Evaluation of dynamic properties of a sandy soil using cyclic triaxial test

Abstract

Dynamic soil properties and liquefaction potential are the key factors to site response analysis and soil modeling programs. The most important dynamic soil properties are cyclic shear strength, shear modulus, and soil damping. Dynamic properties of soils strongly vary with the magnitude of shear strain amplitude. Proper understanding of soil response under dynamic loading is essential not only corresponding to small strain level but also at large strain level. A research has been performed to evaluate dynamic soil properties of a sandy soil deposit. For this research, soil samples have been collected from the site allocated for the Nuclear Power Plant in Ruppur, Iswardi, Bangladesh. Total eight boreholes have been conducted up to a depth of 22 m from ground surface. Standard Penetration Test (SPT) N-value at different depth range from 7 to 45 and average ground water table varies from 6.54 m to 8.80 m at different boreholes from existing ground level. Laboratory tests results have showed that mean particle size (D50), fineness modulus (FM), and fines content (FC) of the recovered samples have varied from 0.12 mm to 0.22 mm, 0.36 to 0.94, and 2.5% to 86% respectively. The minimum and maximum dry unit weight is 13.15 kNm-3 and 17.85 kNm-3 and specific gravity is 2.66. A series of undrained cyclic triaxial tests have been carried out at relative densities of 30%, 50% and 70%, and varying cyclic stress ratios. In the entire test program, harmonic cyclic load has been applied using sine wave with a frequency of 1 Hz. Cyclic triaxial tests results have revealed that the shear modulus decreases and damping ratio increases significantly with increasing shear strain amplitude. It has also been observed that the manner of decreasing shear modulus and increasing damping ratio with shear strain amplitude is almost identical irrespective of the relative density of the soil specimen. It should be noted that, higher relative density has showed slightly lower damping ratio. Relative density, cyclic stress ratio, number of loading cycles, and shear strain amplitude has profound effects on measured cyclic shear strength. From cyclic shear strength curves, it has been observed that cyclic shear strength corresponding to 20 cycles of uniform load application for relative densities of 30%, 50% and 70% are 0.15, 0.21 and 0.24 respectively. An increase in the density results in an increase in the cyclic strength of soil, thereby making it less susceptible to liquefaction. It is expected that the results obtained from this research will be useful to assess dynamic soil response analysis, potential for liquefaction analysis and soil modeling programs for the studied soils as well as similar soils in general, as this is perhaps the pioneering research work in Bangladesh. Key Words: Liquefaction, Shear modulus, Damping ratio, Sandy soil, Cyclic triaxial test