Unsaturated sand in the stability of the cuesta of the Temple of Hera (Agrigento)

Abstract

In the Valle dei Templi in Agrigento seven Doric temples stand lengthwise the crest of a rigid calcarenite cuesta over a layer of carbonate sand which lies along a thick stratum of clays. The environment is highly prone to landslides since topplings of calcarenite blocks often occur. The rock slopes are moving back and the slope edge draw near to the foundation of the Temple of Hera Lacinia contributing to increase their perilous condition. To assess the role of unsaturated sands in the instability processes, after the compositional and textural analysis of the material, direct shear tests and oedometer tests have been carried out on sand samples initially at the natural state, with low or very low values of natural water content, and finally at higher values of the saturation degree.The mechanical behaviour of sands seems one of the main causes of the instability influenced by the presence of an open metastable structure, which develops through bonding mechanisms generated via suctions and/or through cementing material such as clay or salts.

The open metastable structure of a collapsible sand: fabric and bonding

“…2a); (2) clay and silt particles at coarser particle contacts ( Fig. 2b-d) identified by Osipov and Sokolov (1994) as ion-electrostatic forces; (3) cementing agents, such as carbonates or oxides (Fig. 2e).…”

“…Ion electrostatic forces inside the crystals of laminated silicates and between separate clayey particles provide (Nocilla et al 2013) strong bonds between negatively charged structural layers through interlayer cations or water molecules. A particular characteristic of ion-electrostatic forces is their instability in relation to the effects of water (Osipov and Sokolov 1994).…”

“…2e). To the former can be added molecular forces, which are the weakest force, between clayey particles (Osipov and Sokolov 1994).…”

“…Quick settlement with increasing intensity occurs when flooding is applied together with a given vertical stress. When unloading is applied, the irreversibility of the compression is evident: this is due to the structural ''collapse'' (Nocilla 2013).…”

“…In fact, they enable rainwater to reach the underlying sands immediately, triggering further local collapse. As a consequence, conditions are produced that will result in toppling of the calcarenitic blocks, leading to the effect of a progressive cliff recession (Nocilla et al 2013;Ercoli et al 2015).…”