| Author: | He, Jianwei |
| Title: | Second-order analysis and design of flexible barrier system against rockfalls and debris flows |
| Advisors: | Chan, Siu-lai (CEE) Liu, Yao-peng (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Landslides -- Safety measures Debris avalanches -- Safety measures Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 241 pages : color illustrations |
| Language: | English |
| Abstract: | Nowadays, flexible barrier systems (FBSs) have been widely used to intercept rockfalls and debris flows, protecting the public and the infrastructures within the range of threat. Generally, they consist of cable ropes, wire nets, steel posts and energy dissipation devices with a foundation system. Compared to traditional massive interception structures like concrete rigid barriers, FBSs are environmentally friendly and cost-effectively. They are lightweight but can absorb high impact energy from rockfalls and debris flows by use of their flexibility. During the impact process, FBSs exhibit highly nonlinear behaviours such as the sliding motions of cable ropes and wire nets, and the large deflection of the system. As the structural behaviours of FBSs are different from the traditional rigid structures, the conventional finite elements are not able to capture the features of this kind of structures. In addition, there are few design guidelines for the analysis and design of FBSs, which are mainly relied on the full-scale tests. In this project, several finite elements are developed to form a solid foundation of second-order nonlinear analysis and design of FBSs. To simulate the distinctive features of FBSs, a sliding cable element is proposed to capture the multi-node sliding effect of cable ropes in both taut and slack states. The catenary nature of cables is maintained in the proposed cable element, in which the slipping distances of the intermediate nodes are adopted as variables in the computation. Besides, a ring element is developed to model the force-displacement behaviour of wire rings in the cable net under tension. For a typical wire ring, the axial behaviour of its curved segment is modelled by the proposed sliding cable element, while the bending behaviour is represented by using an equivalent stiffness. The performance and accuracy of the proposed elements have been validated through extensive verification examples. With the development of new finite elements, an energy approach is proposed to significantly simplify the analysis of FBSs subjected to dynamic impact from rockfalls for practical design purpose. In this approach, some principal design criteria such as the energy conversion and the deformation limit of FBSs are established. In addition, a load pattern is proposed to represent the impact load from the boulder on a FBS. This energy approach is validated via a full-scale test on a typical FBS. The simulation results from the proposed energy approach agree well with the full-scale test. By using the proposed analysis and design method for FBSs, a new FBS different from the conventional cable-supported structural form is developed in order to improve the stability of steel posts and increase the energy-absorbing capacity. Unlike the traditional system using upslope cables as supporting system, the new system adopts triangular steel frames at the downslope side. The energy-absorbing capacity of the new system is 5000 kJ, which is much higher than many FBS products in the industry. The full-scale test shows that the new system with a specific layout can resist the design impact energy. The test results such as the cable forces and the maximum system deflection can match the design values predicted by the proposed energy approach. Finally, a FBS with a nominal energy capacity of 2000 kJ, which has been verified by a full-scale rockfall test, is adopted in this project for the assessment of the retention capacity against debris flows. A coupled analysis model is explored to explicitly simulate the debris flows and the FBS so that their interaction can be captured for economical design or safety assessment of the FBS. The simulation results reveal that the system can successfully retain the debris volume with the same design energy of 2000 kJ but with different frontal velocities. The debris flows in both the high and low-velocity cases are intercepted and accumulate in front of the FBS. From this study, a FBS originally designed for rockfall-resisting can be scientifically assessed for its extended use against debris flows by advanced numerical analysis without the need of full-scale tests. |
| Rights: | All rights reserved |
| Access: | open access |
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