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

在较高放射性废物的地下处理中，膨润土和砂的混合物有望作为缓冲材料，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类型的储存库中通过压实膨润土和沙子混合物来建造。为了评估储存库构件的结构稳定性，有必要对缓冲区的力学行为进行预测。然而，在本研究开始时，膨润土和砂的混合物的力学行为并不明显。因此，开始了确定膨润土和砂混合物的力学性能的研究，以帮助开发数值模型和确定用于数值分析的实验参数。进行了一系列混合料的三轴压缩试验。结果表明，水和膨润土含量对膨润土/砂混合物的力学性能影响较大：饱和状态下的剪切强度小于最佳含水率状态下的剪切强度；随着膨润土含量的增加，剪切强度迅速下降。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-砂土混合物力学行为的影响。研究结果表明：(1)混合料的刚性随吸力的增大而增大。(2)吸力作用下，土体应变增大，排水量增大。吸力的减少会导致体积压缩(塌陷)。(3)混合料的抗剪强度随吸力的增大而增大。破坏包络图表明，在保持摩擦角不变的情况下，混合物的内聚力随吸力的增大而增大。

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.