This paper presents a critical review of microfabric, physical and chemical indices for scaling and predicting weathering-induced changes in rock properties. The common goal of these indices is mainly to condense complexities to numbers that can be plotted against each other and against engineering properties or weathering grades to determine if trends exist. However, a purely statistical evaluation to establish smooth trends in such highly variable properties is deceptive. No single index is valid or remains unaffected over the whole spectrum of weathered states. Combination of indices aimed at determining grade boundaries may also be misleading unless accompanied by detailed descriptions of microfabric and mineralogical observations. The presented index classifications highlight the nature and scope of indices and provide guidance for compatible and complementary combinations. The meaning and adequacy of criteria used in classifications of residual soils and weathered rocks are reexamined from the point of view of the fundamental differences inherent in their microfabric.

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