The Project

2 October 2023

The underpassing of the Aurelian Walls at Porta Metronia

The underpassing of the Aurelian Walls at Porta Metronia

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Le Mura Aureliane, realizzate alla fine del III secolo d.C. con oltre 18 Km di estensione, un’altezza di circa 6 metri e uno spessore di 3,5 metri, rappresentano il più grande monumento di Roma e una delle maggiori imprese edilizie dell’epoca; sono state completate in tempi relativamente brevi, data l’esigenza impellente di dotare l’Urbe di un sistema difensivo adeguato contro le frequenti incursioni dei popoli germanici.

Le ricerche bibliografiche effettuate da Metro C ScpA, nell’ambito degli studi di interazione Linea-Monumenti, hanno condotto all’individuazione di alcune rilevanti e distinte fasi architettoniche che hanno caratterizzato la costruzione delle Mura:

  1. prima fase: costruzione dell’intera cinta muraria a partire dal 271 d.C;
  2. seconda fase: in seguito ai danni provocati da guerre ed incursioni straniere sono stati eseguiti importanti restauri dal punto di vista strutturale a partire dai primi anni del IV secolo d.C. fino al VI secolo d.C.;
  3. terza fase: attività di manutenzione fino ad una ristrutturazione parziale del complesso monumentale avvenuta nel VIII-XIII secolo d.C.;
  4. quarta fase: inserimento del circuito murario nella nuova conformazione cittadina a seguito di una revisione urbanistico-topografica avvenuta nel XV-XVIII secolo d.C.;
  5. quinta fase: inserimento del complesso nell’ambito di campagne di scavo e di restauro nel XIX secolo.

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Le Mura Aureliane, realizzate alla fine del III secolo d.C. con oltre 18 Km di estensione, un’altezza di circa 6 metri e uno spessore di 3,5 metri, rappresentano il più grande monumento di Roma e una delle maggiori imprese edilizie dell’epoca; sono state completate in tempi relativamente brevi, data l’esigenza impellente di dotare l’Urbe di un sistema difensivo adeguato contro le frequenti incursioni dei popoli germanici.

Le ricerche bibliografiche effettuate da Metro C ScpA, nell’ambito degli studi di interazione Linea-Monumenti, hanno condotto all’individuazione di alcune rilevanti e distinte fasi architettoniche che hanno caratterizzato la costruzione delle Mura:

  1. prima fase: costruzione dell’intera cinta muraria a partire dal 271 d.C;
  2. seconda fase: in seguito ai danni provocati da guerre ed incursioni straniere sono stati eseguiti importanti restauri dal punto di vista strutturale a partire dai primi anni del IV secolo d.C. fino al VI secolo d.C.;
  3. terza fase: attività di manutenzione fino ad una ristrutturazione parziale del complesso monumentale avvenuta nel VIII-XIII secolo d.C.;
  4. quarta fase: inserimento del circuito murario nella nuova conformazione cittadina a seguito di una revisione urbanistico-topografica avvenuta nel XV-XVIII secolo d.C.;
  5. quinta fase: inserimento del complesso nell’ambito di campagne di scavo e di restauro nel XIX secolo.

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The Aurelian Walls, built at the end of the 3rd century AD with over 18 kilometers in length, a height of about 6 meters, and a thickness of 3.5 meters, represent the largest monument in Rome and one of the greatest architectural achievements of the time. They were completed relatively quickly due to the urgent need to provide the city with an adequate defensive system against frequent incursions by Germanic peoples.

Bibliographic research conducted by Metro C ScpA, as part of the Line-Monuments interaction studies, has identified several significant and distinct architectural phases that characterized the construction of the Walls:

  1. First phase: construction of the entire city wall starting from 271 AD;
  2. Second phase: following damage caused by wars and foreign invasions, significant structural restoration work was carried out from the early 4th century AD until the 6th century AD;
  3. Third phase: maintenance activities and partial restructuring of the monumental complex occurred from the 8th to the 13th century AD;
  4. Fourth phase: integration of the city wall circuit into the new urban-topographical layout that took place in the 15th to 18th century AD;
  5. Fifth phase: inclusion of the complex in excavation and restoration campaigns in the 19th century.

The Compensation Grouting intervention consists of the following phases:

  • Drilling: Drilling holes in the ground and installing valved casings to enable subsequent soil consolidation phases.
  • Pre-treatment injection: The first injection phase is necessary to fill the larger voids in the ground where cement and bentonite mixtures can penetrate. This phase is distinct from the subsequent conditioning phase as it involves the use of different mixtures and injection parameters.
  • Conditioning injections: The primary objective of this phase is to consolidate the soil to demonstrate that the injection system is active and that controlled lifting of the structure is achievable. In this way, each subsequent injection generates an immediate response from the ground.
  • Compensation injection: Injections carried out simultaneously with activities that induce settlements to mitigate the development of settlements that could have occurred during the passage of the Tunnel Boring Machines (TBM) excavating the tunnels.

Throughout all phases of the work, geotechnical, structural, and topographic monitoring has been essential to understand the deformation response of the soil and monument structures. This monitoring has been used to calibrate both the injection quantities and the injection zone accordingly.

In the section of the Walls subject to the Compensation Grouting intervention, the following monitoring instrumentation has been installed:

  • 20 ground reference points to level the effects at ground level;
  • 4 piezometers;
  • 48 leveling brackets to measure vertical movements of the monument;
  • 74 continuous-reading mini-prisms from 4 total stations to assess monument movements in 3 directions;
  • 48 continuous-reading leveling cups to continuously measure vertical movements and instantly assess the effects of Compensation Grouting drilling and injections;
  • 7 electric clinometers to evaluate any rotations to which the monument may be subject;
  • 7 joint meters installed at various existing fissures;
  • 5 accelerometers to assess any vibrational impact on the monument resulting from tunnel excavation.
  • Prior to the Compensation Grouting intervention, a field test was conducted from inside the well, in the direction opposite to the Walls, to verify, on a real scale, both the drilling methods and the effects in terms of ground-level settlements, as well as the injection methods of consolidation mixtures based on the deformation response of the soil.

At the location of the Aurelian Walls at Porta Metronia, drilling activities for Compensation Grouting began in July 2018 and were completed in mid-October of the same year. The two Tunnel Boring Machines (TBM) passed under the Walls in March (odd track) and April (even track) 2019, respectively. The efficiency of Compensation Grouting allowed for the compensation of the entire potential subsidence basin that the excavation of the metro line tunnels could have induced on the monument’s structures. The installed monitoring equipment showed practically negligible effects during the passage of both machines. The accelerometers installed did not detect any vibrational impact throughout all phases from drilling to the passage of the two TBMs.