Whenever a hydraulic structure is founded on a pervious foundation, it is subjected to seepage of water beneath structure in addition to all or forces. The water seeping below endangers stability of structure and may cause its failure eir by Piping or Undermining and Direct uplift.

When apron is not sufficiently strong to counteract excessive upward pressure, it may fail by rupture. The uplift pressure is not important for an upstream apron because downward weight of stored water on upstream apron is sufficient to neutralize effect.

Undermining takes place due to excessive percolation of water under pressure below foundation. The seepage water when retains a sufficient residual force at emerging downstream end, it may dislodge and lift up soil particles leading to increased porosity by progressive removal of soil. This leads to failure of structure as it ultimately subsides in hollow so formed.

As per Terzaghi, heave piping may occur within a distance of D/2 on downstream of sheet pile where D is embedment depth of sheet pile. The entire soil mass in zone gets heaves up and is blown out by percolating water when seepage pressure at any level is greater than submerged weight of soil above that level.

Backward erosion piping is an important failure mechanism for water-retaining structure and local disruption of downstream top layer leads to concentrated seepage flow. This results in erosion that continues in upward direction. It is ensured that phreatic lines never touch downstream side of earth dam to avoid failure.

The safety against piping can be ensured by providing sufficient creep length as per Bligh?s ory. The thickness of concrete floor is generally increased by 33% to allow a suitable factor of safety for worst conditions. A reasonably deep vertical cutoff is also provided at downstream end of floor to prevent undermining.

The value of Bligh?s creep coefficient (C) is different for different soil.

For Micaceous sand, safe hydraulic gradient is less than 1/15 and for Coarse-grained soil is 1/12. For light sand and mud, safe hydraulic gradient should be less than 1/8.

The thickness is generally increased by 33% so as to allow a suitable factor of safety. The water on upstream side is more than downstream side and it counterbalances uplift caused on that side. Hence, only a nominal thickness is required.

The hydraulic gradient i.e. HL/L is equal to coefficient of creep 1/C.
Bligh?s coefficient = 12
So, safe hydraulic gradient = 1/12.