Effective stress & Natural stress - Principle of Efective Stress and Importance of Effective Stress

EFECTIVE STRESS

The stress transmission from particle to particle of soil mass through their points of contact are termed as Effective stress or Intergranular stress.

It is known as effective stress since this stress is responsible for the decrease in void ratio or increase in frictional resistance of soil mass.
The effective stress at any point in the soil mass is equal to the total stress minus the pore water pressure.
The effective stress cannot be measured directly in the laboratory. It is only calculate from measurable quantities.

soil

NETURAL STRESS

The stress induced due to filling of pore water in the voids of soil mass which rtrises to separate out the soil grains is termed as Natural stress. It is also known as Pore water pressure. It is denoted by u .

The effect of netural stress is to increase the volume or decreas the frictional resistance of soil mass. It is known as netural stress, because it cannot resist shear stresses.

PRINCIPAL OF EFFECTIVE STRESS-:

The effective stress principle in soil engineering study is one of the most important theories. It described by  Karl Terzaghi in the year 1936. It is the difference between the total stress and the pore water pressure. More features and determination of effective stress in the soil are explained briefly in the below description.

The principle of effective stress can be understood by studying the effect of Intergranular stresses and pore water pressure on a soil mass.

EFFECT OF INTERGRANULAR STRESSES-

Consider a ridig cylinder mouls in which dry sand is filled. Assumed that there is no side friction. Loap is applied at the surface of the soil mass through the piston.

The load applied at the surface is transmitted to the soil grains in the mould through their points of contact. This results in the compression of soil mass in the mould.

The compression of soil mass is mainly due to Elastic compression of soil grains or relative sliding between particles.

The individual particles of soil mass deform as a result of development of contact force. If shear force become larger at the contact surface the particle slide over one another.

When the shape of particle is plate like flaky or elongated, these bend under the action of contact forces to take up new positions.

The effective stress is responsible for the deformation of soil mass. But from the above discussion it is clear that soil mass deforme under the effect of external force partly due to deformation of individuals particles and partly due to relative sliding of particle.

This rearrangement of soil particle within the soil mass is responsible for decrease in the void ratio or increase in the shear strength of soil.

Effective Stress Principle

The effective stress principle can be explaine by studying an example of the soil mass that is in a fully saturated condition as shown in figure.

Fig.1. Saturated Soil Mass

Consider a cross section area  of prism A of soil  . If  weight of the soil mass is P over the soil prism element P is given by the formula:

P = Unit Weigh multiply Volume of the Prism

i.e. P = ( Unit weight multiply  Height of prism(H) multilpy Area of Cross-section of Prism (A))

This gives the relation,

1. Determination of Total Stress

Then total stress is equal to unit weight saturated soil multiply height of the prism.

2. Determination of Pore Water Pressure

The pore water pressure (u) is the pressure caused due to the water molecules present in the voids or porus of the soil. It is given by the relation:

The Pore water pressure can also be designated as a  neutral stress or a natural stress because of the fact that it can not resist the shear forces in the soil mass.

Note: When the pressure is equal to the atmospheric pressure, the pore water pressure is taken as zero(0).

3. Determination of Effective Stress

The effective stress is equal to submerget unit weight multiply by height of prism.

Importance of Effective Stress

The engineering properties of the soil mass are controlled by its respective effective stress. The compression and shear strength of the soil mass is dependent on the effective stress of the soil and hence they can be written as a function of effective stress as expressed in the relation below.
With the increase in the effective stress of the soil, also increases the rate of compression of soil. This results in the settlement of the soil.

The shear strength of the soil also varies with the change in the effective stress. The shear strength of soil affects the stability of slopes. the earth pressures acting against the retaining structures and the bearing capacity of the soil. All these properties of soil mass are affected by the effective stress of the soil.

The soil permeability is dependent on the void ratio of the soil. As, the change in the effective stress of the soil alters that void ratio of the soil, it can be said that the effective stress also affects the permeability of soils.