Deeply weathered granitic rocks are encountered in many engineering works in Hong Kong. To date, the engineering properties and behaviour of these granites, and the soils formed from them, have been poorly related to their basic mineralogical and fabric characteristics. In this paper, a model for weathering of granites is established and, based on this, modifications are proposed to the material and mass weathering schemes commonly adopted in Hung Kong for the characterization of weathered granite in engineering uses. A number of chemical indices have been found to be good indicators of chemical weathering whereas the micro-petrographic indices can be used to characterize most fabric elements of weathered granites. Care in the testing and interpretation of test results is required for soils of granitic origin because of specific mineralogy and fabric that is developed.

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Five stages and three substages of weathering of rock material have been recognized in the granite from Hingston Down, east Cornwall, England. Changes involved in the seven stages have been characterized quantitatively by petrographic modal analysis of thin slices of the rock and determination of a micropetrographic index Ip. A second quantitative index is the microfracture index, If,.

The mineralogical and fabric characteristics of weathered volcanic rocks, which are widespread in Hong Kong, are poorly documented. Consequently, the significant variations commonly observed, even at the same locality, in engineering properties and in situ behaviour of these rocks and the resulting soils cannot be adequately related to their basic mineralogical and fabric characteristics. A model for weathering of volcanic rocks, particularly those of pyroclastic origin, is presented in this paper. Modifications, similar to those recently proposed for granites by Irfan, are proposed to the material and mass weathering schemes commonly adopted in Hong Kong for the characterization of weathered volcanic rocks in engineering use. The appropriateness of various proposed petrographic and chemical weathering indices to generally fine-grained volcanic rocks is examined. A number of modifications are proposed to the standard soil preparation and testing methods for the soils formed from tropical weathering of volcanic rocks in order to obtain meaningful and more repeatable results.