05
Clay particle-water interaction & Index properties
Electrical nature of clay particles a) Electrical charges i) The two faces of all platy particles have a negative charge. Resulting due to isomorphous substitution that is not neutralized by interlayer cation bonding + - - - - - - - -- - - - - - - - - - - + - - - + + + + ii) The edge of clay particles usually have a positive charge at low to moderate ph; Increasing ph lead to a negative charge. iii) The Net charge of clay particles is always negative. Eg. Formation of bentonite cake around peripheries of borehole (electrophoresis)
Electrical nature of clay particles b) Exchangeable Ions Since the soil must be electrically neutral The negative faces attract exchangeable cations (Na+, Ca++, Mg++, etc.) Postive edges attract exchangeable anions (or cations if negatively charged)
Electrical nature of clay particles Surface charge density (σ o ) Number of charges per unit area σ o = CEC/SSA Clay Mineral CEC (meg/100g) SSA(m 2 /g) σ o (C/m 2 ) Kaolinite 5 15 0.32 Illite 28 84 0.29 Na Mont. 100 800 0.18 Meg = milligramequivalents
Structure of clay soils (Fine-grained soils) Forces between clay mineral particles - If two particles (platelet shape) approach each other in a suspension, the forces acting on them are: a) The Van der Waals forces of attraction, and a) The repulsion between the two +vely charged ionized absorbed Adsorbed water layers water layer
Structure of clay soils (Fine-grained soils) At very small separations, the Van der Waals forces are always larger, and particles which approach sufficiently closely will adhere. However, the Van der Waals forces decrease rapidly with increasing separation. If the adsorbed layer is thick, the repulsion will be large at distances from the surface at which the Van der Waals forces Net Repulsion are small. Particles will remain Dispersed (settle independently)
Structure of clay soils (Fine-grained soils) Contact between dispersed particles will only be established if an external force is applied which is large enough to overcome net repulsive force. Attraction (net) If the adsorbed layer is thin, there will be little or no net repulsion at any distance, and random movements of particles will be enough to bring them into contact. This process is called Flocculation. Groups of particles settle together
Net force between two particles in a suspension Net repulsion Low ion concentration in the soil water Net attraction High ion concentration in the soil water Distance between crystal faces
Dispersed Structure of clay soils -The net forces of repulsion are greatest in the case of particles approaching face to face. Lacustrine clays (deposited in fresh water lakes) generally have a dispersed structure. In this case, few of the particles are in direct contact, most being separated by the adsorbed water layers.
Flocculated Structure of clay soils -Marine clays (deposited in sea water in which ion concentration is high, so that the adsorbed water layers are thin) generally have flocculated structure. Void space Flocculated structure has open structure with large void spaces with particles attached to each other with edge to edge and edge to face contacts.
Structure of clay soils (Fine-grained soils) Typical arrangement of platelet particles a) Dispersed b) Flocculated Low ion concentration (P H < 7) High ion concentration
Structure of clay soils (Fine-grained soils) A clay with an undisturbed flocculated structure will possess larger void openings. Silt/sand size particle Un-disturbed flocculent structure of Marine clay
Structure of clay soils (Fine-grained soils) When platelet particles are carried into fresh water lake, they do not flocculate and settle along with silt particles as they do in salt water. Remolded or dispersed structure of Fresh water deposit Remolding of flocculated structure results in dispersed structure
Clay structures Natural clay a)dispersed b) Flocculated c) Bookhouse d ) Turbostratic
Methods to identify soil structure a) Ordinary microscope (valid for coarse grained soils only) Scanning Electron Microscopy (SEM) This is ideally suited for clayey soils, as the resolution is sufficiently high and hence it is possible to go for higher magnifications ( = 1 x 10 5 times) A SEM is a type of electron microscope that images a sample by scanning it with a beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition, and other properties such as electrical conductivity.
SEM of coal ash blended with Na Bentonite
SEM of locally available silt a) b) 500x
Identification of clay minerals No one method is satisfactory for identification. This is partly because (i) Interference of minerals in a mixture and (ii) range of composition and crystal structure of clays from different sources. Three methods: i) X-Ray Diffraction (XRD) ii) Differential Thermal Analysis (DTA) iii) Casagrande s Plasticity Chart Casagradne s Plasticity chart method will be discussed later.
XRD Method The most widely used method of identification of clay minerals is from an X-Ray Diffraction pattern of a powdered sample of the clay size fractions of a soil. Minerals can usually be identified from diffraction lines. Principle: Minerals with regular or repeating patterns of crystal structure diffract x-rays.
Schematic diagram of X-ray diffraction unit for crystal identification Geiger-MÜller counting tube 2θ = Angle of the counter
Typical XRD pattern of Kaolinite, Montmorillonite and Illite Intensity of reflection Degrees, 2θ Different minerals with different crystalline structures will have different x-ray diffraction patterns, and in fact these different patterns help to identify different minerals.
Typical XRD pattern of locally available expansive soil Q Mt Mt Q C Mt-Montmorillonite(48-50%) Q-Quartz(30-32%) C-Calcite(15-16%) A-Anatase(1-2%) A
X-Ray Diffraction spectra for bentonite: Cu-kα radiation
XRD Method Demerits: (i) Not suitable for soils with mixtures of clay minerals, organic matter and nonclay mineral constituents and (ii) Inability to specify proportions of each mineral in a mixture.
