Rapid drawdown
After a condition of steady seepage become established, a drawdown of reservoir level will result in a change in the pore water pressure distribution.
If the permeability of soil is low, a drawdown period measured in weeks may be rapid in relation to the dissipation time and the change in pore water pressure can be assumed to take placed under undrained condition.
Assumed that the total major principal stress at P is equal to the fill pressure
The change in total major principal stress is due to the total or partial removal of water above the slope on the vertical through P.
For a draw down depth exceeding hw: Ds1 = -gw.hw