New approaches to evaluation of chemical weathering of rock materials for geotechnical projects

Abstract

The chemical weathering of rock proceeds by water-rock interaction. Through weathering geochemically mobile elements, alkali and alkali-earth elements are easily leached from rocks. On the other hand, the residual elements with components from the atmosphere form new minerals. The principal assumption in formulating chemical weathering indices is that the behavior of chemical elements is controlled solely by the degree of weathering. The chemical indexes which have been used in previous studies to characterize the chemical weathering do not consider all the chemical weathering processes. In this study the new chemical indices, which take into account all the chemical weathering processes such as leaching and new mineral forming, are explained. These indices are chemical leaching index (CLI), chemical weathering product index (CWPI) and total chemical weathering index (TCWI). Also cation packing index can be used chemical weathering index representing weatherability, weathered state, chemical leaching and weathering product ratio. P wave-velocity is frequently used for defining the weathering grades, predicting the engineering properties of the weathered rocks and weatherability of rocks. In addition, the P-wave velocity in rock-forming minerals is greater than those in weathering products of these minerals. The P-wave velocity in rock without pores and fissures decreases with an increase in the ratio of weathered mineral/ fresh mineral as a result of the enhancement of the chemical weathering. Due to this, it can be concluded that the P wave-velocity may be used to determine chemical weathering state of rock materials. The indices given in this study are applied to the weathering profiles selected from the Kurtun granodiorite, NE Turkey, for estimation of the states of chemical weathering of rock materials