Estimating dry density of bentonite blocks during swelling for geological disposal tunnel closure using actively heated fiber-optic distributed temperature sensing: laboratory validation

Actively heated fiber-optic distributed temperature sensing (a-DTS) is a method that uses heatable fiber-optic cables to measure temperature distributions. In this study, the a-DTS method was applied to monitor the swelling of bentonite blocks and validate its effectiveness. Thermal conductivity was calculated from measured temperature changes, and dry density was estimated from the obtained thermal conductivity. A laboratory experiment was conducted using a cylindrical experimental setup to monitor the swelling of blocks made of two types of bentonite-sand mixtures prepared from Kunigel V1 and silica sand. Dry densities were estimated from measured thermal conductivities using calibration data separately obtained at low dry densities and compared with the values obtained by dismantling and sampling 585 days after the start of water injection. Thermal conductivity measurements and visual observations identified three phases of swelling: the Primary Sealing Phase, in which thermal conductivity decreased as the bentonite-sand mixtures swelled and displaced water; the Secondary Sealing Phase, in which thermal conductivity increased without significant visual changes; and the Slow Phase, in which no significant changes were observed in either thermal conductivity or visual appearance. However, quantitative estimation of dry densities with sufficient accuracy remains difficult. The assumption of using calibration data obtained from saturated specimens was considered reasonable because the injected water volume was sufficient to achieve saturation and the degree of saturation of the dismantled samples was approximately 100%.