GPR and Ultrasonic surveys to investigate the deterioration of roman mosaics: a case study of the house of Venus, Volubilis, Morocco

Volubilis was founded in the third century B.C. and considered to be the largest city on the fringe of the Roman empire. Recently listed as a world cultural heritage by UNESCO, it is one of the most important archaeological sites in Morocco. Volubilis is renowned for its large number of mosaic floors throughout the site. Among the major mosaics of Volubilis, and undoubtedly the most remarkable ones given their aesthetic and the variety of themes they hold are found in the house of Venus. These mosaics have been restored between the 1940s and 1950s but unfortunately, most of these restorations are past due or were not properly done and are triggering the problems seen today. The performed restoration has consisted of replacing the pavement statumen with a reinforced concrete slab. Today, these mosaics are showing floor dropping, cracking, and separation from the wall. Deformation and collapse of these mosaic floors can be triggered by the fact that some of these concrete slabs were laid directly on clay soils, concrete rebars may have been corroded. To help mosaic floor restorers in their interventions, a good knowledge of how mosaic floors were constructed, structural behavior of concrete, and swelling and shrinking of clay soils beneath these floors is required. The objective of this study is to integrate multiple techniques including photogrammetry, Ultrasonic (US) velocity survey, and Ground-penetrating radar (GPR) to examine the state of deterioration of these mosaic floors. The study focuses on the complementarity of these techniques, explains why the concrete slabs are sinking, how void spaces were formed beneath the mosaic floors, and the relationship of these factors to cracking visible at the surface. The adopted methodology showed that the GPR and US are in perfect concurrence, showing how settling of the concrete slab is directly related to the presence of void space beneath the mosaic and the cracking visible on the surface.