Foreword

Landslides and debris flows pose a serious risk to human lives and the built environment in many parts of the world. In Hong Kong alone, an average of around 320 landslides occurred each year from 1945 to 2000, with significant loss of life during the decade 1970–1980. Increasing population has pushed human habitation more and more into potential hazard areas. Even in less densely populated areas, landslides and debris flows may endanger infrastructure networks vital to the community. Increasing precipitation due to climate change is likely to accelerate the occurrence of landslides. Although slope stability analysis is one of the oldest disciplines in geotechnical engineering, there is an increasing demand for new research to better understand the core fundamentals of landslide and debris flow failures, so that we can more accurately predict them in today’s changing climate conditions and are able to design more robust hazard mitigation measures. I am honoured and delighted to introduce five high-quality papers and one technical note on this subject to the readers of the HKIE Transactions theme issue on “Landslides and Debris Flow – Theory and Design, Mitigation, Stabilisation and Monitoring”. The first paper by Cheung et al. discusses the use of advanced numerical analysis to model debris flows and barrier interaction. The analyses include landslide debris mobility, the structural analysis of flexible barriers, and coupled analysis of debris impact on a flexible barrier. Verification of the numerical models has been carried out against actual landslides and impact tests with good success. The article by Kwan et al. presents a very helpful summary of the many research studies concerning technical advancements in the design of both rigid and flexible barriers. The reader can pick up helpful studies, for example, on the performance of cushioning materials for reducing dynamic impact on rigid barriers, or the important aspect of serviceability of flexible barriers when a stream crosses the barrier. Lo et al. present the results of an interesting case study of a large-scale failure that occurred following an intense storm on 21 May 2016. The investigations included detailed engineering geological mapping, ground investigation and analyses to diagnose the probable causes of failure and the likelihood of further large-scale instability. The landslide occurred in difficult mountainous terrain, which presented a major challenge for the investigators. From the many important conclusions of the paper, I would like to mention that the failurewas triggered by rain-induced cleft water pressure and that a three-dimensional slope stability assessment was needed for the complex geometry. Sze et al. introduce a new type of flexible barrier in which the shape of the valley is better accounted for, resulting in more favourable geometry and greater structural effectiveness. The design and analysis of such valley-shaped barriers are presented using a state-ofthe-art analytical force approach. The authors propose lower and upper bound characteristic design curves for the energy-dissipating devices. In the paper by Yifru et al., the performance of a screen-type debris-flow countermeasure is studied in laboratory conditions. The essence of this approach is to reduce the energy of the flow by separating the water from the debris-flow. Different screen types were studied, and an optimum screen opening width identified. Finally, Ip et al. present an interesting note on the challenges encounteredwhen building an overhead line circuit in very difficult terrain, including potential landslide and rockfall areas. I would like to express my sincere gratitude to the authors, reviewers, members of the HKIE Transactions Committee and the HKIE Secretariat for their time and dedication in launching this theme issue for the benefit of the engineering profession inHongKong and the rest of the world.