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Metro C: Excavations of tunnels up to Piazza Venezia completed

On the 29th of August, TBM (Tunnel Boring Machine) work on the line C tunnels between San Giovanni station and the future Piazza Venezia station was completed.  The excavation involved one of the most famous archaeological and monumental areas in the world, which has been declared a World Heritage Site by UNESCO.  As part of the design, specific line-monument interaction studies were carried out to analyse the possible interactions between the construction of Line C structures and the historical and monumental heritage.

Fig. 1 - Tunnels T3

The line-monument interaction study and the interventions carried out.

The line-monument interaction study was developed with an extremely innovative methodological approach, which made it possible to obtain fundamental information and results for the preservation of the pre-existing structures.  The studies were carried out starting from the phase of surveys and investigations through to establishing, for each monument, specific monitoring plans and safeguard interventions where necessary. They were developed in agreement with the Ministry of Cultural Heritage and Activities.

Monitoring of monuments and historical buildings and preservation measures.

The fundamental activity underlying the methodological approach is unquestionably that of surveying and investigating the state of conservation of a monument.  Subsequently, through multidisciplinary analyses and the elaboration of complex two-and three-dimensional numerical modeling, specific monitoring plans and/or safeguarding interventions were decided on for each monument and/or historic building.  Laser and optical techniques, inclinometers, electrical meters, thermometers and accelerometers were used to monitor the vibrations.  The results were entered into a database specially prepared by Metro C SpcA and available in real time to all those involved in the work.  The instrumentation was installed well in advance of the start of the potentially interfering activities – at least 12 months – making it possible to acquire the necessary reference base with respect to the variations resulting from seasonal and daily thermal cycles, verifying the presence and/or evolution of any phenomena in progress.

The main purpose of the extensive monitoring systems was to verify the deformation response of the monuments during the execution of the work, in order to guarantee the control of the job activities and to compare the acquired data with the results acheived with the models developed during the design phase.  In addition, where deemed necessary in relation to the state of conservation or as a resalut of the potential effects of the construction of the line, geotechnical and structural protection measures were implemented for the monuments.

 

N. 2 - Structural consolidation interventions Basilica di Massenzio

N. 3

Safeguard interventions can essentially be put into two categories:

  • temporary or definitive interventions implemented prior to the start of the excavation activities: these are structural consolidation interventions designed to remedy previous problems or a pronounced state of deterioration and therefore not directly correlated to the effects induced by the construction of the Line C.
  • Interventions implemented during excavation to counteract, in real time, induced subsidence by containing it below predetermined thresholds: these are geotechnical interventions such as Compensation Grouting.

N. 4 - Church of San Stefano Rotondo

N. 5

Provisional or definitive interventions.

Structural and/or geotechnical,  provisional and/or definitive – preservation works have been designed and carried out for the following monuments:

  • Aurelian walls at Porta Asinaria: consolidation and restoration, temporary shoring of the walls and construction of a bulkhead of piles to protect Porta Asinaria, and geotechnical intervention to compensate for subsidence (Compensation Grouting) during the passage of the TBM.
  • Church of Santa Maria in Domnica: consolidation and restoration, temporary internal and external shoring and interventions to safeguard the artistic heritage inside the church.
  • Church of San Stefano Rotondo: consolidation and restoration, banding of the outer drum and interventions to protect the artistic heritage inside.
  • Celimontano acqueduct pylon: consolidation, restoration and temporary shoring.
  • Basilica di Massenzio: consolidation and restoration, provisional shoring of the external walls and definitive chaining interventions with ropes and bars in stainless steel.
  • Colonnades of the Foro di Nerva: consolidation, restoration and temporary shoring.
  • Cloaca Maxima: in the section near the Via dei Fori Imperiali consolidation and restoration.

N. 6 - Church of Santa Maria in Domnica

N. 7 - Church of San Stefano Rotondo

N. 8 - Church of San Stefano Rotondo

Work carried out in the excavation phase.

This category includes interventions aimed at counteracting, in real time, any subsidence induced by the construction of the works, limiting it to below predetermined safety thresholds.

These interventions took place using “Compensation Grouting”, a technique that involves the injection of cement mixtures into the ground during tunnel excavation.  This type of intervention was carried out in order to mitigate the possible effects induced by the mechanized excavation of the line tunnels during the undercrossing of the Aurelian Walls at Porta Metronia and of the existing Colosseum Line B station.

N. 9 - Aurelian walls

 

Finally, as far as the Colosseum is concerned, the preventive analyses carried out during the design phase have shown that the effects induced by the excavation activities on the monument are substantially negligible, thus not requiring any type of consolidation and/or mitigation intervention.  However, taking into account the inestimable value of the monument in question, an extensive monitoring system was set up consisting of approximately 260 different instruments through which it was possible to verify the correspondence between the response during construction and what was envisaged in the design phase.

N. 10 Colosseum

 

Conclusions.

 The extensive monitoring system installed ensured constant control in real time of the deformation response of the soil-structure systems, making it possible to verify the parameters and hypotheses underlying the design and highlighting, through specific analysis of the data acquired, the complete absence of damage to the load-bearing structures and the artistic heritage they contain.

The verifications carried out by Metro C SpcA, with the support of the Department of Structural and Geotechnical Engineering of the “La Sapienza” University of Rome, have demonstrated the excellent consistency between the design forecasts and the acquired data.

The work carried out in the design phase has produced excellent results in terms of forecasting the possible effects induced on structures and land by excavation operations.

The restoration and consolidation work carried out, together with the provisional safe-guarding measures, have guaranteed both the complete protection of the monuments, and in some cases, their redevelopment.  The line-monument intervention studies are a virtuous example of how in-depth research and scientific orientation, at the basis of engineering decision-making processes, can ensure a high degree of prediction of the effects potentially induced by excavations in monumental contexts such as the historic centre of Rome.

Metro C S.c.p.A.’s member companies