Abstract: A typical expansive soil in Mengzi, Yunnan Province was selected for the free expansion rate test, and the influences of stirring times and grain size distribution of in the range of <0.5mm, 0.5～0.25mm, 0.25～0.075mm and <0.075mm were analyzed. The test results show that the number of stirring times has a certain influence on the test results, and the grain size distribution has a significant influence on the test results. In order to ensure the reliability of the free swelling ratio test results and enhance the comparability, it is recommended that the number of stirring times should be taken two times, and the grain size distribution should be controlled within the range of 0.5～0.25mm, so as to provide a reference for engineering testing staff. Finally, the mechanism of the swelling and shrinkage of the expansive soil is analyzed from micro and mesoscopic point of view. It is considered that the intrusive water molecules cause the expansion of crystal lattice and the thickening of the bound water film on the surface, which are the internal reasons for the swelling of expansive soil.

Key words: expansive soils, free swelling ratio tests, stirring times, grain size, expansion and shrinkage