Effect of Plastic Fines on Shear Strength of Sands

Natural sands are rarely found as clean sands. They contain considerable amounts of plastic (clay) or non-plastic (silt) fines. If fines are present in sufficient quantity to separate the sand particles, soil behavior will be significantly controlled by the fines. The fabric of such a soil is referred to as floating fabric. Characterization of sand with the presence of fines is important in the design of geotechnical structures. This paper presents a detailed study focused on investigating the effect of plastic fines on the engineering properties of sand-fines mixture. Mixtures of sand and fines are prepared by mixing clean sand with plastic fines (clay). Indian Standard (IS) Grade II sand, widely known under the name of Ennore sand, is used during experimentation. This sand used is classified as poorly graded (SP) as per Unified Soil Classification System (USCS). Plastic fines are obtained locally, and the liquid limit (LL) and the plasticity index (PI) are found to be equal to 65 and 39%, respectively. The effect of plastic fines on shear strength of sands is studied using direct shear testing conducted in displacement-controlled mode. Tests are performed at 0.5 mm/min, and the samples are subjected to three normal stresses-50, 100, and 150 kPa. In addition to determining the shear strength, this study also focused on volumetric changes of specimens during shearing in a direct shear box. Tests are conducted on specimens of sand with various fines content ranging from 0 to 25%, while maintaining a constant total density of 1.7 g/cc throughout. Plots of shear stress versus horizontal displacement of lower box, and vertical displacement versus horizontal displacement are reported. The shear strength envelopes of sand specimens prepared at various percentages of fines are proposed. The test results showed that both friction angle and apparent cohesion decrease with increase in fines content in sand-fines mixture, thereby shifting the shear envelope downwards. In addition, there is a consistent decrease in peak shear strength with increase in fines content.