration is fairly low. Sodium adsorbed onto the clay surface flocculate will not allow the clay to until the total salt concentration is much higher.
Changes in the double layer thickness modifies the soil properties like: the shear strength, compressibility and plasticity.
MODES OF PARTICLE ASSOCIATIONS IN CLAY SUSPENSION 1. Dispersed no face to face association of clay particles. 2. Aggregated face to face association of several clay particles. 3. Flocculated edge to edge or edge to face association of particles or aggregates. 4. Deflocculated no association of particles or aggregates.
PARTICLE ASSOCIATIONS Dispersed and deflocculated Aggregated but deflocculated Edge-to-face flocculated but dispersed Edge-to-edge flocculated but dispersed Edge-to-face flocculated and aggregated Edge-to-edge flocculated and aggregated Edge-to-face and edge to edge flocculated and aggregated van Olphen, 1991
FABRIC IN COHESIVE SOILS Dispersed fabric: formed by settlement of individual clay particles. More or less parallel orientation. Flocculant fabric: formed by settlement of flocs of clay particles. Domain: aggregated or flocculated sub-microscopic units of clay particles. Cluster: domains group to form clusters, can be seen under light microscope. Peds: they are clusters group to form peds, can be seen without microscope.
DOMAIN CLUSTER PED The individual clay particles seem to always be aggregated or flocculated together in submicroscopic fabric units called domains. Domains then in turn group together to form clusters, which are large enough to be seen with a visible light microscope. Clusters group together to form peds and even groups of peds. Peds can be seen without a microscope, and they and other macrostructural features such as joints and fissures constitute the macrofabric system.
FABRIC OF NATURAL CLAY SOILS Domains and clusters with micropores 1.Domain 2.Cluster 3.Ped 4.Silt grain 5.Micropore 6.Macropore Yong and Sheeran (1973) (from Holtz and Kovacs, 1981) Enlargement
Diagram of the fundamental particle units called domains that comprise the building blocks of clay fabric in sediments and rocks. (From Bennett et al., 1991)
MICROFABRIC FEATURES IN NATURAL SOILS 1.Elementary particle arrangements, which consist of single forms of particle interaction at the level of individual clay, silt, or sand particles or interaction between small groups of clay platelets or clothed silt and sand particles. 2.Particle assemblages, which are units of particle organization having definable physical boundaries and a specific mechanical function. Particle assemblages consist of one or more forms of elementary particle arrangements or smaller particle assemblages. 3.Pore spaces within and between elementary particles arrangements and particle assemblages. Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
ELEMENTARY PARTICLES Individual clay platelet interaction Individual silt or sand particle interaction Clay platelet group interaction Clothed silt or sand particle interaction Particle discernible Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
PARTICLE ASSEMBLAGES Collins and McGown, 1974 (from Holtz and Kovacs, 1981)
PORE SPACE TYPES Collins and McGown, 1974 (from Mitchell, 1993)
PARTICLE ASSOCIATIONS IN SOILS Those main groupings can be identified: 1. Elementary particle arrangements, particle interaction of individual clay, silt or sand particles 2. Particle assemblages 3. Pore spaces 4. Intrapedal pores pore within the ped 5. Interpedal pores pores between the ped 6. Transpedal pores the pores that transverse the soil beyond the limits of a single ped. Ped: it is an individual soil aggregate consisting of a cluster of primary particles and separated from adjoining peds by surfaces of weaknesses.
EARLY CONCEPTS OF CLAY FABRIC Minerals of chemically sensitive clays: cardhouse structure. Lambe (1953), particle orientation in a dispersed system is a parallel arrangement (oriented fabric), whereas in a flocculated system, it is random (cardhouse fabric). Cardhouse, of saltwater Cardhouse of freshwater Mitchell (1956) pointed out important differences between dispersed and flocculated clays in relation to their geotechnical properties.
Van olphen proposed various modes of particles association when clay particles flocculate: FF, EF, and EE. EE and EF produce agglomerates (called floc ). FF association is termed aggregation.
Flocculation and aggregation have major effects on engineering properties. Flocculation affects flow behavior. It influences permeability, the ease with which a liquid moves through the soil.
Particles that are dispersed would have less permeability. Flocculation also affects shear strength and compressibility. Soils that have an edge-to-face contact of clay particles (flocculated) are much stronger than soils with a parallel alignment (dispersed).
Changes in hydraulic conductivity (K) also depend on: cementation (removal or formation during leaching), electrostatic forces (increasing or decreasing the flow channel diameter), and additionally on processes like erosion in flow channels and pore clogging.
One effect of the double layer is to cause two clay particles to repel each other when they approach so closely. Repulsive forces caused by overlapping double layers have been used to describe the compression and swelling behavior of clays. Dispersion phenomena is used to explain erosion of clays and tunneling failures in dams.
EROSION AND PIPING IN CLAYS In the past, clay soils were considered to be highly resistant to erosion by flowing water; however, in the recent years it is recognized that highly erodible clay soils exist in nature. Some natural clay soils disperse or deflocculate in the presence of relatively pure water and are, therefore, highly susceptible to erosion and piping.
