The paper analyze the efficiency of rock fall protection fences subjected to the impact of debris flow. Often debris flow protection systems are designed as if it were subjected to a rock fall phenomena, this fact implies strong simplification hypothesis with possible over estimation of fence working rate. For this reason a real debris flow case occurred on the North West Italian Alps has been studied in this work, comparing the different working mechanisms schematized in the case of rock fall impact with those of a debris flow. Analytical methods were first applied for parametrical analysis dedicated to the influence of the rock debris grain size distribution and water content. Obtained results were compared with in situ observations of the damaged fences. The debris flow detachment area has been investigated as well, in order to obtain data regarding the volumes involved as well as the trajectories overtaken by the debris. Finally a 3D FEM was developed to analyze the stresses induced by a concentrated mass simulating the rock fall case and an incremental mass with a certain growing law to simulate the debris flow case. Quantitative comparison concludes the study.
The phenomenon, that has been considered throughout this study, took place in May 2008 in North West Italy (Valsavarenche, AO) after a long lasting raining period. The area that has been studied in this work was characterized by the detachment of material that, after a track of 160–180m and a change in level of 150m impacted with the protective fence. The first part of the slope is characterized by the rock outcrop and it has an inclination of about 55° while the lower part of the slope is made of debris and has a more gentle dip of about 40–42°.