A distinction must be made between the case where Ks is constant with depth and the case where it is variable.In the former case:
Ks=qa/unit subsidence
where qa=qult/FS
qult=limiting load
FS=safety factor (2 or 3 depending on the lithology)
In the second instead:
Ks=40*(c*Nc+0.5*gamma*B*Ngamma)+40*(gamma*Nq*Z)
Nc, Ngamma, Nq e.g. Hansen’s coefficients.