The stability of slopes characterised by colluvium presents major challenges to developers, engineers and government authorities. The risk of slope failures is higher in areas characterised by high altitudes and high intensity and prolonged rainfalls. Unfortunately, the routine methods of geotechnical investigation and analysis normally adopted for steeply sloping sites may not be applicable, largely due to the composition and characteristics of colluvium. Practical experience and innovative engineering is therefore required. This paper presents a discussion on the available site investigation techniques for assessing colluvium slopes. The usefulness of in situ and laboratory tests is reviewed, together with the applicability of the derived soil parameters for the engineering analysis. The effects of rainfalls on the stability of a slope and precautionary measures for ensuring adequate long-term stability are discussed.


Scarcity of suitable development lands within metropolitan areas, and unique views offered by mountainous regions are among contributing factors to hillside developments in many parts of the world. Unfortunately, such regions are usually associated with potential slope failures. The risk of slope instability is significantly increased where the site is characterised by colluvium. There is a general tendency to classify sites characterised by colluvium as unsuitable for residential development. In Australia, and the Gold Coast in particular, the process of site classification by the residential slabs and footings code (AS2870, 1996) may preclude the identification of the presence of colluvium. If appropriately engineered, the presence of colluvium may not preclude development. However, there could be drilling rig accessibility problem due to site topography. Also there could be exploration difficulties associated with subsurface characteristics and material composition of the colluvium.


Lansford (1999) described colluvium as landslide debris, which has slowly accumulated, on the long slopes of mountains

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