7.SEEPAGE

                                     

1.SEEPAGE PRESSURE:

          The pressure exerted on Soil by water through which it percolates is called Seepage pressure. Whenever the water percolates in to the soil some amount of energy transferred through its contact between soil and water. So the energy transferred is actually called seepage pressure.

          If “ h” is hydraulic head “p_s”  is seepage pressure and Y_w is unit weight of water  then

                             P_s= h x Y_w

             _Or               = i . z . Y_w

                                                                         

                                             here         i= Hydraulic gradient

                                                              z= Seepage length

Note:

Seepage pressures always act in the direction of flow.

If the flow occurs downward then Effective stress increased.

If the flow occurs upward then Effective stress Decreased.

          Seepage pressure is cause for occurrence of Quick sand condition

2.QUICK SAND CONDITION:

          When flow occurs in the upward direction of soil, the seepage pressure also acts in the upward direction. So the flow exerts the pressure in upward direction and the effective stress is reduced. When the seepage pressure due to flow become equal to the pressure due to submerged weight of soil then the effective stress reduced to zero. So the cohesion less soil tends to act like a liquid with no shear strength. And all the particles tend to move upward direction with the flow.

          When the quick sand condition or sand boiling occurs the soil will not bear anything on it. Even a lesser weight object also sinks in to the soil.

          The Hydraulic gradient at this condition is called Critical hydraulic gradient.

                     Substituting their values

      = (G-1). (1- n)

                      Note;          

                      Factor of safety   F = i_c / i_exit

                      Sand boiling happen only when   i_c < i

                      Piping occurs when effective stress is zero.

Example Problem:

1.An excavation is to be made in a stratum of clay 9m thick underlain by a bed of sand. In bore hole the ground water is observed to rise up to an elevation of  3m from ground level. With the unit weight of 20KN/cum what depth can be performed without the bottom becoming unstable under uplift pressure due to ground water.?

Ans:

                   Unit weight of soil = 20 KN/cum

                  Excavation is safe when

                 (This means the excavation can be made up to, the weight                      of soil become equal to the pressure due to underground                        water).

                                            20 x (9-y) = 10 x 6

                                                          y = 6 m

                  So the excavation can safely made up to 6 m from ground                       level.

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