The undrained shear strength characteristics of silty sand : an experimental study of the eff ect of fi nes

This laboratory investigation has been conducted to elucidate how the fi nes fraction affects the undrained residual shear strength and liquefaction potential of sand-silt mixtures (Algeria). A series of monotonic and cyclic undrained triaxial tests were carried out on undrained, reconstituted, saturated samples of sand with varying fi nes content ranging from 0 to 50%. These were undertaken in order to evaluate the effect of the fi nes fraction on the undrained residual shear strength and liquefaction potential of loose, medium dense, and dense sand-silt mixtures (Dr = 17%, 53%, 62% and 91%), under an initial confi ning pressure of 100 kPa. The results of the monotonic tests indicate that the stress-strain response and shear strength behaviour is controlled by the percentage of the fi nes fraction and the samples become contractive for the studied relative density (Dr = 17% and 91%). The undrained residual shear strength decreases as the gross void ratio decreases, and the fi nes content increases up to 30%. Above this level of fi nes, it decreases with increasing gross void ratio. Moreover, the undrained residual strength decreases linearly as the fi nes content and the intergranular void ratio increase. Cyclic test results show that for the studied amplitude, the increase in fi nes content leads to an acceleration of liquefaction. The liquefaction resistance decreases with the increase in gross void ratio and the loading amplitude.