Swelling characteristics of compacted GMZ bentonite–sand mixtures as a buffer/backfill material in China

Abstract A mixture of bentonite and sand has been proposed for use as a buffer/backfill material in the disposal of high-level radioactive waste in many countries. In China, it is necessary to quantify the influence of the sand content ratio and the initial dry density on the swelling characteristics of GMZ bentonite–sand mixtures, and to collect test data useful in the design of buffer/backfill materials. To accomplish these goals, this study conducted laboratory tests on the swelling pressure and swelling deformation of GMZ bentonite–sand mixtures consisting of different sand contents of 0, 10, 20, 30, 40 and 50%. The results indicate that swelling occurs in three distinct phases: inter-void swelling, primary swelling and secondary swelling, where both swelling pressure and swelling strain follow a sigmoid relationship with time. With constant initial water content, the maximum swelling pressure presents an exponential increase with increased initial dry density, and the maximum swelling strain increases linearly. As the sand content ratio increases, the maximum swelling pressure decreases exponentially while the maximum swelling strain follows a quadratic decrease. In the introduction of the concept, the effective clay density (ρB), an expression which includes both initial dry density and sand content ratio was derived. New equations for evaluating the relationship between effective clay densities and swelling characteristics are proposed. Furthermore, a prediction method for the swelling characteristics of compacted GMZ bentonite–sand mixtures is presented by combining the expression of effective clay density and the new equations. The applicability of this method was investigated by comparing the fitted curves with laboratory test results of the swelling pressure and swelling deformation of compacted GMZ bentonite–sand mixtures. Experimental data has shown satisfactory results, so these equations can predict the swelling characteristics of GMZ bentonite–sand mixtures with various sand content ratios and initial dry densities within a certain numeric area.