THE BUILDING IN VIA BELLINI N ° 1 IN BOLOGNA
A consolidation project carried out in September 2008 using the “HBC System” is presented below. At the same time as the implementation phase of the plant, a monitoring system of the main crack pattern, general operation and water consumption was started.
Structure of the building and historical vicissitudes
It is a building for residential use, measuring 20.00 x 11.00 m, consisting of three floors above ground and a basement floor used as a basement in the upstream part and shops in the valley portion. The supporting structure is in masonry and consists of the longitudinal external walls of two heads, two spine walls of a head and the transverse walls delimiting the stairwell, also of a head. On the ground floor, in the portion with shops, one of the spine walls has not been built, a masonry pillar and a beam have been built in its place. The foundations, located at 50 ÷ 60 cm from the basement, consist of a poorly cemented gravel bed less than 40 cm wide and about 30 cm thick. The building presented the first static problems in the upstream portion in the mid-1980s. In 1989 the Condominium consulted a Professional who carried out some surveys and identified the characteristics of the foundation ground. The instability is attributed to the soaking of the ground due to the dispersion of rainwater as well as to the poor stiffness characteristics of the foundation. A partial intervention with reinforced concrete poles Φ 400 mm and the collection of metoric waters are recommended and carried out. Starting from 1993, a new cracking phenomenon gradually appeared, with the reopening of old cracks and the appearance of new ones, especially in the portion of the valley not affected by the poles. In 1996 the building was again marked by a whole series of cracks that unequivocally denounce the existence of a settlement phenomenon due to differential subsidence of the ground. Monitoring is recommended but the Condominium decides to postpone and carries out the external restoration of the facades with compensation for all injuries. Between the end of 1996 and the beginning of 1997, following the merger of the two shops on Via Toscana, the supporting structure was changed. The pillar and the section of the spine wall are replaced by four columns and two steel girders. In 2001, new cracks appeared both on the load-bearing walls, on the partitions and on the floors and in particular showy sub-vertical cracks appeared at the beginning of the piling on the external longitudinal walls. From October 2003 to November 2004 the most significant lesions were monitored with crack meters. The readings highlighted the opening of the cracks in summer and autumn and closing in winter and spring. There was a substantial compensation of the failures in the investigation period but the cracking state worsened significantly, also following the appearance of new lesions. The alternation of opening and closing of the cracks confirms that the building is heavily affected by the seasonal volumetric variations of the foundation soils. Below are the survey of the structure and the cracking state, some photographs and the plan of the partial intervention with poles carried out in 1989:
Hydro Buildings Consolidation s.r.l.
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Survey of the structure and the cracking state
Partial plan of the intervention with poles
Geological and geotechnical framework
The area under investigation is located in the eastern area of the city of Bologna and is included in Section 221090 “Bologna Sud” of the regional technical cartography. The site, via Bellini 1, is located in correspondence with a strip of land where the slope breaks between the hilly area and the high plain. In particular, it belongs to the morphological environment of the foothills and the piedmont plain. More precisely, it is placed at the transition between the band of the ancient conoids of the pedecollina interdigitate with those of the recent permeable inter-conoids. The morphological evolution of this sector of the territory of the city of Bologna is closely linked to the dynamics of the minor watercourses that furrow the first hill slopes of the territory between Porta S. Stefano and the locality of S. Ruffillo. These are essentially ditches of minor importance that flow into the Savena stream. The sediments present in the subsoil of the area in question are the result of the transport and storage action of these waterways. They, in the part of the outlet in the plain, have had a similar evolution, with repeated diversions of the riverbed and general displacement of the fan system towards the East. The external portion of the fans has merged and the streams have overlapped several times their riverbed to the abandoned one of the other. In these basins, of limited extension and modest total height difference, the contributions of coarse material are limited. The presence of gravels is in fact generally reported only for very limited powers and extensions. The apexes of the conoid and interconoid bodies rest directly on materials attributable to the Marl Formina del Termina (TER) and the Pliocene Clays Formation (FAA). In particular, the first subsoil is characterized by the presence of a layer of fine materials, in part, deriving from the alteration of the underlying ones (eluvial layer) and, in part, transported by water courses. The following illustrations show an extract of the geological map relating to the area under examination, the detailed stratigraphy detected, the physical and granulometric characteristics of the soil and the variation in humidity with the depth obtained from the analyzes carried out on the samples taken.
Geological map of the area under examination (taken from the Geological and Seismic Service of the Soils of Emilia Romagna
Physical and granulometric characteristics
Variation of the water content with the depth in the soils of the investigated site
Stratigraphy of the site under examination
An examination of the graph relating to the water content with the depth, shown above, shows that in the area between –1.50 and -3.40 m from the ground level, the water content is close to the shrinkage limit. It is evident that in this belt, where the seabed structures are located, there has been a drying of the soil and, therefore, a volumetric shrinkage. Note also the net decrease in humidity on contact with the gravels below. This stratigraphic structure can considerably influence the trend of the water content of the fine materials. It cannot be ruled out, in fact, that the gravels are the site of ephemeral layers and, therefore, can bring and / or drain water.
