Influence of excavation beneath existing building on dynamic impedances of underpinning pile considering stress history

The addition of basement beneath existing building changes the underpinning pile from fully embedded to partially embedded, and thus influences the mechanical properties of pile. In the past, scholars paid attention to the change in the bearing capacity of pile but neglected the difference of dynamic characteristics before and after construction, and potential changes in stress history of remaining soil are also ignored. In this work, a calculation model is built to investigate the influence of excavation on dynamic impedance of underpinning pile considering the effect of stress history. The soil is simulated by the dynamic Winkler foundation, which is characterized by springs and dashpots. Properties of remaining soil after excavation are updated to consider the effect of stress history through modifying the initial shear modulus and related parameters. The dynamic impedance of pile after excavation is obtained based on the transfer matrix method. The parameter study is carried out to evaluate the dynamic impedance with various excavation depths, considering or ignoring stress history effect, and various element lengths. The results show that shallow soil plays an important role to dynamic impedance, and overestimated dynamic impedance is obtained if not considering the stress history effect.