SOIL CONSISTENCE

ADHESION-Molecular attraction that holds the surfaces of two substances (eg. Water and soil particles) in contact

COHESION- Force holding a solid or liquid together, owing to attraction between like molecules. (Soil- Soil particles 

     Soil consistence is defined as “the resistance of a soil at various moisture contents to mechanical stresses or manipulations”.

 Soil consistency is the strength with which soil materials are held together or the resistance of soils to deformation and rupture.

Soil consistence is described at three moisture levels namely ‘wet’, ‘moist’ and ‘dry’.

1. Wet soils: Consistency is denoted by terms stickiness and plasticity

• Stickiness is grouped into four categories namely i) non sticky, ii) slightly sticky, iii) sticky and iv) very sticky

• Plasticity of a soil is its capacity to be moulded  (to change its shape depending on stress) and to retain the shape even when the stress is removed. Soils containing more than about 15% clay exhibit plasticity – pliability and the capacity of being molded. There are four degrees in plasticity namely i) non plastic, ii) slightly plastic, iii) plastic and iv) very plastic.

2. Moist soil: 

Moist soil with least coherence adheres very strongly and resists crushing between the thumb and forefinger. The different categories are

i. Loose-non coherent

ii. Very friable - coherent, but very easily crushed

iii. Friable - easily crushed

iv. Firm - crushable with moderate pressure

v. Very firm - crushable only under strong pressure

vi. Extremely firm -   completely resistant to crushing. (type and amount of clay  and humus influence this consistency)

3. Dry soil:

In the absence of moisture, the degree of resistance is related to the attraction of particles for each other. The different categories are

i) Loose                  - non coherent

ii) Soft                    - breaks with slight pressure and becomes powder

iii) Slightly hard    - break under moderate pressure

iv) Hard                  - breaks with difficulty with pressure

v) Very hard           - very resistant to pressure

vi) Extremely hard - extreme resistance and cannot be broken.

Atterberg’s Constants

Atterberg (1912) studied plasticity from the point of view of moisture range over which plasticity range is maintained. The constants are also known as Atterberg’s limits. There are three limits namely i) shrinkage limit or lower plastic limit, ii) plastic limit and iii) liquid limit or upper plastic limit. From the upper and lower limits, plasticity number or index is calculated. Plasticity number or index is an indirect measure of the force required to mould the soil.

i) Shrinkage limit (or) lower plastic limits (SL)

It is the soil moisture content below which the soil becomes friable. The maximum water content at which a reduction in moisture will not cause a decrease in the volume of the soils. This defines the arbitrary limit between solid and semi-solid states.  

ii) Plastic limit (PL)

Moisture content at which a soil cannot be deformed without cracking. Water content corresponding to an arbitrary limit between the plastic and semisolid states. It is the moisture content at which the soil begins to exhibit plasticity. Soil cannot be deformed without cracking. Soils should bot be ploughed at moisture contents above the plastic limit

iii) Liquid limit (or) upper plastic limit (LL)

The moisture content at which soil ceases to be plastic, becomes semi-fluid and tends to flow like a liquid

iv)  Plasticity number

The difference between the moisture content (consistency) of upper and lower plastic limits.

Factors affecting Atterberg’s constants

1. Clay content: Plasticity is a function of finer soil fractions and moisture content. Because of the plate like shape of clay particles and the lubricating effect of water, the fine soil fractions tend to slide over each other. High clay content increases the moisture contents of different plastic limits and increases the plasticity index / number. (Higher clay content requires more water to exhibit plasticity because of higher surface area)

2. Nature of clay minerals: Quartz and feldspars are non plastic. Kaolinite, illite, talc, muscovite, biotite, vermiculite, montmorillonite clays are plastic.

3. Nature of exchangeable cations: Sodium saturated soils have lower plastic limits than potassium, calcium and magnesium saturated soils.

4. Organic matter content: Organic matter favours plasticity.

Sticky point:

The moisture content at which the attractive power of the soil for water is satisfied. The sticky point moisture percentage is near the liquid limit (slightly higher for less plastic soils and slightly lower for more plastic soils).

Swelling and shrinkage:

Some soils develop cracks - sometimes few feet deep – when dried and the crack closes when wet. Eg. Black soils. This is because of the penetration of water molecules between clay crystals. This property is observed in smectite type clays (eg. Montmorillonite) which allows water molecules to penetrate between two layers of clay crystals and expands itself – causing swelling of soil. When moisture evaporates, the expanded clay crystals shrink, reducing their volume and thereby causing cracks in soil. Shrinkage and plasticity of soil are influenced by the nature of clay mineral, exchangeable bases and humus.

Coefficient of Linear Extensibility (COLE)

Certain soils have the capacity to swell significantly when wet and to shrink and crack when dry, e.g., black soils containing montmorillonitic clay. This property is quantified by determining the Coefficient of Linear Extensibility (COLE) or Potential Volume Change (PVC) or Swell Index. COLE is defined as follows:

                                             COLE = ( Lm/Ld) -1   

Where Lm is the length of wet soil and Ld is the length of the dry soil sample. If COLE exceeds 0.09, significant swelling and shrinkage of the soil is expected. The value of COLE higher than 0.03 indicates the presence of a significant amount of montmorillonitic or swelling clay.