Abstract: Shrinkage is inherent to expansive soil, usually resulting in slope and foundation cracking, but there is little understanding about the influence of structure on shrinkage characteristics. Using an automatic shrinkage test device, comparison tests on shrinkage and scanning electron microscope (SEM) tests for undisturbed and remolded expansive soil were carried out under the same humidity and constant temperature. The results show that compared with undisturbed soil, remolded soil has smaller evaporation rate in the water flow stage, slower shrinkage stability rate in the vapor evaporation stage and larger volume shrinkage strain in the end. For the remolded soil, the linear section of the curve of volume shrinkage versus water content is longer, the slope larger, and the transition between the linear section and the stable section not obvious, while the undisturbed soil is on the contrary. The soil shrinkage characteristic curves (SSC) of remolded and undisturbed soil basically coincide in the section at higher water content; with the decrease of water content, the SSC of remolded soil decreases faster and the corresponding water content range is wider, and when entering the residual-zero shrinkage stage, the void ratio is obviously smaller. Chertkov shrinkage model is suitable for undisturbed expansive soil, but not for remolded one. SEM test results show that the undisturbed expansive soil has stronger primary structure than remolded expansive soil. Under the same initial density and humidity, there exists obvious differences in microstructure, such as particle arrangement, contact mode, cementation state, pore size and distribution characteristics between the undisturbed and remolded expansive soil, resulting in smaller water migration rate and larger matrix suction of remolded soil during evaporation, which is the internal reason why remolded soil shrinks more violently than undisturbed soil. The research results can provide a reference basis for the design of slope engineering protection for expansive soil slopes.

Key words: expansive soil, shrinkage, microstructure, pore, SEM