Creep coefficient of binary sand-bentonite mixtures in oedometer testing using mixture theory

A series of oedometer tests was performed on binary sand-bentonite mixtures considering both the effect of the sand mass fraction and the initial water content of the bentonite matrix. The experimental data reveal that the influence of the initial water content of the bentonite matrix on the overall creep behavior of the mixture is negligible. However, the reference time line (corresponding to 24 h of consolidation) is significantly affected by both the initial water content and the sand mass fraction. The local creep parameter of the bentonite matrix is quite close to that of pure bentonite for a mixture with a sand mass fraction of 50%. However, it decreases with an additional increase in the sand mass fraction due to the increasing heterogeneity of the binary mixtures and the formation of clay bridges between adjacent sand inclusions. An equivalent local creep parameter is defined, and a new structure variable is introduced, which could be approximated by the structure variable responsible for the intergranular structure evolution. Finally, a creep model is formulated using mixture theory. The proposed model has five parameters: one structure parameter that incorporates the intergranular structure effect and four that are dependent on the intrinsic behavior of pure bentonite. Only two conventional oedometer tests need to be done for calibrating the parameters. The model prediction is then compared with experimental data, revealing a satisfactory performance of the proposed model.