时域反射法测量膨胀土含水量试验研究

 通过对探管式TDR仪现场测量膨胀土含水量误差来源的分析，认为膨胀土的黏性以及土体收缩时产生的土与管壁之间的间隙是主要影响因素。在此基础上设计了3种类型的室内试验进行定量分析。试验结果表明：由于膨胀土的黏性大，土中结合水含量高、介电常数小，因此TDR仪测得的体积含水量q 比烘干法得到的qw低，但两者之间具有良好的对应关系，可通过线性拟合获得满意的标定效果；由土体收缩产生的间隙分为环管壁间隙和径向间隙两种，环管壁间隙对含水量测量影响较大，只要存在1 mm的间隙，可导致低估40%的体积含水量，且误差随含水量的增加而增大，而径向间隙对测量结果影响较小，在试验尺寸范围内，最大误差不超过5%，产生这种结果的原因是TDR探头测量敏感度随测量距离增大而迅速衰减。最后通过现场实测数据对试验结果进行验证并提出改进措施。

EXPERIMENTAL RESEARCH ON MEASURING WATER CONTENT OF EXPANSIVE SOILS BY TIME DOMAIN REFLECTOMETRY

Time domain reflectometry(TDR) is increasingly used to estimate soil water content in field and laboratory tests. Errors arising from estimation of soil water content in field are analyzed and their sources are summarized as adhesiveness of expansive soils and gaps between tube and expansive soils. Based on these，three types of laboratory test are designed to quantitatively analyze the corresponding sources of the errors. It is shown that with high adhesiveness of expansive soils，the amount of bound water is large and dielectric constant of soil is small. The measured values of volumetric water content by TDR are smaller than those by oven dry，but there is a corresponding relationship between volumetric water content measured by TDR and that calculated from mass water content. Thus a linear regression equation is proposed to represent the calibration curve. Gaps generated from shrinkage of expansive soils can be divided into gaps around the tube and radial gaps. Errors dramatically increase as gaps around the tube exist. An air gap around the tube about 1 mm will result in an underestimation of 40% of soil water content，and the errors increase with the increase of soil water content. However，there is no obvious influence caused by radial gaps，which only results in errors less than 5%. The reason is that the measurement sensitivity of TDR probe rapidly decreases with the increase of measuring distance. Finally，the data from field monitoring are applied to examine the test results，and measures to reduce errors are put forward.