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.