Collapsible soils and instability processes in the Valle dei Templi (Agrigento)

The sacral complex of the Valle dei Templi in Agrigento, included in the UNESCO Heritage Site List, stands over the crest of a rigid calcarenite cuesta, which overlies a layer of partially saturated carbonate sand. The environment is highly prone to landslides and the calcarenite stratum is often in limit equilibrium conditions. Toppling phenomena threaten some of the more relevant archaeological structures such as the Giunone's Temple, the Vulcano's Temple, the Santuario rupestre and the palaeochristian Necropolis. As highlighted by several previous studies, the instabilities and the recession of slopes have been caused by the undermining of slopes combined with the discontinuity pattern. Recent studies have identified an open and metastable structure, typical of collapsible soils, for the sandy stratum under the calcarenite bench. Relevant vertical strains occur rapidly in the sandy collapsible layer when flooded. By means of in situ investigation, laboratory tests and numerical analyses, the active role of the sandy layer has been highlighted and a new interpretation of instability processes has been proposed.