Raw-earth houses in the Frascheta

In the architectural landscape of the agricultural area around Alessandria, particularly in the lowland zone known as the "Frascheta" (an area that extends mainly across the countryside between Alessandria, Tortona and Novi Ligure, and in particular in the municipalities of Bosco Marengo, Pozzolo Formigaro and Frugarolo), there are many examples of houses built in raw earth, also improperly called "Trunere" or simply, in the Alessandria dialect, "Cà d'tèra". In this area there are numerous examples of buildings constructed in raw earth, from houses to farmsteads, from churches to even schools and town halls.

In the Frascheta, therefore, as in the plain of Marengo, earth has always been used by farmers to build their own houses. The earth used in these areas has a characteristic red colour, due to the strong presence of iron minerals contained in the clay.

Among the main working techniques used to build load-bearing earth walls, especially in the Alessandria area, are "pisé" (rammed earth) and "adobe".

The first, in particular, is based on the construction of walls with moistened and rammed earth — that is, compressed layer by layer inside a wooden formwork — in order to obtain the so-called raw brick. The second technique, on the other hand, was adopted only marginally in the Frascheta area.

Among the factors that most affect the deterioration of walls built with this technique, beyond the classic construction defects (linked to a poor knowledge of the construction process on the part of those who built this type of building in the past), are:

• intrinsic pathologies, that is, those relating to defects in the building materials used, to the construction work and to the type of foundation ground present (which in these areas is mainly of alluvial origin);
• pathologies linked to subsequent modifications, namely: replacements, demolitions or insertions that may have contributed to altering the load paths and reducing the load-bearing capacities of the remaining structures;
• pathologies due to the decay of the structure as a result of very poor maintenance and abandonment of the building.

Current condition of the building

The building in question, for residential use, is located precisely in the province of Alessandria, specifically in the hamlet known as Spinetta Marengo. Its construction undoubtedly predates 1967, although no definite documents have been found to prove this. In its current state, the building had an L-shaped plan, since in all likelihood, compared with the original layout, two separate buildings had been merged, also identifiable by the two separate roofs of different shapes (one hipped and the other pitched). Overall, the building was spread over 3 floors above ground and one basement level.

Structurally, the building was characterised by load-bearing masonry of solid bricks interspersed with portions in raw earth, with floors partly vaulted in masonry and partly with small vaults and iron joists, while the roof is composed of a timber framework and a covering of clay roof tiles. There is no widespread foundation raft, but only a widening of the load-bearing walls below the ground surface.

The inspections carried out on site had highlighted the very poor state of conservation of the building, as a consequence of the poor maintenance carried out during the years in which the building itself had remained uninhabited. In particular, the visual analysis carried out had revealed a series of problems, including:

• the presence of infiltration, problems of rising damp, leaks causing saline efflorescence and sub-efflorescence along the walls of the building;
• wide sub-horizontal cracks along the vaults of the first floor;
• wide sub-vertical cracks along the load-bearing walls of the ground floor.

Structural problems

From a structural standpoint too, the building presented several critical issues, both static and dynamic, which made it vulnerable to seismic actions.

Several walls, in fact (both along the internal and external façades of the building), showed significant cracking, attributable to the poor mechanical characteristics of the masonry, to the absence of tension-resistant elements at the floor levels (ring beams, tie-rods, etc.) and to the presence of thrusting elements (vaults and roof beams), as well as to settlement of the foundation level. The vaults also presented some problems.

In particular, those that thrust against the external walls show cracking on the intrados at the midspan. Again, it was hypothesised that these could be attributed to the poor capacity of the walls on which they rest to counteract the horizontal static thrusts generated by the vaults, thus causing the consequent relaxation of the vaults themselves.

Description of the structural strengthening project

The project, as well as being aimed at architectural renovation as required by the client, first and foremost involved a series of operations aimed at improving the static and seismic conditions of the existing building.

