The use of chemical and textural indices to predict geotechnical properties of granites with different degrees of weathering

Predicting changes in rocks’ physical and mechanical properties due to weathering progress is among the most challenging scientific topics in engineering geology. This study investigated the role of weathering process in changing the geotechnical characteristics of some samples with different weathering degrees from the Shirkuh batholith (west of Yazd, Iran). Based on the results of XRF analysis, some chemical weathering indices were calculated for the samples, and their correlations with physical and mechanical properties were studied. The results indicated that the chemical indices defined based on mobile oxides such as CaO, K 2 O, and Na 2 O show a good correlation (0.98 > R 2 > 0.70) with the geotechnical properties. Also, new mechanical and chemical weathering indices were defined based on the textural characteristics of the rock samples. The mechanical weathering index (MWI), which indicates the degree of quartz fragmentation, showed more ability to estimate geotechnical properties than the chemical weathering index (ChWI), which defines the degree of sericitization of feldspars. In addition, MWI showed strong correlations with the mechanical properties with R 2 values more than 0.80, while the correlations were poor to fair for ChWI ( R 2 < 0.60). Moreover, REE/Al showed a relatively meaningful trend with the progress of weathering; however, due to the dependence of this ratio on factors other than weathering, its use is not recommended to explain the weathering process. Furthermore, some elements and oxides (e.g., CaO, P 2 O 5 , K 2 O, Na 2 O, V, Sr, and Ni) showed good potential to predict the weathering degree.