Problem

When running an eigenvalue (modal) analysis in SACS using a foundation superelement generated by PSI, users may observe that changing the SCOUR depth does not change the extracted eigenvalues or natural frequencies. This behavior is expected when the upper soil strata are defined as cohesive soils (clays).

 

Answer

The behavior is governed by how API General Scour is applied in SACS PSI. According to the SACS PSI Manual, Section 2.4.3.6 – Applying API General Scour Recommendations:

Because the upper layers are clay, the scour depth does not meaningfully reduce the pile–soil stiffness. As a result, the foundation stiffness matrix generated for the superelement remains effectively unchanged, and therefore, the global eigenvalues and natural frequencies also remain unchanged. 

NO SCOUR, cohesive soils:

SCOUR 4.2m, cohesionless strata:

The results:

When the top layers are changed to sand (cohesionless strata), the SCOUR definition starts to reduce the API-generated soil stiffness. This leads to a small change in the equivalent foundation stiffness and, consequently, small changes in the extracted eigenvalues. The fact that the change is small is technically expected, since the affected soil thickness is relatively shallow (about 0.6m) compared to the full pile embedment, and the global modes remain largely structure-dominated.


Suggested Resolutions

1. Review the PSI soil profile: If the top layers are clay, SCOUR will have no effective influence on the superelement stiffness.

2. If you need to study scour sensitivity:

• Define the upper strata as cohesionless soil (sand) where appropriate.
• Use API-generated soil curves (and CPT-based axial resistance if applicable).

3. Regenerate the PSI foundation superelement after changing: Soil type and/or SCOUR depth.

4. Re-run the eigenvalue (DYN) analysis using the updated superelement.

5. Expect small but real changes in eigenvalues if the foundation stiffness contribution is significant.

If your top soil layers are clay, changing SCOUR depth will not change the PSI-generated foundation stiffness, and your eigenvalues will remain unchanged. This is by design and fully consistent with SACS PSI documentation. 

For further details, please refer to the PSI Manual, Section 2.4.3.6, which explicitly describes these limitations and behaviors.