Seismic Vulnerability Checks: existing foundations and intervention criteria

Is the verification of the seismic vulnerability of the foundations of an existing building to be carried out in order to have a complete picture of the building’s seismic safety? Are improvement interventions mandatory?

In the course of our professional activity, we often encountered these questions. Normally we have considered it appropriate to carry out this verification, both as a check of the resistance of the soil, and as a check of the resistance of the foundation structures, especially if in reinforced concrete.

Reading the “Integrated text of Annex 2 – Buildings – to Ordinance 3274 as amended by OPCM 3431 of 3/5/05“, in particular to ANNEX 11.E. – CRITERIA FOR CONSOLIDATION INTERVENTIONS OF MASONRY BUILDINGS, in the paragraph dedicated to foundation interventions we learn that …

It is possible to omit interventions on the foundation structures, as well as the related checks, if all the following conditions are simultaneously present:

  1. In the construction there are no major failures of any nature attributable to subsidence of the foundations and it has been ascertained that failures of the same nature have not occurred even previously
  2. The planned interventions do not involve substantial alterations to the structural scheme of the building
  3. The same interventions do not involve significant modifications of the stresses transmitted to the foundations
  4. Overturning phenomena of the building due to seismic actions are excluded

The inadequacy of the foundations is rarely the cause of the damage observed in post-earthquake surveys.

However, in the event that the foundation rests on soils with inadequate geomechanical characteristics for the transfer of loads, or of localized seabed subsidence in progress, it is necessary to consolidate the foundations, implementing one of the following types of intervention, or an appropriate combination thereof, prior to survey of existing foundations.

Widening of the foundation by means of reinforced concrete curbs or an armed audience. The intervention must be carried out in such a way as to make the existing foundations collaborate adequately with the new ones, paying particular attention to the connection between the new and old foundation in order to obtain a monolithic body capable of spreading tensions homogeneously.

A rigid connection must be made (reinforced and bracketed reinforced concrete beams, steel crosspieces of suitable stiffness, post-tensioned bars that guarantee transmission by friction) capable of transferring part of the loads coming from the superstructure to the new elements.

In the presence of possible differential subsidence of the foundation it is appropriate to evaluate the effects on the entire building, and consequently decide on the necessary extension of the enlargement intervention.

Consolidation of foundation soils. Soil consolidation interventions can be carried out by injections of cement mixtures, resins (eg polyurethanes that expand into the ground), or other chemical substances.

Insertion of deep foundations (micropiles, root piles). The execution of this type of intervention can be carried out as an alternative to the previous one; in the case of settlements affecting individual portions of the building, the intervention can also be carried out limited to the portions concerned, as long as they are homogeneous from the point of view of the underlying problems.

In general, a suitable connection structure between micropiles and existing masonry should be provided (e.g. a reinforced curb rigidly connected to the masonry), unless the micropiles themselves are drilled through the masonry, with a drilling length sufficient to transfer loads to the micropiles for adhesion.

In situations where the seismic activation of slope instability phenomena is considered possible, the problem must be addressed by acting on the ground and not simply at the level of the foundation structures.

Therefore, in the vast majority of cases, verification and intervention would seem unnecessary.

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