Deep geological repository is a favorable choice for the long-term disposal of nuclear wastes. Bentonite–sand mixtures have been proposed as the potential engineered barrier materials because of their suitable swelling properties and good ability to seal under hydrated repository conditions. To investigate the effects of sand grain size on the engineering performance of bentonite–sand mixtures, we prepare five types of bentonite–sand mixtures by mixing bentonite with sand of varying particle size ranges (0.075–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm and 2–5 mm, respectively). We carry out sequential oedometer tests under different simulated repository conditions, including constant vertical stress (CVS), constant stiffness (CS) and constant volume (CV) conditions. The microstructural heterogeneity and anisotropy of these soil mixtures are characterized through the quantitative analysis of micro-CT scanning results. Experimental results reveal that both sand grain size and boundary condition significantly influence the swelling of soil mixtures. Under three conditions, the temporal evolutions of swelling stress and strain follow similar trends that they increase faster at the beginning and gradually stabilize afterward. Comparing the ultimate values, swelling strains follow CVS > CS > CV, while swelling stresses follow CV > CS > CVS. Under CS boundary conditions, as the stiffness coefficient increases, the swelling pressure increases and the swelling strain decreases. CT results further indicate that mixtures with larger sand inclusions are more structurally heterogeneous and anisotropic, resulting in increased inter-particle friction and collision and a higher energy dissipation during the swelling process. Moreover, the non-uniform distribution of bentonite in local zones would be intensified, which plays an important role in compromising swelling behavior. Therefore, soil samples mixed with larger sand particles present a smaller swelling stress and strain values. This study may guide the choice of engineered barrier materials toward an improved design and assessment of geological repository facilities.

深层地质处置库是长期处置核废料的有利选择。膨润土-砂的混合物因其合适的溶胀性能和在水合储藏条件下的良好密封能力而被提议作为潜在的工程隔离材料。为了研究砂粒尺寸对膨润土-砂混合物的工程性能的影响，我们通过将膨润土与不同粒径范围（0.075-0.25 mm，0.25-0.5 mm，0.5-0.5mm的沙子）混合来制备五种膨润土-砂混合物。分别为1毫米，1-2毫米和2-5毫米）。我们在不同的模拟存储库条件下（包括恒定的垂直应力（CVS），恒定的刚度（CS）和恒定的体积（CV））进行连续的里程表测试。这些土壤混合物的微观结构异质性和各向异性通过对微CT扫描结果的定量分析来表征。实验结果表明，沙粒大小和边界条件都显着影响土壤混合物的溶胀。在三种条件下，膨胀应力和应变的时间演变遵循相似的趋势，即它们在开始时增加更快，而后逐渐稳定。比较极限值，溶胀应变遵循CVS> CS> CV，而溶胀应力遵循CV> CS> CVS。在CS边界条件下，随着刚度系数的增加，溶胀压力增加，溶胀应变减小。CT结果进一步表明，夹杂物较大的混合物在结构上更不均质且各向异性，导致在溶胀过程中增加了颗粒间的摩擦和碰撞以及更高的能量耗散。此外，将加剧膨润土在局部区域的不均匀分布，这在损害膨胀行为中起着重要作用。因此，与较大沙粒混合的土壤样品具有较小的溶胀应力和应变值。这项研究可以指导工程隔离材料的选择，以改进对地质处置库设施的设计和评估。与较大沙粒混合的土壤样品的溶胀应力和应变值较小。这项研究可以指导工程隔离材料的选择，以改进对地质处置库设施的设计和评估。与较大沙粒混合的土壤样品的溶胀应力和应变值较小。这项研究可以指导工程隔离材料的选择，以改进对地质处置库设施的设计和评估。