Investigating the foundation of the Amphitheatrum Flavium through the Passage of Commodus

In this work we present a non-invasive investigation of the Amphitheatrum Flavium, executed using the ground penetrating radar (GPR) technique, with the aim to improve the knowledge of the construction materials and techniques employed for building foundation. Therefore, the mail goal of this work is to achieve quantitative and reliable information for assessing the seismic vulnerability of the structure. The GPR survey was performed through the Passage of Commodus, excavated within the foundation for a length of about 60 m. GPR data were acquired on the floor and on the lateral walls, using different antenna frequencies (80, 200, 600, 900 MHz) as they combine good resolution and depth of investigation. On the floor dataset, we detected three equally-spaced anomalies related to old utilities parallel to the passageway, whose roof is located at a depth of 1 m. In addition to this, the GPR radargrams clearly highlight horizontal layers within the foundation, related to the sequential development of works at the time of construction. GPR dataset acquired on the wall allowed us to detect the thickness of the concrete covering the foundation and to locate the extensions of the structural elements underground. Outside the foundation, the passage is built using bricks, with external walls about 1 m thick. Therefore, GPR dataset revealed that the foundation of the Colosseum is a heterogeneous multi-layer element, with the presence of cavity networks and buried elements related to the plinths of the load-bearing structures. This work confirmed that foundation was built over time by means of subdivisions into small sectors, probably both in the horizontal and vertical directions.