Effects of particle shape on shear strength of clay-gravel mixture

Abstract

Soils containing gravel-sized particles attract much less attention from researchers than clay and sands, though they may be the most commonly encountered materials in geotechnical engineering practice, especially in mountainous area. Large direct shear tests are carried out in this study to investigate the shear behavior of such soil mixtures, with emphasis on influences of shape properties (symmetry and smoothness) of gravel particles. The tested samples are prepared by mixing kaolin and gravel-sized particles (2.0 mm < d < 15 mm) of different shapes at various volumetric proportions (40, 70 and 100%). Three types of gravel are used, i.e., glass beads, river cobbles and crushed granite fragments. The applied normal stresses is 150 kPa and the shearing rate is 0.006 mm/min. Results are presented in terms of shear dilatancy, and peak and constant volume friction angle. Based on the analysis of the test results, it is found: (1) increasing gravel content increases both peak and constant volume friction angle; (2) both particle symmetry (quantified by elongation) and surface smoothness (quantified by convexity) play an important role in peak and constant volume friction, though they work in different ways; (3) increasing convexity decreases constant volume friction angle, while increases peak friction angle; and (4) increasing elongation increases constant volume friction angle, but decreases peak friction angle.