The importance of the subject in civil engineering practice was not recognized until the early 1960's when research on piping failure in earth dams due to dispersive clay behavior was initiated in Australia because of many failures of small clay dams (Aitchison and Wood, 1965).
The tendency for dispersive erosion in a given soil depends on variables such as: mineralogy and chemistry of the clay, dissolved salts in the water in soil pores and in the eroding water. Such clays are eroded rapidly by slowmoving water, even when compared to cohesionless fine sands and silts.
When dispersive clay soil is immersed in water, the clay fraction behaves like single-grained particles; that is, the clay particles have a minimum of electrochemical attraction and fail to closely adhere to, or bond with other soil particles.
Thus, dispersive clay soil erodes in the presence of flowing water when individual clay platelets are split off and carried away. Such erosion may start in a drying crack, settlement crack, hydraulic fracture crack, or other channel of high permeability in a soil mass.
SUSCEPTIBILITY TO DISPERSION PIPING One of the properties controlling the susceptibility to dispersion piping is the percentage of adsorbed sodium cations within the clay particles relative to the quantities of other polyvalent cations (calcium, magnesium, and potassium).
A second factor controlling susceptibility of a clay mass to dispersion piping is the dissolved salts total content of in the reservoir or canal water. The lower the content of dissolved salts in the reservoir or canal water, the greater the susceptibility of sodium saturated clay to dispersion.
Any change in the pore solution chemistry that depresses or reduces the double layer leads to a reduction in swell. Calcium ions in the interlayer region compress the double layer, so the sheets are closer together and do not adsorb water and swell as easily.
If DDL Thickness is small swell is small. With sodium ions, the clay swells more easily. Thus the clay mineralogy has a direct effect on its surface chemistry.
Through its effect on surface chemistry, microstructure. clay mineralogy controls The result is the: engineering behavior of soil, its cohesive strength, flow behavior, permeability, and swelling potential.
Dispersed fabrics are more common in clays deposited in fresh water, while flocculated fabrics are typical of seawater deposition. Remolding (disturbance) of soils alters flocculated fabrics to dispersed fabrics.
Chemical factors favoring flocculation (favor structure): High salt concentration Polyvalent cations Low ph Chemical/physical factors favoring dispersion (unfavorable for structure) Low salt concentration Sodium is dominant cation High ph Mechanical disturbance
Saline water applied to soil will allow the clay to flocculate. If the water is saline due to high levels of soluble calcium, the flocculation will persist.
If, however, the water is saline due to high levels of sodium, the flocculation will last only as long as the soil solution remains concentrated. When rain washes excess salts from the soil, the soil solution becomes dilute and the clay disperses.
Saline water is a general term for water that contains a significant concentration of dissolved salts (NaCl). According to United States Geological Survey three categories of saline water: Slightly saline water contains around 1,000 to 3,000 ppm, Moderately saline water contains roughly 3,000 to 10,000 ppm. Highly saline water has around 10,000 to 35,000 ppm of salt. Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 g/l.
Gypsum acts on clay in two ways. Firstly, by raising the level of soluble salts in the soil solution, gypsum allows the clay to flocculate even if the clay has a high percent of exchangeable sodium (this is the electrolyte effect).
Secondly, soluble calcium in the gypsum replaces sodium on the cation exchange sites. The calcium dominated clay will remain flocculated after the free sodium is washed from the soil and the total salt concentration falls. In practice, however, several follow-up applications of gypsum are necessary to maintain the electrolyte effect.
PACKING IN COHESIONLESS SOILS Loose packing Dense packing Holtz and Kovacs, 1981 Honeycombed fabric Meta-stable structure Loose fabric Liquefaction Sand boil
HONEYCOMED Relatively fine sand and silt form small arches with chains of particles. Such soils have large void ratio, e and they can carry ordinary static loads. However under heavy loads or when subjected to dynamic loading, the fabric breaks down causing large settlements.
PACKING -SAND BOIL Loose sand Kramer, 1996
THE RELATIVE DENSITY (D R ) The relative density D r is used to characterize the density of natural granular soil. D r e e max max d max d e e min 100% d d max d min d min 100% The relative density of a natural soil deposit very strongly affects its engineering behavior. Consequently, it is important to conduct laboratory tests on samples of the sand at the same relative density as in the field ( from Holtz and Kovacs, 1981). (compaction) (Lambe and Whitman, 1979)
THE RELATIVE DENSITY (D R ) The relative density (or void ratio) alone is not sufficient to characterize the engineering properties of granular soils (Holtz and Kovacs, 1981). Two soils with the same relative density (or void ratio) may contain very different pore sizes. That is, the pore size distribution probably is a better parameter to correlate with the engineering properties (Santamarina et al., 2001). Holtz and Kovacs, 1981 2 : 1
FABRIC IN COHESIONLESS SOILS Single grained Honey combed Single grained: properties can be studied by uniformly sized spheres. Type of packing Coordination number Porosity (%) Void ratio Single cubic 6 47.64 0.91 Cubical tetrahedral Teragonal & Sphenoidal 8 39.54 0.61 10 30.19 0.43 Pyramidal 12 25.95 0.34 Tetrahedral 12 25.95 0.34