If in a soil sample piping phenomenon occurs then what is the most prominent condition to be satisfied
Answers
Internal erosion is the formation of voids within a soil caused by the removal of material by seepage.[1] It is the second most common cause of failure in levees and one of the leading causes of failures in earth dams,[2] responsible for about half of embankment dam failures.[3]
Internal erosion occurs when the hydraulic forces exerted by water seeping through the pores and cracks of the material in the dam and/or foundation are sufficient to detach particles and transport them out of the dam structure. Internal erosion is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a sand boil can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious.
Piping is a related phenomenon and is defined as the progressive development of internal erosion by seepage, appearing downstream as a hole discharging water.[4] Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed.[5][6]
According to the International Commission on Large Dams (ICOLD), there are four general failure modes for internal erosion of embankment dams and their foundations:[7]
- Through the embankment
- Through the foundation
- Embankment-into-foundation
- Associated with through-penetrating structures
- The process of internal erosion occurs across four phases: initiation of erosion, progression to form a pipe, surface instability, and, lastly, initiation of a breach. Internal erosion is also classified in four types, dependent on failure path, how the erosion initiates and progresses, and its location:
- Concentrated leak: seeping water erodes and enlarges a crack until a breach occurs. The crack may not progress to the exit (albeit failure is still possible), but eventually the continued erosion forms a pipe or a sinkhole.
- Backward erosion: initiated at the exit point of the seepage path, this type of erosion occurs when the hydraulic gradient is sufficiently high to cause particle detachment and transport; a pipe forms backwards from the exit point until breach.
- Suffusion: occurs in soils with a wide range of particle sizes. Finer soil particles are eroded through the voids between coarser particles. Soils susceptible to suffusion are termed internally unstable. Suffusion can only occur if the volume occupied by the finer particles is lower than the available void space between the coarse particles.
- Soil contact erosion: a phenomenon called sheet flow occurs at interfaces between coarse and fine soils. Water seeps along the interface between the two soils, eroding the particles from the finer layer into the coarser layer.