南水北调中线段原状膨胀土抗剪强度试验研究

中文摘要: 膨胀土的抗剪强度除了受含水率、干密度、正压力、干湿循环、裂隙等因素影响外，同时受到黏粒含量和塑性指数的影响。现有研究对于黏粒含量和塑性指数的考虑不足，不能系统、客观地反映某一个地区内膨胀土抗剪强度的规律性变化。以南水北调中线段某一个县的21种原状膨胀土为基础，作者研究了膨胀土中黏粒含量对其自由膨胀率、塑性指数的影响，并利用电动应变控制式剪切仪和ShearTrac Ⅱ反复直剪仪分别以0.8和0.02 mm/min的剪切速率开展了原状膨胀土的快剪（包括快剪、饱和快剪、饱和固结快剪）试验和残余剪试验，分析了黏粒含量、塑性指数对快剪强度摩擦角、残余强度摩擦角的影响。研究结果表明：自由膨胀率随黏粒含量的增加逐渐增大并趋于平缓；塑性指数随黏粒含量的增加逐渐增大，当黏粒含量增大到33%时，塑性指数开始趋于平缓；试样抽气饱和后，强度明显降低，表现为饱和快剪强度摩擦角小于快剪摩擦角，固结对试样起到了“治愈”作用，强度明显提高，表现为饱和固结快剪摩擦角高于饱和快剪摩擦角；随着黏粒含量、塑性指数的增加，快剪摩擦角、饱和快剪摩擦角、饱和固结快剪摩擦角均随之减小，当黏粒含量、塑性指数达到临界值（临界值分别为32%和24%）后，摩擦角变化趋于平缓；残余强度摩擦角随黏粒含量的增加逐渐减小并趋于平缓，但是随着塑性指数的增加，残余强度摩擦角逐渐减小后是否趋于平缓尚有待进一步的研究。
Abstract:The shear strength of expansive soil is not only influenced by the water content, dry density, vertical stress, wetting-drying cycle, crack and other factors, but also influenced by the clay content and plasticity index. However, the existing studies rarely considered the clay content and plasticity index, and thus could not systematically and objectively reflect the regularity of shear strength of expansive soil in a certain area. In order to study the effect of clay content on the free expansion ratio and plasticity index, the effect of clay content and plasticity index on the quick shear strength friction angle (including quick shear, saturated quick shear, saturated and consolidated quick shear) and residual strength friction angle of 21 expansive soils at a county of Middle Route of South-to-North Water Diversion Project, shear tests were conducted on undisturbed soil using electric strain controlled direct shear apparatus at 0.8 mm/min and ShearTrac Ⅱ reversal direct shear apparatus at 0.02 mm/min. It was founded that the free expansion ratio gradually increases with increase of clay content and then changes gently. The plasticity index exhibits a logarithmic growth relation with clay content and then changes gently as clay content increased to 33%. The shear strength of samples significantly reduces after saturating, ie. The friction angle of saturated quick shear is less than the quick shear friction angle. There is a "cure" effect of consolidation on the samples, the strength is obviously improved, ie. The friction angle of saturated and consolidated quick shear is larger than the saturated quick shear friction angle. The friction angle of quick shear, saturated quick shear, saturated and consolidated quick shear decreases with increase of clay content and plasticity index, and then changes gently with critical value about 32% and 24%,respectively. The friction angle of residual strength decreases with increase of clay content and then changes