Experimental Study of Undrained Shear Strength of Silty Sand: Effect of Fines and Gradation

Two strong earthquakes occurred in the region of Chlef (north western part of Algeria) during the last century. From the geological context, there were several great masses of sandy soil ejections on to the ground surface level and severe damages to civil and hydraulic structures. These damages were due to the soil liquefaction phenomenon. The objective of this laboratory investigation is to study the effect of low plastic fines and gradation characteristics on the undrained shear strength (liquefaction resistance) response of sand-silt mixture samples. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with different fines content ranging from 0 to 50 % at two initial relative densities (D_r = 20 and 91 %). The initial confining pressure was kept at 100 kPa. The evaluation of the data indicates that the undrained shear strength at the peak (q_peak) can be correlated to the undrained residual strength (S_us), the excess pore pressure (Δu), the fines content (F_c) and the intergranular void ratio (e_s). The test results indicate also that the undrained shear strength at the peak decreases with the increment of the coefficient of uniformity and fines content as well as with the decrement of the mean grain size in the range of 0–50 % fines content for both relative densities (D_r = 20 and 91 %).