Physical Properties of sediments and water sediment mixture

Lecture 3

Jyoti KhatiwadaRoll no. 10

Mass Density and specific wt of solid particles

• The particle density or true density of a particulate solid or powder, is the density of the particles that make up the powder, in contrast to the bulk density, which measures the average density of a large volume of the powder in a specific medium (usually air).

• These mass and volume definitions can be used to define the concepts of soil particle density, bulk (dry) soil density, and total (wet) soil density.

• Density represents weight (mass) per unit volume of a substance.• Density = Mass / Volume• Soil density is expressed in two well accepted concepts as particle density

and bulk density. In the metric system, particle density can be expressed in terms of mega grams per cubic meter (Mg/m3). Thus if 1 m3 of soil solids weighs 2.6 Mg, the particle density is 2.6 Mg / m3 (since 1 Mg =1 million grams and 1 m3 =1 million cubic centimeters) thus particle density can also be expressed as 2.6 g / cm3.

• Particle Density: The weight per unit volume of the solid portion of soil is called particle density. Generally particle density of normal soils is 2.65 grams per cubic centimeter. The particle density is higher if large amount of heavy minerals such as magnetite; limonite and hematite are present in the soil. With increase in organic matter of the soil the particle density decreases. Particle density is also termed as true density.

Textural classes

Particle density ( g/ cm3)

Coarse sand 2.655Fine sand 2.659Silt 2.798Clay 2.837

• Bulk Density: The oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk density. The bulk density of a soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 g / cm3, whereas that of organic matter is about 0.5. Bulk density normally decreases, as mineral soils become finer in texture. The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of the porosity of the soil. Bulk density is of greater importance than particle density in understanding the physical behavior of the soil. Generally soils with low bulk densities have favorable physical conditions.

• Factors affecting bulk density• 1. Pore space: Since bulk density relates to the combined volume of the solids and

pore spaces, soils with high proportion of pore space to solids have lower bulk densities than those that are more compact and have less pore space. Consequently, any factor that influences soil pore space will affect bulk density.

• 2. Texture: Fine textured surface soils such as silt loams, clays and clay loams generally have lower bulk densities than sandy soils. This is because the fine textured soils tend to organize in porous grains especially because of adequate organic matter content. This results in high pore space and low bulk density. However, in sandy soils, organic matter content is generally low, the solid particles lie close together and the bulk density is commonly higher than in fine textured soils.

• 3. Organic matter content: More the organic matter content in soil results in high pore space there by shows lower bulk density of soil and vice-versa.

Bulk or Mass density of different class

Textural classBulk density (g/cc) Pore space (%)

Sandy soil 1.6 40

Loam 1.4 47

Silt loam 1.3 50

Clay 1.1 58

Specific wt of particles• The specific weight (also known as the unit weight) is

the weight per unit volume of a material. The symbol of specific weight is γ (the Greek letter Gamma)

Submerged unit weight• Submerged unit weight, which is defined as the

difference between the saturated unit weight and the unit weight of water. It is often used in the calculation of the effective stress in a soil.

Specific gravity

• Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the s ame given volume.

Grain size classification

Fall Diameter and nominal diameter

• Standard Fall Diameter - The standard fall diameter of simple fall diameter, of a particle is the diameter of a sphere that has a specific gravity of 2.65 and has the same standard fall velocity as the particle.

• Nominal Diameter - The nominal diameter of a particle is the diameter of a sphere that has the same volume as the particle.

Gradation coefficient

• The Coefficient of Gradation is defined as:• where

Probablity plots and various measure of size distribution

• The particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Significant energy is usually required to disintegrate soil, etc. particles into the PSD that is then called a grain size distribution.

• The normal probability plot is a graphical technique to identify substantive departures from normality. This includes identifying outliers, skewness, kurtosis, a need for transformations, and mixtures. Normal probability plots are made of raw data, residuals from model fits, and estimated parameters.

Normal probablity plot

Measure of grain size distribution• I. Grain Size Analyses Since particle diameters typically span many orders

of magnitude for natural sediments, we must find a way to conveniently describe wide ranging data sets. The base two logarithmic f (phi) scale is one useful and commonly used way to represent grain size information for a sediment distribution. A tabular classification of grain sizes in terms of f units, millimeters, and other commonly used measurement scales is included for purposes of comparison

Shape factor , form , sphericity and roundness

• The roundness classes are based upon another Wadell roundness index given by;

• ρ =r/R• where r is the radius of curvature of the largest inscribed circle

and R is the radius of the smallest circumscribing circle. The index ranges from 0 to 1, with 1 indicating a perfect circle. The roundness classes are based upon a logarithmic scale because the distinction of differences at the high roundness end of the scale is more difficult than at the low roundness end of the scale. The class between 0.00 and 0.12 is excluded, because natural particles generally have roundness values greater than 0.12.

Sphericity

• Sediment Maturity refers to the length of time that the sediment has been in the sedimentary cycle. Texturally mature sediment is sediment that is well rounded, (as rounding increases with transport distance and time) and well sorted (as sorting gets better as larger clasts are left behind and smaller clasts are carried away. Because the weathering processes continues during sediment transport, mineral grains that are unstable near the surface become less common as the distance of transport or time in the cycle increases. Thus compositionally mature sediment is composed of only the most stable minerals.

