Capacity design of embedded retaining structures

An earth-retaining structure consists of structural elements that interact with a soil mass. The study of the soil–structure interaction becomes especially complex under seismic conditions, because the inertial forces reach a portion of soil that is already mobilising part of its strength to sustain the excavation: a suitable soil model would need to include, in addition to non-linearity and damping, the progressive mobilisation of the soil strength. However, for design purposes it may be unnecessary to predict the detailed dynamic behaviour of the system: it could be sufficient to endow the system with features that will ensure a desirable behaviour under a severe seismic event. In this paper, it is maintained that a desirable behaviour for cantilevered and singly propped retaining walls subjected to a severe earthquake is to mobilise a plastic mechanism deriving from the attainment of the strength in the volume of soil that directly interacts with the wall, while preserving the integrity of the structural members. Following this line of thought, it is shown that it is possible to use relatively simple pseudo-static tools, essentially based on the strength properties of the soil, to study the plastic mechanism associated with the desired behaviour, and to derive the internal forces that the structural elements are called to resist in order to ensure that the plastic mechanism will be maintained at its full strength during the seismic event. This approach is applied to a number of schematic retaining structures; results of full dynamic analyses of the soil–structure interaction are employed to validate the proposed approach and to evidence its limitations.