Impact of gradation on the shear strength-dilation behavior of well graded sand-gravel mixtures

A total of 27 large scale direct shear tests have been conducted to investigate the shear strength and dilatancy of well graded sand-gravel mixtures. This paper focuses on the differences in behavior between three soils: sandy gravel with a specified gradation as the base soil, and two other ones with equivalent scalped or parallel gradations. Direct shear tests were conducted on three considered soils. Maximum grain size is 25.4mmin base soil, and is limited to 12.5mmfor equivalent scalped and parallel gradations. Test results are considered in terms of frictional and dilatancy contributions to the shear strength as a function of relative density and applied surcharge pressure. According to the results, the gradation of tested soils affects their shear strength by a change in maximum friction angle, which can be related to both dilatancy at failure and the constant volume friction angle. Scalped gradation is found as a better approximate gradation compared to the parallel one for determination of peak shear strength for coarse grained soils. Finally, empirical equations are developed to relate the shear strength characteristics of the base gradation to scalped and parallel ones.