Laboratory Study of Young Modulus of Granular Soils Using Cyclic Triaxial Test with Local Strain Measurement

Young's modulus (E) is one of the important parameters in soil mechanics that is commonly used in geotechnical projects. This parameter is very important because it directly indicates the hardness of the material. Elastic constants (Young's modulus and Poisson's ratio (ν)) are used as the main parameters of soil mechanical properties for the analysis and design in construction projects. Two common methods for extracting the Young's modulus of soil materials are the static method and the dynamic method. In the static method, the Young's modulus (E50), which is usually obtained from the stress-strain curve in element tests such as the triaxial test. The dynamic elastic modulus (Ed) can be obtained by measuring the compressional wave velocity (Vp) in situ wave propagation experiments or by using bender element in element tests. In this study, using a triaxial device, Young's modulus over a wide range of strains have been studied. The effect of grading characteristics such as gravel content, anisotropic consolidation, sample preparation method, confining stress and relative density on the Young's modulus of granular soil has been evaluated. Using local strain measurements, the Young's modulus of granular materials is investigated. In conventional triaxial machines, load plate errors and system capability effects usually limit shear strain measurements to values greater than 0.01%, although local strain measurements can be used to make accurate measurements. The materials used in the present study were 161 Firoozkooh silica sand and Mesutak gravel. Firoozkooh sand 161 has a golden yellow color and has a uniform granulation. The reconstructed specimens in this study are mainly based on the wet tamping (WT) method. It is noteworthy that in order to evaluate the fabrication method in the last stage, two samples with water deposition (WP) method and air deposition or dry precipitation (AP) method have also been studied. The results show that the Young's modulus increases at small strain levels with the addition of gravel to the host sand at a certain stress level and relative density, but its effect at large strain levels is almost negligible. On the other hand, as the confining stress increases, the effect of gravel content on the Young's modulus decreases. The results show that the Young's modulus has an increasing trend with increasing confining stress or relative density. At small strain levels, sand mixed with 50% gravel has the highest Young's modulus. The results related to the studied sand show that in this particular type of sand, sand mixed with 50% gravel has the highest degradation in hardness. This indicates that in this amount of sand, the combination of sand and gravel is such that the most slippage or rolling occurs between the grains, which further reduces the hardness. The reason for this is due to the special arrangement of sand grains in samples containing 50% gravel. In fact, in 50% gravel samples, the grains of sand are placed between the grains of sand like tiny balls, and the application of force causes the grains of sand to move and slip. Also, the results of anisotropic tests show that in the consolidated specimens under anisotropic consolidation, the Young's modulus increases with increasing the initial deviatoric stress compared to the consolidated specimens in the isotropic condition. The results show that different methods of samples preparation have a significant effect on Young's modulus.