6. EFFECTIVE STRESS

1.Effective stress:

It is  the vertical pressure transmitted through all their points of contact through soil mass. It can be calculated using pore water pressure and total stress.

                   It is also called inter granular pressure.

          So the effective stress is equal to Total stress - pore water                       pressure

2.Capillary rise:

Capillary rise is the rise of water against gravity through the soil pores or capillary tube above the free water due to the presence of surface tension.

The height of rise of capillary water is mainly depends on the diameter of pores or tube and value of surface tension.

The capillary rises of water do not affect the pore water pressure.

 

           

                        In Soil Capillary rice = 0.3/d                             d= Dia. Of pores

So the capillary rice is inversely proportional to the diameter of pores. So It state that the capillary rice is small in Coarse grained soils and large in fine grained soils.

Rise in temperature reduces the Surface tension of liquid and also reduces the height of rise.

          Note:

          Capillary rise has no effect on pore water pressure.

                    Surface tension of mercury = 2.45x10^-6 KN/cm (Max)

Example Problem: (GATE)

1 1.For the Following data..

Water content= 20%

Specific gravity = 2.7

Unit weight of water = 10KN/m³

Water table@ ground level

i)                 Saturated unit weight of sand(KN/M³ ) is?

                                        

           We need to find out the voids ratio of soil at this water content

   e.S = G.w

   e.1 = 2.7 x 0.20

      e = 0.54

Submitting the value of voids ratio ,we get

                 

                           

                                                       = 21 KN/ M²

ii)              The total stress and pore water pressure and effective stress (KN/m²) at the point “P” are

Ans:

                                                           = 21*5

                                                           = 105 KN/m²

                                                               = 10 x 5

                                                               = 50 KN/m²

               

So the Effective stress is 

              

                                                                = 105 - 50

                                                                = 55 KN/m²

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