Impact Model Tests and Simplified Analysis for Flexible Pile-Supported Protective Structures Withstanding Vessel Collisions

Flexible pile-supported protective structures have been used extensively in the protection of bridge piers against vessel collisions. Their success is dependent on their ability to absorb impact energy through large deflection and yielding. A large soil tank ($15\times 5\times 6\mathrm{m}$) was used to conduct large-scale model tests on protective structures in silt. A lateral static load test on a single pile was carried out to derive $p\mathrm{\text{-}}y$ curves for the piles on the basis of the measured bending moment of the pile shaft. Impact tests with different initial impact energies were then conducted on the protective structures and on a single pile to study the energy transfer mechanism during a collision. Soil damping and the inertia force of the pile shaft have obvious effects on the load-displacement curves of single piles, especially in the initial phase of impact. However, their influence on the maximum bending moment of the pile shaft was weak. Compared with protective structures that consist of free-standing piles or a pile row, a novel protective structure with two rows of piles has a relatively larger lateral stiffness and a much larger energy-absorbing capability. A simplified energy-based analysis method is proposed to estimate the lateral deflection of the flexible pile-supported protective structures that are subjected to a given impact energy. Comparison of the calculations with the test results shows that this simplified analysis method gives conservative results approximately 30% for the energy-absorbing capability.