Report No. : GEO Report No. 78

Report Title : Report on the Ching Cheung Road Landslide of 3 August 1997 (1998), 142 p. (Bilingual)

Author : Halcrow Asia Partnership Ltd.

Abstract

On 3 August 1997, a landslide occurred on cut slope No. 11NW-A/C55 adjacent to Ching Cheung Road. The landslide debris completely blocked a 50 m section of Ching Cheung Road, trapping a private car travelling on the westbound carriageway. The driver of the car was uninjured. The landslide was the final stage of a progressive process of failure and collapse that began on 7 July 1997 in cut slope No. 11NW-A/C55.

A comprehensive investigation into the landslide was carried out for the Geotechnical Engineering Office (GEO) during the period August 1997 to February 1998 by GEO's landslide investigation consultants, Halcrow Asia Partnership Ltd. This detailed study included review of documentary information, analysis of rainfall records, interviews with witnesses to the landslide, site survey, ground investigation, theoretical stability analyses and diagnosis of the causes of failure.

The investigation concluded that the landslide of 3 August 1997 was probably the result of the progressive deterioration of the slope following an initial failure on 7 July 1997. The early movements caused disruption of surface drainage channels and ground deformation thereby allowing surface water infiltration during later intense rainstorms which contributed to the subsequent collapses.

The investigation has identified a long history of instability on the slope following initial large-scale failures caused by quarrying below the site from before 1924 up to 1954. The failed mass has remained largely in place throughout the subsequent history of the slope and indeed is still there.

The previous failures at the site may have contributed to the 1997 landslide, which is shown from photogrammetric analysis to be largely coincident in plan area with failures seen in 1954 and 1963 aerial photographs. It is concluded, however, that the main cause of the 1997 failure was the adversity of groundwater conditions which developed following extremely severe rainfall.

A high, transient perched water table probably developed prior to the failure, particularly beneath a natural drainage line at the head of the failed slope. The natural subsurface groundwater pathway seems to be largely controlled by a system of natural pipes, associated partly with decomposed basalt dykes within the local granite, but the pipes may also be exploiting zones of previous disturbance. A system of raking drains, which was installed to reduce groundwater pressures following a failure at the same location in 1972, was unable to prevent the development of critical water pressures, which caused the 1997 landslide.

Details of the investigation and its findings are given in this report on the landslide

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