There are two problems which make the calculation of time-based consolidation settlement inaccurate for rigid loads in Settle3D. First, Settle3D implements an uncoupled solution for the load and excess pore water pressure. The rigid load creates very high load concentration at the boundary of the load which accelerates the process of pore water dissipation but will not trigger any load redistribution. Second, Settle3D uses a 1D consolidation formulation. This means that there is no redistribution of the excess pore water pressure in the x-direction. Both these issues will create problems for time-dependent consolidation analyses.
Note that Settle3D solves the rigid foundation as a load with full contact to the soil underneath. The solution for semi-rigid with no tension is more complicated and an alternate numerical technique such as the finite element method may be more suitable for problems that require that type of analysis.