Can raw penetration-test values be used directly to size a wind foundation?

The short answer

Raw penetration readings are measurements, not ready-made foundation design values. SPT and CPT results require interpretation appropriate to the test, soil and design mechanism. Corrections, soil classification and correlation uncertainty can materially change derived strength or stiffness. An unqualified blow count or cone resistance should not directly determine a wind footing size.

Identify the test and its measurement chain

SPT blow counts depend on the test arrangement and energy delivered as well as the ground. CPT measurements distinguish cone resistance, sleeve friction and, for suitable equipment, pore pressure. These quantities are not interchangeable and their depth references must match the interpreted soil profile.

Check equipment, procedures and relevant corrections in the geotechnical report. A spreadsheet column labeled bearing capacity may conceal several interpretation steps that deserve separate review.

Match the correlation to the intended parameter

A correlation developed for a particular soil and stress condition may not apply to different material. Gravel, cemented layers, sensitive soils or unusual fill can complicate interpretation. Agreement with a regional rule of thumb is not independent verification of the full ground model.

Derived friction angle, undrained strength and deformation modulus serve different calculations. A relation used to estimate one should not silently be reused as evidence for all three.

Request interpreted values with their limits

The geotechnical deliverable should identify characteristic or design parameters, the relevant layers and the applicable loading or drainage assumptions. Ask how uncertainty affects foundation bearing, settlement and rotation, rather than demanding a single apparently precise number.

If a small parameter change alters the selected foundation, targeted additional evidence may have value. The structural designer should use the accepted interpretation with consistent load and resistance conventions. Neither a penetration-test number nor a turbine’s kW rating establishes a universal concrete volume, embedment or anchor arrangement for a MARSTON installation.