Mechanical Analysis of Soil: Meaning, Process and Principles

After reading this article you will learn about:- 1. Meaning of Mechanical Analysis 2. Process of Mechanical Analysis 3. Principles 4. Stokes’s Law 5. Practical Implications.

Meaning of Mechanical Analysis:

The mineral component constitutes the largest volume of soil mass and plays a vital role in determining most of the physical properties of soil. This mineral portion consists of particles of various sizes. Soil scientists usually separate them into convenient groups according to size.

The various groups like gravels, sands, silts, clays are termed as soil separates or fractions. The analytical procedure by which the particles are separated into various size groups from the coarsest sand, through silt, to the finest clay is called a mechanical analysis. It is one of the most important laboratory determination made in soil studies.

Process of Mechanical Analysis:

During mechanical analysis, all larger rocks, pebbles, roots and other rubble are removed (and measured) by screening the finer soil parts through a 2 mm sieve before analysis. Humus is removed from the soil sample by destroying it with an oxidizing chemical (e.g. hydrogen peroxide) before particle size separation is done.

Other binding materials are also removed from the soil using certain chemicals like, sodium hydroxide, sodium hexametaphosphate (Calgon solution), sodium oxalate etc. In the absence of complete dispersion, the lumps or aggregates of clay of the same size as sand would be wrongly reported as sand in the results of mechanical analysis.

There are several methods of mechanical analysis, of which the Pipette Method and Bouyoucos Hydrometer Method are important. Both methods are based upon the differential rate of settling of soil particles in water, and the accuracy of the methods depends upon various conditions and assumptions.

Principles of Mechanical Analysis:

Soil consists of particles of various sizes and since the fundamental objective of a particle-size analysis is to determine the percentage distribution of those above mentioned particles (sand, silt and clays) in the soil mass.

The rate of fall of particles in a viscous medium depends upon the size, density and shape of the particle. In a medium like water, larger particles settle more rapidly as compared to smaller ones with the same density and consequently settle out of suspension very quickly. This principle serves as the basis of practically all mechanical analysis.

Stokes’s Law about Mechanical Analysis:

G.G. Stokes (1851) suggested the relation between the radius of a particle and its rate of fall in a liquid. He stated that the resistance offered by the liquid to the fall of the particle varied with the radius of the sphere and not with the surface.

According to formula, the velocity of fall of a particle with the same density is directly proportional to the square of the radius and inversely proportional to the viscosity of the medium.

V = 2/9 (d_p – d) gr²/Æž

where V = velocity of fall (cm/sec)

g = acceleration due to gravity (cm/sec²)

d_p = density of the particle (g/cc)

d = density of the liquid (g/cc)

r = radius of the particle (cm)

Æž = absolute viscosity of the liquid (poise or m poise)

Derivation of Stokes’ Law:

If a solid body (soil particle) is moved through a liquid, the thin layer of liquid in immediate contact with the solid is virtually at rest, just as in the flow of liquid through a tube; as a result of viscosity a viscous drag is exerted on the moving body. In order to maintain a uniform velocity a steady force must, therefore, be applied to overcome the influence of the viscosity of the liquid.

It has been found that if a small sphere or soil particle of radius r travels at a velocity V through a fluid, gas or liquid, having a coefficient of viscosity Æž, the force applied f, which just balances that due to viscosity given by Stokes’s law,

F =6Ï€r Æž V …(1)

Liquids having low co-efficient of viscosity are said to be mobile, since they flow readily whereas the liquid with high viscosity, does not flow-easily.

If the sphere or soil particle is falling under the influence of gravity force, the constant downward force is,

4/3 Ï€r³ (d_p– d)g …(2)

Where d_p = density of solid sphere or soil particle

d = density of the medium through which the particle falls

g = acceleration due to gravity

r = radius of sphere or soil particle

According to the opposing force of viscosity increases with increasing rate of fall of solid body or soil particle and eventually a constant speed will be attained when the viscous force (1) is exactly equal to the gravitational pull (2), that is

4/3 Ï€r³ (d_p– d) g = 6 Ï€ r Æž V

V = 2/9 (d_p– d) gr²/Æž

This form of Stoke’s law is applicable to a solid sphere or soil particle falling through a liquid or gas or to a drop of liquid falling through a gaseous medium.

