Seismic Design Approach for Large Counterfort Wall Retaining Structures

The use of factor of safety design approaches based on limiting equilibrium is conventional 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. 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 meters (74 feet) in height, the counterfort structures posed a significant challenge when evaluating stability against bearing failure under seismic loading, based on factor of safety 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 failure modes and helped to dispel fears of seismically induced global bearing failure. It also served to justify the use of Newmark sliding block analyses to characterize dynamic wall behavior. Based on equivalency of 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 factor of safety design approach.