Report No. : GEO Report No. 94

Report Title : Report on the Rock Slope Failure at Cut Slope 11NE-D/C7 along Sau Mau Ping Road on 4 December 1997 (1999), 69 p.

Author : B.N. Leung, S.C. Leung & C.A.M. Franks


On 4 December 1997, a rock slope along Sau Mau Ping Road failed. The rock slope was the lower part of a cut slope of reference 11NE-D/C7, the top portion of which had been removed under an ongoing Hong Kong Housing Authority site formation contract.

The failure completely destroyed a section of the protective fence along the toe of the slope. The failure debris blocked the entire four lanes of the Sau Mau Ping Road, covering 25 m of its length. While there was no injury, the road had to be closed for a period of 17 days, during which the failure debris was cleared, the slope inspected, the fence repaired and other works undertaken to ensure safety prior to re-opening of the road.

A comprehensive investigation of the slope failure was carried out. The investigation included a review of relevant documentary records, examination of aerial photographs, interviews with eye-witnesses to the failure, topographic survey, geological (including rock discontinuity) mapping, analysis of ground vibration monitoring data due to blasting and diagnosis of the failure.

The investigation concluded that the blasting which took place on 4.12.1997 close to the crest of the slope caused the slope failure. The location of the nearest blast area was assessed from the evidence collected. The closest distance between the blastholes in this area and the crest of the failed slope was found to be within 3 m. While this area was within the permitted blasting area limits, the amount of explosives used was found to have exceeded the permitted value stipulated in the Blasting Permit. Theoretical analysis has indicated that the blast-induced ground vibration alone could not have resulted in complete detachment of the largest rock block from the slope. However, with blasting carried out at this close distance from the slope, it is considered that the failure could have been triggered by the shock waves and gas pressures generated by the blast.

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