Influence of X-Ray beam exposure on the development of gas bubbles during triaxial testing of sand using 3D synchrotron micro-computed tomography

The constitutive behavior of granular materials is highly influenced by the degree of saturation and whether a saturated or unsaturated framework is adopted to model the behavior of granular materials. Conventional axisymmetric triaxial compression (CTC) testing of saturated specimens tends to assume that the specimen remains saturated during shearing. X-ray computed tomography (CT) has allowed for 3D in-situ measurements beyond the global scale to investigate microscale and localized events such as fabric evolution. The literature reported several CT-coupled geotechnical experiments of saturated specimens; however, no study examined how X-ray exposure might affect the specimen, specifically the radiolysis of pore water. In this study, we present the synchrotron micro-computed tomography (SMT) results of experiments performed on sand specimens without shearing (acrylic tubes) and with shearing (CTC experiments) to assess the influence of X-ray exposure on the development of the gas phase of saturated sand. The observed phase changes were dependent on the initial pore water pressure and duration of exposure to the X-ray; the behavior of individual gas bubbles was dependent on the bubble’s surrounding sand grains and pore throat sizes leading to changes in the degree of saturation.