Evaluating the response of piles subjected to static and multiple dynamic axial loads

Abstract

Piles often move horizontally during earthquakes. In case of superstructure interactions, accurate prediction of pile response becomes more challenging due to vertical propagation of shear wave during the earthquake that sways the superstructure causing vertical axial movement. To effectively design the pile foundation, a series of large-scale 1-g model pile tests were performed in sand and clay. Multiple axial load tests (Dynamic Load Test (DLT) and Static Load Test (SLT)) were conducted on the fully instrumented piles to assess their stiffness and capacity. Mainly, the characteristics of loading rate effect, loading test sequence effect, and cyclic effect were experimentally evaluated. The axial force, skin friction distribution, and velocity response along the pile, and pile's response in cohesive and non-cohesive soils under multiple dynamic loads with similar load amplitude were examined. It was found that the dynamic capacity of the pile exceeded the static capacity (by 30–40% for sand, and 60–80% for clay). In sands, the dynamic capacity of the pile obtained from the first DLT is 45% smaller than that of the second DLT, whereas in the latter case, it is 20% larger than the second DLT. It is also notable in SLT that, the secant stiffness of the pile plummeted comparatively more in clay. In general, the stiffness and capacity of the pile-soil system are largely governed by the loading rate and sequence of loading tests, depending on the soil properties. Also, the results show that the axial force along the pile shaft decreased nonlinearly with depth in both sand and clay; however, it lessened rapidly in the clay. The skin friction of the pile body increased gradually with hammer blows in sand, while a different trend is observed in the case of clay. The velocity response of different locations on the pile is almost identical. Furthermore, in sand, under a similar dynamic load amplitude, the normalized velocity (vn) of the pile slowly lowers at higher number of blows, whereas in clay, the vn of each blow is almost similar.