In the recent decades, destruction of hydraulic structures founded on gypsiferous soils, has created serious problems for designers, contractors, and operators, specially, in the Middle East Countries.In this research it has been tried to investigate the behaviour of gypsiferous soils with increasing temperature and to evaluate the effects of drying temperature at (105 C). Another objective of this study was to evaluate the suitability of the current partice on grain size analysis methods in gypsiferous soils. Since the gypsiferous soils are widely spread in Iran, three sites namely Saveh, Isfahan and Shiraz have been chosen for investigations.The soil samples taken from the above three sites had (19-26%), (21-32%) and (28-34%) gypsum content respectively and are classified in different groups of SC, SM and ML, according to the Unified Soil Classification System. The electrical conductivity of the samples were 18.2, 15.3 and 3.05 (dS/m) respectively and all of the tests accomplished according to the ASTM standards. Results of experiments for determinations of water content, compaction and Atterberg limits tests showed that the drying temperature of (105 C) for measurement of true water content of gypsiferous soils is incorrect, since the possibility of liberation of ciystallized water from the gypsum composition is not considered. It was found that the water content of gypsiferous soils, must be determined by diying at the temperature of (60C) and adjusted for (105 C).Determination of gypsum content based on liberation of crystallized water under varying temperature can be used in geotechnical tests. Chemical methods for determination of gypsum content such as measurement of sO4 2 and precipitation with acetone have some limitations and takes longer time.Using coating of barium sulphate, in grain size analysis of gypsiferous soils, can prevent floculation and dissolution of gypsum particles in hydrometer test.
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