Grain size distribution and particle shape effects on shear strength of sand – gravel mixtures

This paper focuses on the grain size distribution and shape effects on shear strength of three sand - gravel mixtures from Switzerland. A total of 28 direct shear tests in a large direct shear box apparatus have been performed to investigate the strength and dilatancy of sand - gravel mixtures. A relation between the grain size characteristics, the shape and the shear resistance has been found. For each mixture, the void ratio extent (emax−emin), and angle of repose, ϕcv are determined following the standard ASTM procedure. The experimental results are analysed in terms of the frictional and dilatant contributions to the strength of mixtures as a function of the grain size distribution, shape effects and their relative density. The particle shape effects are evaluated using the criteria used by Cho et al. (2006) for natural and crushed sands. The results show the dependency of the dilation gradient on the grain size distribution and the shape of the particle. The minumun and maximum void ratios are also dependent on the grading and the shape of the particles. The shear tests are interpreted in terms of friction angle as function of the dilation angle both evaluated from the tests at peak value and at the end of the shear phase. A useful empirical equation has been developed to evaluate the friction angle at constant volume and the gradient of dilation.