Some clarification needed about boiling, heave and piping

Hi all,

In the sheetpile deep excavation seepage is possible which induces hydraulic failures. In the surface of the cohesion less soil at the exit position of the excavation there will be a boiling and we can find its FOS using ic/iexit. I need some explanation of this terms regarding sheetpile deep excavation

1. Boiling 2. Heaving 3. Piping (due to the removing of soil particles from downside)

Basically boiling occurs due to the effective shear strength becomes zero and there is a having of particles. Then what is the reason for using Terzhagi body for heave check (In dams there is no any embedment depth like this and there we can consider Ic/iexit). Why failure not considered at the last flow net instead of considering full body (terazhgi body).

My understanding is in cohesionless sand boiling occurs due to heaving (effective stress is zero) and following by piping and in cohesive soil we need to check for heaving and if the movement of water is blocked by less permeability soil or structure then uplift comes into action.

Some other questions are?

01. does piping occurs in the c-phi soil with c=2 and phi =30? (we need to check by including cohesion inside the heave equation whether the soil is moving or not and decide piping will be occurred due tot the movement of the soil mass)

02. In soldier pile steel plate deep excavation system, sheet plate is progressively installed for each stages of excavation. each excavation will remain for some hours. So is it required to do hydraulic failure analysis in each stages? or is any time limit there to start the hydraulic failure in soil ?