Introduction —
When a building is constructed, its weight is transmitted to the ground through its foundation, thus creating a stress in the underlying level. These induced stresses can cause problems such as excessive settling or shear failure and are thus important to geotechnical engineers.
Tension in sub-soil: —
The stress in the sub-soil occurs due to the following reasons: -
(i) Self weight of soil.
(ii) structural load on the soil.
The stresses developed in saturated soil are: ––
(i) Direct shear strength test
(ii) effective stress
(iii) neutral stress
(iv) Total stress.
(I) Direct shear strength test —
The direct shear test is one of the oldest strength tests for soils. In this laboratory, a direct shear device will be used to determine the shear strength of a cohesionless soil (i.e. angle of internal friction (f). From the plot of the shear stress versus the horizontal displacement, the maximum shear stress is obtained for a specific vertical confining stress. After the experiment is run several times for various vertical-confining stresses, a plot of the maxi mum shear stresses versus the vertical (normal) confining stresses for each of the tests is produced. From the plot, a straight-line approximation of the MohrCoulomb failure envelope curve can be drawn, f may be determined, and, for cohesionless soils (c = 0), the shear strength can be computed from the following equation:
s = s tanf.
(II) Effective stress : —
Karl Terzaghi was the first to recognize the importance of effective tension. It is the stress transmitted through the soil mass from grain to grain at the point of contact. It is also known as inter-granular stress. It is denoted by '. When the soil mass is loaded. The load is transferred to the soil gain through their point of contact. If at the point of contact, the applied load exceeds the resistance of the grain, compression will occur in the soil mass.
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| Load transmission |
This compression is partly due to the elastic compression of the grains at the points of contact and partly due to the relative sliding between the particles. This load per unit area of soil mass, responsible for the deformation of the soil mass, is called the effective stress.
(III) Neutral Tension: —
This orifice is the tension or pressure transmitted through the fluid. This is also called the hole pressure and is denoted by u. In saturated soil, the pores of the soil mass are filled with water. When saturated soil mass is loaded, the load is not transmitted through the grain. The load is transferred to the orifice water. Since water is
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| Neutral stress |
incompressible, a pressure develops in the pore water. This pressure is called pore pressure or pore water pressure. There is no measurable effect of pore pressure on the mechanical properties of soil such as void ratio, shear strength, etc. This pressure or tension is called neutral stress.
(IV) Total Tension: —
The total stress is equal to the sum of the effective stress and the neutral stress. denote it by.
= + u
The effective stress in the field cannot be measured with any instrument. It can be calculated only after measuring the total tension and pore pressure. Thus the effective stress is not a physical parameter, but only a very useful mathematical concept for the determination of the engineering behavior of soils.
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| Total stress |
Importance of Effective Stress in Engineering Problems:
Effective tension plays an important role in:
(i) Soil disposal
ii) Shear strength of soil
Soil disposal:
The phenomenon of gradual reduction in soil volume due to removal of water from the soil pores is called soil consolidation or compression or settlement. the compression curve of the soil. It is a curve between the effective stress and the zero ratio e. It is clear from the graph that as e increases, e decreases i.e. due to increase in effective stress the compression of soil will increase.
Soil Shear Strength:
Many geotechnical engineering problems require the estimation of shear strength, including:
(a) Structural Foundations:
The load from a structure is transferred to the ground through the foundation. This produces shear stress and compressive stress. If the generated shear stress is greater than the shear strength of the soil, shear failure occurs which causes the structure to collapse.
(b) Earth Slope:
On sloping land, gravity creates shear stress in the soil. If these stresses exceed the shear strength, a landslide occurs.
(c) Highway Pavement:
The wheel load from the vehicles is transferred to the ground through the pavement. These loads generate shear stress which causes shear failure.
you are know ––
The value of K in the x-direction is equal to the value of the y-direction for a flat surface.
The shear strength of the soil is calculated by the formula
S = body
where = effective stress
= effective friction angle
For a given soil, f is constant. The shear strength is then directly proportional to the effective stress. So the strength increases with an increase in the effective stress. If the shear strength of the soil is high, the shear failure will be low.
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