Experimental and statistical analysis of the block orientation effect on the mechanical behaviour of bimrocks

Abstract

The mechanical characterization of block-in-matrix rocks (bimrocks) remains a significant challenge in geotechnical engineering, primarily due to the difficulty of obtaining undisturbed specimens. While the Volumetric Block Proportion (VBP) is a recognized controlling factor, the specific role of block orientation is not well quantified. This study employs a systematic experimental approach, augmented by Response Surface Methodology (RSM) and Analysis of Variance (ANOVA), to investigate the effect of block orientation angle relative to loading direction (α) on the uniaxial compressive strength (UCS), deformation, and failure mechanisms of bimrock. A total of 87 synthetic specimens with VBP from 0% to 50% and α from 0° to 90° were tested. Statistical analyses confirm that both VBP and α, as well as their interaction, are statistically significant for both UCS and Elastic modulus. An increase in VBP reduces UCS, though this effect reduces with increasing α. Failure analysis reveals that higher VBP increases crack density, while an increase in α from 0˚ to 90˚ forces failure paths to become more tortuous. The Elastic modulus increases with VBP, with the most pronounced stiffening effect at α = 0° (vertical alignment). This effect diminishes with an increase in α and approaches that of the pure soil for α = 90°. In contrast, Poisson’s ratio showed no systematic trend, underscoring its sensitivity to local heterogeneity of Bimrock. This research conclusively demonstrates that block orientation is a critical, independent governing parameter. Consequently, predictive models must account for the interaction between block proportion and orientation to accurately simulate bimrock behaviour.