Evaluation of cliff recession in the Valle dei Templi in Agrigento (Sicily)

The sacral complex of the Valle dei Templi in Agrigento, added in the UNESCO Heritage Site List, stands over thecrest of a rigid calcarenite cuesta, which overlies a layer of partially saturated (S = 6–12%) carbonate sand. Inturn, the sand stratum lies on a thick stratumof clays. The environment is highly prone to landslides as highlightedby several previous studies that identify the undermining of slopes and the discontinuity pattern, occurred inthe late Neotectonic phase, as causes of cliff failure. For wider and less exposed areas, where the undermininglocal instabilities can be present because of the specific morphology of the site, a recent research has proposeda new interpretation of the failure general mechanism. The new hypothesis here proposed would ascribe tothe collapsible behaviour of the sand; both the failure mechanisms of the cliffs and the development of the discontinuitiesin the rock mass. A unitary framework is then proposed. A series of direct shear and oedometer testswas performed on the collapsible sand samples and values of collapse potentialwere estimated. As typical for collapsiblesoils when flooded, vertical strains occurred rapidly causing an increase of shear stresses in the uppercalcarenite, characterised by low values of yielding and low values of tensile strength. As a consequence, additionaldiscontinuities occur, volumes of rock fall and cliffs move back. Both by means of in situ surveys and theanalyses of involved rock deformability and strength characteristics, a soilmodelwas considered and a modellingprocess developed. The model results were compared with measurements of displacements as well as dimensionsof fallen blocks and consequently the comprehension of the effect of sand collapse was then possible.The identification of the new mechanism key factors allows the definition of the soil parameters that need tobe monitored. In order to prevent the cliff recession for the archaeological site, the work proposes a plan of monitoringmethods. These methods will be able to estimate the cliff recession, to localize “sensitive areas” and toquantify the parameters that could activate the instabilities.