How is seepage analysis performed under hydraulic structures?

Seepage analysis uses flow nets - orthogonal grid of flow lines and equipotential lines satisfying Laplace's equation. Seepage quantity Q = k*H*(Nf/Nd) where k = permeability, H = head loss, Nf = number of flow channels, Nd = number of equipotential drops. Exit gradient ie = delta_h/(distance between last equipotentials). Critical gradient for piping icr = (Gs-1)/(1+e) approximately 1.0. Factor of safety against piping = icr/ie should exceed 4-6. Modern analysis uses finite element seepage software.