Report No. : GEO Report No. 132

Report Title : Report on the Investigation of Kaolin-rich Zones in Weathered Rocks in Hong Kong (2002), 75 p.

Author : S.D.G. Campbell & S. Parry

Abstract

In 1995, two large landslides, at Fei Tsui Road and at Shum Wan Road, both of which caused fatalities, were influenced by kaolin-rich zones. Following the landslides, the Geotechnical Engineering Office (GEO) carried out a series of studies to improve understanding of the origin, distribution and characteristics of kaolin-rich zones in weathered rocks, which may adversely affect slope stability. The studies, of engineering geology, clay mineralogy, shear testing and downhole geophysical techniques, were completed in 2001 and have been separately documented in reports published by the GEO. They have formed the basis of Technical Guidance Notes issued by GEO to assist in identifying kaolin-rich zones.

Kaolin-rich zones in Hong Kong comprise layers within which kaolin veins and infills occur within weathered volcanic and granitic rocks, and occasionally sedimentary rocks. The kaolin infills and veins occupy discontinuities, including joints, faults, shear zones and lithological contacts. Manganese oxide and variably kaolinized highly to completely decomposed volcanic and granitic material, are common within the kaolin-rich zones. Kaolin infills also occur in some superficial deposits.

The main process controlling the formation and redistribution of kaolin in Hong Kong is weathering. Hydrothermal effects are only locally important. With respect to vertical distribution, kaolin concentrations mainly occur in relict discontinuities in saprolite (Grades V and IV material), and kaolin is usually restricted in amount in Grade III rock or better. The thickest kaolin-rich zones and kaolin infills are associated with low-angle discontinuities dipping sub-parallel to natural slopes, especially within a few metres vertically of the weathering front, (typically the upper bounding surface of PW90/100 rock mass) particularly where overlain by PW0/30 rock mass. The main lateral controls of kaolin distribution include faults and shear zones, zones of more closely-spaced discontinuities and depressions in the weathering profile, all of which may host kaolin-rich zones.

Mineralogical evidence indicates that the kaolin comprises halloysite and kaolinite in varying proportions, and is mainly transported in solution. The kaolin varies depending on whether it formed as a result of weathering or hydrothermal processes. In the case of weathering, the kaolin further reflects the primary rock-forming minerals from which the kaolin was derived, the rate at which they altered, whether shearing occurred, and whether there was drying during kaolin formation. As kaolin infills commonly show evidence of shear deformation, there may be a relationship between shearing and kaolin concentration.

Shear strength testing of kaolin infills is commonly carried out using a direct shear box. However, obtaining representative shear strength data of kaolin infills is difficult. This is due to such factors as the limited amounts of infill, the alignment of the infill during shearing and volume changes during loading. Consequently, interpretation of the data requires considerable care. The shear strength also varies due to the morphology of the two main kaolin minerals found in Hong Kong. Kaolinite, which has a typically platy morphology, tends to align during shearing, whilst halloysite, which has a tubular morphology, does not. Hence, kaolinite typically has both a lower peak and residual shear strength than halloysite.

Of the available downhole geophysical techniques assessed, gamma density and spectral gamma ray methods can be used as supplementary ground investigation techniques to help to identify weak layers, including kaolin-rich zones.

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