Re-assessment of chemical weathering indices: case study on pyroclastic rocks of Hong Kong

Quantitative and objective characterization of weathered states of rock by means of chemical weathering indices has significant potential in ground investigation practice. This paper reviews more than thirty different chemical indices and re-assesses their variations along a weathered profile developed over pyroclastic rocks under subtropical conditions. Chemical weathering indices are principally based on the basic assumption that distributions of chemical elements as well as loss on ignition (LOI) content along the profile are mainly regulated by the degree of weathering. However, this study clearly demonstrates that the behavior of chemical elements and LOI during weathering depend not only on the degree of weathering, but also on microenvironmental conditions that control the type and abundance of clay minerals and sesquioxides and thus determine the level of heterogeneity of the profile. It is suggested that signatures of chemical weathering should be used to establish a framework for the interpretation of the heterogeneity level and the prediction of variability in engineering properties rather than for quick classifications using chemical weathering indices.