Investigating the Effect of Sample Preparation Method and Plastic Fines Content on the Behavior of Sands

Previous investigation has been shown that there are several sample preparation method to model the sand or silty sand behaviors. Methods like Dry Funnel Deposition (DFD), Water Sedimentation (WS), Slurry Deposition (SD), Water Pluviation (WP), Moist Tamping (MT) and Air Pluviation (AP) are some of the mentioned methods. Each method may be useful to model the behavior of some special deposited soils. For example, DFD is the best method to model the natural deposition of silty sands, while the WS method is suitable for modeling the sands natural deposition in the rivers. MT method is the best method to simulate the behavior of compacted embankments while SD method is suitable to prevent the segregation of particles in respect to other methods like Water Pluviation. Effect of sample preparation methods on the sands and silty sands behavior is investigated to some extent, while the clayey sands are rarely investigated. In order to investigate about the effect of sample preparation method and also clay content on the behavior of sands, some specimens were prepared using air pluviation and moist tamping method and tested under undrain consolidated condition using a monotonic triaxial device. To have uniform specimens with good repetition in sample preparation, a new sample preparation device using air pluviation method was invented by the authors. Specimens of clean and clayey sand mixtures were prepared using different drop height and flow rates. The dry unit weight of all specimens was increased with increase of drop height, while increase of flow rate leads to the decrease of dry unit weight. Also, some specimens were prepared using moist tamping method in order to compare the results. A modified Ladd technique was applied to have the uniform specimens in the moist tamping method. In modified Ladd technique (Under compaction method), the initial layers are compacted with lower density, while they reach the desired density during the taping of the upper layers. This method prevents the more compacted lower layers, which is usually occurred in the usual moist tamping method. Based on the results, it was concluded that the air pluviated specimens usually behave in strain hardening manner with dilative properties, while moist tamped specimens tends to behave in strain softening manner. In addition, some valuable results are presented about the variation of liquefaction potential. It was seen that the steady state line of sand mixed with 10 percent clay is lower than the steady state line of clean sand in the e-Lnp’ plan, which is the sign of more liquefaction potential in clayey sand mixture. Also, the internal friction angle was found to be lower in clayey sand mixture in respect to clean sand. The interpretation of the results shows that two interaction manner can be distinguished between sand and clay particles; one of them is easy slippage between sand particles due to locating clay particles between them and the other is increasing the friction and cohesion between sand particles due to plasticity of clay particles. The latter will decrease the slippage between sand particles. Effect of each interaction is investigated separately using the obtained results.