• For example a poorly sediment containing glassy angular volcanic fragments, olivine crystals and plagioclase is texturally immature because the fragments are angular, indicating they have not been transported very far and the sediment is poorly sorted, indicating that little time has been involved in separating larger fragments from smaller fragments. It is compositionally immature because it contains unstable glass along with minerals that are not very stable near the surface - olivine and plagioclase.

• On the other hand a well sorted beach sand consisting mainly of well rounded quartz grains is texturally mature because the grains are rounded, indicating a long time in the transportation cycle, and the sediment is well sorted, also indicative of the long time required to separate the coarser grained material and finer grained material from the sand. The beach sand is compositionally mature because it is made up only of quartz which is very stable at the earth's surface.

Shape factor• Shape factors are dimensionless quantities used in image

analysis and microscopy that numerically describe the shape of a particle, independent of its size. Shape factors are calculated from measured dimensions, such as diameter, chord lengths, area, perimeter, centroid, moments, etc. The dimensions of the particles are usually measured from two-dimensional cross-sections or projections, as in a microscope field, but shape factors also apply to three-dimensional objects.

• Shape factors are often normalized, that is, the value ranges from zero to one. A shape factor equal to one usually represents an ideal case or maximum symmetry, such as a circle, sphere, square or cube.

forms

A simple classification by zingg in 1935 used the ratio of width to length (b/a) and thickness by breadth (c/b) . These ratios are called forms indices. Every shape particle applies from indices to define form of particles. Four different shape nomenaclature are Tabular(rod), equant, bladed and prolate (disc)

Sorting packing and orientation of particles

• Sorting describes the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Very poorly sorted indicates that the sediment sizes are mixed (large variance); whereas well sorted indicates that the sediment sizes are similar (low variance).

• The terms describing sorting in sediments - very poorly sorted, poorly sorted, moderately sorted, well sorted, very well sorted - have technical definitions, and semi-quantitatively describe the amount of variance seen in particle sizes

• The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

Sediment consisting of

well sorted grains (left) poorly sorted grains (right).

Packing and orientation of grains

• Elements of the sediment show dimensional relations among them, expressed by various kinds of contact of adjacent grains. This kind of relationship, for the grains forming a grain skeleton, is called PACKING. It is a feature specifying dimensional density of grains in a sedimentary rock. Besides packing, grains can show directional arrangement in space, which is called a GRAIN ORIENTATION. Especially long and flat grains can be arranged in a rock in a way that is an oriented structure

Packing of grains

Porosity

PorosityPorosity is the quality of being porous, or full of tiny holes. Liquids go right through things that have porosity. Go back far enough and you'll find that porosity stems from the Greek word poros for "pore," which means “passage.” So something withporosity lets things through

Void ratio

• e = (V_v) / (V_s) • Where V_v is the volume of

the voids (empty or filled with fluid),

• and V_s is the volume of solids.

• Void ratio is usually used in parallel with soil porosity (n) , which is defined as the ratio of the volume of voids to the total volume of the soil.

Dry specific weight

The specific weight (also known as the unit weight) is the weight per unit volume of a material. The symbol of specific weight is γ (the Greek letter Gamma).Not to be confused with specific gravity Dry specific weight =Dry wt of sediments /total volume Dry specific mass =Dry mass of sediments/total volume

Newtonian mixture

Newtonian Fluids

• Newtonian fluids are named after Sir Issac Newton (1642 - 1726) who described the flow behavior of fluids with a simple linear relation between shear stress [mPa] and shear rate [1/s]. This relationship is now known as Newton's Law of Viscosity, where the proportionality constant η is the viscosity [mPa-s] of the fluid:

• Some examples of Newtonian fluids include water, organic solvents, and honey. For those fluids viscosity is only dependent on temperature. As a result, if we look at a plot of shear stress versus shear rate we can see a linear increase in stress with increasing shear rates, where the slope is given by the viscosity of the fluid. This means that the viscosity of Newtonian fluids will remain a constant) no matter how fast they are forced to flow through a pipe or channel (i.e. viscosity is independent of the rate of shear).

viscosity

• Viscosity is a property arising from collisions between neighboring particles in a fluid that are moving at different velocities. When the fluid is forced through a tube, the particles which compose the fluid generally move more quickly near the tube's axis and more slowly near its walls; therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between particle layers to keep the fluid moving. For a given velocity pattern, the stress required is proportional to the fluid's viscosity.

Dynamic viscosity

Dynamic viscosity of newtonian mixture

dynamic (shear) viscosity

• The dynamic (shear) viscosity of a fluid expresses its resistance to shearing flows, where adjacent layers move parallel to each other with different speeds. It can be defined through the idealized situation known as a Couette flow, where a layer of fluid is trapped between two horizontal plates, one fixed and one moving horizontally at constant speed u.

Kinematic viscosity

Kinematic viscosity of newtonian mixture

• The kinematic viscosity (also called "momentum diffusivity") is the ratio of the dynamic viscosity μ to the density of the fluid ρ. It is usually denoted by the Greek letter nu .

Volumetric sediments concentration

The End.

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