掺砂率对膨润土与砂混合物膨胀特性的影响

Influence of sand mixing ratio on swelling characteristics of bentonite-sand mixtures

膨润土与砂混合物的膨胀特性是评估核废料深层地质处置工程长期性能的重要指标。对比不同膨润土及其与砂混合物的膨胀试验结果可知,对纯膨润土及其低掺砂率混合物,浸水膨胀完成后蒙脱石孔隙比e_m与竖向应力δ_v在双对数坐标内呈惟一的线性关系,利用该线性关系可预测浸水完成后不同竖向应力下的体积变化量及不同初始状态对应的膨胀力;对高掺砂率混合物,在砂骨架形成之前,e_m-δ_v线性关系成立,随着竖向应力的增大,砂骨架形成,对应的e_m值脱离e_m-δ_v线性关系,混合物中掺砂率越大,脱离该线性关系时的竖向应力就越小。砂骨架形成前,砂颗粒被蒙脱石包围,外力由蒙脱石承担,最终变形量由试样中单位体积蒙脱石的含量决定;砂骨架形成后,竖向应力最终由砂骨架和蒙脱石共同承担。利用砂骨架孔隙比的概念可确定各种不同掺砂率混合物形成砂骨架时的应力起偏点。同时,还可确定混合物能够形成砂骨架的掺砂率范围。

The swelling property of bentonite-sand mixtures is important to evaluate the long-term performance of deep geological repository of nuclear waste. Through the comparison of swelling test results of different kinds of bentonite-sand mixtures, we can conclude that, for pure bentonite and bentonite-sand mixtures with low sand mixing ratio, montmorillonite void ratio em and vertical stress δv relation is a unique line in their bilogarithmic coordinates, and we can predict the volumetric strain after water uptake under a given vertical stress and the swelling pressure at constant volume for mixtures with different sand mixing ratios; while, for the mixtures with high sand mixing ratio, the em-δv linear relation is satisfied only before sand skeleton is formed. When the stress is larger than deviation starting stress, the em-sv relation deviates from the line. Moreover, the larger the sand mixing ratio, the smaller is the deviation starting stress. Before sand skeleton comes into being, sand particles are surrounded by montmorillonite, hence the vertical stress is mainly burdened by montmorillonite, and the swelling quantity at full saturation including swelling deformation and swelling pressure is determined by the montmorillonite content per unit volume; after the sand skeleton is formed, the vertical stress is burdened by montmorillonite and sand skeleton. The deviation starting stress when sand skeleton is formed can be determined for mixtures with different sand mixing ratios according to the concept of sand skeleton void ratio, and the range of sand mixing ratio in which the sand skeleton may be formed can also be obtained.