WHAT ARE THE ENGINEERING PROPERTIES OF SOIL?

The following properties of soil are taken into consideration while dealing with soil as a construction material.

Cohesion 
Angle of internal friction 
Capillarity 
Permeability 
Elasticity 
Compressibility 

1. COHESION

It is the internal molecular attraction which resists the rupture or shear of a material. Cohesion is derived in the fine grained soils from the water films which bind together the individual particles in the soil mass. Cohesion is the property of the fine grained soil with particle size below 0.002 mm. cohesion of a soil decreases as the moisture content increases. Cohesion is greater in well compacted clays and it is independent of the external load applied.

2. ANGLE OF INTERNAL FRICTION

The resistance in sliding of grain particles of a soil mass depends upon the angle of internal friction. It is usually considered that the value of the angle of internal friction is almost independent of the normal pressure but varies with the degree of packing of the particles, i.e. with the density. The soils subjected to the higher normal stresses will have lower moisture contents and higher bulk densities at failure than those subjected to lower normal stresses and the angle of internal friction may thus change. The true angle of internal friction of clay is seldom zero and may be as much as 26⁰. The angle of internal friction fro granular soils may vary in between 28⁰ to 50⁰.

3. CAPILLARITY

It is the ability of soil to transmit moisture in all directions regardless of any gravitational force. Water rises up through soil pores due to capillary attraction. The maximum theoretical height of capillary rise depends upon the pressure which tends to force the water into the soil, and this force increases as the size of the soil particles decreases. The capillary rise in a soil when wet may equal as much as 4 to 5 times the height of capillary rise in the same soil when dry.

Coarse gravel has no capillary rise; coarse sand has up to 30 cm; fine sand and soils have capillary rise up to 1.2 m but dry sand have very little capillarity.

Clays may have capillary rise up to 0.9 to 1.2 m but pure clays have very low value.

4. PERMEABILITY

Permeability of a soil is the rate at which water flows through it under action of hydraulic gradient. The passage of moisture through the inter-spaces or pores of the soil is called ‘percolation’. Soils having porous enough for percolation to occur are termed ‘pervious’ or ‘permeable’, while those which do not permit the passage of water are termed ‘impervious’ or ‘impermeable’. The rate of flow is directly proportional to the head of water.

Permeability is a property of soil mass and not of individual particles. The permeability of cohesive soil is, in general, very small. Knowledge of permeability is required not only for seepage, drainage and ground water problems but also for the rate of settlement of structures on saturated soils.

5. ELASTICITY

A soil is said to be elastic when it suffers a reduction in volume (or is changed shape & bulk) while the load is applied, but recovers its initial volume immediately when the load is removed. The most important characteristic of the elastic behavior of soil is that no matter how many repetitions of load are applied to it, provided that the stress set up in the soil do not exceed the yield stress, the soil does not become permanently deformed. This elastic behavior is characteristic of peat.

6. COMPRESSIBILITY

Gravels, sands & silts are incompressible, i.e. if a moist mass of those materials is subjected to compression; they suffer no significant volume change. Clays are compressible, i.e. if a moist mass of clay is subjected to compression, moisture & air may be expelled, resulting in a reduction in volume which is not immediately recovered when the compression load is withdrawn. The decrease in volume per unit increase of pressure is defined as the compressibility of soil, and a measure of the rate at which consolidation proceeds is given by the ‘co-efficient of consolidation’ of the soil. Compressibility of sand & silt varies with density & compressibility of clay varies directly with water content & inversely with cohesive strength.