Spring Constant k=F/s
F = the static force
s= the displacement
k=spring constant
k=12EI/L^3
E = Young's Modulus,
k=spring constant
I = the moment of inertia, and
L = the Length of the beam.
for raft on piles,
Axial stiffness ,k=AE/L of take from load-settlement curve.
Bending stiffness, k=12EI/L^3, L is the equivalent cantilever length of pile.