Abstract: Two ideal models, i.e. a perfect well model and an imperfect well model, used for calculating deformation initial period, are set up, and their development processes are reprerented by FEM. The computed results obtained from the perfect well model indicate that: the distribution of piping in plane is not in succession; in addition to the near area of the well, the percolation deformation near the riverbed may also occur within a certain extent; and drift sands chiefly come from one side near river, with the continuous driftage of sands, the percolation deformation will develop as a long narrow strip, and may connect with the percolation deformation near river bed to form a concentrated percolation passage under certain conditions. In the imperfect well model, the expansion of drift sands in depth is slowly and ultimatly stops. So, drift sands in the perfect well modol are of developmental property, but these in the imporrfect well model aren't of this property.

Key words: bank foundafion , seepage deformation , model of well flow , seepage gradient