Assumptions in Stokes’ Law:

There are number of fundamental assumptions upon which the validity of this formula is based:

(a) The particles must be large in comparison to liquid molecules so that Brownian movement will not affect the fall.

(b) The extent of the liquid must be great in comparison with the size of the particles. The fall of the particle must not be affected by the proximity of the wall of the vessel or of adjacent particles.

(c) Particles must be rigid and smooth. In fact it is difficult to fulfill with soil particles. It is highly probable that the soil particles are not completely smooth over their entire surfaces. Soil particles are not spherical or round shaped. These are generally irregularly shaped with a large number of plate shaped clay particles.

Due to such diversification in shape, soil particles fall with different velocities. In order to overcome this difficulty in this law, the term “equivalent or effective radius” of a soil particle is used and it may be defined as the radius of a sphere or soil particle which would fall with the same velocity.

(d) There must be no slipping between the particle and the liquid.

(e) The velocity of fall must not exceed a certain critical value so that the viscosity of the liquid remains the only resistance to the fall of the particle.

(f) Particles greater than silt size fractions of a soil mass cannot be separated accurately with the help of this Stokes’ law.

Limitations of Stokes’ Law:

(i) The effect of different particle shapes (i.e. irregular flake, roundish, sub-rounded rod and disc etc.) on the settling velocities of clay particles is a major limitation for the accuracy of this law.

(ii) During mechanical analysis based on this principle, it is necessary to maintain a known constant temperature because the rate of fall varies inversely with the viscosity of the medium which changes with the change in temperature.

(iii) The density of the soil particle is another factor that affects the accuracy of this law. Density depends upon the mineralogical and chemical constitution of the particles and their degree of hydration. Generally, the value of particle density 2.65 g/cc will give more or less exact figure for the determination of mechanical analysis.

Practical Implications of Mechanical Analysis:

(i) The mechanical analysis is not of much significance unless stone and gravel are present in large quantities exceeding 10 per cent. If they are present beyond 10 per cent but not exceeding too large then facilitate drainage and tillage.

(ii) It helps in deciding the textural class names like sand, sandy loam, clay loam etc. by determining the percentages of different size groups of particles.

(iii) By mechanical analysis one can easily understand the physical properties as well as colloidal behaviour of soils.

(iv) It can help cultivation of soil by giving an idea of ‘light’ and ‘heavy’ properties of soil.

The use of the terms ‘light’ and ‘heavy’ refer to ease of tillage and not to soil weight.

Classification of Soil Particles:

A number of different classifications have been devised. They are:

(i) System developed by United States Department of Agriculture (USDA),

(ii) System developed by British Standard Institution,

(iii) System developed by International Union of Soil Science (IUSS) and

(iv) United States Public Roads Administration (Fig. 4.1).

Out of these systems, the international system for the classification of soil particles is commonly followed in India.

Problem:

A sample of soil was sieved and had the size separates in material smaller than 2 mm determined by particle-size (mechanical) analysis, with the following results:

Determine the textural class name.

Whole soil = coarse fragments + fine earth (< 2 mm)

Solution:

Textural names consider only the less than 2 mm portion. The coarse fraction name is given if over 20 per cent of the mineral soil weight is coarse material.

The coarse fraction = 53/253× 100 = 20.9% and the size is gravel.

Thus, the term gravel will be added to the textural class name as determined below:

Percentages of sand, silt and clay are based only on the less than 2 mm fraction, so subtract the coarse material weight.

The percentages are:

140/200 × 100 = 70% sand

38/200× 100= 19% silt

22/200× 100= 11% clay

Using the textural triangle (Fig. 4.2), place the triangle with 100% clay at the top and read across parallel with the base along the 11% line. Keeping this line in mind, turn the triangle so 100% silt is now at the top, and read across parallel to the new base of the triangle along 19% line.

The 11% clay and 19% silt lines intersect in the sandy loam. The percentage sand value could have been used as easily as either clay or silt values. The correct total textural name is gravelly sandy loam.