Landslide Studies by the Geotechnical Engineering Office

Information Note 08/2008 May 2008Landslide Studies by the Geotechnical Engineering Office Key Message:Systematic studies of landslides by the GEO improve theunderstanding of slope failures in Hong Kong, provide new ideasfor reducing landslide risk and innovative design of slopes, andassist Government’s 10-year (2000-2010) Extended LandslipPreventive Measures (LPM) Project and the post-2010 LandslipPrevention and Mitigation Programme (LPMitP) by identifyingareas for improved practice in slope design, construction andmaintenance.IntroductionSince the early 1980s, the Geotechnical Engineering Office (GEO) has beencollecting data and conducting annual reviews of rainfall and landslides in Hong Kong.Landslides were selected for detailed study, on the basis either of their serious nature or forpurposes of advancing the understanding of landslides in Hong Kong.Professor N R Morgenstern of the University of Alberta was engaged by theGovernment to carry out an independent review of the 23 July 1994 Kwun Lung Laulandslide. One of Professor Morgenstern's recommendations was for Government tosupplement its LPM Programme by a process that involves a more integrated stabilityassessment through review of landslides. The Administration decided that, as a startingpoint, a 3-year trial implementation should be carried out, through the assistance ofconsultants, to develop a new methodology for use in long-term implementation. Thetrial was completed in 1999. A new landslide investigation (LI) methodology wasdeveloped and tested in the trial and was found to be useful. The trial also enabled theGEO to assess the resources required and balance the work programme with the resources.Starting from 2000, landslide investigation work has become part of the LPM Programmeto make the best use of the investigation results and supplement the conventional,catalogue-based slope stability assessment process. Systematic LI is also an integral partof the LPMitP, which has been launched by Government in late 2007.Benefits and Arrangement of Landslide InvestigationsThe benefits of systematic landslide investigations include:(a) identification of slopes in need of early attention under the LPMProject and LPMitP and review of earlier studies,(b) provision of forensic evidence in cases of landslides that may involvecoroner’s inquest, legal action or financial dispute,(c) provision of data for reviewing the performance of Government’sslope safety system and identifying areas for improvement, and(d) improvement in knowledge on the causes and mechanisms oflandslides in Hong Kong so as to formulate new ideas for reducing

- 2 -landslide risk and enhancing the reliability of landslidepreventive/remedial works.On average, about 300 landslides are reported to the GEO every year. Alllandslides reported to the GEO are examined and screened in order to identify cases thatwarrant follow-up study. Worthy cases are studied in detail to document the failure andrelevant background information, establish the probable causes, and identify the necessaryfollow-up actions, for example, arranging out-of-turn investigations and any necessaryupgrading works at the landslide site and any adjoining areas with generic instabilityproblems.There are dedicated resources in the GEO (i.e. Landslip Preventive MeasuresDivision 1) to coordinate the work on landslide investigations. The Landslip PreventiveMeasures Division 1 carries out an overall diagnosis of the landslide data and findingsfrom landslide studies yearly to consolidate experience and make recommendations toenhance slope engineering practice in Hong Kong.The GEO continues to undertake in-house investigations of selected landslides.For example, investigation of significant landslides may be carried out by the LandslipPreventive Measures Division 1, the Standards and Testing Division or the PlanningDivision. Input to landslide investigations is also provided by the Hong Kong GeologicalSurvey (HKGS) through provision of specialist geological advice, by the GeotechnicalProjects Division through provision of ground investigation services and by the Standardsand Testing Division through provision of laboratory testing services. However, in-houseresources are insufficient to study all the slope failures and assistance from consultants isneeded. The use of consultants also provides a mechanism for external independentreview.Two consultants have been engaged to assist in landslide investigations, withone covering Hong Kong Island and outlying islands and another covering Kowloon andthe New Territories. Both consultants can mobilise large teams of experienced anddedicated geotechnical engineers who are on standby round the clock. The investigationteams can also be backed up by overseas landslide experts where necessary. For casesinvolving conflict of interest by the concerned LI consultants, provisions have been madein the consultancy agreements for the other term LI consultants to carry out theinvestigation.Output of Landslide InvestigationsOver the years, the GEO has undertaken detailed and forensic landslideinvestigations using in-house teams. Forensic investigations completed include the fatallandslides at Baguio Villa and Kennedy Road of 8 May 1992, the fatal landslide at CheungShan Estate of 16 June 1993, the fatal landslide at Kwun Lung Lau of 23 July 1994, thefatal landslide at Castle Peak Road of 7 August 1994, and the fatal landslides at Fei TsuiRoad and Shum Wan Road of 13 August 1995.Notable examples of work completed under the LI consultancies to dateinclude about 720 landslide inspections (out of more than 2,890 records examined), about190 landslide studies, and six forensic investigations, namely the fatal landslides at KauWa Keng and Ten Thousand Buddhas' Monastery on 4 June 1997 and 2 July 1997

- 3 -respectively, the Ching Cheung Road landslide on 3 August 1997, the fatal debris flow atSham Tseng San Tsuen on 23 August 1999, the Shek Kip Mei landslide on 25 August 1999,and the fatal landslide at Fu Yung Shan Tsuen on 20 August 2005. In addition, with theassistance of the LI consultants, the GEO has carried out several major reviews of batchesof old slopes of potential concern, slopes affecting catchwaters, annual diagnostic reviewsof landslides since 1997, as well as compilation of landslide statistics and updating of thelandslide database within the GEO’s Slope Information System (SIS). In 2007, the GEOalso carried out a review of landslides involving soil-nailed slopes and made suggestionsfor enhancing the design practice.Examples of technical and system improvements arising from landslideinvestigations in the last few years include amendments to Geoguide 5 (Guide to SlopeMaintenance), input to the drafting of the Highway Slope Manual, improved guidelines forthe GEO Emergency Manual on landslide inspections, and promulgation of lessons learntto enhance the slope engineering practice through GEO Technical Guidance Notes. Areasof improved technical knowledge on landslides include slope deformation prior tolandsliding, mobility of landslide debris and mechanisms of natural terrain landslides.Reports on all forensic investigations have been published and made availableto the geotechnical profession. The detailed landslide investigations are documented in aseries of Landslide Study Reports (LSR) and Special Project Reports (SPR). Up to theend of April 2008, 135 LSR and 32 SPR on landslide investigations have been issued.These are lodged in the Civil Engineering Library and are accessible by the general public.To further promulgate the study findings, one-page summaries of selected significantlandslides, including lessons learnt, are uploaded onto the CEDD website, and selectedstudy reports are reprinted as GEO Reports, which are available at the CEDD website andfor sale through the Government Publications Centre.Prepared by Landslip Preventive Measures Division 1Geotechnical Engineering OfficeCivil Engineering and Development DepartmentMay 2008