Experimental accuracy of the initial specific volume

Many advanced soil models rely on the current state relative to normal and critical state lines to describe soil behavior. The position of these lines, therefore, requires an accurate estimation of the specific volume or void ratio. A series of one-dimensional compression tests was performed both on a coarse and a fine grained soil to investigate the experimental accuracy of the initial specific volume. This was obtained comparing independent calculations of the initial specific volume that were based on redundant measurements of height and weight of the specimen, both at the beginning and at the end of the test. The redundancy in the measurements was a key factor to obtain independent calculations. It was found that the excess water, such as may be stored in the filter papers, was the main cause of inaccuracy, when gross errors did not occur. Two novel confining rings having a closed-base were designed to reduce this effect. Although this was possible for the coarse grained soils tested, the fine grained soils retained more water due to the higher suction at the end of the test and water adsorption could not be avoided. The assumption of saturation is shown to be far from accurate, meaning that both the bulk unit weight and the water content should be measured independently to obtain a reliable measurement of the specific volume. The specific volume of the intact soil was found to be less accurate than when reconstituted. The experimental scatter was compared with the theoretical accuracy obtained from the error propagation theory. Good agreement was found between the theoretical and experimental accuracy. © ASTM Int'l.