Should a wind foundation use drained or undrained soil strength?

The short answer

The appropriate strength model depends on soil permeability, drainage paths and the timescale of loading. A brief loading event in saturated fine soil may be assessed as undrained, while long-term conditions require an effective-stress assessment. A foundation can need both checks; neither model should be chosen merely because it gives greater resistance.

Compare loading time with drainage time

Water in soil pores cannot always escape as quickly as loading changes. Where drainage is restricted, changes in pore pressure influence the short-term response. Over longer periods, dissipation and consolidation can produce a different strength and displacement condition.

A soil description such as clay or sand helps frame the question but does not replace investigation. Layer thickness, drainage boundaries and the actual stress path affect how quickly the loaded ground can respond.

Keep strength parameters within their model

Undrained strength is used in an appropriate total-stress assessment. Effective-stress parameters describe resistance in relation to effective normal stress and the adopted pore-pressure condition. Mixing an undrained strength with unrelated friction assumptions can count resistance inconsistently.

Request the test basis, sample quality and loading conditions behind each parameter. A laboratory result from one drainage condition does not automatically represent every wind-loading case, particularly where repeated actions change the ground response.

Check the cases that could govern different decisions

The geotechnical designer should distinguish extreme overturning, sustained dead load, construction stages and any repeated loading requiring additional assessment. Short-term resistance can govern stability while longer-term consolidation governs alignment or settlement.

Provide the foundation load history and duration information available from the tower designer. Ask which drainage assumptions control the selected foundation and whether a credible alternative changes its size or stiffness. The outcome is a consistent set of design cases, not a choice between two globally preferred soil-strength labels.