GEOTECHNICAL ENGINEERING

ACE ACADEMY IES ESE HANDWRITTEN CLASSROOM NOTES

ENGINEERING SERVICES GOVT EXAMINATION NOTES

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Geotechnical Engineering IES ESE Exam Ace Academy Notes Handwritten Free Download PDF

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Geotechnical Engineering IES ESE Exam Ace Academy Notes

PLASTICITY OF FINE SOILS

Plasticity is an important characteristic in the case of fine soils, the term plasticity describing the ability of a soil to undergo unrecoverable deformation without cracking or crumbling. In general, depending on its water content (defined as the ratio of the mass of water in the soil to the mass of solid particles), a soil may exist in one of the liquid, plastic, semi-solid and solid states. If the water content of a soil initially in the liquid state is gradually reduced, the state will change from liquid through plastic and semi-solid, accompanied by gradually reducing volume, until the solid state is reached. The water contents at which the transitions between states occur differ from soil to soil. In the ground, most fine soils exist in the plastic state. Plasticity is due to the presence of a significant content of clay mineral particles (or organic material) in the soil. The void space between such particles is generally very small in size with the result that water is held at negative pressure by capillary tension. This produces a degree of cohesion between the particles, allowing the soil to be deformed or moulded. Adsorption of water due to the surface forces on clay mineral particles may contribute to plastic behavior. Any decrease in water content results in a decrease in cation layer thickness and an increase in the net attractive forces between particles.

Toughness test :

A small piece of soil is rolled out into a thread on a flat surface or on the palm of the hand, moulded together and rolled out again until it has dried sufficiently to break into lumps at a diameter of around 3 mm. In this condition, inorganic clays of high liquid limit are fairly stiff and tough; those of low liquid limit are softer and crumble more easily. Inorganic silts produce a weak and often soft thread which may be difficult to form and readily breaks and crumbles.

Dilatancy test

A pat of soil, with sufficient water added to make it soft but not sticky, is placed in the open (horizontal) palm of the hand. The side of the hand is then struck against the other hand several times. Dilatancy is indicated by the appearance of a shiny film of water on the surface of the pat; if the pat is then squeezed or pressed with the fingers the surface becomes dull as the pat stiffens and eventually crumbles. These reactions are pronounced only for predominantly silt size material and for very fine sands. Plastic clays give no reaction.

Dry strength test :

A pat of soil about 6mm thick is allowed to dry completely, either naturally or in an oven. The strength of the dry soil is then assessed by breaking and crumbling between the fingers. Inorganic clays have relatively high dry strength; the greater the strength the higher the liquid limit. Inorganic silts of low liquid limit have little or no dry strength, crumbling easily between the fingers.

Organic soils contain a significant proportion of dispersed vegetable matter which usually produces a distinctive odour and often a dark brown, dark grey or bluish grey colour. Peats consist predominantly of plant remains, usually dark brown or black in colour and with a distinctive odour. If the plant remains are recognizable and retain some strength the peat is described as fibrous. If the plant remains are recognizable but their strength has been lost they are pseudo-fibrous. If recognizable plant remains are absent, the peat is described as amorphous.