Flexible barriers against rapid events: interaction with moving masses and structural behavior control

Abstract

Nowadays, hydrogeological instability is a very critical issue for our country because of its impact on population, infrastructures and economic and productive sectors. Italy, due to its particular geological, geomorphological and hydrographic conformation, is naturally predisposed to failure phenomena. After the Second World War, the intense urbanization, which took place without considering the areas where dangerous and potentially harmful hydrogeological events could occur, led to an increase of the exposed and vulnerable elements and, therefore, of risk. In addition, climate changing caused an increase of the frequency of extreme weather events and, consequently, of dangerous and destructive natural phenomena. This makes essential to use more advanced and reliable defense technologies, which are able to guarantee high performance and reliability. Flexible and permeable structures are ideal for stopping natural phenomena characterized by rapid moving masses, thanks to their high deformation capacity and their water permeability. Differently from rigid walls, they distributed the impact energy over longer impact duration and thus reduce the peak impact force (Boetticher et al. 2011). In recent years, the use of flexible barriers has become very frequent; the reason is that, compared with rigid structures, these systems can be easily installed in irregular and narrow mountain areas, with less costs for transport and lower impacts on natural environment. Rigid barriers were widely used in the past for mitigation of risk connected to several phenomena like rockfalls and debris flows. Therefore, thanks to a large engineering experience, their designed method and behavior are well known. This does not apply to flexible systems, design of which is still characterized by a high level of uncertainty. For this type of structures, which are often installed in sites difficult to reach, is essential to guarantee high performances in terms of structural efficiency and reliability, but also to have some information about their functioning over time. The latter purpose can be made possible only by automatic monitoring systems, which can register the structural changes of the barriers and send remotely the data without any direct intervention of technicians, saving time and money. This study is born in conjunction with Incofil Tech Srl, a company that, for several years, has intensified its activities into slope consolidation systems and protection against unstable rock blocks and avalanches through flexible barriers. This project was made possible also thanks to the Autonomous Province of Trento that, through Law n. 6/1999, has provided a substantial financing to support these research activities. The main objective of the present work is to improve the existing protection barriers through both the development of a new energy dissipation device, which guarantee higher structural performances, and the implementation of an innovative monitoring system, that allows to control remotely these structures after their installation. With this new monitoring technology, it will be possible to obtain information regarding the wear status of the barrier and indications about some natural phenomena in progress. The improved monitoring database deriving from data acquired from this system will be useful for planning territory safety interventions; this aspect is very important especially for public bodies, because it represents a tangible support to choose where is better to invest money for slope stabilization and when to carry out maintenance works. A widespread use of these solutions will give the possibility to have an updated slope scale mapping of the natural events occurring. In this way, the territory will be characterized by different space-time risk coefficients, allowing statistical elaborations and forecast model implementation. Flexible barriers are usually identified based on the boulder energy they can withstand. This approach is appropriated for rockfall phenomena but it is not applicable to debris flows, which are rapid mass movements composed by a mixture of solid particles of various sizes and water, generally originated from collapses (landslide, erosions etc.) and associated with extreme meteorological events. Because of their multi-phase nature, in which solid, fluid and air always interact and coexist, debris flows are a very complex phenomenon, difficult both to investigate and to simulate. Due to its velocity and unpredictability, it can cause loss of human lives and damages to environment and structures. In order to safeguard the infrastructures and prevent victims is important to understand the behavior of the protection system under the influence of these flows. For this purpose, Brighenti et al. (2013) proposed a simplified analytical model with the aim of estimating the tensions acting on the supporting cables, knowing the geometry and the mechanical features of the flexible barrier and the characteristics of the impacted debris flow. The most difficult aspect is the application of the simplified model regarded the calibration and validation of the model parameters, due to the limited amount of data available in the literature. This study presents some results obtained from laboratory tests regarded the impact of a simulated debris flow against a scaled physical model of the barrier. A granular flow composed of aggregates of known particle size was released in a channel with variable inclination and known length and height. Using this experimental setup, several tests were carried out by varying the inclination of the channel and the geometry of the barrier; the flow velocity was monitored using a PIV camera (Particle Image Velocimetry), the height of the flow was monitored using ultrasonic level sensors and the forces on ropes were recorded using load cells. In order to determine the required mechanical parameter for the barrier, a test procedure similar to the one proposed by Ferrero et al. (2015) on site was carried out in laboratory. Different static load combinations were imposed on each structural cable of the scaled barrier and the induced deformations were measured using a theodolite. This allowed studying how the load on each single cable influenced the deformation of the others. The results of this experimental study are very useful and can be taken as a good starting point for the application of the simplified analytical model for the analysis of real cases.