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Dispersive soils are clayey soils, which erode easily under concentrated seepage. Specific characteristic of these soils is existence of high amounts of sodium
ions in their pore water. Identification of dispersive soils is impossible through ordinary soil mechani_s tests and for this purpose, conducting four specific tests namely Crumb, Double hydrometer, Pinhole
and Chemical tests is suggested by most of
researchers and test codes. Sherard's chemical criterion is one of the commonly used criteria for identifying dispersive soils. For using this criterion, it is suggested by Sherard to extract pore water from the soil close to its liquid limit. However, through former studies on dispersive soils encountered in different regions if Iran, we found that in most cases, pore water extraction around liquid limit is not possible using ordinary S.M. laboratory equipment and requires special extraction device. In
this study, a specific device was designed for pore water extraction in low moistures around liquid limit. Then, to determine the effect of extraction moisture content on the evaluation of dispersivity according to Sherard's chemical criteria, necessary identifying tests were conducted on eight soil samples, which were collected from different parts of Iran. The results of these tests showed that by variation of extraction water content, the amount of Sodium, Potassium, Calcium and Magnesium ions and also the ratio of sodium to the total dissolved salts may change notably. In other words, using Sherard's criterion disregard
to his suggestion on extraction water content can lead to inaccurate or even incorrect results.