Influence of flood-induced scour on dynamic impedances of pile groups considering the stress history of undrained soft clay

Flood-induced scour causes loss of lateral support at bridge foundations and thus modifies their dynamic impedances, which will in turn influence the dynamic behavior and the seismic response of bridges located in seismically active flood-prone regions. When evaluating the seismic performance of scoured bridges, however, few efforts have been made to analyze the dynamic impedances of their foundations, and possible changes in stress history of the remaining soils are also ignored. In reality, the remaining soils around pile foundations undergo an unloading process due to scour and their over-consolidation ratios are increased, which can be expected to change the properties of the remaining soils. Therefore, a three-dimensional numerical model, in which the pile-soil-pile interaction and the radiation problem are accounted for by means of the elastodynamic Green's functions that permit the mutual interactions between all the piles of the group consistently and simultaneously, is employed in this paper to investigate the influence of flood-induced scour on the dynamic impedances of pile groups in soft clay. Accordingly, the key soil parameter of the elastodynamic Green's functions, i.e., Young's modulus of soft clay, is modified in this model to take into account the stress history effect due to scour. A case study to evaluate the dynamic impedances of 3×3 and 4×4 pile groups with various scour depths and pile spacing, considering or ignoring stress history effect, is discussed in this paper. The results show that ignoring the effect of stress history of the remaining soft clay will overestimate the scour affected dynamic impedances of pile groups, which are found to be reduced significantly as the scour depth increases.