Issue 47

M. Marchelli et alii, Frattura ed Integrità Strutturale, 47 (2019) 437-450; DOI: 10.3221/IGF-ESIS.47.34 446 C1 C2 C3 D0 0 (A0) 0 (A0) 0 (A0) D1 1 (A0) 2 (A0) 3 (A1) D2 2 (A0) 3 (A1) 5 (A2) Table 6 : Table of the scores and the maintenance level (in square brackets) attributed to each entry of the check lists reported in Tabs. 3 and 4, depending on the level of damage and the class of importance. S TUDY CASES he residual efficiency assessment procedure herein proposed was tested in various sites located in the Northwestern Italian Alps. The campaign serves both for verifying the influence of the single damages on the overall functioning of the rockfall mitigation structures and for testing the capabilities of a matrix approach, evaluating if the proposed procedure is able to minimise the subjectivity, which is intrinsic in such man-made survey operations. The investigated structures are directly placed above local and national roads. Their selection was based on the possibility to reach the top of the drapery net. The sites are located at an altitude ranging from 300 to 1100 m a.s.l. The slope angle of the cliffs onto which the surveyed drapery nets are installed is variable: gentle slopes as well as overhanging cliffs were examined. A large heterogeneity in the characteristics of the protective structures was observed. In detail, the size of the nets is between 20 to 500 m in length and 5 and 30 m in height. As expected, various net types and damage levels were observed. In the following, the relevant aspects of the survey campaign are reported. Three study cases are illustrated (Tab. 7 details the scores attributed to each component). Fig. 5.a shows a simple drapery net system (Case A) installed in a debris slope, in which a few anchors of the top rope are unthreaded. The assigned damage level at “Top rope – Unthreading of the anchors” entry of Tab. 3 is D2 since the element is missing, i.e. at least one anchor is unthreaded. Considering the importance class of this damage, i.e. C3, the maintenance level raises up to A2, requiring urgent intervention works. This outcome of the procedure is confirmed by the fact that the system is no more capable to drive the debris at the foot of the slope in a controlled way. The spacing between two fixing points is large and the top rope is slack engendering an unsuited deformation of the wire mesh, which loses its adhesion to the slope. In this particular case, both signs of corrosion on top and bottom ropes and a significant amount of debris accumulated in the foot of the slope were observed. Fig. 5.b shows a reinforced drapery system (Case B) installed on a vertical gneiss cliff. The visual inspection reports a well- preserved anchoring system. The local damages affect the support ropes, with a small degree of corrosion, the connection between the wire mesh panels. The wire mesh is slightly corroded, with vegetation interacting with it. The global maintenance level is A1, i.e. medium plan intervention, while the global score is quite high (32). The observed situation is well represented by the outcomes of the procedure, since the system preserve a certain degree of functionality. Fig. 5.c shows a reinforced drapery system (Case C) located in a debris slope. The visual inspection reports that no corrosion problems emerge, with a global well-preserved situation. The presence of vegetation interacting with the mesh represents the only critical situation. With the adoption of the proposed method, the global maintenance level is A1, i.e. long term intervention works, and the global score is low (2). This result is in complete agreement with what observed. It is worth noting that Case A and Case B have similar overall score, meaning that the overall damage level is similar, i.e. in both cases several elements are damaged. Anyway, in Case A only the damage precludes the correct functioning of the protection system, i.e. the maintenance levels are different. As an overall comment, the driving idea of classifying the components into main categories helps in dealing with the heterogeneity of techniques within the same site. For example, different mesh types (wire mesh and cable net) are sometimes combined, see Fig. 6.a, or different anchoring devices are used. More frequently, the two investigated different drapery net systems occur in sequence, i.e. a simple and a reinforced drapery nets, as shown in Fig. 6.b. In this case, since the check list is system-dependent, two different analyses must be considered, as the influence of the local damages on the global efficiency varies. T