Effect of Fines and Confining Stress on Undrained Shear Strength of Silty Sands

A silty sand is considered as a delicate composite matrix containing a sand-grain-matrix and a silt-matrix. Large strain undrained shear strength (S_us) data in triaxial compression for a particular host sand mixed with different amounts of nonplastic fines are presented. Results indicate that the intergranular void ratio, e_s, which is the void ratio of the sand-grain-matrix [given by (e+fc)/(1 −fc), where fc is the silt content fraction by weight], plays an important role on S_us of silty sands. At the same void ratio, e, a silty sand shows low S_us compared to that of the host sand. However, when compared at the same e_s, provided that it is less than the maximum void ratio of the host sand, e_max,HS, both the silty sand and the host sand show similar S_us that is fairly independent of the initial confining stress. When e_s of the silty sand is in the vicinity of or exceeds e_max,HS, the S_us depends on the initial effective confining stress. At such “loose” states, S_us normalized with respect to the initial consolidation stress, $(S_{\mathrm{us}}/{\sigma }_{\mathit{v}o}^{\prime })\text{,}$ is very low and it decreases with a further increase in e_s. At a fines content greater than about 30%, a silty sand or sandy silt is expected to behave as a silt at an interfine void ratio, e_f, defined as the void ratio of the silt-matrix (given by e/fc), unless the silty sand or sandy silt is very dense. A criterion is presented for the undrained shear strength behavior of silty sands in terms of relative percent compaction for field applications. Preliminary recommendations are presented for the choice of undrained strength for seismic stability evaluation against large deformations.