Liquefaction Behavior of Sand-Gravel Mixtures based on Cyclic Triaxial Tests

Most of the previous studies on liquefaction of soils have been concentrated on sandy soils since it was traditionally believed that only clean sandy soils are prone to liquefaction and those containing fine or coarse portions are unliquefiable. However, the evidences during past earthquakes have shown that sandy-gravel composites can also liquefy under certain circumstances. Although a number of recent laboratory studies have considered the liquefaction behavior of sand-gravel mixtures, some aspects of this behavior have not fully been understood yet. In this study, a series of undrained cyclic triaxial tests was performed to characterize the liquefaction behavior of sand-gravel mixtures with different gravel contents (GC) and relative densities (D_r). The tested soils comprised of Firoozkooh silica sand (no. 161) mixed with different amounts of a river gravel obtained from Tehran. In order to characterize the behavior of the mixtures in small strain range, shear wave velocity (V_s) of all tested specimens was measured using bender element tests. Combining all testing data, the effects of GC and D_r values on the liquefaction resistance and shear wave velocity of the samples were quantified. The tested samples had a diameter of 100 mm and a height of 200 mm and were prepared according to the wet tamping technique. The tests were performed on samples with three different D_r values of 10%, 30%, and 50% and five different GC values of 0%, 10%, 30%, 50% and 75%. In addition, for any combination of D_r and GC values, the tests were repeated with three different cyclic stress ratio (CSR) values, namely, 0.1, 0.15 and 0.2. The testing procedure included sample preparation, saturation, consolidation under a confining stress of 100 kPa, V_s measurement with bender elements and application of a cyclic deviatoric stress at the desired CSR value. The results of bender element tests show that for a constant GC value, V_s of the sample increases with its relative density. In addition, at any constant D_r, V_s increases as the GC value in the mixture increases up to 50%. In order to quantify the liquefaction behavior of the mixture, the variation of cyclic stress ratio (CSR) versus number of cyclic stress cycles (N) required to cause initial liquefaction in the samples defined by a double amplitude strain value of 5% or an excess pore pressure ratio (r_u) of 1.0 were also scrutinized. The results demonstrate that N decreases with increasing CSR values. For all GC values, as the relative density of the mixture increases, the liquefaction resistance increases accordingly. The variation of liquefaction resistance of the mixture at a constant D_r value with gravel content shows that as GC increases up to 10%, the liquefaction resistance increases as well; however, with further increasing GC to 50%, the liquefaction resistance decreases and further increase in GC up to 75% increases the liquefaction resistance of the mixture. In other words, the highest liquefaction resistance is observed for a GC value of 75%. It should be added that at higher D_r values, the effect of GC on liquefaction resistance was found to be more significant. To interpret this behavior, it was further showed that the liquefaction behavior of the mixture can be well illustrated by dividing its behavior into two distinct parts, i.e. the sand-controlled and the gravel-controlled parts. For a sand-controlled part, the interfine void ratio (e_f) can be used to characterize the liquefaction behavior of the mixture while within the gravel-controlled portion, the interangular void ratio (e_c) is better to be adopted. These two definitions of void ratios can be substituted for D_r, which was conventionally used to quantify the liquefaction behavior of the soils. Finally, the correlation between  the liquefaction resistance of the mixtures with their V_svalues was evaluated. The observations describe that for a
constant GC value, as V_s of the mixture increases, the liquefaction resistance increases as well whereas for a constant V_s value, with increasing GC up to 50%, the liquefaction resistance decreases. The reason is that in order to keep V_s constant during increasing the GC value, the sand matrix of the mixture should be kept looser and therefore, the mixture whose behavior in this case is profoundly controlled by the sand matrix, shows a lower liquefaction resistance.