With what certainty can pile lengths be determined by PIT?
The accuracy of the determined length is as accurate as the accuracy of an assumed wave speed. This is because TIME is being measured. TIME = 2L/c, where L is length and c is wave speed. The TIME equation makes L vary directly with c.
An engineer can determine the wave speed within an accuracy of 15%, and likely within 10%, based on concrete strength. Furthermore, if a pile with known length has a clear reflection from the toe, the wave speed can be determined. An engineer, with moderate confidence, can use the same wave speed on other piles at the same site if the concrete is consistent.