Relaxation of Driven Pile Resistance Assessed by Dynamic Pile Load Tests

Driven piles are frequently used as deep foundations to support structures when soils at shallower depths are not suitable to provide the resistance needed. Depending on soil types at a project site and pile driving conditions, driven piles may experience setup, i.e., increase in pile resistance, or relaxation, i.e., decrease in resistance, after installation. This paper presents the investigation performed on piles experienced relaxation with time after installation. The piles were driven as foundations to support roadway bridges. Dynamic pile load tests were performed to assess the pile resistance during installation and at a later time after installation. The investigations showed that significant relaxation in pile tip resistance can occur, resulting in relaxation of pile total resistance. The properties of the piles used in the study, soil properties where the piles were driven, and pile load test results are presented. Potential correlations between the soil/pile properties and pile relaxation behavior are explored. The results show that the piles experienced relaxation were installed in predominantly coarse-grained soils, and majority of these piles had coarse-grained material at the pile tip. In addition, the steel closed-ended pipe piles, on average, experienced more relaxation compared to the H-piles.