Abstract: 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.

Key words: unsaturated soils, axis translation technique, matric suction, micropore, quartz sand, soil-water characteristic curves (SWCC)