A second interaction effect, tied to mechanical properties of soil, is the sinking of foundations, worsened by a seismic event.
Neglecting SSI is reasonable for light structures in relatively stiff soil such as low rise buildings and simple rigid retaining walls.
[3] Hence, the modern seismic design codes, such as Standard Specifications for Concrete Structures: Seismic Performance Verification JSCE 2005 [4] stipulate that the response analysis should be conducted by taking into consideration a whole structural system including superstructure, foundation and ground.
It is conventionally believed that SSI is a purely beneficial effect, and it can conveniently be neglected for conservative design.
The main idea behind the provisions is that the soil-structure system can be replaced with an equivalent fixed-base model with a longer period and usually a larger damping ratio.
Using rigorous numerical analyses, Mylonakis and Gazetas [9] have shown that increase in natural period of structure due to SSI is not always beneficial as suggested by the simplified design spectrums.
[2] At low level of ground shaking, kinematic effect is more dominant causing the lengthening of period and increase in radiation damping.
However, with the onset of stronger shaking, near-field soil modulus degradation and soil-pile gapping limit radiation damping, and inertial interaction becomes predominant causing excessive displacements and bending strains concentrated near the ground surface resulting in pile damage near the ground level.
Yashinsky [10] cites damage in number of pile-supported bridge structures due to SSI effect in the Loma Prieta earthquake in San Francisco in 1989.
Extensive numerical analysis carried out by Mylonakis and Gazetas [9] have attributed SSI as one of the reasons behind the dramatic collapse of Hanshin Expressway in 1995 Kobe earthquake.
The second interaction effect, tied to mechanical properties of soil, is about the lowering (sinking) of foundations, worsened by the seismic event itself, especially about less compact grounds.
The methods most used to mitigate the problem of the ground-structure interaction consist of the employment of the before-seen isolation systems and of some ground brace techniques, which are adopted above all on the low-quality ones (categories D and E).