Abstract: To investigate the expansion and shrinkage characteristics of the Lushi expansion rock, experimental study on the expansive rock expansion and contraction characteristics under dry-wet cycle was carried out. After experiencing dry and wet cycles, the expansive rock was analyzed by scanning electron microscopy (SEM) and nitrogen adsorption experiment (NA). The phenomenon of swelling and water shrinkage of expansive rock was analyzed from the microscopic point of view, and the reason for the change of expansion and contraction characteristics was explained. The results show that the expansion rate of expansive rock increases with the increase of dry and wet circulation, and the absolute expansion rate increases by 25%. The contraction curve shows an evident contraction inflection point, which usually occurs at 20% of the total shrinkage time. At this time, the water loss state of the expansion rock changes from the free water loss to the combined water loss. Cracks appear during the first expansion and contraction of the expansive rock, which are transfixion. In the later expansion and contraction process, the cracks appeared shallower gradually stabilized with the increase of dry and wet circulation. After the number of cycles reached 6-8, the expansion and shrinkage rate of Lushi expansion rock reached a stable value, with the absolute expansion rate stable at 17% and the absolute contraction rate stable at 9%. As the number of dry and wet cycles increased, the aggregation of clay particles in the microstructure of expansive rock changed from compact state to loosen. In addition, the pore characteristics of the sample show that total pore volume gradually increases, the pore diameter gradually decreases, and the specific surface area gradually increases with the increase of the number of dry and wet cycles.

Key words: swelling rock, wetting-drying cycle, swell-shrink characteristic, microstructure, pore distribution