Abstract: To analyze the mechanical property changes of the black soil under dry-wet and freeze-thaw cycles, the fissure observation, volumetric strain measurements, direct shear tests and unconfined compression tests are performed on black soil samples under the two cycling modes. The results show that: (1) Black soils are characterized by the wet swelling and the dry shrinkage, the frost swelling and the melt shrinkage. After the first dry-wet cycle, the net volumetric strain is 12.5% and obvious cracks occur; After the first freeze-thaw cycle, the net volumetric strain is 0.45% without obvious cracks. (2) The internal friction angle of black soil samples decreases the most after the first dry-wet cycle, from 25.2° to 23.7°, and is basically stable after the third cycle. The internal friction angle decreases from 25.2° to 23.4° after the first freeze-thaw cycle and then fluctuates around 23.4°. In both cycling modes, the cohesion decreases with the increase of the number of cycles, and the decay of the cohesion is greater after the dry-wet cycles. (3) After the first dry-wet cycle, the unconfined compressive strength of the black soil samples decreases by 32.3%. After the first freeze-thaw cycle, the unconfined compressive strength deceases by 38.2%. The unconfined compressive strength of black soil samples is stabilized in the third cycle.

Key words: Black Soil, Dry-Wet Cycles, Freeze , Thaw Cycles, Shear Strength , Volumetric Strain