EFFECTIVE STRESS IN SOIL : Geotechnical Engineering

EFFECTIVE STRESS OF SOIL

The effective stress principal in soil engineering was given by the KARL TARZAGHI in the year 1936.

According to the KARL TARZAGHI, The effective stress in soil engineering is the diiference between the Total stress and The Pore water water.The defination of pore warer pressure and Total stress are disscused below:-

TOTAL STRESS

Definition of Total stress:-

Total stress is equal to the sum of effective stress and and pore water pressure.

Calculation of Total stress

Total weight =ϒZA

here,

ϒ = Unit weight

Z = depth

A = cross section area

Stress =  𝛔 = weight /A

𝛔 = ϒZ km/m²

PORE WATER PRESSURE

Definition of Pore water pressure:-

Pore water pressure is referred as the pressure of water molecules held in pores of soil or rocks.

Pore water pressure below PHREATIC LEVEL is measured with the help of piezometer. In unsaturated soil the Pore water pressure is determined by the capilarity and also known as TENSION OR MATRIC PRESSURE.

Pore water pressure is very essential for calculating the effective stress of soil in Geotechnical engineering from  Tarzaghi’s expression.

Calculation of Pore water pressure:-

Pore water pressure is denoted by = µ

µ = hY_w

CALCULATION OF EFFECTIVE STRESS

Effective stress is the difference between the Total stress and Pore water pressure.

E S  = T S  – pore water pressure

                           =   𝛔 – µ

 

ANALYSIS OF EFFECTIVE STRESS

There are four cases for analysis of effective stress are described below :-

1)      When there is no water (DRY SOIL)

2)      When water table at any depth

3)      When the water table at Ground Surface

4)      When the water table is above the ground surface.

 

1)WHEN THERE IS NO WATER :-

2)WHEN WATER TABLE AT ANY DEPTH:-

As the water table increases in the soil the effective stress decreases in the soil and vice-versa.

3)WHEN THE WATER TABLE AT GROUND SURFACE :-

4)WHEN WATER TABLE IS ABOVE THE GROUND :-

Above the ground surface specially in case of Rivers,Oceans ,etc.If water table fluctuates there is no change in the effective stress because the Total stress and Pore water pressure below the water varies equally due to fluctuation of water level.

Hence there is no change in effective stress.

EFFECT OF CAPILLARITY IN EFFECTIVE STRESS

Capillarity action is the ability of liquid to flow in a narrow spaces.it occurs becausse of intermolecular forces between the and surrounding surfaces.Cohesion and Adhesion two phenomenon that causes the capillarity action in soil.

COHESION:

Cohesion is the property of material that ius caused between two similar molecules.

That attract two each other due to cohesive force.

ADHESION:

Adhesion is the tendency two particle or surfaces to climg together between to unlike particles.In case of liquid wetting agent adhesion causes the liquid to cling to surface on which it rest.

 

EFFECT OF CAPILLARITY ON EFFECTIVE STRESS

Capillarity action is good phenomenon  to increase the strength as well as Effective stress of soil

Due capillarity action in the soil,effective stress increases. Effective stress and capillarity are directly proportional to each other  that means as the capillarity increases in the soil the effective stress also increases.

It is difficult to walk away from the sea or river but it became easier to walk near the river or  sea because of capillarity action the effective stress is more in near by soil.

 As the effective stress increases the shear strength of soil also increase.

EFFECT OF SEEPAGE ON EFFECTIVE STRESS:-

DEFINITION OF SEEPAGE

Water seepage is the process by which the water flow from one place to another through small holes or porous material.It generally occurs in the rainy season , after the heavy rainfall Groundwater level rises and additional soil in water creates Hydrostatic pressure against the home foundation.

EFFECT OF SEEPAGE ON EFFECTIVE STRESS:-

When the starts to flow through the soil particle,the pore water pressure increase within the soil and overall effective stresses decreased and the shear strength of soil reduced.
Flow of water through the soil particle effect the effective stress of soil,
When the flow is upward,The current tends to lift the soil particle sand reduced the intergranular contact pressure.
When the flow is downward the current drags the particles of the soil and increases the intergranular pressure,
Due the seepage when the effective stress is reduced to zero then the condition is known as the QUICK SAND CONDITION.