Mitigation of Seismic Accelerations by Soft Caissons

Earthquake accelerations cause many problems for structures on the ground surface. Ground improvement may be carried out to reduce or otherwise modify shear waves before they reach the structures, which can be more cost-effective than structural strengthening. This paper investigates the response of a simple structure whose foundation has been completely enclosed by a layer of soft material. Physical and numerical models of this are presented. Based on these, it is seen that not only is linear acceleration significantly attenuated by such a system, but the foundation-structure system is able to rock in antiphase to the translational motion to further reduce acceleration. A simple two degree of freedom spring model is presented that can match the behaviour of the more sophisticated models. Recorded strong motion data applied to this simple model suggests that improvement can be achieved provided caisson modes and structural modes occur at different frequencies.