The Basilica of Maxentius: the Safeguard Intervention
The T3 section of the C line currently under construction runs from the Stazione San Giovanni to the Stazione Fori Imperiali. Along the route the tunnels interact with monuments of great value such as the Colosseum, the Aurelian Walls, the church of San Stefano Rotondo and the Basilica of Maxentius. To ensure the preservation of a unique heritage of the world protected by UNESCO, specific studies have been developed since 2008, aimed at analysing the possible interactions between the construction of the line and the historical and monumental heritage. The results of the interaction study developed for the Basilica of Maxentius undoubtedly represent one of the most interesting examples of the methodological approach followed in the aforementioned studies.
The Basilica of Maxentius is located on the via dei Fori Imperiali, between the Fori Imperiali and Piazza di Venezia stations. In this area the line layout is almost straight and sub-parallel to the longitudinal axis of the monument. The ground level of the Basilica is about 27 metres above sea level, while that of the via dei Fori Imperiali, below which the tunnel passes closest to the monument, is variable between 22.5 and 23.5 metres above sea level.
The difference in level between the Basilica and via dei Fori Imperiali is supported by the ancient Roman retaining wall of the Velian Hill, excavated to make room for the via dei Fori Imperiali. The foundations are of a continuous type with a width varying between 4 and 6 metres and a ground level varying between 23 metres above sea level on the Colosseum side and 13 metres above sea level on the Carinae side.
The tunnels of the Line C are sub-parallel to the facade of the Basilica, with a distance of 16 metres between the axis of the ‘pari’ tunnel closest to the monument and the monument itself in correspondence to the Colosseum side and 35 metres to that of the Carinae side. The axis of the tunnels is located at a depth of about 25.5 metres from the Stazione Fori Imperiali within the layer of recent paleotevere floods. The monument is a considerably heavy structure that transfers a very high compression load to the ground below.
The foundations are very superficial and consist of a simple extension of the bearing walls. The construction of the Basilica began in the northern area of the Roman Forum under the emperor Maxentius in 308 A.D and was completed in 312 A.D by Constantine the First after the defeat of Massenzio in the battle at Ponte Milvio. The Basilica had a rectangular plan with a dimension of 100×65 metres with the longitudinal axis oriented in an east-west direction that originally corresponded to the main axis. The internal space was divided into three naves of which today only the one on the northern side remains, which runs along the via dei Fori Imperiali.
During the preliminary studies the state of conservation of the structure was examined through a survey of the cracked and sensitive condition together with a detailed survey with cutting-edge technologies of the material characteristics and the three-dimensional geometric reconstruction of the monument. Geotechnical investigations have constituted a fundamental phase of these studies. The geotechnical characterization of the foundation soils was carried out using the results of a series of in situ and laboratory tests on the basis of which shows that the tunnels near the Basilica of Maxentius are contained in the granular layer of the medium-fine soils of the Paleotevere.
The use of the materials and the masonry technique are based on the traditional construction systems of the Roman building using the opus testaceum and the opus caementicum techniques. At the end of the preventive studies, on the basis of the investigations carried out and the analysis of the copious historical documentation found, a rather widespread situation of structural suffering of the Basilica has been highlighted. The fact that two thirds of the original monument has collapsed is a clear indication of congenital structural deficiencies.
The causes of this collapse can also be reasonably linked to the density of vertical bearing elements with respect to the covered surface that is significantly lower than that of other monuments of similar importance. The ratio of the bearing structural section to the entire covered area of the Basilica is about 12%, compared to 23% of the Pantheon and 26% of the Basilica of St. Peter. Furthermore, the original covers were in many cases exerting forces on the ancient walls. The study of the interaction between the line tunnels and the monument was carried out through numerical analyses executed in two and three-dimensional conditions. These analyses have shown that, due to the high strains on its foundations, the field displacement induced by the construction of the tunnels is negligible. In fact, with the imposed subsidence for the excavation of the tunnels we get to a maximum deformation value of the ancient walls of the same order of magnitude under its own weight. Therefore the disturbance caused to the monument is not worsened by the excavation of the line tunnels.
This conclusion, together with the current static conditions of the monument and the structural suffering presently shown in the Basilica, led to the decision to effectuate definitive safeguard interventions, to be carried out before the construction of the two line tunnels, aimed at reducing the pushing of the large barrel vaults. The structural interventions carried out on the monument starting in 2014, consist of a conservative restoration of the ancient walls by structural injections and masonry restoration together with the installation of a system of suitably preloaded metal bars, in order to reduce the unbalanced thrusts of the two large barrel vaults on the Colosseum side and the Carinae side respectively.
This last intervention was characterized by the installation of 4 levels of longitudinal and transversal Dywidag bars with a diameter of 40mm and weight varying from 40 to 60 tons for each bar and a encircling of the Constantinian apse with a couple of ropes in stainless steel.
Together with the aforementioned intervention, a series of provisional shoring was put in place which will be duly dismantled at the end of the tunnel construction. A large and complex monitoring system was designed and installed on the monument to verify the congruence between the project forecasts and its actual behaviour during the excavation of the tunnels.
Monitoring is a fundamental part of the observational method which, the way in which it operates, is an essential tool for verifying the correctness of the design concept, for the eventual refinement of the models and the design parameters-applying the methods of back analysis-and not least for the verification of the effectiveness of any project action to be made during the executive phases.
Eliano Romani – Engineering Manager