Large-scale physical modelling study of impact mechanisms of rockfall and debris flow on a flexible barrier

Abstract

Rockfalls and debris flows in mountain areas normally carry enormous kinetic energies and cause catastrophic damages to human lives, buildings, and infrastructures in the influenced areas. As a potential mitigation countermeasure, flexible barriers have been increasingly utilized to mitigate those disastrous and frequent natural geohazards. However, it is still an open question on how to design the flexible barriers economically and effectively to withstand the impact loads. The major objectives of this study are to reveal the interaction mechanisms of rockfalls and debris flows with a flexible barrier and to improve the design approach of debris flow-resistant flexible barriers based on the interaction characteristics at different impact stages. Findings of this study will help deepen the understanding of the impact mechanisms of boulders and debris flows on a flexible barrier and improve the design of flexible barriers for debris flow mitigation. Large-scale physical modelling is adopted as the main methodology in this study considering the scale effects and the complex structures of a flexible barrier. A large-scale physical modelling facility was designed and built to perform a series of impact experiments of a single boulder, dry granular flows, and debris flows. A dynamic monitoring system was established to capture the dynamic responses of key components of a flexible barrier during impacts. Two high-speed cameras were used to trace the motions and the interactions of the impact mass with the flexible barrier. The experiment results are comprehensively presented and analysed in this thesis. The interaction between a rockfall and a flexible barrier is investigated by two impact tests using boulders with two different diameters (400 mm and 600 mm). The impact forces on the flexible ring net directly and transferred to the supporting structures are analyzed and compared based on measured data. From the comparison, a parameter named Impact Reduction Rate (IRR) is defined to quantify the impact force attenuated by the flexible ring net. It is found that the IRRs of two boulders with different diameters are both around 30%. A simple method is proposed to estimate the impact force from a rockfall on a flexible barrier considering the stiffness of the flexible barrier and the impact area of the boulder. This simple method is calibrated and verified by the measured impact forces in this study. The interaction between dry granular flows and a flexible barrier is studied by two consecutive granular flow impact tests. The motions, depositions, and impact behaviours of dry granular flows were recorded during testing and are analysed in this thesis. The impact forces on the flexible ring net directly and transferred to the supporting structures were measured during testing and are analysed and compared with values calculated using several existing simple methods. It has been found that the dynamic method can properly predict the impact force on the flexible ring net, and the hydro-static method can be used to calculate the impact force on the supporting structures. Four debris flow impact tests were conducted to investigate the interaction mechanisms of debris flows with a flexible barrier. In these tests, the man-made debris flows mixture was composed of gravel, CDG, and water. The deposition and impact behaviours of debris flows on a flexible barrier with different initial conditions are investigated from the test results of three consecutive debris flow impact tests. Another debris flow test was performed to study the interaction behaviour of a debris flow impacting, filling, and overflowing a flexible barrier in a short period (less than 1 second). Force distribution on the flexible ring net during the impact process is analysed and presented based on measured data. It is found that the impact loading from a debris flow is not uniformly distributed on the flexible ring net, and the impact pressure on the central area is much larger than that on the side areas. Based on the findings of the experiment results, a simple method is derived to calculate the impact force of a debris flow on a permeable flexible barrier considering the passing-through of slurry and small particles during the impact process. This method simplifies the debris flow as a two-phase flow: the debris phase that can be retained by the flexible barrier and the slurry phase that can pass through the flexible barrier. This simple method is verified by the data of the large-scale physical modelling tests and well-documented laboratory tests in the literature. Finally, a developed design approach for flexible barriers in debris flow mitigation is proposed. Two steps are identified in this approach: firstly, the retaining capacity of a flexible barrier is determined based on the deposition characteristics and the retaining rate of potential debris flows; secondly, a new load approach is proposed and utilized to determine the impact forces on the flexible ring net and on the supporting structures separately. In the load approach, the impact process of a debris flow is divided into three stages: the first thrust, the debris filling stage, and the overflow stage. Relevant equations are derived to calculate the impact forces at different stages based on the findings in this study. With the application of this design approach, the flexible barriers in debris flow mitigation can be designed using basic parameters and active volumes of potential debris flows in the protection area. Based on the above works, a summary of findings and conclusions are then presented. Recommendations for further studies are also suggested.