Effect of Initial Density, Particle Shape, and Confining Stress on the Critical State Behavior of Weathered Gap-Graded Granular Soils

Weathered gap-graded soils are a common geological body in mountainous regions, and they are widely used as construction materials. The shear strength is the controlling parameter for the design of civil projects; however, there is still a controversy on the coarse fraction effect on the shear strength of gap-graded soils. To this end, 22 triaxial shear tests are performed on gap-graded soils, and the factors affecting the coarse fraction effect have been analyzed, including the confining stress, the particle shape of aggregates, and the initial density of sand matrix. Partial contacts and sand bridges between aggregates are responsible for the transmission of loading and thus affect the coarse fraction effect. The results of triaxial tests reveal: (1) The overall shear strength of sand-beads mixtures is rather independent of the confining stress and the coarse fraction, and even the volume of aggregates is as high as 44.5%. (2) The effect of the shape of aggregates is effective only at a high coarse volume fraction (44.5%) for loose-sand-gravel mixtures, where the partial contacts between aggregates play an important role in forming the interaggregate structure. (3) The overall shear strength of gap-graded soils with a denser matrix increases continuously with a rising coarse fraction. The formation of a densified sand bridge is correlated with the initial density of sand matrix, contributing to the loading transmission in the interaggregate structure and, in turn, affecting the overall critical state behavior of gap-graded soils. The insights drawn from this study provide a reference for assessing the deformation behavior of weathered residual soils.