Structure of the building and historical vicissitudes
The building, L-shaped and maximum dimensions 24.00x12.00 m, consists of a basement and five floors above ground for a total height in the eaves of approximately 18.00 m. The structure is mixed, load-bearing masonry and concrete conglomerate frames reinforced with brick-concrete floors and covered with wood. Despite being a single body of the building, three portions can be identified structurally organized in different ways. In the west wing the load-bearing lines are parallel to Via Marco Emilio Lepido and consist of the external walls, a spine wall on the street side and a four-span reinforced concrete frame on the courtyard side. The corner portion between Via M. E. Lepido and Via Correnti instead has the floors warped in the opposite direction and the load-bearing lines are constituted by the external wall on Via Correnti, by the wall parallel to it that delimits the Computer And Service s.r.l. and a central frame with three spans in reinforced concrete. Finally, the north wing has a masonry structure consisting of external walls parallel to Via Correnti and a spine wall. No information on foundations is available; given the structural typology of the building and given its age, it is possible to hypothesize ribbon-like foundations for the walls and plinth foundations for the pillars in reinforced concrete. However, since the genesis of the crack phenomenon is clear, it was not considered necessary to make tests at this stage. When the condominium decides to intervene with a consolidation, it will be necessary to carry out some surveys on the foundations in advance, calibrated on the type of consolidation adopted, to establish their effective consistency, dimensions and depth of installation. The crack pattern found, and in particular the diagonal cracks present at all floors on the walls orthogonal to the west end of the building, testify to the existence of a localized phenomenon of subsidence affecting precisely the extremity of the west wing of the building. There is substantially a subsidence of the foundation of the wall bordering the garden of number 13 of Via Marco Emilio Lepido which also involves the ends of the orthogonal structures. In the rest of the building no injuries attributable to static instability emerged.
Hydro Buildings Consolidation s.r.l.
Via Alessandro Tiarini, 27/A - 40129 - Bologna - Italia
C.F. / P.Iva: 02836401204
Tel: 342 832 8237 - 051/6350845 - Fax: 051/6354238
Hydro Buildings Consolidation s.r.l. © 2022 - Powered by Logos Engineering
Ground withdrawal of the west courtyard
Geological and geotechnical framework
The investigations carried out on the ground show the existence of a lithologically homogeneous subsoil consisting of a slightly sandy clayey silt, at least up to about six meters from the ground level, a depth at which the probe stopped due to its remarkable compactness. The laboratory investigations on the samples taken have made it possible to establish that these are materials that show strong variations in volume when their water content changes. In fact, the linear shrinkage value determined for the two tested samples (RL≈15%) means that in the event of complete drying, the surface of the soil would drop by about 15 cm for each meter of depth affected by the phenomenon. This is obviously a limit situation which however gives an idea of the order of magnitude of the failures. In these situations, variations in humidity of any origin (climatic variations, evapotranspiration, water infiltration, etc.) affecting the layers of soil below part of the foundations of a building cause localized volume variations of the soil and, of consequently, compulsive states (stresses) on the structure. From the comparison of the graphs relating to the humidity values found in the two surveys carried out, a strong difference in the water content between the west end of the building and the facade on Via Correnti is revealed. In particular, from the west side (Assay n. 1) the measured humidity is largely below the shrinkage limit (green line). On the contrary, on the side of Via Correnti (Saggio n. 2) the water content is close to the plastic limit (yellow line). It is therefore evident that at the west end of the building the land has undergone a strong drying and a notable decrease in volume. These phenomena are also denounced by the evident lowering of the level of the garden at number 13 of Via Lepido which is more than 20 cm lower than the base of the plaster of the facade completely redone 17 years ago. Below is a reasoned comparison of the humidity graphs relating to the two surveys performed. The relief of the crack state tells us that there are cracks only in the west end of the building and that said cracks are attributable to foundation subsidence of the same portion of the building. Likewise, the geotechnical investigations carried out show that the foundations of the west end of the building rest on a layer of soil that has undergone considerable shrinkage over time.
Consequently, the situation is clear, the static instability is due to the drying of the foundation soil of the west end of the building. Furthermore, examining the external situation we realize that the building is completely surrounded by bituminous pavements (asphalt) except on the west side where it borders a park with several tall trees and large diameter, some of which are very close to the wall. It is therefore evident that drying is essentially due to evapotranspiration phenomena.
Stratigraphy of the site under examination
Boundary conditions and probes positioning
Physical and granulometric characteristics
Change in humidity with depth
Project of the intervention
Examining the available consolidation interventions very briefly, we arrive at the following considerations. A sub-foundation intervention does not make sense as it would be necessary to deepen the laying surface beyond the depth subject to drying which at present exceeds six meters from the ground level and which will increase over time. An intervention by injection of resins would not be decisive as this technique is not able to stop the phenomena of shrinkage and swelling of the land. Consolidation using micropiles is the classic solution which, however, being rather invasive, presents high costs and considerable inconvenience as it is necessary to access the basements with bulky machines, given the spaces and access routes available. Furthermore, a partial intervention using micropiles is not recommended as it would create a strong discontinuity in the soil-structure interaction between the consolidated part and the rest. In practice, the piling part becomes fixed while the rest of the building may be subject to slight excursions due to variations in humidity. The HBC consolidation technique, on the other hand, consists in the construction of a system capable of restoring and maintaining the humidity of the foundation soil of the damaged portion of the building in order to avoid subsidence and differential swelling and, consequently, to heal the static instability from them. originated. Although a global intervention on the entire building is always advisable so as not to create discontinuity in the behavior of the land, in this case, apart from the anomaly that occurred at the west end of the building, the context is rather homogeneous so that a localized system that restores the initial humidity conditions. Basically, since on the side of Via Correnti the soil moisture is close to the saturation value, as the surrounding conditions are the same on all sides (except the west) and since the rest of the building is intact and healthy, it is only possible to intervene on the west side. The consolidation project involves the installation, near the west side wall, of an HBC system consisting of a line of vertical diffusers, suitably built and spaced, connected to a control and management device, which allows to supply water to the ground if, when and where it is needed regardless of the causes that generate the variations in humidity. The optimal solution would be to build the system outside the building in the neighboring property (permission from the neighbor is required). The system would be totally underground and the inspection pit could be located in the condominium property. The alternative is positioning inside the building working in the cellars, obviously with higher costs. The water could be derived from the laundry while the check could be done mechanically saving the electrical system.
Plan of the HBC plant