Undrained monotonic shear behavior of sand mixed with a small amount of fines content

There exists no clear consensus about the effect of fines on the undrained monotonic shear behavior of mixed soil. The contribution of fines in the force transfer mechanism differs compared to sand. There have been conflicting conclusions regarding the role of fines in the undrained shear strength of mixed soil. Some preceding studies completely ignored the role of fines regarding fines as void, whereas some studies applied a factor to consider the role of fines. Therefore, a series of undrained monotonic triaxial tests have been performed on sand containing a small amount of fines content (f(c) = 0-20%) to scrutinize the effect of fines on the shear behavior of mixed soil. The test results show that the steady/critical-state lines (SSL/ CSL) for sand move downwards with increased fines content in terms of global void ratio, e. But, as the void ratio is expressed in terms of equivalent intergranular void ratio (e(c))(eq), all the SSL/CSL gets normalized. The contribution factor, b, is computed as 0.21-0.28 for plastic and non-plastic fines, respectively, via the back-analysis method. This suggests that the fines contribute differently to the force transfer mechanism of the mixed soil. Also, fine particles in sand affect the critical internal angle, M, differently. The result reveals that the critical internal angle, M, decreases for plastic fines and increases for non-plastic fines. Finally, triaxial test data from several studies have been combined with the data from this study to propose a simplified method to compute the contribution factor, b. The proposed method is based on the void size distribution of host sand and fine particle, termed grain size ratio, chi. The grain size ratio is a ratio of D-10,D-hostsand to D-50,D-fines. Finally, an empirical equation, b = 0.8883 -0.207 ln(chi), has been proposed to compute the b value.