Ground truth, control and design of driven piles: implementing old ways with a new twistKeynote Address

Piling design and verification is a fraught and risky business. The spread of pile capacity estimates submitted to conference predictions exercises is often staggering and sobering. This underlines why design of driven piles does not stop at the design engineer’s desk but continues through construction, and relies on the valuable information provided by the installation process. Each installation blow is a test – a test of the ground response to hammer input delivered into the pile. Traditionally, pile capacity has been interpreted from this input-response relationship through various and many pile driving formulae. Five decades ago, measurement systems were first used to measure and interpret the stress waves in piles generated from the hammer inputs and reflected from the ground response to infer capacity in a more sophisticated and reliable way using wave mechanics principles. Today, PDA testing and wave matching are routinely accepted practice. However, each PDA test has direct relevance only to the individual pile which is tested. This paper will argue that our fundamental task as designers and supervisors is to establish ground truth, by synthesizing the results of PDA tests into a locally-evidenced and locally-targeted dynamic formula. Therefore, only dynamic formulae, properly modified and correlated,mustbethevehiclefordeliveringlocalgroundtruthandultimatelybeingthebasisforsign-off. On a foundation-wide basis, the role of PDA tests is critical but subservient, and principally to provide the evidence on which a correlated dynamic formula is developed. Consequent implications for the foundation sign-off process, and for a proposed new approach to establishing capacity reduction factors for driven piles will also be discussed.