轴平移技术在基质吸力测控中的局限性和误差分析

为了揭示非饱和土在自然环境和轴平移环境两种条件下，基质吸力测试数据存在差异的原因，分别研究了表面张力系数和难充水微孔隙在相同吸力作用下对含水率的影响。一方面，表面张力系数随压力的增大而小幅减小，同一基质吸力条件下采用轴平移技术时对应的含水率较自然状态有偏小的趋势；另一方面，由于一端封闭微孔隙的存在，较高的孔隙气压力必然促使土中水进入部分难充水微孔隙，从而同一基质吸力条件下轴平移技术对应的含水率较自然状态又有升高的趋势。因此，在特定基质吸力条件下，轴平移方法得到的含水率较自然状态偏大还是偏小取决于上述两方面的综合效应。毛细上升试验和针对一端封闭微孔隙模型的研究表明，一端封闭微孔隙的存在对土-水特征曲线的影响远远大于表面张力系数变化的影响。针对石英砂、砂土和黏土的两种SWCC测试结果表明，随土颗粒逐渐变细，轴平移技术对SWCC的影响越来越大。从而进一步印证，张力计法和轴平移方法在测试黏土土-水特征曲线方面存在差异的原因在于一端封闭微孔隙的存在。

Limitation and error analysis of axis translation technique for measuring and controlling matric suction

To explain the differences between matric suction measurements under natural environment and axis translation conditions, the influences of surface tension coefficient and micropores, which are hard to be saturated on water content under the same suction, are studied. It is found that while the axis translation technique can depress water content since the surface tension coefficient decreases slightly with pressure increase, it increases the water content due to the existence of closed micropores which can hold some amount of water at high pressure. As a result, the water content in axis translation conditions may be larger or smaller than that in natural environments at certain matric suction depending on the competition of the above two mechanisms. The capillary rise test and the study of one-end closed pore model show that the influence of the closed micropores on the soil-water characteristic curve (SWCC) is far greater than that of the surface tension coefficient. Based on the experimental results of quartz sand, sand and clay, it is shown that as the soil particles become finer, the effect of axis translation technique on SWCC becomes more and more obvious. Hence, it is clear that the difference of SWCC, measured by axis translation technique, is owing to the micropore closed at one end.