Report No. : GEO Report No. 13

Report Title : Mineralogical Assessment of Creep-type Instability at Two Landslip Sites (1992), 136 p. (Reprinted 1995)

Author : T.Y. Irfan


A major landslip occurred on 31st August 1988 behind the Island Road Government School, Aberdeen. The failure took place in a relatively low-angle (c. 30°) slope in weathered coarse ash tuff during the construction of landslip preventive works. A detailed study of the landslip revealed that creeping of the upper soil had taken place in part of the slope and that a well-defined shear plane had developed prior to the commencement of investigation or preventive works.

This report documents the findings of this study. It also covers a similar but less intensive study of a slow-moving landslip in weathered sedimentary rocks and colluvium at Ch. 8 + 200 on the Tai Lam Chung Catchwater, Tsuen Wan.

The case histories studied illustrate that creep-type movements can occur in soils formed from the insitu weathering of volcanic and sedimentary rocks as well as other clayey soils. These creep-type movements, and discrete shear planes formed as a result of creep, can be contributory factors in slope failures, but can go undetected in conventional detailed ground investigation. It is recommended that a mineralogical and microfabric study should be undertaken in cases where it is suspected that creep has contributed to a slope failure or where creep movements are occurring in an existing slope.

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