Numerical modeling and analysis of flexible barriers subjected to the impact of rockfalls/granular flows in comparison with test data

Abstract

Rockfall and debris flow hazards have occurred before and still impose a great threat to human life, property and infrastructures all over the world due to their unpredictability and destructive power. Flexible barriers, as one of the effective retention systems among mitigation measures, have been extensively adopted in Hong Kong and many other regions and countries to intercept falling rocks and debris flows. A great number of field tests and experimental programs were carried out in the past few decades to study the dynamic behavior of flexible barrier systems. However, as the key component of a barrier system, the tensile forces developed on interception structure have rarely been well studied, particularly for the circular ring net. Meanwhile, there is a lack of a better understanding of the interaction between rockfalls/granular flows and flexible barriers. On the other hand, the numerical approaches have become a powerful and efficient tool to study the dynamic response of flexible barriers. However, the dynamic and non-uniform loading on flexible barriers induced by rockfalls/granular flows, the highly non-linear property in different structural components and the interaction mechanisms between solid and barriers collectively pose a great challenge for numerical analysis. Therefore, in view of these problems, this thesis aims to build both an innovative physical model and newly developed numerical models for studying the performance of a flexible barrier system subjected to the impact of rockfalls and granular flows. Firstly, a newly developed numerical ring model based on the discrete element method (DEM) has been built to simulate the behavior of a circular wire ring net. The mechanical behavior of a wire ring element is analyzed from measurements collected during quasi-static tensile tests. A novel systematic calibration approach on bond parameters of this ring model is elaborated based on a parametric study of tensile tests and experimental data from the literature. With calibrated DEM parameters from the tensile tests, the model reliability is assessed by reproducing tensile tests carried out on a steel ring under different boundary conditions. The capability of the new ring model is further tested to replicate the tests results of dynamic rockfall impact on a squared ring net. Secondly, a specially designed large-scale physical model testing facility has been built to study the impact mechanisms between rockfalls/granular flows and a flexible barrier and to further validate the developed DEM model. The design of the physical model is elaborated in detail. Both the single boulder impact test and dry granular flow impact test are introduced with respect to experimental setups, instrumentations and testing procedures. Thirdly, a new flexible ring net barrier model is developed based on the DEM to simulate single boulder impact tests. Different components of the barrier system such as steel strand cable and shackles are considered in the model. All the input mechanical parameters are calibrated by means of laboratory tests. The effectiveness and overall performance of the discrete element model are evaluated by comparison with the experimental results. In addition, the effects of boulder size, impact position, barrier inclination, and barrier initial slack on barrier response in a parametric study are investigated. Finally, the dry granular flow impact test is modeled with the DEM. The implementation of the secondary mesh net makes all the components of the barrier system have been fully described with limited simplification. A series of laboratory tests were carried out for model calibration including the granular material and barrier system. The capabilities of the numerical model are evaluated by comparing the results from the experimental test. A good agreement with experimental data can be found regarding to flowing velocities, net elongations, filling heights, and tensile forces developed on net. Moreover, the impact force acting on barrier is analyzed from the perspectives of dead zone mass and particle size. The results are also compared to the rigid barrier. Major conclusions and recommendations for further studies are presented in the last chapter of the thesis.