Definitions

ground ways

Ground-structure interaction

Design

The interaction between ground and structure consists of an exchange of mutual stress between the structure itself and the foundations ground. This means that the movement of the whole ground-structure system is influenced by the type of ground as well as by the type of structure. The main types of foundations, based upon several building characteristics, are:

- 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 A: homogeneous rock formations;

- Category B: compact granular or clayey grounds;

- Category C: quite compact granular or clayey grounds;

- Category D: not much compact granular or clayey grounds;

- Category E: alluvial surface layer grounds (very low quality 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.

Mitigation

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). The most diffused techniques are the jet-grouting technique and the pilework technique. The jet-grouting technique consists of injecting in the subsoil some liquid concrete by means of a drill. When this concrete hardens it forms a sort of column which consolidates the surrounding ground. This process will be repeated on all the area of the structure. The pilework technique consists of using piles which, once inserted in the ground, serve the function of supporting the foundation and then the building, by moving the loads or the weights towards ground layers which are deeper and therefore more compact and resistant.

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