Report No. : GEO Report No. 86

Report Title : Report of the Independent Review Panel on Fill Slopes (1999), 36 p. (Reprinted, 2002)

Author : J.L. Knill, P. Lumb, S. Mackey, V.F.B. de Mello, N.R. Morgenstern & B.G. Richards

Abstract

The Report of the Independent Review Panel on Fill Slopes was originally prepared following the Sau Mau Ping disaster in 1976. It is now re-published as GEO Report No. 86. A summary of the Report is given below.

Slope failures in natural and artificial slopes during periods of heavy rainfall are not unusual in Hong Kong. However, at Sau mau Ping major failures have repeatedly occurred in fill slopes within the same estate. Seventy-one persons were killed in a mud avalanche in June 1972 and this disaster was reported on by a Commission of Inquiry. On 25 August 1976 a further fill slope failed, burying the ground floor of Block 9 and killing eighteen people. These failures resulted from the development of a seepage condition within a wetted zone as water penetrated into the face of the slope. Consequent loss of strength in the fill resulted in downhill movement and an almost instantaneous conversion of the slope into a mud avalanche with considerable destructive energy. Investigations have demonstrated that the face of the slope at Sau Mau Ping was formed by end-tipping of fill in a loose condition. In such circumstances, the wetted-zone can develop to a significant depth relatively quickly within the duration of heavy rainstorms such as occur every four or five years in Hong Kong. A limited study of other fill slopes in public housing areas in Hong Kong has demonstrated the presence of loose materials forming the slope surfaces. It is apparent, therefore, that the fill conditions at Sau Mau Ping are not unique, nor are the intense rainfall conditions rare. Although adequate specifications for the construction of fills have been available in Hong Kong since at least 1966, there is little evidence to suggest that these specifications have been applied to any significant extent in slopes. It is the view of the Panel that construction of fill slopes using inadequate compaction, and involving end-tipping, may have been general in Hong Kong. Thus, conditions such as those at Sau Mau Ping may be widespread; all fill slopes may be composed of loose material; and so are suspect. A detailed programme of investigation to identify such slopes is required as a matter of urgency. Potentially unstable fill slopes, once identified, can be stabilized satisfactorily by re-compacting a surface layer of fill, of a minimum thickness of 3 m, to a slope of 1 on 1.5, and providing suitable drainage, surface protection, and safeguards against leaking services. In view of the probable widespread occurrence of potentially unstable slopes in Hong Kong, in both public housing and other areas, and the need to ensure that proper standards of placement of fill are adopted in the future, the Panel is of the view that a special control organisation, with appropriate powers, should be formed within the Government.

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