- with isolated plinths (not feasible at the moment);
- with plinths connected by foundations beams;
- with reverse beams;
- with a plate (used for low-quality grounds).
The filing of foundations grounds takes place according to the mechanical properties of the grounds themselves: in Italy, for instance, according to the new earthquake-proof norm – Ordinanza 3274/2003 – you can identify the following categories:
- Category C: quite compact granular or clayey grounds;
- Category D: not much compact granular or clayey grounds;
The type of foundations is selected according to the type of ground; for instance, in the case of homogeneous rock formations connected plinths are selected, while in the case of very low quality grounds plates are chosen.
For further information about the various ways of building foundations you can see foundation (architecture).
Both grounds and structures can be more or less deformable; their combination can or cannot cause the amplification of the seismic effects on the structure. Ground, in fact, is a filter with respect to all the main seismic waves, as the stiffer ones foster the way to high-frequency seismic waves, while those which are less compact foster the way to lower frequency waves. Therefore a stiff building, characterized by a high fundamental frequency, will suffer an amplified damage when built on a stiff ground and then subjected to higher frequencies.
For instance, we suppose to have two buildings: they share the same stiffness (we suppose it to be high); they stand on two different grounds, the first of which is very stiff (rocky), the second deformable (sandy). If they are subjected to the same seismic event, they suffer different kinds of damage. That on the stiff ground will suffer the greater damage.
The second interaction effect, tied to the mechanical properties of grounds, is about the lowering (sinking) of foundations worsened by the seismic event itself, especially about less compact grounds. This phenomenon goes under the name of soil liquefaction.