Report No. : GEO Report No. 103

Report Title : Report on the Kwun Lung Lau Landslide of 23 July 1994 (2000), 365 p.

Author : N.R. Morgenstern & Geotechnical Engineering Office

Abstract

This GEO Report comprises two volumes printed separately when first published in November 1994. Volume 1 contains the independent findings of Professor N.R. Morgenstern as to the cause of the Kwun Lung Lau landslide and the lessons to be learnt from it. Volume 2, prepared by the Geotechnical Engineering Office of the Civil Engineering Department, presents the detailed findings of the landslide investigation. The contents of Volume 2 were reviewed and agreed by Professor Morgenstern who relied on them in his own assessment given in Volume 1.

VOLUME 1 EXECUTIVE SUMMARY

A landslide occurred below Block D of Kwun Lung Lau, Kennedy Town on 23 July 1994. The incident resulted in five fatalities and three other people were injured. The Geotechnical Engineering Office (GEO) has carried out a comprehensive investigation into the cause of the landslide and Professor N.R. Morgenstern has conducted an independent review of this investigation. Both parties are in agreement as to the cause of the Kwun Lung Lau landslide.

Professor Morgenstern had observed that the Kwun Lung Lau landslide was preventable had its eventuality been foreseen. There were opportunities for various parties to have anticipated problems at Kwun Lung Lau but they were thwarted. The provision of false information with respect to retaining wall thickness on the site formation plan approved for the construction of Kwun Lung Lau misled subsequent screening studies undertaken by the GEO to assess stability of the retaining wall and the slope above it. In addition, Professor Morgenstern was of the view that consulting practice in Hong Kong with respect to the evaluation of slope stability was excessively influenced in a restricting manner by the slope catalogue used to provide an inventory of potentially hazardous locations. This practice was not sufficiently responsive to indications of potential problems at a project, estate, or development scale.

Professor Morgenstern had provided the following recommendations to the Government on the adequacy of its approach to slope safety in Hong Kong:

1. The GEO should implement a program of measuring masonry wall thickness in all cases where they had relied on the estimate of wall thickness from old drawings.
2. The Government should develop a program for direct monitoring and repair of buried services at housing estates and other developments in all cases where leakage might impact on slope stability.
3. The GEO should introduce a more integrated approach into the slope stability assessment process following the guidelines set out in the Report.
4. The GEO should undertake and support elsewhere in Hong Kong research into improved means of site characterization focussed on the factors that affect slope instability in Hong Kong.
5. The GEO should consider appointing an external Technical Review Board to assist management in enhancing technical quality improvement, keeping abreast of international standards of risk-taking and other aspects associated with the discharge of due diligence.

VOLUME 2 EXECUTIVE SUMMARY

A comprehensive investigation into the Kwun Lung Lau landslide was carried out by the Geotechnical Engineering Office (GEO). This detailed study included review of documentary information, analysis of rainfall records, interviews with witnesses to the landslide, site surveys, physical site investigations, examination of the underground drainage systems and engineering analyses. Details of the investigation and its findings are contained in this volume of the Report.

The key findings of the investigation are:

1. the landslide was triggered by soil saturation resulting from the ingress of a large volume of water into the ground behind the masonry wall, which was appreciably thinner than was usual for walls of this type,
2. leakages from defective underground water-carrying services (both stormwater drains and the foulwater sewer) within the lot boundary of Kwun Lung Lau were the principal sources of the water, and
3. the prolonged and heavy rainfall preceding the landslide played a contributory part in resulting in a significant quantity of water having entered the ground, initially through the defective stormwater drainage system.

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