GNGTS 2014 - Atti del 33° Convegno Nazionale

106 GNGTS 2014 S essione 3.1 On the basis of these stereonets, Markland Test (1979) allows identifying the unstable mechanisms, with respect to the internal friction angle of the rock mass. In fact, the graphical representation of the discontinuities (as poles or cyclographics), slope (as a cyclographic) and friction angle (as a circle) leads to the delineation of “critical areas” in the stereogram. Each discontinuity, whose pole falls inside of these areas, is to be considered unstable. Safety factors have been calculated, for the main unstable mechanism, by the limit equilibrium analysis, where the full shear strength is assumed to be mobilized simultaneously (Hoek et al. , 1973): 3 γ w γ w FS = ––– ( c A X + c B X ) + ( A – ––– X ) = tan ϕ A + ( B – Y ––– ) = tan ϕ B (4) γ H 2 γ 2 γ where C A and C B are the cohesive strengths of planes A and B along the base of the rock wedge; ϕ A and ϕ B are the internal friction angles of planes A and B along the base of the rock wedge; γ is the unit weight of the rock mass; γ w is the unit weight of water; H is the total height of the rock wedge; X, Y, A and B are dimensionless factors depending upon the geometry of the rock wedge and the slope. Results. The spectrum of the noise recording N1, located in the northern sector of the cultural site, shows a clear peak at a high frequency (41.88 Hz) with an amplitude of the H/V spectral ratio greater than 4. On the contrary, the spectra of the recordings N2 and N3, respectively located in the central and southern portions of the site, show two peaks at different frequencies. The first has an amplitude of the H/V ratio of about 3, at frequencies between 4 and 4.5 Hz; the other has an amplitude ranging between 2 and 3 at frequencies around 40 Hz. The comparison of the results obtained from noise measurements and the stratigraphic succession (Fig. 2) indicates the presence of one peak in the N1 spectrum, corresponding to transition between low fractured calcarenites and medium fractured calcarenites. On the other hand, spectra N2 and N3 show a low frequency peak (4 Hz) corresponding to the transition between intensely fractured and low fractured calcarenites (Fig. 2). These results are comparable with the stratigraphic succession reconstructed through mechanical drilling S1-S2-S3 (Imposa et al. , 2010, 2014). Thus, it can be stated that noise survey, followed by H/V analysis can give information on the stratigraphy (Amorosi et al. , 2008), since it allows detecting the depth of impedance contrast related to lithological transitions, if the shear wave velocity is known. Results of geostructural analysis allowed identifying 5 main discontinuity sets at SL-1 and 4 systems at SL-2. These showed a close spacing, modest aperture and slight roughness. Sometimes discontinuities are filled with soft material. Geomechanical classification, according to Bieniawski (1976), allowed the estimation of the RMR index, which classifies the rock mass as “Fair Rock” (III class). Calculated values of cohesion and internal friction angle range between 200 and 300 kPa and 25° to 35° respectively. Kinematic analysis and Markland Test (1979) showed that planar sliding, wedge sliding and toppling are to be considered potential failure mechanisms. In particular planar sliding are mainly possible at SL-1, at those discontinuities with direction matching to the slope. The main unstable mechanism, at both stations, is the wedge sliding, occurring along ��� ������������ two intersecting discontinuities, with a plunge of the intersection lower than the dip angle of the slope face. Calculated safety factors (FS) for the main failure mechanism (wedge sliding) are lower than 1, both in static and pseudostatic conditions, highlighting the high instability of the slope. It has to be noted that the pseudostatic conditions are the most reliable in this case, since the high seismic activity of the study area. Conclusions. ���� ����� ���� �� �������� ��� ������ �� ����������� �� ���� �� ��� ��������� This study aims to evaluate the degree of fracturing, as well as the stability of a rock cliff in an area with a high seismic activity. The Convent of Friars Minor, considered among the cultural heritage sites of Ispica Village, stands on the top of the cliff, which is bordered by an important tectonic alignment striking NE-SW. Several rockfalls occurred in