Identification and Quantification of Pile Relaxation-Paper

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The paper examines “pile relaxation,” a time-related reduction in driven-pile capacity that contrasts with the more common increase known as setup. Using dynamic testing and signal-matching analyses, the authors assess 36 piles at 26 sites and distinguish actual resistance loss from apparent reductions caused by changes in hammer performance.<br /><br />The cases involved H-piles, pipe piles, prestressed concrete piles, and combinations of concrete piles with H-pile stingers. Relaxation occurred mainly in dense to very dense sands and weak, often weathered or fractured rock, especially shale. In the sand cases, restrike toe resistance averaged 62% of end-of-driving resistance—a mean loss of 38%. For shale, restrike resistance averaged 65%, a mean loss of 35%. Individual losses varied considerably, and some piles required substantial additional driving to regain ultimate capacity.<br /><br />Dynamic measurements help identify relaxation by comparing impact force, mobilized resistance, and their ratio (ROF) between initial driving and restrike. CAPWAP signal matching is used to assess changes in toe and shaft resistance separately, since shaft setup can mask toe relaxation.<br /><br />The authors recommend anticipating capacity loss in relaxation-prone materials and designing the pile–hammer system to permit driving beyond the required ultimate capacity. A reserve corresponding to roughly 20–45% potential toe-resistance loss would have covered most cases in the database, but larger losses occurred in some piles. Therefore, restrike dynamic tests with signal matching, or static load tests, are recommended where relaxation could affect design. The authors also emphasize that the available data are limited and that site-specific testing is important.

Keywords

pile relaxation

driven pile capacity

dynamic pile testing

CAPWAP signal matching

restrike resistance

toe resistance loss

shaft setup

pile hammer performance

dense sand

shale