The effective stress principle in soil engineering is one of the important theories put forward by Karl Terzaghi in the year 1936. It is the difference between total stress and pore water pressure. More features and determination of effective stress in the soil are explained briefly below.
Effective Stress Principle
The effective stress principle can be explained by studying an example of soil mass that is in a fully saturated condition as shown in figure-1 below.
Consider a prism of soil in the soil mass with an area of cross-section 'A' . If P is the weight of the soil over the soil prism element, 'P' is given by the formula:
P = Unit Weight x (Volume of the Prism)
i.e.P = Unit weight x Height of prism(h) x Area of Cross-section of Prism (A)
This gives the relation,
1. Determination of Total Stress
Then total stress acting on the prism is given by,
2. Determination of Pore Water Pressure
The pore water pressure (u) is the pressure caused due to the water molecules present in the pores or voids of the soil. It is given by the relation:
Pore water pressure can also be designated as neutral pressure or neutral stress because of the fact that it cannot resist the shear forces in the soil mass.
Note: When the pressure is equal to atmospheric pressure, the pore water pressure is taken as zero.
3. Determination of Effective Stress
The effective stress is given by the formula:
For saturated soil,
Effective stress is given by the equation-5 as:
Importance of Effective Stress
Most of the engineering properties of the soil are controlled by its respective effective stress. Say, the compression and shear strength of the soil 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, the rate of compression of soil also increases. This results in the settlement of the soil.
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The shear strength of the soil also varies with the change in 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. Hence, all these properties 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 the void ratio of the soil, it can be said that the effective stress also affects the permeability of soils.