Evaluating the combined influence of anisotropy and weathering on strength behavior of gneissic rocks in Himachal Pradesh, India: Insights from chemical weathering indices

Abstract

The present study examines the combined effect of anisotropic foliations and weathering on the strength and deformation characteristics of the three grades of weathered Himalayan gneiss (fresh, slightly, and moderately weathered) from Kullu, Himachal Pradesh, India. The study's novelty is that it links the triaxial strength of weathered gneiss with suitable chemical weathering indices (Plagioclase Index of Alteration (PIA), Chemical Index of Weathering (CIW), and Loss of Ignition (LOI)). The study reports the output of extensive experimental work containing uniaxial, triaxial compression, Brazilian tensile, and point load strength testing on three grades of weathered gneiss at five different anisotropic foliations (0°, 30°, 45°, 60°, and 90°). The observations report that anisotropic foliations and weathering affect the strength of gneissic rocks under unconfined and confined conditions. The weathering decreases gneissic rock's uniaxial, Brazilian tensile, and triaxial strengths. The U-shaped strength anisotropy curve is noticed in all grades of weathered gneiss under both uniaxial and triaxial states. The linear relations were observed between the triaxial strength and appropriate indices of chemical weathering. These relations help to obtain the strength of the anisotropic rock at different anisotropy angles using data from powder XRF. The powder XRF tests are relatively easier to execute as sample preparations have no physical dimensions restrictions. In the absence of the required number of perfect cores (specimens) of L/D ratio two, the correlations enable approximate strength assessment for rock engineering projects. Owing to these relationships, the study identifies the suitable modified Hoek-Brown anisotropic rock failure criterion that is modified to consider the effect of weathering using appropriate indices of chemical weathering for the Himalayan gneiss. The coefficient of determination (R²) was calculated to be 0.93 for the proposed modification.