Influence of weathering-induced iron precipitation on properties of sandstone in a tropical environment

Abstract

Weathering-induced iron dissolution and precipitation in rock masses lead to reduction in shear strength of discontinuities, amongst others, while precipitation leads to an increase, and precipitated iron may reduce the susceptibility to future weathering. This effect is investigated in fieldwork campaigns with detailed rock mass characterization and chemical analyses in Kota Kinabalu, Malaysia. Many cut slopes in this area consist of different zones in which iron in various forms is either dissolved or precipitated. Iron-coated and -cemented discontinuities have a marked higher shear friction, while staining has little influence on friction. Discontinuities with iron-coating and -cementing reduce the susceptibility to weathering of a rock mass as weathered material is less easily eroded and underlying material is protected from further weathering. On the other hand, dissolution zones, which are mostly found below the precipitation zones, have considerably lower rock mass quality. In engineering works the latter are invisible before excavation. Just assuming the continuation downward of the surface rock mass with precipitated iron, or, worse than this, assuming a less weathered rock mass with better characteristics, may be very erroneous. This study emphasizes that forecasting the susceptibility to weathering should include the influence of morphology, environment and climate.