Seismic behaviour of anchored steel sheet pile walls in sand
Anchored Steel Sheet Pile (ASSP) walls are frequently adopted as retaining structures in port facilities. Many coastal areas in the world are highly seismic regions, meaning that ASSP walls are likely to experience earthquakes during their lifespan. ASSP retaining walls are frequently designed for seismic actions using pseudo-static methods. Their displacements can be estimated using a Newmark's sliding block approach. A deeper understanding of the seismic behaviour of these structures is required to identify the main parameters affecting their maximum internal forces and displacements.
In this research project, the seismic behaviour of ASSP walls in dry and saturated sand was investigated. A series of dynamic centrifuge tests showed that, in dry conditions, the maximum internal forces were driven by the peak horizontal acceleration of the earthquake in the backfill. The systems accumulated the majority of the permanent displacements during the very initial cycles of shaking, whereas much less displacement occurred during later cycles, showing an overall increase of their seismic capacity.
In saturated conditions, excess pore water pressures drove the maximum internal forces and were responsible for a significant deamplification and phase lag in the horizontal accelerations, as these travelled through the soil deposit. The accumulation of permanent displacements in the systems was gradual and uniform over the duration of the earthquakes. In this case, the tendency of ASSP walls to increase their seismic capacity during earthquake shaking was inhibited by the generation of excess pore water pressures.
The causes of the progressive increase of the overall capacity of ASSP walls subjected to seismic shaking were identified as: (i) densification of the sand, (ii) rotation of the walls, (iii) reduction of the retained height, and (iv) the progressive mobilization of the passive resistance of the soil. In saturated conditions, these were counterbalanced by the generation of excess pore water pressure. Accounting for these aspects in limit equilibrium pseudo-static solutions improved the predictions of the failure mechanisms and the maximum internal forces for ASSP walls, especially in dry conditions. Besides, it allowed to develop an effective method for the prediction of earthquake-induced displacements of these systems, based on the Newmark's sliding block approach, which accounts for the failure mechanisms and the initial deformability of the system, and the geometrical changes of the layout of these structures.