Abstract: Gravelly soils formed at the base of mountains exhibit a coarse-grained orientation due to multi-stage water scouring, complicating the understanding of their mechanical behavior. Existing research indicates a strong correlation between coarse-grained orientation and the mechanical properties of gravelly soils; however, a systematic understanding of these effects remains lacking. To clarify the effect of coarse-grained orientation, we considered typical field gravelly soils with a coarse-grain content of 30% and cohesion in the matrix soil. A series of large-scale indoor triaxial tests was conducted to analyze the nonlinear variation of shear strength and its parameters. The results indicated that (1) the coarse-grained oriented structure attenuated the shear strength of clayey matrix gravelly soil by 5.99% to 24.88%. When the orientation angle of coarse grains is no more than 45°, the shear strength of gravelly soil under various confining pressures decreases most significantly at an angle of 0°, and the attenuation effect of coarse particle orientation weakens as confining pressure increases. When the orientation angle exceeds 45°, the influence of coarse-grained orientation becomes negligible. (2) The coarse-grained oriented structure attenuateds the shear strength of sandy matrix gravelly soil by 11.84% to 20.46%. When the orientation angle is less than 45°, the shear strength decreases almost linearly, reaching its minimum at 45° under various confining pressures. When the orientation angle exceeds 45°, the shear strength increases with the angle. (3) By examining the development of the main shear plane and the rotation of coarse particles, we analyzed the effect of coarse-grained directional structure on the shear strength of gravelly soil. It was found that the shear strength of coarse-grained directional gravelly soil in cohesive and sandy matrices is primarily influenced by the rotation of coarse particles and the development of the main shear plane, respectively. Furthermore, the influence of coarse-grained directional structure on sandy matrix gravelly soil was found to be more significant than on cohesive matrix gravelly soil. Additionally, we analyzed the reasons for the differences in matrix regulation. We modified the Mohr-Coulomb equations for gravelly soils with coarse-grained oriented structures under different matrix conditions by introducing orientation angles. The conclusions of this study will serve as a reference for the scientific assessment of stability in mountain front slopes or sites.

Key words: gravelly soil, large-scale triaxial test, coarse grain orientation, matrix regulation, shear strength