Investigation on the undrained shear strength of loose sand with added materials at various mean diameter ratios

This study aims to investigate the undrained shear strength of sand with added materials at different mean diameter ratios D₅₀/d₅₀ (D₅₀ and d₅₀ respectively represent the mean diameter of host sand and added materials) in loose state by performing a series of undrained triaxial tests. For this purpose, a very low density index of sand matrix I-Dmat equaling to 0.00 was employed and the fines of C500 silt, Illite and Speswhite with distinct mean grain sizes were respectively added into the host HN31 sand at different contents F_c (5%, 10% and 15%). For comparison and further analysis, undrained triaxial tests were also carried out on Sand-Sand mixtures, created by adding the HN31 sand into the host HN31 sand at F_c of 5%, 10%, 14% and 18.7%. Results showed that, for low value D-50/d(50) (e.g., 1 for Sand-Sand), the undrained shear strength S u increased as F_c increased, which might be attributed to the increase of occlusal friction, and in that case the sliding of host sand became difficult. By contrast, in the case of higher value D-50/d(50)(e.g., 58.33 for Sand-0500, 159.09 for Sand-Illite and 500 for Sand-Speswhite), S_u decreased as F_c increased. This was explained by the fact that, with increase of F_c, the quantity of added materials with small size increased, and in that case the sliding of host sand became easy. A critical value of D-50/d(50) (14.5) was thus deduced, which theoretically verified the negligible effect of F_c on S_u. Furthermore, for sand mixed with fines (e.g., C500 silt, Illite and Speswhite clays), F_c has an insignificant effect on the peak value of Δu, while the increase of D-50/d(50) facilitated the generation of excess pore water pressure after reaching about Δu_max). These findings provided useful information for analyzing the mechanical behaviour of sand with added materials in geotechnical practice. (author)