Evaluation of Mechanical Characteristics during Long Period of Artificial Geo-material under Deep Underground Condition

在高放射性废物的地下处置中，期望使用膨润土和砂的混合物作为缓冲材料，起到低渗透层的作用。混凝土筒仓周围的缓冲层和墙将在筒仓式储存库中通过压实膨润土和沙子混合物来建造。为了评价储库构件的结构稳定性，有必要对缓冲带的力学行为进行预测。然而，在本研究开始时，膨润土和砂混合物的力学行为并不明显。因此，开始研究确定膨润土和砂混合物的力学特性，以协助建立数值模型并确定数值分析的实验参数。对混合料进行了一系列三轴压缩试验。结果表明，膨润土和砂混合料的力学性能受水和膨润土含量的影响较大：饱和状态下的抗剪强度小于最佳含水量状态下的抗剪强度；抗剪强度随膨润土含量的增加而迅速降低。强度特性在很大程度上取决于围压。此外，还进行了吸力控制试验，研究了吸力对膨润土-砂混合料力学性能的影响。结果表明：(1)随着吸力的增大，混合料的刚度增大。(2)吸力加载时，体积应变和排水量增大。吸力的减少引起体积压缩（塌陷）。(3)混合料抗剪强度随吸力增大而增大。破坏包络表明，在保持一定摩擦角的情况下，随着吸力的增加，混合料的黏聚力增加。

In the underground disposal for relatively high radioactive wastes, bentonite and sand mixtures are expected to be used as a buffer material which acts as a low permeable layrs. Buffer layrs and walls surrounding concrete silo will be constructed by compacting bentonite and sand mixtures at the silo type repository. To assess the structural stability of repository component, it is necessary to predict the mechanical behavior of the buffer zone. However the mechanical behavior of the bentonite and sand mixtures was not apparenet at the beginning of this study. Thus studies to determine the mechanical properties of bentonite and sand mixtures were initiated to assist in developing a numerical model and in determining the experimental parameters for numerical analysis. A series of triaxial compression tests of the mixtures was performed. It was found that the mechanical properties of the bentonite and sand mixtures are strongly influenced by water and bentonite contents : shear strength in a saturated state is less than that in an optimum water content state ; shear strength decreases rapidly with increasing bentonite content. Strength properties are much dependent on the confining pressure. In addition, the suction controlled tests had been carried out to investigate the effect of suction on the mechanical behavior of bentonite-sand mixtures. The following results could be obtained : (1) The stiffness of the mixtures increases with increasing suction. (2) The volumetric strain and water drainage increase on suction loading. The reduction in suction induces a volumetric compression (collapse). (3) The shear strength of the mixtures increases with increasing suction. The failure envelopes show that the cohesion of the mixtures increases with increasing suction while maintaining a constant friction angle.