Soil consistence - Cohesion - Adhesion - Plasticity - Atterberg’s constant - Upper and lower plastic limits, plasticity number

1. SAC 101 Fundamentals of Soil Science (2 + 1) 2006-07
Lecture 23. Soil consistence - Cohesion - Adhesion - Plasticity -
Atterberg’s constant - Upper and lower plastic limits,
plasticity number
Plasticity, Cohesion, swelling, shrinkage, dispersion and flocculation are some of the
important soil physical properties related to soil solids and water.
Soil Consistence
Soil consistence is defined as “the resistance of a soil at various moisture contents to
mechanical stresses or manipulations”.
It combines both the ‘cohesive’ and ‘adhesive’ forces, which determine the ease with
which a soil can be reshaped or ruptures.
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
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 1 of 4
http://www.bhaskaran.net.in<<<</unoconv/20170419150332_e6f98681240a84fc26838470e4a6cb4e17a064ba/23soilconsisten
ce-170419150313.doc>>>>
Adhesion
Molecular attraction that h
olds the surfaces of two
substances (eg. Water and
soil particles) in contact
Cohesion
Holding together: force holding a solid or
liquid together, owing to attraction
between like molecules. Decreases with
rise in temperature

2. SAC 101 Fundamentals of Soil Science (2 + 1) 2006-07
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 limit (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
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 2 of 4
http://www.bhaskaran.net.in<<<</unoconv/20170419150332_e6f98681240a84fc26838470e4a6cb4e17a064ba/23soilconsisten
ce-170419150313.doc>>>>

3. SAC 101 Fundamentals of Soil Science (2 + 1) 2006-07
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
i) 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)
ii) Nature of clay minerals: Quartz and feldspars are non plastic. Kaolinite, illite,
talc, muscovite, biotite, vermiculite, montmorillonite clays are plastic.
iii) Nature of exchangeable cations: Sodium saturated soils have lower plastic
limits than potassium, calcium and magnesium saturated soils.
iv) Organic matter content: Organic matter favours plasticity.
Cohesion
The tendency of clay particles to stick together is due to cohesion – attraction of the clay
particles to water molecules and between them. Hydrogen bonding between clay surfaces
and water and also among water molecules is the attractive force responsible for
cohesion. Smectites and fine grained micas exhibit more cohesion than kaolinite clays or
hydrous oxides. Humus tends to reduce the attraction of individual clay particles for each
other.
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 cracks closes when wet. Eg. Black soils. This is because of the penetration
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 3 of 4
http://www.bhaskaran.net.in<<<</unoconv/20170419150332_e6f98681240a84fc26838470e4a6cb4e17a064ba/23soilconsisten
ce-170419150313.doc>>>>

4. SAC 101 Fundamentals of Soil Science (2 + 1) 2006-07
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.
Dr.A.Bhaskaran, Assistant Professor (SS&AC), ADAC&RI, Tiruchirapalli - 620 009 Page 4 of 4
http://www.bhaskaran.net.in<<<</unoconv/20170419150332_e6f98681240a84fc26838470e4a6cb4e17a064ba/23soilconsisten
ce-170419150313.doc>>>>