Report No. : GEO Report No. 233

Report Title : Detailed Study of the 21 August 2005 Debris Flow on the Natural Hillside near Fei Ngo Shan Service Reservoir (2008), 123 p. plus 1 drg.

Author : Maunsell Geotechnical Services Limited

Abstract

This report presents the findings of a detailed study of a landslide (Incident No. 2005/08/0381) that occurred on the natural hillside near the Fei Ngo Shan service reservoir, at about 6 a.m. on 21 August 2005. The major landslide involved a source volume of about 3,350 m³. The debris reached a natural drainage line further downhill and developed into a channelised debris flow. The fast moving debris flow travelled downhill along the drainage line, resulting in complete blockage of the access road to Fei Ngo Shan service reservoir and Fei Ngo Shan Road and deposited on Pak Fa Lam Road, some 290 m (plan distance) from the landslide crown. Four registered man made slopes (Nos. 11NE-B/C655, 11NE-B/F231, 11NE-B/C789 and 11NE-B/F234) were affected by the landslide/debris flow. No casualties were reported as a result of the debris flow.

The key objectives of the detailed study were to document the facts about the landslide, present relevant background information and establish the probable causes of the failure. The scope of the study comprised desk study, site reconnaissance, detailed field mapping and engineering geological mapping, ground investigation and laboratory testing, together with theoretical engineering analyses. The study also included a brief review of two smaller landslides which occurred in the vicinity of the natural hillside, which provided some insight to the detailed study of the major landslide. Recommendations for follow-up actions are presented separately.

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