Investigation of an unusual landslide at Sai Kung Sai Wan Road, Sai Kung, Hong Kong

ABSTRACT

An intense rainstorm on 21 May 2016 resulted a total of 34 natural terrain landslides in the vicinity of Sai Kung Sai Wan Road in the Sai Kung East Country Park, Hong Kong. One of these landslides was a large-scale failure with a source volume of about 2100 m³. It was almost an order of magnitude larger than all the others. The debris severely eroded a road embankment, undermining the road and resulting in road closure. The nature of the landslide was unusual in that it was a structurally controlled rockslide, with a basal failure surface formed from a persistent, planar sheeting joint within columnar jointed tuff rock with a maximum depth of about 12 m, while the other landslides that occurred in the country park during the same rainstorm involved only shallow failures within the regolith. An investigation of this unusual landslide was undertaken to diagnose the probable mechanism and causes of the failure. As there was concern about further large-scale instability occurring in the adjacent hillside where the sheeting joint was inferred to extend into, the potential for further large-scale instability was assessed as part of the investigation. The key components of the investigation included detailed engineering geological mapping, ground investigation and engineering analyses. This paper presents the findings of the investigation.

KEYWORDS:

• Investigation
• unusual
• potential for further large-scale instability
• engineering analyses
• planar sheeting joint

Acknowledgments

This paper is published with the permissions of the Head of Geotechnical Engineering Office and the Director of Civil Engineering and Development of the HKSAR Government.

Additional information

Notes on contributors

Dominic O K Lo

Ir Dr Dominic O K Lo graduated from The University College London in 1984 and was awarded the University College London Exchange Postgraduate Scholarship to study at the University of Illinois where he received an M.Sc. and a Ph.D. degree. He was the first recipient of the Ralph B Peck Fellowship awarded by the University of Illinois in 1987. He worked in the New Jersey Office of Golder Associates Inc. prior to joining the Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department (CEDD), where he conducted landslide investigation, research and development in piling, quantitative risk assessment, and design methodology for landslide debris-resistant barriers. He is currently the Chief Geotechnical Engineer of the Landslip Preventive Measures Division 1 and leads a team of professional and technical staff in the management of the landslip prevention and mitigation studies and works, the selection and management of consultancies for the Landslip Prevention and Mitigation Programme, and in the implementation of the systematic landslide investigation programme.

Rachel H C Law

Ir Rachel H C Law obtained her BEng degree from The University of Hong Kong and her master degree from the Massachusetts Institute of Technology (MIT). She is the winner of the 2012 HKIE Fugro Prize. She joined the GEO, CEDD in 2001 and was responsible for research and development studies on landslides and slope safety for the past few years. Currently, she is a Senior Geotechnical Engineer in the Landslip Preventive Measures Division 1 of the GEO, and leads a team of professional and technical staff in managing a number of landslide investigation consultancies and providing technical direction in undertaking the examination of all reported landslides in the territory and in-depth landslide studies of selected significant landslides. Other areas of her recent work include slope design and geotechnical checking.

Roland C T Wai

Ir Roland C T Wai obtained his BEng degree in civil engineering and his M.Sc. degree in geotechnical engineering from The University of Hong Kong in 2005 and 2010 respectively. He has a wide range of geotechnical experience, through local and overseas projects, in slope engineering, deep excavation, site formation and foundation. He joined the GEO, CEDD in 2012 and was responsible for undertaking landslide studies, diagnosis of slope performance, management of landslide investigation consultancies and provision of geotechnical advisory services.

Axel K L Ng

Ir Dr Axel K L Ng obtained his Ph.D. in geotechnical engineering at The University of Sydney. He has over 25 years of geotechnical experience, through projects in Hong Kong and other parts of Asia; in slope engineering, deep excavation, site formation, tunnel, reclamation, foundation and landfill. His experience in landslide investigation started in 2005. At present, he is the Project Manager of the ongoing Landslide Investigation Consultancy for which he oversees the investigations of landslides in Kowloon and the New Territories between 2016 and 2018. He is currently an Executive Director of AECOM Asia Company Limited and has been a member of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) Technical Committee on Geotechnical Infrastructure.

Steven J Williamson

Steven J Williamson is a Technical Director of AECOM Asia Company Limited with over 25 years’ experience in the fields of engineering geology and geotechnical engineering. He is a Chartered Engineer and Chartered Geologist, and leads a team of professional and technical staff in undertaking a variety of geotechnical works including geohazard and risk assessment both in Hong Kong and overseas. With his extensive experience in landslide investigation going back to 1999, he has been a main contributor to several notable Landslide Investigations in Hong Kong.

Jason K S Lee

Ir Jason K S Lee is an Associate of AECOM Asia Company Limited and has been working in the field of geotechnical engineering for more than 20 years. Particularly, he has extensive experience in landslide investigation, stability assessment of slopes and slope works design. He is currently the Project Co-ordinator of the 2016–2018 Landslide Investigation Consultancy (Kowloon and New Territories) and leads a team dealing with landslide inspection, examination and overall review of landslides.

Y M Cheng

Ir Dr Y M Cheng has worked on many underground constructions, rock and soft-ground tunnelling for MTR Corporation, submerged tunnel construction for Eastern Harbour Crossing (EHC) design of Island Eastern Corridor (IEC) links, various geotechnical works and marine works before joining The Hong Kong Polytechnic University. Currently, he is the editor of HKIE Transactions and the international journal Computers and Geotechnics, as well as a member of several engineering and educational committees. He has published 168 papers and seven books, and has received best-paper awards from HKIE Transactions and Computers and Geotechnics. Currently, his research work is related to slope stability analysis, pile driving analysis, plasticity theory, innovative soil nail, distinct element analysis, Internet of Things (IOT) monitoring and cloud computing in geotechnical works. Based on the research work, several patent programmes have been developed, namely SLOPE 2000, SLOPE3D, GeoSuite, PLATE, SEEP and other programmes. Besides research works, he has also provided consultancy advice to large-scale and difficult engineering works in Hong Kong, Macau, Australia, Egypt, European countries and the Mainland China and has worked with research institutes/universities in Europe and the Mainland China consultancy firms, and contractors as well as the HKSAR and Macau Government.