DTA Method Differential Thermal Analysis determines the temperature at which changes occur in a mineral when it is heated continuously to a higher temperature. -The intensity of change is proportional to the amount of the mineral present. -Clays lose water or go through phase changes at specific temperatures. -The temperatures at which these reactions occur are characteristic of the mineral and can, therefore, be used for identification.
DTA apparatus with associated recording and control mechanisms For the DTA measurement, a sample of clay and a sample of inert material are slowly heated in a furnace. Calcined Aluminium Oxide or Ceramic are used as the reference material. When a temp. is reached at which the clay looses water by vaporization, the sample temp. will drop below that of inert material.
Review 1. Particulate arrangement in coarsegrained soils and fine-grained soils 2. Loose and Dense sand deposits Relative Density 3. Forces between clay mineral particles 4. Dispersed and Flocculent structures
Index Properties Index Properties refers to those properties of a soil that indicate the type and condition of the soil, and provide a relationship to structural properties such as the strength and the compressibility or tendency for swelling, and permeability. Can be divided into two categories, namely -Soil grain properties -Soil aggregate properties
Index Properties The development of the ability to think of soils in terms of numerical values of their index properties should be one of the foremost aims of every engineer who deals with Soil Mechanics.
Soil Grain Properties The soil grain properties are the properties of the individual particles of which the soil is composed, without reference to the manner in which these particles are arranged in a soil mass. For e.g., Mineralogical Composition, Specific Gravity of Solids, Size and Shape of Grains.
Soil Aggregate Properties Which are dependent on the soil mass as a whole and thus, represent the collective behaviour of soil. Soil aggregate properties = f (Stress history, mode of soil formation and the soil structure) Aggregate refers to soil itself- It may differ in Porosity, Relative Density, Water and Air Content and Consistency.
Soil Aggregate Properties Although soil grain properties are commonly used for identification purposes, the engineer should realize that the soil aggregate properties have a greater influence on the engineering behaviour of a soil Because: Engineering structures are founded on natural deposits or undisturbed soil mass.
INDEX PROPERTIES Grain Size Distribution Consistency Limits Mechanical Analysis (Coarse Grained Soil) -Dry Method - Wet Method Hydrometer Analysis (Fine Grained Soil) -Liquid Limit -Plastic Limit -Plasticity Index -Shrinkage Limit (Fine Grained Soil)
Grain Size Distribution (GSD) -In Soil Mechanics, it is virtually always useful to quantify the size of grains in a type of soil. -Since a given soil will often be made up of grains of many different sizes, sizes are measured in terms of grain size distributions. - GSD assists in providing rough estimates of soil engineering properties.
Grain Size Distribution (GSD) A subject of active research interest today is the accurate prediction of soil properties based largely on GSDs, void ratio, and soil particle characteristics. When measuring GSDs for soils, two methods are generally used: For grains larger than 0.075 mm sieving is used. For grains in the range of 0.075 mm > d > 0.5µm, the hydrometer test is used.
Sieve Analysis (For coarse-grained soil with d > 0.075 mm) opening size Large Sieves by their opening size #8 per inch 64 per sq. inch Size < 1/8 inch (width of the wire) Particle (grain) size d
Typical Grain Size Distribution Curves Idealized Fuller Packing Fuller Theoretical Curve
Sedimentation Analysis (for fine-grained soils: 0.5 µm < d < 75µm) -It is assumed, as a first approximation, that finegrained soil particles can be idealized as small spheres. -Spherical particle falling in a liquid of infinite extent and all particles have the same unit weight. -Particles reach constant terminal velocity within a few seconds after it is allowed to fall. Although clay particles are far from spherical, the application of Stroke s law based on equivalent diameters provide a basis for arriving at GSD of fine-grained soils (Sufficiently Realistic).
Sedimentation Analysis According to Stroke s Law, the viscous drag force F D on a spherical body moving through a laminar fluid at a steady velocity v is given by F D = 3πµvd Where µ is the viscosity of the fluid v is the steady velocity of the body d is the diameter of the sphere
Sedimentation Analysis If we drop a grain of soil into a viscous fluid, it eventually achieves a terminal velocity v where there is a balance of forces between viscous drag forces, gravity weight forces, and buoyant forces, as shown below: Fg-F b = (1/6) (G s -1)γ w πd 3
Sedimentation Analysis For equilibrium of the soil grain: F D = F g F b From this equation, we solve for the equilibrium or terminal velocity v of the soil grain as: v = ( G s 1) 18 µ γ w d 2 Observe: v d 2 Strokes law After Sir George Strokes (1891) Thus, the larger a soil grain is, faster it settles in water. This critical fact is used in the hydrometer analysis to obtain GSDs for fine-grained soil.
Process of Sedimentation of Dispersed Specimen Theory of sedimentation is based on the fact that the large particles in suspension in a liquid settle more quickly than small particles, assuming that all particles have similar densities and shapes. If all the particles were of a single size, with effective diameter d, by knowing terminal velocity v, we can calculate t d : The velocity which a falling particle reaches is known as terminal velocity t d ( G 18µ z 1) = γ 2 s wd
Limitations of Stroke s law -Soil particles are not truly spherical and sedimentation is done in a jar (For d > 0.2 mm causes turbulence in water and d < 0.0002 mm Brownian movement occurs (too small velocities of settlement) --- Can be eliminated with less concentrations. -Floc formation due to inadequate dispersion -Unequal Sp.Gr of all particles (insignificant for soil particles with fine fraction)