Displacement Based Seismic Design For Large Gravity Retaining Walls

The use of factor of safety (FOS) design approaches based on limiting equilibrium is conventional practice for designing retaining structures, but experience has shown that there can be significant uncertainty and a wide range of performance achieved when using force-based design approaches for seismic design. Such shortcomings were highlighted during the design of large counterfort wall retaining structures for the Port Botany Expansion project in Sydney, Australia. Extending 21.5 metres in height, the counterfort structures posed a significant challenge when evaluating stability against bearing failure under seismic loading, based on FOS criteria alone. In order to better understand the apparent stability concerns, recourse to dynamic finite element modeling was made to more accurately assess potential failure modes and wall performance.

The dynamic modeling provided greater insight into more realistic failure modes and helped to dispel fears of seismically induced bearing failure in the traditional sense of a global mechanism. It also served to justify the use of Newmark sliding block analyses to characterize dynamic wall behaviour. Based on equivalency of the dynamic finite element and Newmark sliding block results, an appropriate pseudo-static force to represent seismic loading was established. This rationalized approach to seismic design demonstrated overall adequacy in design, overcoming the initial shortcomings of the FOS design approach.