EFFECT OF GRADING CHARACTERISTICS ON THE UNDRAINED SHEAR STRENGTH OF SAND-SILT MIXTURES M. Belkhatir 1 , A. Arab 1 , N. Della 1 , H. Missoum 2 , T. Schanz 3 1 Laboratory of Materials Sciences & Environment, University of Chlef (Algeria), abelkhatir@yahoo.com 2 Laboratory of Construction, Transport and Environment Protection, University of Mostaganem (Algeria) 3 Laboratory of Foundation Engineering, Soil and Rock Mechanics, Bochum Ruhr University (Germany) ABSTRACT The liquefaction susceptibility of saturated medium sand-silt mixture samples is evaluated by monotonic undrained triaxial laboratory tests that were carried out on reconstituted specimens at various relative densities (Dr = 17 and 91%) and a constant confining pressure (σ 3 ’= 100 kPa). The test results were used to conclude on the effect of grading characteristics and other parameters on the liquefaction resistance of the sand-silt mixtures. The test results indicate that the undrained residual strength can be correlated to the coefficient of uniformity (Cu) and the average diameter (D 50 ). Indeed, it decreases linearly with the increase of the uniformity coefficient and decrease of the average diameter. It is found that a relationship between the static liquefaction resistance and any of the diameters (D 10 or D 50 ) and the coefficient of uniformity (Cu) would be more realistic than to build a relation between the coefficient of gradation (C c ) and the static liquefaction resistance. Keywords: liquefaction resistance, mean size, fines content, uniformity coefficient, gross void ratio.