Effect of Gradation on the Strength and Stress-Dilation Behavior of Coarse-Grained Soils in Drained and Undrained Triaxial Compression

Geotechnical engineering practice commonly estimates the properties and behavior of well-graded soils using methods and relationships developed based on the behavior of clean, poorly graded coarse-grained soils. The influence of gradation on the strength and stress-dilatancy behavior of coarse-grained soils is complex because it depends on the soil state, which is complicated by the use of parameters that can bias and obscure the effects of gradation (i.e., void ratio versus relative density or state parameter). This study examines the effect of gradation and particle size on the drained and undrained triaxial compression behavior of pluviated poorly graded and well-graded soils sourced and sieved from a single deposit. A series of 69 triaxial tests were performed on soil specimens with a range of initial state parameters. Wider gradations resulted in a reduction in the slope and intercept of the critical state line (CSL), whereas increasing particle size for poorly graded soils resulted in an increase in the CSL slope and intercept. The results indicate an increase in peak friction angle and maximum dilation angle for soils with wider gradations for any given state parameter. The peak drained and undrained strengths at any given state parameter are shown to increase with range of particle sizes as a result of the increase in dilative tendencies. However, the contribution of the maximum rate of dilatancy to the difference between the peak and critical state friction angles appears to be unaffected by gradation. Finally, the Bolton framework was found to underpredict the difference between peak and critical state friction angles at any given relative density.