Progressive collapse of a drilled-shaft bridge foundation under vessel impact

• The Jiujiang Bridge was impacted by a vessel in 2007. Three piers collapsed.
• The failure mechanisms of the bridge under vessel impact are investigated.
• The failure proceeded progressively from one pier to another.
• The failure of the adjacent piers was not directly caused by the impact load.
• Instead, they failed due to the pulling forces from the falling bridge deck.

On 15 June 2007, a pier of the 1675 m-long Jiujiang Bridge near Guangzhou, China was impacted by a vessel. The pier being impacted and two adjacent piers collapsed in a sequence and approximately 200 m of the bridge deck fell into the river. Each of the three piers was supported on a 1.7 m diameter, 2×2 drilled shaft group. The mechanisms of the progressive failure of the bridge foundation under vessel impact are investigated through nonlinear finite element analysis. The simulation model includes all three drilled shaft groups as an integrated system connected by a bridge deck. A possible system progressive failure mode is proposed based on the analysis. The bending moment, displacement and soil reaction along the drilled shafts are computed. The pier foundation being impacted collapsed first. The failure of this pier was initiated at the connections between the pile cap and the drilled shafts and between the pile cap and the pier columns. The collapse of the adjacent pier foundations occurred one by one in a sequence, which was not directly caused by the vessel impact load but by the centrifugal forces from the rotation of the adjacent bridge deck when it fell about one end.