The main purpose, therefore, was to reduce the building's intrinsic vulnerabilities, restoring the pre-damage configuration of the deteriorated parts and also increasing the strength and/or ductility characteristics of the damaged elements.

In addition, given the previous settlements that had occurred, another significant aspect concerned the safety of the building in order to prevent future local collapses, especially differential ones.

The structural works carried out therefore involved:

• The construction of a shallow foundation, not present at the current stage, characterised by a reinforced concrete raft 0.20 m thick, fitted with a ventilated crawl space created using igloo-type formwork. The raft will be bordered by reinforced concrete ring beams 0.50 m thick, built against the existing walls of the building and reinforced with B450C improved-bond bars and stirrups. The function of these perimeter ring beams is to provide suitable support for the new columns that will flank the existing masonry walls, redistributing their loads. In order to suitably connect the new raft foundation slab to the existing perimeter walls of the building, promoting the box-like behaviour sought to achieve good performance against dynamic seismic actions, structural anchorages will be adopted, represented by injected B450C improved-bond bars 16 mm in diameter, 100 cm long and bent at 45° inside the masonry. The bars will be arranged at a spacing of 1 m from one another, after creating a suitable pre-hole of at least 22 mm in diameter, subsequently filled with structural geo-mortar such as Geocalce;
• The construction, in elevation, of a new load-bearing reinforced concrete frame structure composed of columns and beams that will flank the existing masonry walls. In particular, the building's load-bearing structure in its current state is represented by walls of solid-brick masonry interspersed with portions in raw earth. In this project, the existing masonry will be flanked by a reinforced concrete frame structure, based therefore on the so-called "cage" system, composed of vertical structural elements, which are the columns, and horizontal elements represented by beams. In particular, the columns (14 in total) will all have the same cross-section of 25×40 cm, while the beams of the first floor and the attic floor have a section of 50×24 cm (main beams) and 30×24 cm (secondary beams);
• The replacement of the existing vaulted floors or small-vault-and-joist floors, as they showed a high degree of cracking and in some cases did not guarantee the minimum height, with new composite floors (steel beams, hollow clay blocks and a reinforced concrete completion casting), capable of allowing the minimum height to be achieved in all the internal rooms. As with the raft foundation, suitable connections with the building's perimeter walls will also be made for the floor slabs, using structural anchorages represented by injected B450C improved-bond bars 12 mm in diameter, 100 cm long and bent at 45° inside the masonry. The bars will be arranged at a spacing of 1 m from one another, after creating a suitable pre-hole of at least 18 mm in diameter, subsequently filled with structural geo-mortar such as Geocalce. The stratigraphic and structural characteristics of the new floors (on the first floor and the attic floor) are composed of:

• • Main beams in S275 steel – IPE 160 section
  • Hollow clay blocks 120×60×6 cm;
  • High-density insulation – thickness 10 cm
  • Reinforced concrete completion casting – thickness 6 cm
  • Beam-slab connectors consisting of equal-leg angles 40×4 mm welded to the main beams.

• Complete reconstruction of the roof. In particular, the existing roof was split into two portions: one pitched, with an inclination of 24°, and the other hipped, with a pitch inclination of 21°. Both had a Lombard-style framework with solid timber beams of variable dimensions and a covering of clay tiles. As part of this project, the unification of the two roof portions into a single L-shaped one was planned, providing for the removal and complete replacement of the existing frameworks with new solid timber elements;
• The construction of a reinforced-masonry top ring beam, 0.20 m thick, counteracted by fibre-reinforcement strips, in order to improve the transmission of the horizontal actions, to connect the upper walls — where the masonry is less cohesive — and to improve the interactions with the roof structures, thus ensuring good box-like behaviour of the entire building.

For the calculation of the stresses and for the verification of the reinforced concrete beams, columns, floors and foundations, a finite element solver was used, employing as the calculation program: DOLMEN WIN (R), version 23 of 2023, produced, distributed and supported by CDM DOLMEN srl, based in Turin, Via Drovetti 9/F.