title: Experimental and numerical characterization of Double-Twisted hexagonal meshes for rockfall protection
titleType: Other
Creators: name: Previtali, Marco
Publisher: name: University of Dundee
Year: 2023

description: Flexible protection systems represent the most widespread and economic passive mitigation structure for rockfall hazard. Originally adapted from anti-torpedo meshes from World War 2, they have seen very little evolution in the past decades. The practical design and dimensioning for a given site are still based on arbitrary decisions and the experience of the engineer assigned to the job, while a series of guidelines for manufacturers to assess and compare barrier performance has only been released in recent years (ETAG027). However, this is only useful for comparison purposes, as the effective barrier capacity on the field appears to depend on a variety of parameters, not accounted for in the guidelines. While research on the topic is advancing, with innovative monitoring systems and sophisticated numerical models being developed, the procedures used to characterise barrier behaviour and the underlying constitutive laws for the numerical models stay the same.

In this thesis, a combination of numerical modelling and laboratory testing is used to improve the current practices in the numerical simulation of double-twisted hexagonal meshes. First, a series of Discrete Element simulations is carried out to investigate the influence of the assumptions typically adopted in literature. The numerical model is then used to quantify the energy dissipation occurring during impact tests, the load on the structural elements and the effect of repeated loading cycles. It was found that the geometry discretisation approaches typically adopted in the literature produces an overestimation of the mechanical stiffness of the system, while the effect of multiple overlapping types of mesh designs changes according to the loading conditions and rates (i.e. quasi-static vs dynamic). Depending on the impact position, the percentage of energy dissipated through different processes changes (i.e. friction, mesh plasticisation, etc..), while the direction of the loads acting on the structural elements changes even within the same test, inducing bending, compressive and tensile actions.

Next, the wire-scale barrier behaviour is characterised through a combination of laboratory testing and Finite Element modelling, in order to provide a more robust dataset for successive studies. An image analysis procedure was developed, calibrated and used to quantify wire slippage, correcting the experimental data. Overall, a much stiffer material behaviour, characterised by both necking and shearing failure modes, was observed (compared to the limited literature data available).

The numerical model was validated against the experimental results, compared to analytical and semi-analytical solutions and used to carry out hybrid tensile/bending tests, difficult to reproduce experimentally. The data was summarised in a novel contact model for DEM, developed within a macro-element framework. The model is based on isotropic plastic hardening, with a non-associated flow rule. Non-linear plasticity is integrated by means of the third-order Runge-Kutta method. The model has been implemented in the commercial code PFC3D by means of a dynamic link library, compiled in C++. In order to aid model validation, an experimental procedure was developed to obtain continuous mesh deformation through low-cost, consumer grade instruments. The procedure was validated against LIDAR data and then used to record mesh-scale barrier impact tests. Finally, the data has been compared to the numerical results obtained using the contact models presented here and those available in the literature. While the latter were also able to capture the peak displacement at the barrier centre, i.e. what is typically used for validation, only the novel model was produced a realistic deformation field. This characteristic is important because it affects the energy repartition during the impact, changing the direction and modulus of the forces acting on the structural elements.