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The calcarenite shows relatively high values of the
porosity [9] (Table 1) related to the low diagenetic
process, the low cementation (spathic calcite with
meniscus structures) and to the textural characters
(equi-dimensional, well sorted, loosely packed, low fine
grained matrix). The value of the compressive strength
offered in literature, and confirmed through some field
measurements with a Schmidt hammer, indicates a
weakly cemented carbonate rock.
There are three systems of faults that displace both the
Mesozoic-Tertiary and Pleistocene deposits. A very
recent tectonic activity involves Tyrrhenian sediments
and more recent continental deposits, as revealed by
kinematic indicators of NW-SE, NE-SW and W-E strike-
slip fault [10] and of differential uplift [11].
The study is focused on three bays, “Cala Rossa”,
“Cala del Bue Marino” (Figure 2) and “Cala Azzurra”,
located in the eastern side of the island. In these three
bays, the Pleistocenic biocalcarenites form some cliffs
with height ranging from few meters (Cala Azzurra and
central part of Cala Rossa) up to over 30 meters. These
areas are classified as high and very high landslide
hazard [12].
The cliffs of Cala Rossa and Cala del Bue Marino
are separated from the sea by a narrow strip of talus
deposits, whereas at Cala Azzurra a low angle slope and
a beach separate the old cliff from the present shoreline, The geomechanical investigations were focused on
assuming respectively the shape of a seasonal sea
cliffs and a coastal slope [13]. In the west side of Cala
Rossa and Cala Azzurra, this wide rock slab lies on the
plastic clays belonging to the Pliocene formation. The
contact between the two formations can be recognised
here above the sea level, while in the east side of the
two bays and in Cala del Bue Marino area the surface
FIGURE 2 Rock blocks fallen at Cala del Bue Marino
62 EAI Energia, Ambiente e Innovazione 4/2015
is presumably below sea level. The overlapping of
hard rock masses on a more plastic substratum leads
to mechanical instability due to the diverse response
of the materials to perturbations, such as seismic input,
weathering, erosion or man-made excavations [14]. The
resulting mass movements can be classified into two
different but strictly interconnected typologies: lateral
spread and rock blocks fall [15].
The good resistance brought the biocalcarenite,
improperly called “tuff”, to be extracted in several
hypogeal and open air quarries and used as building
stone. The exploitation of the Favignana sandstone
is ancient, but it reached its maximum development
mainly between 1700 and 1950. Many buildings were
constructed in Tunis with the “tuff” of Favignana, and
Messina was rebuilt with it after the 1908 earthquake.
After the World War II the “tuff” went out of the market
and the mining areas were abandoned to a degradation
process which increased the risk of block collapse.
Methodology
A geomechanical analysis and multi-system monitoring
of the cliffs of Cala Rossa, Cala del Bue Marino and Cala
Azzurra were carried out in this study between April
2012 and April 2014.
the recognition of the rock mass joint setting and of
the lithotecnical characteristics of the calcarenites in
the three coves. The geomechanical characterization
of the calcarenite was carried out via a traditional
geomechanical field survey performed according
to the ISRM standard. Twenty-five geomechanical
stations were completed in the three coves (7 in the
west side of Cala Rossa, 13 in the east side of Cala
Rossa, 3 at Cala del Bue Marino and 2 in Cala Azzurra).
Field mapping of rock discontinuities is the most
common approach for the analysis of cliffs shaped on
hard rock. This classical geomechanical investigation
allows to characterize the main joints in terms of dip,
dip direction, spacing, opening, presence of gouge
material, persistency, relationship with average slope
face orientation and other factors. The poles of the
measured joints were plotted using the “Georient”
open source software and the Schmidt equi-areal