Calibration of two filter papers at different temperatures and its application to GMZ bentonite

It is worth to study the hydro-mechanical behavior of unsaturated soils at different temperatures since non-isothermal conditions are inevitable in the practice of geotechnical engineering, e.g., unsaturated buffer/backfill materials in the nuclear waste disposal repository experience increasing temperature due to the dissipation of the radioactive energy. Suction is the key variable in predicting unsaturated soil’s hydro-mechanical responses, and the filter paper method is a simple, but effective method to measure the suction of unsaturated soils. This study presents the results of a series of tests examining the effect of temperature on the calibration curves of Whatman No. 42 and Double Circle No. 203 filter papers, and the application of these curves to the measurement of the total suction of compacted unsaturated bentonite at different temperatures. The calibration curves between the suction and water content for the two filter papers were determined at temperatures of 20, 40, 60 and 80 °C by the vapor equilibrium technique using fifteen types of saturated salt solutions. The test results show that calibration curves decreases with increasing temperature. The calibration equations for the filter papers are proposed at temperatures of 20, 40, 60 and 80 °C. The calibration curves were used to determine the soil–water characteristic curve (SWCC) of Gaomiaozi calcium bentonite, a possible buffer material for the deep geological disposal of high level radioactive waste in Chinese repositories, at these temperatures. It is found that the SWCCs of the bentonite are similar between the two filter papers, and decrease clearly with temperature, especially above 60 °C. Not only does the bentonite show a lower soil–water characteristic curve at higher temperature, but the volume of the soil sample shrinkage was also more significant.