For the purposes of sizing the HBC system, it is essential to determine the radius of influence of the diffusers to be set up. Unfortunately, the analytical way is very complicated because, as previously mentioned, in the case of unsaturated soils the parameters of Darcy's law, which interprets the filtration process, are functions of the suction forces present. Consequently, we opted for on-site experimentation using a borehole instrumented with a Norton-type piezometer used as a diffuser element. The piezometer was powered for a month by imposing a piezometric level at approximately -4.00 m from the ground level. To determine a useful radius of influence of the diffusion, we proceeded to compare the initial water content determined in the first investigation phase (figure 7) with two other profiles detected respectively adjacent to and 1.00 m away from the diffuser after the feeding period. The comparison of the water content after the soil imbibition phase is shown alongside.
As can be seen from the results obtained, the area located below the piezometric level showed an increase in the water content, going from percentages close to 19% to values around 25%. The water contents between 24% and 27% represent the total saturation of the material under examination.
Project of the intervention
Consolidation was carried out in 2008 by installing a soil moisture control system (patented HBC system). As can be seen from the plant design, shown in the following illustrations, four sub-horizontal pre-drilled polyethylene pipes were installed at a suitable depth. They have the function of spreading the water necessary to keep the humidity constant regardless of any external conditions that may intervene in the ground of the building. Obviously, the installation does not affect the part of the building that was consolidated with poles at the time. The water supply was derived from the condominium meter located near the right corner of the building, while the necessary electricity was taken from the condominium meter panel located in the stairwell. Each diffuser is equipped with an HBC sump, which can be inspected for maintenance, located in the square on Via Toscana which contains the electrical and electronic equipment designed to control, monitor and maintain the projected piezometric level of the water. Given the presence of the square on Via Toscana it was possible to lay the speakers with the use of a horizontal drilling machine (No-Dig) without intervening in the slightest inside the commercial establishment located on the ground floor. The "Directional drilling" is a drilling system based on the execution of a drilling, by means of an orientable head and a system to locate it. The advancement of the drilling head, for the execution of the "pilot hole", occurs through the combination of the thrust and rotation movements exerted by the machine. In the case carried out, it was sufficient to carry out four excavations, two in the cellars and two in the courtyard, for the interception of the probe. Below are the plan and the section of the HBC plant built, and some photos relating to the construction phases.
Section of the HBC plant built
Plan of the HBC plant created
Realization of the no-dig perforations
Construction phases of the plant
Detail of the HBC cockpit
Monitoring and results obtained
The plant was activated in September 2008 and at the same time a monitoring was implemented which consists in the control of the most significant lesions by means of twilight stations, in the measurement of water consumption by means of a meter and in the operation checks of the lines by means of a control unit. control electronics. The results obtained were excellent. As can be seen from the graphs shown, the movements, after an initial adjustment phase of a few months, have completely stopped. The measurements refer to a critical crepemetric station as it is located on the main lesion on the external wall at the beginning of the piling. The need for water was minimal as assumed in the design phase. In particular, the latest reading, carried out on 30/09/2010, indicates a total consumption of 143 cubic meters in line with the permeability characteristics of the land. By examining the trend in water consumption, it can be seen how, after the initial settlement period, the consumption hints at a pseudo-sinusoidal trend as evidence of the seasonal influence on the process.
Total water consumption: 140 square meters in two years.
(Less than a family's consumption)
What is illustrated in the previous paragraphs shows how it is possible to solve situations of static instability of buildings due to clay shrinkage and swelling phenomena by installing a system capable of maintaining constant soil moisture. Up to now, this type of instability has been solved by transferring loads to deeper layers of soil, and therefore not affected by variations in humidity, creating pilings or sub-foundations of various kinds. These interventions are very expensive and invasive as they generally affect all the load-bearing structures of the building and require accessibility even with impressive equipment and, often, from both sides of the walls. Furthermore, since most of the damaged buildings have foundations that are not reinforced and / or of insufficient dimensions, to connect the piling to the masonry it is necessary to create perimeter curbs of reinforced concrete. As a result, extensive excavations, demolition and reconstruction of sidewalks, courtyard and interior flooring, sewers, partitions and systems are often required. More recently, methods based on injections of resins under pressure have also been adopted, but often the interventions carried out in clayey soils have not led to the hoped-for success. If the static instability of a building is caused by shrinkage and / or swelling of the foundation soil, the patented HBC system, compared to traditional systems, offers considerable advantages in terms of economy and invasiveness. In fact, it requires small diameter perforations with limited depths and reduced excavations, does not require interventions on the foundations and involves construction sites of very limited size and duration. In addition, if the external spaces allow it, it is possible to operate with the horizontal drilling technique without intervening at all inside the building. As far as maintenance is concerned, an annual operational check is sufficient and any replacement of the plant control equipment involves negligible costs. Water consumption, as the case of Via Bellini demonstrates, is lower than that of a family, or a few cubic meters per year. The HBC system is also profitable made preventively, in the case of new buildings, where the foundation level falls to depths that are still affected by seasonal variations in water content (see point 6.4.2 NTC 2008). In fact, it eliminates the uncertainties related to determining the thickness of the active blanket, which varies over time, and avoids the adoption of particular measures in the foundation that are economically more burdensome. It also protects and preserves the building from possible differential subsidence triggered by all those accidental factors due to external causes (sucking due to trees, leaks from sewers, systems, breakage of downspouts, etc.) that often occur.