Influence of phase difference between kinematic and inertial loads on seismic behaviour of pile foundations in layered soils

Abstract

A series of dynamic centrifuge experiments was conducted on model pile foundations embedded in a two-layered soil profile consisted of soft-clay layer underlain by dense sand. These experiments were specifically designed to investigate the individual effect of kinematic and inertial loads on a single pile and a 3×1 row pile group during model earthquakes. It was observed that the ratio of free-field soil natural frequency to the natural frequency of structure might not govern the phase relationship between the kinematic and inertial loads for pile foundations as reported in some previous research. The phase relationship obtained in this study agrees well with the conventional phase variation between the force and displacement of a viscously damped simple oscillator subjected to a harmonic force. Further, as expected, the pile accelerations and bending moments can be smaller when the kinematic and inertial loads act against each other compared to the case when they act together on the pile foundations. This study also revealed that the peak kinematic pile bending moment will be at the interface of soil layers for both single pile and pile group. However, in the presence of both kinematic and inertial loads, the peak pile bending moment can occur either at the shallower depths or at the interface of soil layers depending on the pile cap rotational constraint.

Seismic response of piles in layered soils: Performance of pseudostatic Winkler models against centrifuge data

“…This is due to the phase difference between the kinematic and inertial loads. For the same tested pile foundations, Garala and Madabhushi [18] has shown that there is a significant phase difference between the kinematic and inertial loads for the single pile during BE2, BE4 and BE5 excitations and hence the pile accelerations in K+I flight are smaller than those in K flight. For all other cases, the kinematic and inertial loads act together or with smaller phase difference, leading to larger pile accelerations in K+I flight compared to K flight.…”

“…An air hammer device, a small actuator that can act as a source to induce waves within the soil model [17], was placed Published in Soil Dynamics and Earthquake Engineering 153 2022 107110 at the bottom of the model on a 10-15 mm thick sand layer during sand pouring. The detailed model preparation procedure and equivalent prototype characteristics of a single pile can be found in Garala [16] and Garala and Madabhushi [18]. The unit weight of the saturated clay and the sand is 16.2 kN/m 3 and 20.4 kN/m 3 , respectively.…”

“…The significant phase difference between the kinematic and inertial loads also leads to lower pile bending moments as the piles are vibrating with smaller acceleration amplitudes. More details about the phase difference between the kinematic and inertial loads and its influence on pile accelerations and bending moments can be found in Garala and Madabhushi [18]. A's shallow and deep failure criteria is a minimum of 10% (during BE2) and maximum of 27% (during BE3) for the single pile.…”

“…The studies of Adachi et al [47] and Tokimatsu et al [48] also recommend that whether kinematic or inertial loads dominate is a function of the relevance between the natural frequencies of the soil and the pile-supported superstructure. However, recently, Garala and Madabhushi [18] concluded that whether kinematic or inertial loads dominate pile response is independent of the natural frequency of the soil and the phase relationship between the kinematic and inertial loads follows the conventional force-displacement phase variation for a viscously damped simple oscillator excited by a harmonic force. In this study, to keep the analysis simple, the kinematic and inertial loads are assumed to act together on the pile foundations, indicating in-phase loading conditions.…”

“…Published in Soil Dynamics and Earthquake Engineering 153 2022 107110 loads (BE4 and BE5 excitations), see Garala and Madabhushi [18] for more details about the influence of phase difference between kinematic and inertial loads on pile bending moments. Further, the maximum percentage difference between Method A (shallow criteria) and Method B in the pseudostatic models is only 5.5% (during BE5) for the single pile.…”