Evaluation and prediction of the swelling pressures of GMZ bentonites saturated with saline solution

Gaomiaozi (GMZ) bentonite has been chosen as a potential buffer/backfill material for deeply buried geological disposal system to isolate high-level radioactive waste (HLW) in China. The Beishan area located in Gansu Province has been identified as the most suitable area for a national repository for radioactive waste. Since the groundwater in this region contains the total dissolved solid (TDS) with as cation Na+ and anions Cl− and SO42 −, NaCl–Na2SO4 was chosen as a TDS in the solution saturating GMZ sodium and calcium bentonites (called as GMZ Na- and Ca-bentonites respectively). In this paper, one-dimensional constant-volume tests were carried out on GMZ bentonite samples saturated with saline solutions with different TDS concentrations. Based on the concept of the montmorillonite (Mt) void ratio, the swelling characteristics of GMZ bentonites under fully saturated conditions were studied in distilled water and the saline solution conditions. The test results showed that the relationships of the Mt void ratio versus the swelling pressure could be represented by a series of straight lines in the double logarithmic coordinator system and these lines (corresponding to different concentrations of TDS) were parallel to each other. The experiment results revealed that the swelling pressure on a logarithmic scale changed linearly with the TDS concentration. A simple new method with different decay factors was proposed to describe the variation in the Mt void ratio in different saline solutions under the same stress. In addition, a microscopic prediction model was presented. The swelling pressures predicted by the proposed macroscopic method showed a good agreement with those obtained by the microscopic model.