Report No. : GEO Report No. 125

Report Title : Assessment of Global Landslide Risk Posed by Pre-1978 Man-made Slope Features: Risk Reduction from 1977 to 2000 Achieved by the LPM Programme (2002), 63 p.

Author : W.M. Cheung & Y.K. Shiu

Abstract

The technique of Quantitative Risk Assessment has been applied to evaluate the performance of the Government's Landslip Preventive Measures (LPM) Programme since the establishment of the Geotechnical Control Office (renamed as the Geotechnical Engineering Office (GEO) in 1991) in 1977. The components of this study are:

1. determination of the global landslide risk in 1977 arising from pre-1978 man-made slope features registered in the GEO's New Catalogue of Slopes;
2. determination of the reduction in global landslide risk from 1977 up to the end of September 2000 attributed to LPM action on the selected slope features; and
3. determination of the percentage risk reduction from 1977 to 2000 attributed to the LPM Programme.

This study deals with landslide risk reduction and risk minimisation attributable to the LPM Programme only. Risk reduction and risk minimisation due to other actions such as squatter clearance under the Non-development Clearance Programme, issue of Landslip Warning, slope removal or upgrading under public works and private development projects, advisory services on land-use planning and checking of new slopes, etc, are not examined. Also, this study focuses only on risk-to-life. Other types of risk such as economic risk and social impact are not considered.

The approach of global landslide risk assessment adopted in this study includes the determination of the frequencies of landslides and analysis of the potential consequences of landslides in terms of fatalities. The frequencies of landslides are estimated from historical records on failures of man-made slopes reported to the GEO during the period from 1984 to 1998. The consequence analysis considers the characteristics of the slope feature, the distance of the affected facilities from the slope feature, the size of the landslide and the vulnerability of the affected facilities. In assessing the global landslide risk, the residual risk of those slope features that have been upgraded under LPM Programme and confirmed to meet the required safety standards is accounted for. Sensitivity analysis is also carried out.

The results of the risk assessment are expressed in terms of Potential Loss of Life (PLL). F-N curves have also been derived. The risk assessment indicates that about 90% of the landslide risk is attributed to 'major' failures (i.e. failure with a volume >= 50 m³). The results of this study also show that the LPM Programme has been effective in reducing landslide risk in Hong Kong. Up to the end of September 2000, it is estimated that the global landslide risk has been reduced to about 50% of the level in 1977.

The landslide risk results (both PLL and F-N curves) presented in this Report are not true risk values in the absolute sense due to a number of uncertainties. As such, they should only be used for comparative purposes. The uncertainties affecting the calculated risk values can be divided into two categories, viz. parameter uncertainty and model uncertainty. These uncertainties are briefly discussed in the Report.

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