石英砂掺量对混合型缓冲回填材料抗剪强度的控制机制

高放废物深部地质处置工程中,向膨润土中加入一定比例的石英砂可以优化缓冲回填材料的热传导性和可施工性。从"混合土"的概念出发,设计干密度和含水率相同、掺砂率不同的压实膨润土-砂混合物试样,通过剪切试验揭示掺砂率对抗剪强度的控制机制。试验材料选用内蒙古高庙子膨润土(GMZ001膨润土),石英砂按照质量0%～50%的比率添加。剪切试验结果表明:随着掺砂率的增大,压实混合物由应变软化型向应变硬化型过渡,黏聚力及内摩擦角逐渐减小,即抗剪强度降低。基于混合土孔隙结构假说,通过类比分析,对黏土-砂混合物抗剪强度的界限掺砂率进行估计,提供孔隙结构的扫描电镜证据。引入有效黏土密度和有效含水率的概念,描述石英砂颗粒之间的黏土基质的物理状态,阐释石英砂掺量对混合型缓冲回填材料抗剪强度的控制机制。

CONTROLLING MECHANISM OF QUARTZ SAND CONTENT ON SHEAR STRENGTH OF BENTONITE-SAND MIXTURES AS BUFFER/BACKFILL MATERIAL

A certain proportion of quartz sand is added to pure bentonite in deep geological disposal project of high-level radioactive wastes(HLW) to optimize the thermal conductivity and constructability of buffer/backfill materials. Proceeding from the concept of composite soil，the compacted bentonite-sand mixture specimens with the same dry density，water content and different sand ratios are designed to reveal the controlling mechanism of sand ratio to shear strength by shear test. Gaomiaozi bentonite from Inner Mongolia(GMZ001 bentonite)，mixes with quartz sand in a weight ratio of 0%–50%，is selected as test specimens. The shear test results indicate that，with an increase in sand ratio from 0% to 50%，the compacted mixtures yield a shear behavior from strain-softening to strain-hardening；and the cohesion and internal friction angle decrease correspondingly，which means that the shear strength decreases. Based on a pore structure hypothesis，according to analogy analysis，the bounded sand ratio governing the shear behavior of clay-sand mixture are estimated from the published data；and the supporting scanning electron microscope(SEM) pictures of pore structure are provided. By introducing the effective clay density and effective water content，the physical states of clay matrix among quartz sand particles is described，the controlling mechanism of quartz sand content to the shear strength of bentonite-sand mixtures as buffer/backfill material is reasonably explained.